Die Basics
By Art Hedrick,
When I conduct conferences, it isn't unusual
to have one or two attendees who are new to
the stamping die and pressworking world.
Some are young new hires trying to learn
about stamping, and others are individuals
who have been transferred from a different
department and thrown to the wolves in the
stamping department.
This article is the first in a series intended to
introduce beginner toolmakers, die
maintenance technicians, engineers, and press
technicians to stamping. The series will define
Figure 1
a die as well as a stamping operation. It will
also discuss cutting and forming operations, components and functions, and different methods
used to stamp parts.
What Is a Stamping Die?
A stamping die is a special, one-of-a-kind precision tool that cuts and forms sheet metal into a
desired shape or profile. The die's cutting and forming sections typically are made from
special types of hardenable steel called tool steel. Dies also can contain cutting and forming
sections made from carbide or various other hard, wear-resistant materials.
Stamping is a cold-forming operation, which means that no heat is introduced into the die or
the sheet material intentionally. However, because heat is generated from friction during the
cutting and forming process, stamped parts
often exit the dies very hot..
Dies range in size from those used to make
microelectronics, which can fit in the palm
of your hand, to those that are 20 ft. square
and 10 ft. thick that are used to make entire
automobile body sides.
The part a stamping operation produces is
Figure 2
called a piece part (see Figure 1). Certain
Typical Cut Edge of a Stamped Part
dies can make more than one piece part per
cycle and can cycle as fast as 1,500 cycles (strokes) per minute. Force from a press enables
the die to perform.
How Many Die Types Exist?
There are many kinds of stamping dies, all of which perform two basic operations cutting,
forming, or both. Manually or robotically loaded dies are referred to as line dies. Progressive
and transfer dies are fully automated.
Cutting
Cutting is perhaps the most common operation
performed in a stamping die. The metal is severed
by placing it between two bypassing tool steel
sections that have a small gap between them. This
gap, or distance, is called the cutting clearance.
Cutting clearances change with respect to the type
Figure 3
of cutting operation being performed, the metal's
Trimming
properties, and the desired edge condition of the
piece part. The cutting clearance often is expressed as a percentage of the metal's thickness.
The most common cutting clearance used is about 10 percent of the metal's thickness.
Very high force is needed to cut metal. The process often introduces substantial shock to the
die and press. In most cutting operations, the metal is stressed to the point of failure, which
produces a cut edge with a shiny portion referred to as the cut band, or shear, and a portion
called the fracture zone, or break line (see
Figure 2).
There are many different cutting
operations, each with a special purpose.
Some common operations are:
Trimming The outer perimeter of the
Figure 4
formed part or flat sheet metal is cut away
Notching
to give the piece part the desired profile.
The excess material usually is discarded as scrap (see Figure 3).
Notching Usually associated with progressive dies, notching is a process in which a cutting
operation is performed progressively on the outside of a sheet metal strip to create a given
strip profile (see Figure 4).
Blanking A dual-purpose cutting operation usually performed on a larger scale, blanking is
used in operations in which the slug is saved for further pressworking. It also is used to cut
finished piece parts free from the sheet metal. The profiled sheet metal slug removed from the
sheet by this process is called the blank, or starting piece of sheet metal that will be cut or
formed later (see Figure 5).
Piercing Often called perforating, piercing is a metal cutting operation that produces a
round, square, or special-shaped hole in flat sheet metal or a formed part. The main difference
between piercing and blanking is that in blanking, the slug is used, and in piercing the slug is
discarded as scrap. The cutting punch that produces the hole is called the pierce punch, and
the hole the punch enters is called the
matrix (see Figure 6).
Lancing In lancing, the metal is sliced or
slit in an effort to free up metal without
separating it from the strip. Lancing often
is done in progressive dies to create a part
carrier called a flex or stretch web (see
Figure 7).
Shearing Shearing slices or cuts the
metal along a straight line. This method
commonly is used to produce rectangular and square blanks (see Figure 8).
All forming operations deform sheet
material by exposing it to tension,
compression, or both. Most part
defects, such as splits and wrinkles,
occur in forming operations.
Successful sheet metal forming relies
heavily on the metal's mechanical
properties. The metal being formed
must have the ability to stretch and
compress within given limits. It also
must be strong enough to satisfy the
Figure 1
part's fit and function. This balance
Embossing
between formability and strength
often is hard to achieve.
Most forming operations involve at least two basic components: a punch, representing the
male portion of the die, and the cavity, representing the female portion.
Common Forming Die Types
Although many die types exist, this article focuses on those used in the most common forming
operations.
Embossing Dies
Embossing dies use tension to stretch metal into a shallow depression. The dies primarily are
comprised of a punch and a cavity. The metal's thickness and mechanical properties, along
with the forming punch geometry, determine the depth that can be achieved (see Figure 1).
Solid Form/Dead Hit Dies
Solid form/dead hit dies also called crash forming dies deform the metal using only a
punch and cavity. These dies do not control metal flow and cannot prevent the metal from
wrinkling or buckling. They are used to form simple parts, such as brackets and braces, made
from thick, stiff metals that are more wrinkle resistant than thinner metals. Because this
operation also uses tension to form the part, attempting to solid-form difficult part geometries
using thin metal often results in severe failure (see
Figure 2).
Coining Dies
Coining dies create the part's shape by squeezing the
metal under extreme pressure. Coining also can
reduce the metal thickness. Coins (metal currency)
are created with the coining process. A simple round
metal slug is placed into the die and forced to flow
into a given shape by compressing it (see Figure 3).
Figure 4
Simple Bending
Restrike Dies
The restrike die operation fundamentally is a solid forming operation. The main difference is
that a restrike die is used after most of the major forming already has been performed. The
restrike die's function is to finish forming features that could not be obtained in a previous
operation. Restrike dies add details such as sharp radii and small embosses. They also help
compensate for springback that occurred during the initial forming.
A restrike die operation often follows a drawing or
trimming operation. These dies, also referred to as
qualifying dies, usually use tension to re-form the
part; however, compression also can be used.
Bending Dies
Bending can be defined simply as a forming operation
in which the metal is deformed along a straight axis.
Items such as tabs and channels are created using the
bending process. Achieving the correct bend angle in
a bending operation can be very difficult.
Among the various bending methods are wipe
bending, V bending, and rotary bending. All three are
very popular, and each has its advantages and
disadvantages. Both compression and tension occur
Figure 5
during bending. Compression occurs on the inside
Bending
radius, while tension occurs on the outside radius.
Figure 4 shows the compression and tension. Figure 5 shows the three basic bending types.
Flanging Dies
Flanging is bending metal along a curved axis. Two basic types of flanges are tension, or
stretch, flanges, and compression, or shrink, flanges. Tension flanges are susceptible to
splitting, and shrink flanges are susceptible to wrinkling.
Flanges are created using a flanging die that wipes the metal
between a punch and a lower die section. Both tension and
compression occur during the flanging process (see Figure
6).
Drawing Dies
Drawing dies are the most impressive forming dies. Oil pans,
automobile doors and fenders, cookware, and door knobs are
Figure 6
just a few parts manufactured by drawing.
Flanging
Draw dies create the part shape by controlling metal flow into a cavity and over the forming
punch. Draw dies utilize a special pressure-loaded plate or ring called a draw pad or blank
holder to control the metal's flow into the cavity. This plate prevents the metal from wrinkling
as it flows into the cavity. Increasing or decreasing the pressure exerted under the pad also
controls how much metal feeds into the die. Although compression can occur when the metal
is drawn, drawing uses mostly tension to obtain the part geometry (see Figure 7).
Ironing Dies
Ironing dies are similar to coining dies
in that they deform the metal with
compression. However, unlike
conventional coining, ironing squeezes
metal along a vertical wall. This highly
compressive process unifies a wall's
thickness and increases the drawn
vessel's length. Items such as beverage
and soup cans are made using an
Figure 7
ironing process. Ironing allows an
Drawing
aluminum can's wall thickness to be
reduced to as little as 0.002 in. (see Figure 8).
Extruding Dies
In extruding, the metal is flanged around the perimeter of a prepierced hole. Like during
stretch flanging, the metal is susceptible to splitting during forming. Extrusions also are
referred to as hole expansions or continuous stretch flanges. Often extrusions are tapped for
holding fasteners used in the part assembly process (see Figure 9).
Among the many factors to consider when
choosing a production method are the production
speeds necessary to produce the required quantity
within a given time frame; the material
consumption needed for each part; the production
method cost; preventive maintenance
requirements; equipment availability; and the part
shape, size, and geometric tolerance specified.
Line Dies
Line dies are tools that typically are hand or
Figure 1
robotically loaded. Often each station that forms
Tandem Line Presses
or cuts the sheet metal represents a single
Photo courtesy of APT.
operation die. Hand- (human-) loaded line dies
usually lend themselves to low-production parts or those that are too big and bulky to handle
with automation. Several line dies usually can be placed within a single press. This allows the
operator to transfer the parts from die to die to with a minimal travel distance.
Larger line dies often are placed in individual presses close together in a line, an arrangement
referred to as tandem line presses (Figure 1).
Some line die advantages are:
1. They often cost less than more complicated dies.
2. They can be timed to run together in a common press.
3. The operation's simplicity allows the part to be turned over or rotated in any axis by
the operator or robot if necessary. This often allows for more complex geometries to
be created.
4. Smaller individual tools are lighter and can be handled with lower-cost die handling
equipment.
5. Maintaining a single station does not require removing all the dies.
Common line die disadvantages are:
1. They often cannot compete with production speeds achievable with other methods,
such as progressive dies.
2. They require expensive robots or human labor.
3. They often require several presses to manufacture a single part.
Transfer Dies
Transfer dies are special line dies that are timed together and properly spaced an even distance
apart in a single press. The distance between each die is referred to as the pitch, or the
distance the part must travel between stations.
Unlike with conventional line dies, the piece
parts are transferred by special traveling rails
mounted within the press boundaries. These rails
most commonly are mounted on each side of the
dies. During the press cycle, each rail travels
inward, grabs the part with special fingers, and
then transfers it to the next die.
Transfer systems can perform numerous motions.
However, the two basic types are 2-D (two-axis)
and 3-D (three-axis). Two-axis transfers move
inward, grip the part, and slide it forward to the
Figure 2
next station. Three-axis transfers move in, grip
Transfer Rails
the part, pick it up vertically, move it to the next
station, and lower it down onto the die. This third-axis movement allows the part to be placed
within the perimeter gauging boundaries. Transfer systems are popular for manufacturing
axial-symmetrical (round), very deep-drawn parts (Figure 2).
Some transfer system advantages are:
1. Large parts can be handled at fairly rapid speeds.
2. Stamped parts can be turned over or rotated during the transfer process.
3. Servodrive-type transfers can be programmed to accommodate a large variety of parts,
press speeds, and stroke lengths.
4. Transfer dies do not tie each part together, often allowing for material savings.
5. Large volumes of parts can be produced in a fairly short time frame.
Some transfer system disadvantages are:
1. They often are quite costly.
2. They often require sophisticated electronics and mechanical finger motion to function
properly.
3. They require more die protection sensors.
4. They require a blank destacking
system.
Progressive Dies
The progressive die is one of the most
common, fastest methods available for
producing piece parts. Unlike line or transfer
dies, progressive dies tie the parts together by
a portion of the original strip or coil, which is
called a strip carrier. Different types of parts
require different carrier designs.
Figure 3
Progressive Die Strips
Progressive dies can produce as few as seven
Sample strips courtesy of SURE Tool.
or eight parts per minute or as many as 1,500
parts per minute. Unlike transfer or line dies,
all necessary stations are mounted on a single common die set. These stations are timed and
sequenced so that the piece part can be fed ahead a constant given distance called the
progression or pitch. Many parts can be tied together allowing many parts to be made with
each single press stroke.
Progressive dies most commonly are coil-fed, and if they contain the proper sensing system,
they often can run unattended. It is not uncommon for a single press operator to run two or
three progressive dies. The coil material typically is pushed through the die; however, systems
that can pull and push the coil material through the die are available. Progressive dies usually
require the use of a coil feeder and stock
straightener (Figures 3 and 4).
Progressive die advantages are:
1. They can produce a great volume of
parts very quickly.
2. They often can run unattended.
3. They require only one press.
Progressive die disadvantages are:
Figure 4
Progressive Die and Strips
1. They usually cost more than line or
transfer dies.
2. They often require precision alignment and setup procedures.
3. They require a coil feeder system.
4. They require an open-ended press to allow for the metal to feed into the die.
5. Damage to a single station requires removing the entire die set.
6. They often are much heavier than single-station line dies.
The production method you choose depends on many factors. Carefully consider items such
as the required volume of parts, your labor rates, and your existing equipment before choosing
a production method for your stamped parts.
While many specialty components can be used in manufacturing dies, most dies contain
certain common components.
Die Plates, Shoes, and Die Sets
Die plates, shoes, and die sets are
steel or aluminum plates that
correspond to the size of the die.
They serve as the foundation for
mounting the working die
components. These parts must be
machined so that they are parallel
and flat within a critical tolerance.
The machining methods are milling
and grinding. Although grinding is
the most popular, a milled surface
now can be obtained that is as
accurate as a ground surface.
Most die shoes are made from steel.
Figure 1
Aluminum also is a popular die shoe
Various die set types
material. Aluminum is one-third the
weight of steel, it can be machined
very quickly, and special alloys can be added to it to give it greater compressive strength than
low-carbon steel. Aluminum also is a great metal for shock adsorption, which makes it a good
choice for blanking dies.
The upper and lower die shoes assembled together with guide pins create the die set. The
lower die shoe often has machined or flame-cut holes that allow slugs and scrap created in the
die to fall freely through the die shoe onto the press bed. The holes also may serve as
clearances for gas springs and other die components.
The die shoe thickness is based on how much force can be expected during cutting and
forming. For example, a coining die, one that compresses metal by squeezing it between an
upper and lower die section, requires a much thicker die shoe than a simple bending die
(Figure 1).
Guide Pins and Bushings
Guide pins, sometimes referred to as guide posts or pillars, function together with guide
bushings to align both the upper and lower die shoes precisely. They are precision-ground
components, often manufactured within 0.0001 in. Although numerous specialty mounting
methods can be used to install these components, there are only two basic types of guide pins
and bushings friction pins and ball bearing-style pins.
Friction pins are precision-ground pins that are slightly smaller than the guide bushing's inside
diameter. Pins are made from hardened tool steel, while bushings often are made from or
lined with a special wear-resistant material called aluminum-bronze. The aluminum-bronze
may contain graphite plugs that help to reduce friction and wear that occur to the pins and
bushings.
Friction pins also help to heel the die shoes and prevent them from moving from side to side.
Figure 2
CVarious guide pins and bushings
Precision or ball bearing-style guide pins comprise precision-hardened pins, ball cages, ball
bearings, and bushings. Unlike friction pins, these pins ride on a series of ball bearings
contained in a special aluminum ball cage that permits the bearings to rotate without falling
out. These pins have several advantages. First, friction is reduced so the die can run at faster
speeds without generating excessive friction and heat. Second, they allow the diemaker to
separate the upper and lower die shoes easily. Third, because they use ball bearings, they can
be manufactured with greater accuracy than friction pins (Figure 2).
Remember, guide pins are meant to align the upper and lower die shoes, not to align a poorly
maintained or sloppy ram in a press! Some companies try to compensate for a poorly
maintained press by adding oversized guide pins or grinding the guide pin ends to a cone
shape. Care must be taken when flipping die shoes over so that the guide pins are not bent.
Heel Blocks and Heel Plates
Heel blocks are special steel blocks that
are precision-machined, screwed, doweled,
and often welded to both the upper and
lower die shoes. They contain components
called wear plates and function to adsorb
any side thrust that may be generated
during the cutting and forming process.
They are especially important if the
generated force is one-directional. Too
much force generated from one direction
only can cause the guide pins to deflect,
which results in misalignment of critical
cutting and forming components.
Figure 3
Most heel blocks have steel heel plates,
Heel blocks
and the heel block on the opposite shoe has
a wear plate made from aluminum bronze or some other dissimilar metal. The plate selection
process is critical. Using two opposing plates made of the same metal type can result in high
friction, heat, and eventually galling or cold welding of the wear plates.
Heel blocks can be used to heel the die in any or all directions. Box heels often are used to
heel the die in all directions (Figure 3).
Screws, Dowels, and Keys
Screws fasten and secure the working components to both the upper- and lower-die shoes.
The socket head cap screw is the most popular fastener used in stamping dies. This hardened
tool steel screw, often referred to as an Allen head screw, offers superior holding power and
strength.
Dowels are hardened, precision-ground
pins that precisely locate the die section or
component in its proper location on the die
shoe. Although dowels have much heeling
ability, their main function is to locate the
die section properly.
Keys are small, rectangular blocks of
precision-ground steel that are inserted into
a milled pocket in the die shoes and
sections called keyways. Keys locate and
Figure 4
heel die sections and components (Figure
Keys, dowels and screws
4).
While these are the most common, other components can be used in manufacturing stamping
dies.
Many specialty components can be used in dies, but the most commonly used are die plates,
shoes, die sets, guide pins, bushings, heel blocks, heel plates, screws, dowels, and keys all
of which were already explained. This article focuses on other common components pads,
retainers, and springs.
Pads
A pad is simply a pressure-loaded plate,
either flat or contoured, that holds,
controls, or strips the metal during the
cutting and forming process. Several types
of pads are used in stamping dies.
Depending on their function, pads can be
made from soft low-carbon steel or
hardened tool steel. Contoured pads must
fit very closely to the mating die section.
Precision requirements determine whether
Figure 1
the pads are positioned with guide pins and
bushings or left unguided.
Stripper Pads/ Plates. Stripper pads are flat or contoured, spring-loaded plates that pull, or
strip, the metal off the cutting punches. When it's cut, metal naturally tends to collapse around
the body or shank of the cutting punches; this is especially true during piercing. The stripper
pad surrounds the cutting punches and mounts to the upper die shoe. As the punch exits the
lower die, the spring-loaded pad holds the metal down flush with the lower die section, which
allows the cutting punches to withdraw from the sheet metal or piece part.
Often stripper pads are inserted with a small block of steel called a pad window. This pad
window usually is small and lightweight and can be removed easily to allow the die
maintenance technician to remove the ball lock-style pierce punch from the retainer without
removing the entire stripper pad. Stripper pads also function to hold the metal flat or to the
desired shape during the cutting process (Figure 1).
Pressure Pads/ Plates. During the wipe bending process, the metal must be held down tightly
to the lower die section before the forming punch contacts the metal. Pressure pads must
apply a force that is at least equivalent to the bending force. Most pressure pads use high-
pressure coil or gas springs (Figure 1). When loaded with very high-pressure springs,
contoured or flat pads also can form sheet metal. These pad types often are referred to as
power punches (Figure 2).
Figure 2 Figure 3
Draw Pads. Draw pads control metal flow during the drawing process. In drawing, the
amount of pressure, or downward force, exerted on the sheet metal determines how much
metal is allowed to flow and enter the draw die cavity. Too much pressure may stop the metal
from flowing and cause splitting; too little downward force may allow excess metal to flow
inward and cause loose metal or wrinkling.
Draw pads, often referred to as binders or blank holders usually are made from hardened tool
steel. They can be flat or contoured, depending on the piece part shape. Most drawing dies use
a single draw pad; however, in special cases, some use two (Figure 3).
Spools, Shoulder Bolts, and Keepers
Spools, shoulder bolts, and keepers are used to fasten pads to the die shoes while allowing
them to move up and down. They are secured to either the top or bottom die shoe with screws
and often dowels for precision location. Of all of the components used for securing pads,
spools are the most common, especially in larger dies (Figure 1 and Figure 4).
Figure 4
Retainers
Retainers hold or secure cutting or forming die components to both the upper and lower die
shoes. One of the most popular retainers is a ball-lock retainer, a high-precision, accurately
manufactured die component that secures and aligns both cutting and forming punches. It uses
a spring-loaded ball bearing to locate and secure the punches, which feature a precisely
machined teardrop or ball seat. The spring-loaded ball bearing locks into the teardrop shape
and prevents the punches from coming out of the retainer.
Figure 5
The advantage of ball-lock retainers is that they allow the die maintenance technician to
remove and reinstall punches quickly. The punch is removed by depressing the spring-loaded
ball bearing and pulling up on the punch. Specialty retainers also can be made to hold and
align irregular punch shapes, as well as headed-style punches and pilot pins (Figure 5).
Springs
Springs supply the force needed to hold, strip, or form metal. Many different springs are used
in stamping dies. Spring selection is based on many factors, including the required force and
travel, the spring's life expectancy, and, of course, cost. Among the most popular are gas
springs, which, when filled with nitrogen, can supply a great deal of force. They also have an
excellent life expectancy.
Figure 6
Other types are coil and urethane springs, often called marshmallow springs (Figure 6). Coil
springs are very popular when a reasonable amount of force is needed and budget constraints
are present. Urethane springs work well in short-run or prototype stamping operations. They
also are inexpensive.
Previous articles in this series discussed common stamping die components. This article
focuses on less common specialty components found only in certain dies, most of which are
available from various suppliers.
In-die Tapping Units
Many dies produce parts that contain holes or
extrusions that will be tapped or threaded to
hold a fastener. These holes often are tapped
in the die rather than in a separate, offline
operation.
In-die tapping units use a series of helix-style
shafts and gears to transfer linear motion
(press ram) into rotary motion. The
Figure 1
mechanical rotary motion can be press ram-
Inidie Tapping Units
driven, or it can be created by special
Image courtesy of Danly IEM.
electronic servo-drive motors. Besides moving
downward, the tap spins and creates the threaded hole.
Unlike a regular cutting tap, an in-die tapping unit uses special roll forming taps. Instead of
removing chips, roll forming taps gradually deform the metal into the shape of a thread. Using
a standard cutting tap in an in-die tapping unit would create a cutting chip removal problem.
Because the work hardens during the metal deformation process, an in-die tapped hole's
strength can be similar to a standard cut thread's strength. The difference is cost using an in-
die tapping unit instead of an offline tapping process can reduce costs significantly (see
Figure 1).
Rotary Benders
Rotary benders, often referred to as rocker
benders, are specialty metal bending units that
feature a rotary action-producing V-grooved
cylinder. This cylinder is spring loaded and
secured into a special retainer called a saddle.
As the die closes and the cylinder makes
contact with the sheet metal, it rotates about
its centerline and creates the bend. Rotary
benders can be used to create straight-line
bends only.
Unlike conventional metal bending
equipment, rocker benders require no
additional pressure pad. Rocker benders can
Figure 2
be easily adjusted and require less force than
Image courtesy of Danly IEM
conventional bending methods. When inserted
with a special hard plastic, they are nonmarking and can overbend the metal to create an acute
or less than 90-degree angle. They also can create double bends (Figure 2).
Pierce Nut Units
Fasteners, such as screws, nuts and rivets, can be inserted into a stamped part in various ways.
Using a pierce nut unit currently is a common method. This special mechanical unit (Figure
3) both pierces a hole and fastens a threaded nut to the stamped part.
Figure 3
Pierce Nut Installation Unit
Image courtesy of Multifastener Corp.
Pierce nut units can feed fasteners in several different ways and can be incorporated easily in
progressive, line, and transfer dies. Unlike tapping, in which the hole relies on the amount of
thread engagement that can be achieved by the specific extrusion length, pierce nut units can
work with a variety of nut sizes, strengths, and thread series.Pierce nut units can be used in
almost any hole-piercing operation and are very popular in both the automotive and other
industries.
HYDROCAMs
Activated by press ram-driven hydraulic cylinders, HYDROCAMs (Figure 4) pierce holes
and create special forms in die areas that are inaccessible using standard cams. Using
HYDROCAMs can reduce the number of stamping operations necessary, as well as the die
cost.
Figure 4
HYDROCAM Assembly
Image courtesy of Ready Technology.
The drive unit can be placed almost anywhere beneath the press ram and can be used to
activate one of several cams. Because these cams run on hydraulics, they can achieve a great
force. HYDROCAMs also can be adjusted easily to fine-tune the timing to execute specialty
cutting and forming operations.
Thread-forming Punches/Buttons
Thread-forming punches and buttons (Figure 5) both pierce and form the metal into a special
shape. The specially shaped pierced hole functions to hold a variety of screws and increases
the force necessary to pull the screw out of the sheet metal.
Figure 5
Image courtesy of Danly IEM.
The punches and buttons can be incorporated into standard ball lock retainers, or they can be
the headed type. Because the metal simply is being pierced and formed, no press speed
reduction is necessary.
Holes created with special thread-forming punches and buttons have improved holding ability
over putting a screw into a flat piece of sheet metal.
Metal cutting and forming methods are virtually endless and limited only by the imagination.
Each die has its own special function. To list all commercially available and custom-made die
components available would be nearly impossible.
Previous articles in this series focused on stamping dies and production methods. This article
discusses stamping materials both ferrous and nonferrous.
To process, design, and build a successful stamping die, it is necessary to fully understand the
behavioral characteristics of the specific material to be cut and formed. For example, if you
are forming 5000 series aluminum and you follow the same process you use for deep drawing
steel, the operation most likely will fail not because aluminum is bad, it's just different from
steel.
Each metal has its own unique mechanical characteristics. The metal type that the die is
forming and cutting often determines the tool steel that must be used, as well as how many
operations are required. In addition, different metal types require different lubricants, press
speeds, and capacities. Because stampers are end users of metals, this article focuses on
selecting and understanding the end-product behavior only and not the metal-making process.
Two Metal Types
Although there are literally thousands of metals that can be stamped, all fall within two basic
categories ferrous and nonferrous. Ferrous metals contain iron, and nonferrous metals are
those without iron. Steel is a classic ferrous metal because it is derived essentially from iron
ore. Aluminum, however, contains no iron and is classified as a nonferrous metal.
With the exception of a few exotic specialty metals, ferrous metals are magnetic and
nonferrous metals are nonmagnetic. Because nonferrous metals do not contain iron, they are
less likely to deteriorate through oxidation or rusting. Some commonly stamped nonferrous
metals are aluminum, brass, bronze, gold, silver, tin, and copper.
Aluminum is a very popular metal for applications in which strength, weight, and corrosion
resistance are factors. Aluminum is approximately one-third the weight of steel. Although
hundreds of alloyed steels exist, plain carbon steel is by far the most commonly stamped
ferrous metal.
Steel Basics
Carbon is a basic element of the steelmaking process. In its raw form, carbon could be
described as a chunk of coal or pencil lead. A piece of coal buried a mile or so beneath the
surface of the earth and subjected to intense heat and pressure for about a thousand years
yields what? A diamond. A diamond is nothing more than pure, compressed carbon. (Yes,
"Carbon is a girl's best friend." Just make sure that it's natural, highly compressed carbon that
you are giving her.)
From this basic knowledge of carbon, it is easy to deduce that the more carbon present in the
steel, typically the stronger and less formable it will be. For example, tool steel used in
manufacturing dies contains far more carbon than the sheet metal being processed. Keep in
mind that the carbon content of a particular metal does not fully determine the metal's
mechanical properties. Carbon content is only one factor.
Alloys
An alloy is a homogeneous compound or mixture of
two or more metals that enhances the metal's
chemical, mechanical, or physical properties. When
combined, the metals must be compatible and resist
separation under normal conditions. For example, two
common alloys added to steel are chrome and nickel.
Chrome is very hard and resists oxidation, and so
does nickel. Adding chrome and nickel to steel
produces stainless steel. These added alloys enable
Figure 1
the stainless steel to resist oxidation.
If you have purchased stainless steel flatware recently, you may have noticed different grades
are available. These grades usually are designated as good, better, and best. The main
difference in the quality depends primarily on the alloy content. The numbers that you see on
the packaging, such as 18/8 or 18/10, refer to the percentage of chromium (18 percent) and
nickel (8 percent or 10 percent) in the stainless steel. Chromium is known for its stain
resistance, and nickel is known for its high luster and shine. Higher alloy numbers mean
higher quality and cost.
Alloys can be introduced into both ferrous and nonferrous metals. Many aluminum alloys are
available today. A very common steel type used in the automotive industry is high-strength,
low-alloy steel (HSLA). Alloys are combined with medium carbon steel to give the metal
good load-carrying ability and reasonable formability. These mechanical properties make
HSLA a good candidate for frame rails and other automotive structural parts that require
strength.
The number of alloyed metals used in stamping
are far too numerous to mention in this article. The
thing to remember is that alloyed metals are a
combination or mixture of two or more metals that
create a new metal with special characteristics.
Plain Carbon Steel
Plain carbon steel can be defined as pure steel,
meaning that it contains no intentionally added Figure 2
alloys. Plain carbon steel among the most
popular steel types used in stamping today usually is assigned a four-digit number, such as
1006, 1020, 1050, and 1080. To determine the steel's carbon content, simply place an
imaginary decimal place between the four digits and read the last two digits as a percentage of
1 percent. For example, 1010 steel contains 10 1/100 of 1 percent carbon, or 0.10 carbon (see
Figure 1).
The more carbon in the steel, the harder it will be to cut and form. Metals with increased
carbon can be hardened further by heating them to a critical temperature and cooling them
quickly in the proper quenching medium. Processing harder metals requires dies made from
tougher, more wear-resistant tool steels. Also, greater force is needed to cut and form the
metal. Knowing the metal's carbon content can help you make a better decision about the
appropriate tool steel and press capacity. Figure 2 shows a few typical applications with
respect to the steel's carbon content.
This article covered very basic metal types and properties only. The next article in this series
will discuss the mechanical characteristics of different metals in more detail. It also will
explain how the metal selection affects the die processing method and die materials.
Previous part of this series introduced two basic types of metals used to manufacture stamped
parts ferrous, metals that contain iron, and nonferrous, metals that do not contain iron. This
article discusses the specific mechanical properties of these metals in more detail.
The metal's mechanical properties greatly influence the process chosen to transform the flat
sheet metal into the finished part's shape and profile. The mechanical properties often
influence the tool steel and lubricants used to form and cut the sheet metal. They also
determine if offline processes, such as annealing or hardening, are necessary.
Literally thousand of metals are used in stamping today, and it would be nearly impossible to
cover each material's specific mechanical properties. This article explains the fundamental
properties they all share and discusses methods for testing and defining some of these
properties.
Tensile Testing
Among the numerous methods used to test metal's mechanical capabilities, the most widely
used and accepted is the tensile test. In a tensile test the metal is carefully cut to a specific
shape according to a given testing standard. The cut sample is called the test coupon.
The test coupon then is placed into a special machine called a tensile tester, which grabs each
end of the coupon and stretches it. The metal is stretched until it fails (breaks). Factors such as
how much the metal stretched, how it thinned out, how it changed shape, and how much force
was required throughout the entire forming process are carefully measured and documented.
Mechanical properties such as elongation percentage, tensile strength, yield strength and n
and r values can be obtained using this test (Figure 1).
The tensile test also can generate a special graph called a stress/strain diagram (Figure 2).
This diagram shows the relationship between the force that is needed and the deformation that
occurs. In short, it shows how the metal behaves when being deformed.
Ductility
Ductility is a very broad term that describes a
metal's ability to change shape without fracture. In
flat-rolled steel, ductility usually is measured by
hardness or mechanical properties in a tensile test.
Generally speaking, the more ductile the metal is,
the more it can be deformed. However, keep in
mind that metal can be deformed in more than one
way. Better defining how ductility affects the
forming process requires first defining a few
important properties that are obtained when the
metal is subjected to a tensile test.
Elongation Percentage. Elongation percentage is Figure 2
one of the properties that affect metal ductility.
Elongation can be described simply as a numerical expression of how much the metal
stretched within a given boundary. The most commonly used boundary is 2 inches.
The metallurgical definition and mathematical equation for elongation can be expressed as the
extension of a uniform section of a specimen expressed as a percentage of the original gauge
length:
Elongation, % = (Lx - Lo) / Lo x 100,
where Lo is the original gauge length and Lx is the final gauge length.
For example, a material having 42 percent total elongation stretched 42 percent of its
beginning length within a 2-in. boundary before it fractured.
Tensile Strength. Tensile strength can be defined as the maximum stress that a material can
withstand. In tensile testing, the measurement is the ratio of maximum load to the original
cross-sectional area. Often it is also referred to as the metal's ultimate tensile strength (UTS).
Another definition of tensile strength is the maximum stretching that a material is capable of
withstanding without breaking under a gradually and uniformly applied load. Simply, it is the
measurement of the breaking or rupturing force.
Yield Strength. A metallurgist may describe yield strength as the point at which material
exhibits a determined deviation from the proportionality of stress to strain. While this most
certainly is a true statement, it is not one that's easy to understand. Think of yield strength as
the measurement of the force necessary to deform the material permanently.
Remember, before a material can permanently change its shape, it must first go through a
transition from elastic deformation (not permanent) to plastic deformation (permanent). Think
of it like this: Imagine suspending a flat piece of sheet metal that is 0.062 in. thick, 12 in.
wide, and 24 in. long between your arms. The sheer weight of the metal will cause it to sag
slightly in the center. This change in shape is the result of elastic deformation, meaning that
although you have witnessed a change in the metal's shape, the change is not permanent. This
can be proven by placing the metal on a flat table, at which point it will return back to a flat
sheet.
However, if you severely bow the metal
sample and apply enough force, it will begin
to take the shape of the bow. The point at
which the metal permanently changes its
shape is its yield point. Yield strength is a
measurement of how much force it took to get
the material to deform permanently, give up,
or "yield." Yield strength usually is expressed
in pounds per square inch (PSI), or
megapascals.
Hardness. Hardness, which can be defined
simply as a measurement of the metal's
penetrability, usually is tested with a special
Figure 3
machine. The most common hardness testing
machine is a Rockwell/Brinell tester (Figure 3). This device applies a load or weight to a point
that penetrates into the steel's surface. The deeper the penetration, the softer the material. By
measuring the applied force and the penetration depth, we can obtain a numerical value that
expresses the metal's hardness.
Although hardness alone does not give enough data to determine the metal's formability, it
can be used for comparative analyses. Generally, with the exception of metals such as
aluminum, the softer the metal, the more ductile it will be. Materials such a copper, brass,
gold, titanium, and many other nonferrous metals often are categorized by their hardness.
This article discussed only a few mechanical properties that both ferrous and nonferrous
metals have. The next article in this series will cover even more properties.
Figure 1
Strain and Thickness Distribution
Previous part of this series discussed some of the specific mechanical properties of metals
ductility, elongation percentage, tensile and yield strength, and hardness and how to derive
these properties. This article describes other important mechanical properties, as well as a few
behavioral characteristics.
Strain
Strain can be defined simply as a measurable deformation of the metal. In other words, metal
must be "strained" in order to change its shape. Strains can be positive (pulling the metal
apart, or tension) or negative (pushing the metal together, or compression.) Strains also can be
permanent (plastic) or recoverable (elastic). The result of elastic straining commonly is
referred to as springback, or elastic recovery.
Remember, every metal type wants to return to its original shape when it's deformed. The
amount the metal springs back is a function of its mechanical properties. When engineers
refer to part areas that are "high strain," they typically are referring to areas that have been
subjected to substantial stretch or compression. Figure 1 shows a simulation image of a part
that has been stretched. Each color represents a different type and amount of strain. Some of
the strains are positive and others are negative.
Stress
Stress is simply the result of straining the metal. When subjected to stress, metal incurs
internal changes that cause it to spring back or deform nonuniformly. Trapped stresses within
a part often result in a loss of flatness or other geometric characteristics. All cut or formed
parts incur stress.
Stretch Distribution
Stretch distribution is a very important
mechanical property. A metal's stretch
distribution characteristics control how much
surface area of the stretched metal is
permanently deformed. Stretch distribution is
determined primarily by checking the metal's
thickness when it's deformed in tension
during the tensile testing process. The more
uniform the thickness distribution, the better
the stretch distribution. Stretch distribution
also is partially expressed in the metal's n
value. Figure 2 shows different stretch
distribution results. The red areas of the
Figure 2
sample test coupon represent areas that have
Stretch Distribution / Tensile Test
been stretched.
n Value
To understand n value, otherwise known as the work or strain hardening exponent, you must
understand that every time metal is exposed to permanent deformation, work hardening
occurs. It's the same thing that happens when you bend a coat hanger back and forth. As you
bend the hanger, it gets harder and harder to bend. It also becomes more difficult to bend it in
the same place. This increase in strength is the result of work or strain hardening. However, if
you continue to bend the hanger in the same spot, it will eventually fail.
Ironic as it may seem, materials need to work-harden to achieve both good stretchability and
stretch distribution. How they work-harden is the key. The n value of a material can be
defined fundamentally as the metal's stretchability; however, it also is an expression of a
material's stretch distribution characteristics.
Perhaps one of the most important mechanical properties to consider if the stamped part
requires a great deal of stretch, the n value is expressed numerically in numbers from 0.100 to
0.300 and usually is carried out two or three decimal places. The higher the number, the
greater the metal's stretchability and stretch distribution. Higher-strength metals, such as
spring steel, have very low n values, while metals such as those used for making oil pans and
other deep-formed parts usually exhibit higher n values.
The metal's n value also is a key mechanical value used in creating forming limit diagrams.
(This will be discussed in subsequent parts of this series.)
r Value
The metal's r value is defined metallurgically as the plastic strain ratio. To understand this
concept, you must clearly know the difference between stretching and drawing. Stretching is a
metal forming process in which the metal is forced into tension. This results in an increase in
surface area. Items such as most automobile hoods and fenders are made using this process.
Drawing is the displacement of metal into a cavity or over a punch by means of plastic flow
or feeding the metal. Items such as large cans, oil pans, and deep-formed parts usually are
made using this process.
Figure 3
Plastic Strain Ratio r Value
The metal's r value can be defined simply as the metal's ability to flow. It also is expressed
numerically using a value from 1 to 2, which usually is carried out two decimal places. The
greater the r value, the more drawable the metal (Figure 3).
The metal's r value is not uniform throughout the sheet. Most metals have different r values
with respect to the metal's rolling direction. Testing for a metal's r value requires tensile
testing in three different directions with the rolling direction, against the rolling direction,
and at 45 degrees to the rolling direction. The test results usually are averaged and expressed
as the r bar, or average of the r values.
Differences in the plastic strain ratio result
in earring of the metal when being drawn.
For example, when drawing a round shell
from a round blank, the results will be a
near square bottom on the flange of the
cup. This effect (Figure 4) is caused by
different amounts of metal flow with
respect to the metal's
Surface Topography
A metal's surface topography, defined
Figure 4
simply as the metal surface finish, is
Earring Caused by Differences in the Metal s r
created mainly during the metal rolling
Value
process. Surface topography is an
important metal characteristic. When being drawn, metals often require a surface finish that
has the ability to hold lubricant. Surface topography is determined with a measuring tool
called a profilometer.
This wraps up the discussion of sheet metal characteristics. The next article in this series will
focus on metal cutting.
This article, part of a series covering stamping die fundamentals, begins an in-depth look at
the metal cutting process. It covers piercing and cutting clearance and discusses some
common piercing misconceptions.
Cutting is the most severe metalworking process that takes place in a die and shouldn't be
taken lightly.
Cutting Basics
Cutting metal requires great force. For example, it takes approximately 78,000 lbs. of pressure
to cut a 10-in.-diameter blank from 0.100-in.-thick mild steel. Consequently, the punch, die,
and press must absorb overwhelming shock.
Overshocking the press and die components usually is what causes them to fail prematurely.
If you work in a shop that blanks heavy metals, you know what I mean. You can hear and feel
the press shock. Doing everything you can to reduce the unnecessary loading and shocking is
important. Factors such as cutting clearance and shear angles contribute significantly to the
amount of force required. They also affect the amount of shock that is generated.
Piercing Misconceptions
If you participated in a tool and die apprenticeship, you probably were taught the following
rules for piercing punches:
" The punch determines the hole size.
" The cutting clearance always should be even (equal) around the punch.
" 10 percent of the metal's thickness is a good cutting clearance for each side of the
punch.
These are good starting guidelines for cutting, but they aren't entirely true. Let's examine each
misconception.
The punch determines the hole size Although the punch produces a hole that is very close
to its actual diameter, altering the clearance between the punch and the button (sometimes
referred to as the matrix) also affects the hole size. The simple truth is that a hole can be made
slightly larger or smaller than the punch diameter by increasing or decreasing the cutting
clearance. This is because of the way that the metal deforms before the cutting actually takes
place.
Think of the metal that you're cutting as Silly Putty® or a rubbery plastic. If the clearance
between the cutting punch and the button is insufficient, it will cause the metal to compress or
bulge out away from the punch before the cutting takes place. After the slug is created, the
metal grips the punch sides. This increased friction between the sides of the punch and the
metal raises the amount of force necessary to strip or pull the punch from the metal.
The insufficient clearance between the punch and the button means that a greater force is
needed to create the hole. Inadequate clearance also increases the load on the edges of the
punch and the matrix, which causes premature edge breakdown.
After the punch is removed, the metal that once was compressed decompresses and collapses
around the void area (the hole). The result is a hole that is smaller than the punch's diameter
(Figure 1).
Figure 1
Insufficient Cutting Clearance
If the clearance between the punch and button is increased, the metal is pulled inward in slight
tension into the button. After the slug is created, the metal pulls away from the edges of the
punch, resulting in a hole that is slightly larger than the pierce punch.
Increasing the cutting clearance also reduces the cutting force needed to create the hole. In
addition, because the hole is slightly larger than the punch, the force needed to strip the metal
from the punch is greatly reduced (Figure 2).
Figure 2
Using Increased Cutting Clearance
Keep in mind that changing the clearance does not affect the hole size to a great extent
about 0.001 in. to 0.002 in. Although it might seem small, this change can reduce the friction
generated during punch withdrawal significantly and extend the punch life.
The cutting clearance always should be equal around your punches Once again, unless
you are piercing only round holes, this statement is not entirely true.
Cutting clearances should change around the punch perimeter with respect to the punch
geometry. Let me explain using this example: If you are piercing a square hole, you may
notice that the corners of the punches are the first areas to break down. Once the corners break
down, the entire punch must be sharpened. Ever wonder why the corners break down first?
It's because this is the area that is subjected to the highest cutting loads. Very simply,
wherever there is a small radial feature in a cut (nothing is worse than a dead sharp corner),
the compressive forces will be greater.
Excessive compression can be compensated for by increasing the cutting clearance in areas
with small radial features or sharp corners. Increasing the clearance in these areas helps to
increase punch and button life and reduce the probability of a large corner burr. A good rule
of thumb is to increase the clearance in the corners to approximately 1.5 times the normal
clearance. An even better scenario is to avoid dead sharp corners whenever possible (Figure
3).
Figure 3
Increasing Cutting Clearances in Corners
10 percent of the metal's thickness is a good cutting clearance for each side of the
punch Once again, this statement isn't always true. While 10 percent is by far the most
popular cutting clearance used, it most certainly is not always the ideal cutting clearance.
Cutting clearances can range from as little as 0.5 percent up to as much as 25 percent of the
metal's thickness per side. Among the many factors that determine the best cutting clearance
are the metal's thickness and hardness and the punch size and geometry. For example, the
ideal cutting clearance for piercing a 0.500-in.-diameter round hole in a sheet of 0.100-in.-
thick 300 series stainless steel is about 13 percent of the metal's thickness per side, or 0.013
in. per side. This calculates to a total clearance of 0.026 in.
However, changing from a 0.500-in.-diameter punch to a 0.100-in.-diameter punch requires
more cutting clearance, from 13 percent to 20 percent per side. This is because the smaller
punch has a smaller radius, and compressive forces congregate at the smallest radial feature of
a cut (just as in the rectangular punch example noted above).
Metal type also affects cutting clearance selection. Harder, higher-strength materials require
more cutting clearance, while softer metals, such as aluminum, require smaller cutting
clearances.
As you can see, metal cutting is slightly more complicated than often perceived.
Understanding the many variables and how they affect the cutting process are key.
The next article in this series will continue the discussion about cutting. Topics to be covered
in the next and subsequent articles include methods for preventing slug pulling and punch
breakage; specialty cutting methods, such as pinch, breakout, and shim trimming; and tricks
for piercing multiple layers and piercing on an angular surface.
This article, Part XI of a series covering stamping die fundamentals, offers an overview of
slug pulling a problem that can damage parts and tools significantly and explains the
different causes. Part XII will cover corrective and preventive actions. Descriptions of and
links to the first 10 parts in this series can be found at the end of this article.
Slug pulling is a serious problem in a stamping operation. Addressing the issue requires first
understanding why the slugs are pulling.
What Is Slug Pulling?
When a pierce punch creates a hole, it also produces scrap, usually referred to as a slug. Slug
pulling occurs when the slug sticks to the punch face upon withdrawal and comes out of the
button, or lower matrix.
If a slug falls off the punch and onto the strip or part, it can damage the part and die. Keeping
the slug down in the matrix or, better yet, completely pushing it out of the die is the desired
scenario.
What Causes Slug Pulling?
Many factors contribute to slug pulling. Among them are trapped air; large cutting clearances;
extremely fast piercing operations; sticky lubricants; improperly demagnetized punches; and
fatigued or insufficient spring ejectors.
1. Trapped air/ vacuum pockets The slug generated during the piercing process has some
curvature. The curved, void areas where air is trapped, creating a vacuum action. During the
perforating process, a tight seal is maintained around the punch perimeter. When the punch is
withdrawn, this seal prevents the slug from coming off the punch (Figure 1a). Keep in mind
that the only portion of the piercing punch that makes contact with the metal is a localized
zone around the punch's outside diameter. Even punches with angularity make only localized
contact with the metal (Figure 1b).
Trapped air must be allowed to escape to reduce the amount of vacuum. This is done by
creating a small air vent in the center of the pierce punch, which allows the otherwise trapped
air to exhaust itself from the vent hole and reduce the suction. Losing suction breaks the seal
between the slug and the pierce punch and allows the slug to fall (Figure 2a).
When piercing punches that are too small to vent are used, other means of addressing slug
pulling most likely will be necessary. Also keep in mind that addressing the trapped air
probably won't solve the slug pulling issue completely, but it will certainly help.
2. Larger cutting clearances Although using engineered or larger cutting clearances can
result in much greater punch and matrix life, there is one drawback to doing so. As the
clearance gets larger, compression on the slug decreases, which increases the chances of slug
pulling.
When smaller cutting clearances are used during the perforating process, both the slug and
metal outside the slug are forced into compression. After the slug is cut free, it decompresses
and remains in the matrix. This is because the decompressed slug now has an interference or
press fit into the matrix.
In simple terms, when greater cutting clearances are used, the slug will be slightly smaller
than the hole in the matrix, which means it may be pulled from the matrix by the punch,
resulting in slug pulling. Reducing the cutting clearance certainly can help this problem, but it
also can shorten punch life and increase sharpening frequency. Rather than reducing the
cutting clearance, it is recommended stampers try a few methods that will be discussed in the
next part of this series (Figure 2b).
3. Oil / lubricant problems Using heavy, thick, highly viscous oils and deep-drawing
lubricants only adds to slug pulling problems. Unfortunately, these compounds often are
necessary for forming dies to perform correctly.
Over time heavy oils and compounds can become coagulated and sticky. Thick, sticky
compounds can cause slugs to stick to punches. Periodically cleaning the cutting components
can help to resolve this sticky residue problem. Other methods to resolbe this problem will be
discussed in Part XII.
Figure 3
Using a flat surface grinder
4. Magnetized punches Punches and die sections often are sharpened with a surface
grinder. Most surface grinders secure the sections and punches to be ground by a high-power
electro- or conventional magnet (Figure 3). Any ferrous metal that comes in contact with this
magnet becomes slightly magnetized.
After the die components have been ground, they then must be demagnetized fully. This
process is accomplished by using a commercially available demagnetizing unit. Magnetized
pierce punches and die sections can cause slugs and other magnetic debris to be picked up and
carried through the tool.
5. Weak or fatigued spring ejectors Spring ejectors often are used in piercing and cutting
punches. These small, spring-loaded pins push the slug from the punch face after cutting has
taken place. If the spring behind the punch fails or fatigues, slug pulling can occur.
Periodically inspecting and replacing springs is a necessary part of a good die maintenance
program (Figure 4).
Figure 4
Spring Ejectors
Slug pulling can have disastrous consequences. A single slug carried through a progressive
die can damage every tool in the station. The next part in this series will discuss methods for
resolving slug pulling problems. Subsequent articles will cover specialty cutting methods,
such as pinch, break out, and shim trimming, as well as tricks for piercing multiple layers and
piercing on angular surfaces.
Slug pulling, which occurs when scrap metal the slug sticks to the punch face upon
withdrawal and comes out of the button, or lower matrix, is a serious problem that can
damage parts and dies. Various methods can help reduce the occurrence of slug pulling.
Air Vents
Putting air vents in cutting and piercing sections most likely will not completely stop cutting
slugs from pulling, but it's a good start. This is because trapped air that creates vacuum
pockets is a major cause of slug pulling. It is good die-building practice to drill air vents in all
cutting punches whenever possible, especially if they are piercing punches.
Spring Pins
Figure 1
Spring Ejectors and Air Vents
Images courtesy of Dayton Progress
A common, popular method for preventing slugs from pulling is to use a pierce punch with a
spring-loaded ejector pin. However, this method is effective only if the punch is large enough
to accept a spring pin.
The spring-loaded pin pushes slugs from the punch point and into the matrix. Keep in mind
that to maximize the spring pin's effectiveness, it must be accompanied by an air vent. This
can be achieved by drilling an oversized hole for the pin and allowing the trapped air to
escape around the spring pin.
Spring pins work well in large dies containing large pierce punches, but they do not lend
themselves well to small-die, high-speed operations. Many commercial punch manufactures
can provide these types of punches for you. Some commercially available punches even have
a special wire retainer that allows the maintenance technician to depress the spring pin, lock it
in place with a special retention pin, and grind the punch with the spring depressed. This
capability allows the punch to have the same amount of spring travel as a new punch (Figure
1).
Reduce the Punch-to-Die Clearance
Although reducing the cutting clearance shortens the life of the punch and matrix, it helps
minimize slug pulling. This is because reducing cutting clearance forces the slug in
compression during cutting. After the cutting is completed, the slug decompresses in the
matrix for an interference fit.
For short-term runs and low-production parts, reducing the clearance may be your answer;
however, for high-production dies, it is recommended that you use an engineered cutting
clearance combined with an alternate method for slug retention.
Special Die Inserts, Buttons, and Matrix Alterations
Many commercially available inserts orbuttons can help address slug pulling problems. Some
common commercial names are "slug huggers" or "slug-control buttons" (Figure 2).
Figure 2
Commercial Slug Control Buttons
A slug-control button consists of two small slots machined at an angle in each side of the
matrix. These slots cause a burr to be generated on the slug. The burr is forced downward at
an angle, wedging the slug in the matrix.
A slug-hugger button has barbs in the matrix that impale themselves into the slug. Both of
these methods work well and are highly recommended.
A reverse-tapered "bell mouth" button also works well. Most die buttons have a bell mouth
taper machined into them, with the hole diameter increasing toward the bottom of the button.
Although it may seem strange to use a button with a hole in the matrix that gets slightly
smaller as it nears the clearance opening, this is an effective slug retention method. The
reverse taper holds the slugs in compression in the matrix. Keep in mind that in most piercing
operations, 0.0005 inch to 0.001 in. is more than sufficient taper. Too much taper and
compression can cause the matrix to split (Figure 3).
Figure 3
Alternate Slug Control Buttons
Vacuum Units
Commercially available vacuum units can be incorporated in your piercing operation. These
units create a vacuum and pull the slug downward into the matrix. In a pinch, try a simple wet
and dry vacuum. In my experience, it works fairly well. However, keep in mind that these
vacuums typically are not meant to run for hours and hours. Even the higher-quality models
burn up quickly.
Other Ideas
Although it may be somewhat crude, using a weld spatter technique on the inside of a button
can be a relatively effective slug-pulling remedy. Commercially available deposit machines
work best to execute this application. These special deposit machines deposit tiny barbs on the
inside of the button. These barbs impale themselves into the slug and help prevent it from
pulling upward.
These portable application machines have significant advantages over ordinary weld spatter.
First, they can deposit tungsten or vanadium carbide on the button surface, which decreases
button wear and increases slug-retention life. Second, the deposits can be made accurately
with as little heat as possible. This helps to reduce tool steel and button damage. Deposit
amounts can be carefully controlled.
Keep in mind that each cutting and piercing operation may require a different slug pulling
method. The key is to remember that one pulled slug is one too many. Even a single pulled
slug can result in extensive die damage. Don't risk ignoring the issue: An ounce of prevention
is worth a pound of cure!
Future articles in this series will cover specialty cutting methods, such as pinch, breakout, and
shimmy trimming, as well as tricks for piercing multiple layers and on angular surfaces.
Various specialty metal cutting methods are used in stamping operations. Among them are
pinch, breakout, and shimmy.
Pinch Trimming
Pinch trimming is a special method in which the vertical walls of a drawn or stretched vessel
are cut by pinching the metal between two hardened tool steel die sections. In most cases, the
clearance between the die sections is as little as possible (Figure 1).
Unlike a conventional metal cutting process, no shearing or fracturing takes place in pinch
trimming. Items such as deep-drawn cans often are pinch trimmed.
Although pinch trimming is a very popular method, because the metal basically is pinched
off, a very sharp burr usually remains on the part (Figure 2). This burr must often be removed
by tumbling the parts in a tub containing
abrasives.
Pinch trimming also places a great load on the
sides of the die sections, which results in high
wear. Most pinch trimming operations require
a great deal of maintenance.
Breakout Trimming
Breakout trimming is a specialty metal
trimming process in which the metal is forced
to fracture or break free from the vessel's
Figure 2
flange. If you are accustomed to conventional
Result of pinch trimming
cutting operations, this process most certainly
may look harebrained to you. Unlike a conventional metal cutting process, the lower die
section has a 45-degree angle ground on its edge. This angle has two basic functions: first, to
allow the cup to fully nest in the lower die, and second,. to force the flange to bend upward
slightly.
The cutting clearance also is much greater in breakout trimming than the clearance commonly
used in conventional cutting operations. This additional clearance causes leverage action, but
does not allow for the metal to be bent into a vertical wall. However rest assured, this process
works well, especially for metals that severely
work harden (Figure 3).
Breakout trimming takes advantage of the
metal's work hardening and reduced thickness
in a given localized zone. This method works
best for round or axial symmetrical drawn
parts.
For breakout trimming to work effectively,
the drawn cup must be properly prepared for
the process. The inside radius on the cup's
Figure 3
flange must be reduced to a dead sharp corner
Break out trimming
before using this method. This is achieved by
drawing the cup deeper in the drawing
operation or compressing it back over a dead sharp corner on the die section (Figure 4).
Doing so reduces thickness in the radius and
allows work hardening to take place.
After the cup has been prepared properly, it
can be introduced into the breakout trimming
process, in which the cup flange will be
forced upward, causing the metal to break at
the dead sharp corner (Figure 5). Because the
cup flange is round, as it is pushed upward it
is forced into radial compression. This
compression works to your advantage by
forcing the cup to be fractured out of the
flange.
Figure 4
Breakout trimming does not produce a burr as
Creating a small radius
large as that produced by pinch trimming.
Also, because the loads on the tool steel sections are minimal, the die requires less frequent
maintenance. However keep in mind that this method can be used only in situations in which
the metal must be cut at the intersection of the flange and the cup's vertical wall.
Shimmy Trimming
Shimmy trimming is a unique metal trimming process in
which a series of specially designed cams are used to force
the part to move side to side. Unlike conventional cam
trimming, the part does not remain stationary, but rather
moves horizontally in the die. It moves in such as fashion
that it can be trimmed true to the surface of the vessel.
Trimming 90 degrees or true to surface results in a much
cleaner cut and considerably lower burrs than pinch
trimming.
Figure 5
A great advantage of shimmy dies is that they can cut the
entire perimeter of a part in a single press stroke. Unlike conventional pinch trimming, a
shimmy trimming operation is not restricted to straight line cuts. Features such as notches,
curved cuts, as well as a various other cuts can be made. Many common items such as
cigarette lighters and gun shells are made
using the shimmy trimming process.
Shimmy trimming operations also can be
designed to cut metal as thick as 0.250 in.
and, unlike conventional pinch trimming, can
keep the original metal thickness in the
trimmed area. Not like conventional cutting
operations, shimmy dies require a pressure
system such as a press cushion or a nitrogen
gas manifold (Figure 6).
Figure 6
Which metal cutting operation a die design
Parts trimmed with a shimmy trim die
engineer chooses is based on many factors,
Images courtesy of Vulcan Tool Corporation
including allowable burr height, parts
volumes, metal type and thickness, and trim line geometry. No single trimming operation is
best for all scenarios. The next article in this series will continue the discussion of metal
cutting.
Although fineblanking and GRIPflow® often are categorized as metal cutting operations, they
more closely resemble a cold metal extrusion process that creates what appears to be a
blanked part. The processes can be defined simply as methods in which a part is squeezed
from the strip.
Figure 1
Results of Conventional Cutting
Unlike parts made with conventional metal cutting methods, the parts made using
fineblanking and GRIPflow have little or no fracture zone (Figure 1). In other words, these
parts appear to have smooth, square machined edges.
These processes also can produce parts with very close flatness and dimensional tolerances
and roughness of about 2 to 3 µm, which means that, in many cases, postprocessing
operations such as grinding and milling can be eliminated.
Parts commonly made using fineblanking and GRIPflow include gears and parts that require
close flatness tolerances or a square cut edge. These processes also can pierce holes with
diameters as small as one-third of the metal's thickness and very close to the part's edge.
Before these methods were available, the metal had to be shaved in one or several shaving
operations to achieve a smooth cut edge. Shaving in a die often produces slivers and debris
that can create tool problems and product defects.
Fineblanking
Invented in Switzerland in the 1920s, fineblanking, unlike conventional stamping methods,
utilizes a special triple-action hydraulic press called a fineblanking press. Fineblanking
requires the use of extreme-pressure pads. These high-pressure pads hold the metal flat during
the cutting process and keep the metal from plastically deforming during punch entry.
In fineblanking presses, a V-ring is incorporated into one of the high-pressure pads. This V-
ring also is commonly referred to as a stinger or impingement ring.
Before the punch contacts the part, the V-ring impales the metal. It surrounds the part
perimeter and functions both to trap the metal from moving outward and push the metal
inward toward the punch. This action reduces the rollover that occurs at the part's cut edge.
Using high-pressure pads combined with the stinger ring and close clearances keeps the metal
from fracturing and creates a smooth edge (Figure 2). Because the part is held extremely tight
between the high-pressure pads during cutting, part distortion is minimal.
Figure 2
Fineblanking Process
Unlike conventional cutting operations that use approximately 10 percent of the metal's
thickness for the cutting clearance, fineblanking operations usually use clearances less than
0.0005 in. per side. This small-clearance requirement combined with high pressure also
contributes to the fully sheared part edge.
Once again, don't confuse fineblanking with a cutting operation. It's not a cutting operation at
all; it is more like a cold extruding process. The slug (part) is pushed or extruded from strip
held so tightly between high-pressure holding plates and pads that the metal cannot bulge or
plastically deform during the process. These high-pressure pads fit precisely around all cutting
components. Fineblanking can be used to produce parts as thick as 0.5 in. from a variety of
metals.
GRIPflow
Not to be confused with fineblanking, the GRIPflow process does not use a stinger or
impingement ring to stop outward metal movement but relies solely on hydraulically applied
pressure to the blank. The pressure is applied through precision-guided pressure pads.
Figure 3
GRIPflow Part
Source: Ebway Corp.
Think of the GRIPflow process as similar to compound blanking. However, unlike a
compound blanking operation, GRIPflow uses very small cutting clearances between each of
the cutting components. This small clearance, combined with high blank holding pressures
and precision clearances between all moving components, produces a smooth-edged part that
can be held to very tight dimensional tolerances (Figure 3).
Once again, keep in mind that GRIPflow is not a metal cutting process but a cold extruding
process. The cutting sections do not have cutting shear ground on them.
It is difficult to tell the difference between a part that was fineblanked and one made using the
GRIPflow process just by looking at them. Unlike fineblanking, GRIPflow does not require a
triple-action press. Because it uses hydraulic cylinders mounted in the die, the process is best-
suited to a hydraulic action press.
Both fineblanking and the GRIPflow process now are being used to produce many parts
previously made by more costly processes, such as casting, forging, and machining. Because
other minor forming operations can be combined with these special processes, they both lend
themselves to many geometries. Keep in mind that each process has its own advantages and
disadvantages.
Metal bending often is perceived as the simplest metal forming operation. This article
describes wipe and V bending and discusses the advantages and disadvantages of both. It also
addresses ways to reduce springback. Descriptions of and links to the first 14 parts in this
series can be found at the end of this article.
Part II of this series presented a basic overview of metal forming operations, such as bending,
flanging, drawing, ironing, coining, curling, hemming, and embossing. This and future
installments discuss these operations in more detail. We will look at factors controlling the
success of each operation, as well as tooling design guidelines. Let's begin with metal
bending, a process often perceived as the simplest.
Bending
Bending can be defined simply as a forming operation in which the metal is deformed along a
straight axis. Both compression and tension occur when bending sheet metal. The inside
radius of the bent metal is in compression, or being squeezed together. The outside bend
radius is in tension, or being stretched.
Because of the metal's elastic properties, it wants to decompress on the inside radius and
return to its flat shape on the outside radius (Figure 1), which causes springback. Also know
as elastic recovery, springback is present in all metal bending operations.
Figure 1
For a 90-degree bend angle, the metal must be bent to an acute angle (less than 90 degrees)
and allowed to spring back to its finished position, a difficult task given the fact that a press
ram typically travels vertically only. In addition, because of natural mechanical variability in
the material from coil to coil and from the beginning of the coil to its end, attempting to
achieve a consistent, precision angularity in a bending operation can be very difficult.
If ultraprecision bend angles are required, one of the best things you can do to achieve them is
to design the tool so that it can be adjusted quickly, safely, and effectively to compensate for
incoming variables.; Changing the product design also can help reduce springback and
inconsistency problems. Incorporating darts, ribs, or gussets into the part design will enhance
stiffness and reduce the amount of springback (Figure 2).
Figure 2
Photos courtesy of Batesville Tool and Die Company.
Bending Methods
Several basic types of bending methods can be incorporated into a stamping operation wipe
bending, V bending, and rotary bending. All three are popular, and each has its advantages
and disadvantages.
Wipe Bending One of the most common methods used, but not always the most effective,
is simple wipe bending. Unfortunately, this method does not allow for much overbending
other than the very slight acute angle that can be achieved by wiping the side extremely tight
(Figure 3). Even though wipe bending effectively creates a bend, controlling the bend angle
is very difficult. This method is not well-suited to bending high-strength metals or for parts
requiring precision bend angle tolerances. Wipe bending can be improved by capturing the
outside profile of the radius with the forming die section.
Figure 3
Simple wipe bending method
Figure 4 shows a coin relief method. This process allows the outside radius to be coined, or
squeezed, near the lower tangent point, which causes under- or over -bending to take place.
The coin relief method works best when the metal is bent over a radius that is equal to or less
than one metal thickness, and the metal is not ultrahigh-strength. One advantage to this
method is that by vertically shimming the forming section up and down, the gap, as well as
the amount of coining, can be adjusted. This allows for easy adjustments to be made for
variability in the metal's thickness and mechanical properties.
Figure 4
Coin relief bending method
Figure 5 shows a pivot-style bending method, which incorporates both wipe bending and cam
motion. This design works well both to create the bend and to adjust the amount of overbend.
Overbend is created by adjusting the lower driver block vertically up and down.
Figure 5
Pivot style bending design
V Bending A very good method for obtaining a given bend angle, V bending is
undoubtedly the most common method used with press brake bending. An acute angle ground
on both the punch and die can provide adequate overbending of the metal. Also, the bending
amount can be altered by adjusting the amount of coining the metal undergoes at the bottom
of the press stroke (Figure 6).
Figure 6
V bending method
One disadvantage to V bending is that it often requires the part to be rotated in such a manner
that sometimes is difficult to incorporate in a progressive die. An advantage is that it often
requires less force to create the bend compared with conventional wipe bending.
Future articles will discuss rotary bending, punch relief bending, and multistation bending
methods.
This article continues the discussion of bending in stamping operations. It focuses on rotary
and reverse U bending and addresses the advantages and disadvantages of rotary bending.
Descriptions of and links to the first 15 parts in this series can be found at the end of this
article.
Part XV of this series about stamping die fundamentals described several bending methods
wipe, coin relief, pivot, and V bending. It also discussed springback and how to compensate
for it when using these methods. This article focuses on other bending processes. Keep in
mind that the key to success is to design the bending process so that it can be easily, quickly,
and safely adjusted to allow for material variables.
Rotary Bending
Rotary bending perhaps is one of the most popular and effective ways of creating a precision
bend. Rotary benders, also known commercially as Ready Benders® or Accu-Bend"!
benders, have many advantages over conventional wipe bending methods. First, let's examine
how they work.
Rotary or rocker benders consist of a foundation block, often referred to as the saddle. The
saddle has a spring-loaded V-shape component called the rocker. This rocker rotates about its
centerline and performs the bending action. It acts as both a holding pad and the bending
mechanism.
Although this type of bender can be installed in almost any direction with respect to the ram
travel, it most commonly is fastened to the upper die shoe. As the bender moves down, the
rocker makes contact with the sheet metal. One contact point acts as a holding pad, while the
opposite contact point rotates, creating the bending action. After the bend is completed and on
the press's return stroke, the spring forces the rocker to return back to its original or idle
position (Figure 1).
Figure 1
Advantages. Rotary bending has some advantages over other methods. The most
advantageous feature is the simplicity of adjustment. Changes in the bend angle can be made
simply by shimming or grinding the height of the assembly. Doing so takes very little time,
and time is money.
Rotary benders can bend as much as 120 degrees and are well-suited to bending high-strength
material. One company in Sweden has successfully created two 90-degree return bends in
steel with yield strength of 980 mega pascals. This translates into steel that by U.S. standards
has a yield strength of more than 142,000 pounds per square inch (PSI) five times stronger
than low-carbon steel. Attempting to make such a bend in a conventional wipe-bending
operation most certainly would be impossible.
Another advantage is that, unlike conventional wipe bending, rotary benders require much
lower forces to create the bend. Anywhere from a 40 percent to 80 percent reduction in force
can be expected. This makes this method ideal for producing long, heavy-gauge, large parts,
such as truck and semi frame rails.
You can expect less hole distortion in rotary bending. Consider a hole that is pierced in a flat
blank and later bent into a vertical wall. During conventional bending, this hole can be
subjected to a great deal of tension, which causes the hole to distort. Because rocker benders
fold the metal around the punch, hole distortion is eliminated (Figure 2).
Figure 2
Inserting rockers with a special hard plastic called Delrin® can make them nonmarking,
which is desirable when bending cosmetic-quality stainless steel or prepainted materials.
Rotary benders can used to bend up or down. They also can be placed on cam slides.
Disadvantages. Despite the many advantages, rotary benders do have some disadvantages.
First, they can be quite expensive; however, consider the advantages of the reduction in
downtime and frustration. Overall, they often pay for themselves in a short period of time.
Figure 3
Poor Candidate for Rotary Bending
Also consider that you most likely will not need an external pad, which reduces die cost.
Often the true cost of designing and building a conventional wipe bending die is much greater
than the rocker bender. Don't confuse cost with value. In my opinion, rotary benders are worth
every penny.
Because these benders have moving parts, there is a risk of galling up and failing to rotate.
This can be prevented by periodically cleaning and lubricating them.
Remember that rotary benders can be used for straight-line bending only. Avoid using them to
bend special-shaped trim lines that do not allow for simultaneous punch contact. Angled
corners are not good candidates for rocker benders (Figure 3).
Overall, I highly recommend using rotary benders for appropriate applications. They are
available commercially from a few reputable suppliers.
Reverse U Bending
Reverse U bending is a unique but effective way of obtaining either a 90-degree bend or a
bend with a slight negative angle. This process utilizes a high-pressure pad with an insert that
can be adjusted in height by shimming or grinding it. The insert causes the part to bow
upward in the center of the punch where a void has been created. Raising or lowering the
insert changes the severity of the bow. Keep in mind that this bow must be created with the
pressure exerted by the pad. This often requires the use of high-pressure gas springs.
After the bow has been created, the pad moves downward and the bends are established. Upon
punch removal, the part has a tendency to spring back in the center, which causes the bends to
"toe in." This method works well with materials that exhibit a great deal of springback. If the
metal permanently deforms in the center bowed area, it may be necessary to push the part
back flat in order to achieve a 90-degree angle (Figure 4).
Figure 4
Reverse U Bending Process
Remember that the true key to bending success is to design the tool in such a fashion that it
can be quickly, safely, and accurately adjusted with respect to ever-changing incoming
variables. Avoid using the grinding and welding process whenever possible.
This article in a series about stamping fundamentals is an introduction to deep drawing. It
defines drawing and differentiates between drawing and stretching. It also lists and explains
basic drawing components. Descriptions of and links to the first 16 parts in this series can be
found at the end of this article.
Part XVI of this series wrapped up the discussion of bending in stamping operations. This
article focuses on drawing.
What Is Drawing?
Figure 1
Click image to view larger
Drawing is a metal forming process in which a product is made by controlling sheet metal
flow into a cavity and over a punch. The process of deep drawing means that the part must be
taller than its minimum width.
Many people confuse drawing with stretching. True drawing results in very little stretching of
the metal. Drawing requires metal flow, while stretching does not. It is only through the
drawing process that objects such as oil pans, beer kegs, and oil filters can be made.
Drawing can be better defined as the process of displacing pre-existing surface into an
alternate-shaped vessel containing nearly the same surface area. Stretching can be defined as
the increase of surface area that results in a product with more surface area than the original
surface area.
Drawing requires the metal to feed inward toward the punch. Very little or no metal flow
takes place during stretching. However, keep in mind that because drawing does require
tension to pull the metal inward, some stretching occurs during drawing.
The key in deep drawing is to limit the amount of metal stretching and thinning that take
place. Items such as oil pans require significant drawing and stretching. Achieving a deep-
drawn product that exhibits very little metal thinning requires extensive knowledge of sheet
metal properties, drawing ratios, radii, and friction. Figure 1 shows the drawing process.
Basic Drawing Components
Figure 2
Click image to view larger
The deep-drawing process is not directional-specific. In other words, the direction in which
the drawing takes place really doesn't matter. You can draw a part up or down into a cavity.
You can even draw a part vertically using cams or special vertical-motion presses.
Please keep in mind that I am in no way indicating that process engineers or die designers
don't pay close attention to the direction in which are drawing. Drawing direction must be
given careful attention because it affects the ability to move, cut, and eject the part in the die.
If drawing is incorporated into a progressive die, the drawing direction also may affect the die
and strip carrier design.
Figure 2 shows a section view of a very simple single-action drawing die. This die is
designed to produce a round cup with a small flange. A basic drawing die consists of the
following components:
1. Die set or foundation. This could be made of mild steel cast iron or aluminum. It serves as
the guided foundation on which all of the metal forming sections will be mounted.
2. Draw cavity. The draw cavity represents the drawing die's female portion. Usually made
from tool steel or solid carbide, it serves as the cavity in which the metal is formed.
3. Ejectors  knockouts. These pressure-loaded components serve to push or eject the part
from the draw cavity. A high-pressure knockout must be timed properly so that it pushes the
part out of the cavity after the die has fully separated. If the knockout is timed incorrectly, the
part can be crushed during the return stroke of the press.
An alternate method to using a knockout is to use a small ejector pin and a lightweight spring.
This spring must have enough force to eject the part adequately but not deform it during the
press's return stroke. The pin and spring method does not require specific timing. However,
keep in mind that certain part geometries require a great deal of force to eject from the cavity.
In such cases, a timed high-pressure knockout may be necessary.
4. Air vents. Air can be trapped during drawing. This trapped air must be vented out of the
tool. Not venting the air can cause defective parts, splitting, and wrinkling, as well as make it
difficult to strip the drawn part from the cavity.
It is critical that both the cavity and punch contain air vents. Air vents in the cavity allow
trapped air to escape during the downstroke of the press; air vents in the punch allow air to be
pulled into the punch, which prevents suction during the part-stripping process.
5. Die face. The die face is the surface surrounding the cavity. It can be a flat or a contoured
surface. This surface interfaces with the sheet metal and keeps it from wrinkling during the
drawing process. The die face typically is made of tool steel or carbide and is highly polished
in the direction of metal flow.
6. Draw punch. This component represents the male shape of the drawn part geometry. Like
the cavity, it usually is made of tool steel. In most cases, it is polished to a mirrorlike surface.
However, there are times when a rough surface is desired.
Figure 3
Click image to view larger
7. Blank holder / draw pad / binder. This pressure-loaded plate, which serves to keep the
metal from wrinkling during the drawing process, typically is loaded with gas springs.
However, certain drawing dies can achieve the force needed to control metal flow through the
use of a press cushion.
8. Pressure system. The pressure system supplies the force necessary to control metal flow. It
may consist of gas, coil, hydraulic, or urethane springs. Certain drawing dies utilize a press
cushion to obtain the needed pressure. A press cushion is a plate or series of vertically moving
thick, flat plates mounted beneath the press's bolster plate. These plates transfer the force to
the bottom of the draw pad using a cushion pin (Figure 3).
9. Equalizer block. This block functions to maintain a specific gap between the die face and
the draw pad surface. It also allows for minor adjustments to be made with respect to how
much pressure is being applied to the blank.
More advanced drawing components, such as draw beads and bars, will be discussed in future
articles in this series. The next installment will discuss how a drawing operation works and
will begin exploring the factors that control drawing die success.