Application of HSS Cutting Tools in the Mechanical
Engineering Industry
Prof. Dr.- Ing. Diethard Thomas
FETTE GmbH, 21493 Schwarzenbek, Grabauer Straße 24, Germany
Fette is a member of the Leitz Metalworking Technology Group LMT
1. INTRODUCTION
More than 90% of the worldwide consumption of taps and more than 60% of all hobs
are made of HSS. The real competition in cutting material occurs in the milling cutter
section. In this field HSS looses more and more market share against carbide. The
present market share of HSS milling cutters is roughly 30%. The advantage of HSS
compared with carbide and other modern cutting materials is the higher toughness
and the lower price. That allows the economical and also ecological application in a
field of lower speed and feed combined with instable cutting conditions. We find
these conditions often in the mechanical engineering industry also in the export
market, with older machine tools and smaller order batch sizes.
2. LIFE CYCLE COST AND PERFORMANCE
The basic step will be to define the real application field of HSS milling cutters. It
depends on different boundary conditions and can be calculated for a typical
example. Figure 1 shows a comparison of HSS- and solid carbide milling cutters.
Figure 1: Life cycle cost and life cycle performance of HSS- and solid carbide (SC)
end mills.
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This application example shows a roughing operation of Tool Steel 1.2312 with a
tensile strength of 1.200 N/mm2. Basing on the cutting data the life cycle cost and the
life cycle performance is calculated. The life cycle cost depend on the tool price, the
tool life, the number of regrinding cycles and the regrinding cost. In this case the life
cycle cost are 98 Ź for the HSS end mill and 238 Ź for the SC end mill.
The life cycle performance depends on the total tool life (tool life per life cycle) and on
the rate of metal removal. In this case the life cycle performance is a machined chip
volume of 10.260 cm3 for the HSS tool and 103.050 cm3 for the SC tool.
Figure 2: Life cycle cost and life cycle performance of HSS- and SC end mills with a
diameter of 20 mm, roughing tool steel.
Figure 2 shows the results of this calculation and also the  Break Even Point of the
solid carbide end mill with 25.000 cm3. This  Break Even Point marks the corner of
the HSS application field and separates it from the carbide field.
Figure 3 shows the  fight between HSS and carbide. Both cutting materials want to
extend there areas. HSS fights with new PM materials and with modern coatings.
Solid carbide penetrates the typical HSS field for example with micro tools in the left
side with the smallest speed and the smallest stiffness of all possibilities.
But there is one area in this picture, which is rational not realizable for the carbide
milling cutters because of technology (cutting data) and because of cost. This is the
very important HSS- place for  Special Application
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Figure 3: Application fields for HSS- and carbide milling cutters
3. SPECIAL APPLICATIONS FOR HSS MILLING CUTTERS
3.1 Instable machining conditions
Extremely instable machining conditions are described in the both following
examples:
The rails of older railways need special holes for mounting the electrical connection.
Therefore people walk along the railway with a special equipment on their shoulders.
The milling cutter is driven by a combustion engine over a flexible shaft and tracked
by hand. The power of the engine and the cutting speed are very low. Only special
HSS milling cutters (in this case: slot drills, diameter 20 mm) are able to do this
operation. The tool life travel is 20 holes.
A patient needs a new hip-joint made of stainless steel. At the beginning of the
operation the surgeon has to mill a conical hole into the hip-bone. He uses a tapered
HSS- bone-milling cutter with a length of cutting edge of 300 mm. The resulting chips
are bone- meal. The special milling cutter is connected with the engine by using a
flexible shaft and the tool is tracked by hand.
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These are no typical examples for the mechanical industry, but they show also the
special performance of HSS-tools.
Figure 4 shows the stability conditions of milling cutters and work pieces in general.
The stability conditions can be separated in four different classes. In the three
instable classes the performance of HSS-milling cutters is better than carbide milling
cutters because of the higher toughness.
Figure 4: Stability conditions of milling cutters and work pieces
In the next application example (Figure 5) we find a stability situation in which the
milling cutter is stable (small overhang) and in which the work piece is instable.
Besides is the machining of the work piece material (Inconel 718) very difficult and
needs big cutting forces. The customer wanted to machine a slot into this work piece
with a carbide tipped end mill, but he had no success. The cutting edge is broken
several times in the beginning of the machining process. Therefore he changed the
tool an took a PM-HSS milling cutter, diameter 20 mm with a cutting speed of 10
m/min and a feed per tooth of 0,15 mm. The result was a tool life travel of 3,5 m.
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Figure 5: Machining Inconel 718 with HSS at instable machining conditions
3.2 Difficult cutting tool geometry
One of the main advantages of HSS is the economic realization of difficult cutting tool
geometries for complicated work pieces. The foot of turbine blades for example has
not only a complicated geometry, also material and stability conditions are very
difficult for machining operations. Figure 6 shows pictures from the turbine blades
and from six combined HSS form milling cutters with a diameter of 350 mm. The final
turbine wheel has a diameter of 6 m. The work piece material is X12 CrNiMoV12-1.
The cutting data of the milling operation are: max. speed = 33 m/min; rpm = 30;
feed= 0,04 mm/rev. (roughing) and 0,02 mm/rev (finishing).
Figure 6: Machining slots with HSS form milling cutters into a foot of turbine blades
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Types of typical and more known form milling cutters are t-slot milling cutters, form
relieved convex and concave milling cutters, angle and double angle milling cutters,
form milling cutters for gears and ball nose milling cutters.
The next application example in figure 7 shows a HSS shell type angle milling cutter
for machining a conical work piece made of 1.2346. The technical data of the tool
are: Max. diameter 135 mm; 18 cutting edges with nonuniform pitch; cutting material
PM-HSS, coated with TiAlN. The cutting data are: Max. speed = 42 m/min; rpm=
100; feed = 180 mm/min; feed per tooth = 0,1 mm.
The cutting conditions are instable because of big overhang (280 mm).
Figure 7: Machining of a conical steel part with a PM-HSS shell type angle milling
cutter
3.3 HSS indexable inserts
There are a number of modern work piece materials which are very difficult for
machining. Sometimes carbide milling cutters are not suitable for these materials.
The following example describes a problem with machining Aluminium alloy AlMgSi2.
The user wanted to solve this problem with a carbide inserted milling head. His
machine tool could realize a maximum cutting speed of 120 m/min for this tool. He
tested coated and uncoated carbide inserts. The result was in both cases the same
(figure 8): the material glued on the inserts.
For the next step the user changed the inserts and machined with uncoated PM-HSS
inserts. The result was much better, the tool life increased.
The best results were achieved with special coated and polished HSS-inserts. The
coating was nano composite Si3N4 with a TiAlN layer.
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Figure 8: Material glued on when machining Aluminium alloy with a carbide inserted
milling head at lower cutting speed (Reference: GFE e.V., Schmalkalden)
3.4 Dry Milling with HSS
Even under ecological milling conditions- without coolant lubrication- HSS milling
cutters run very well.
Figure 9: Comparison of dry and wet milling with PM-HSS end mills
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Figure 9 shows a comparison of dry and wet milling with PM  HSS end mills.
Remarkable for this example is, that
- the dry milling operation has less wear than the wet milling operation,
- in both cases high cutting speeds of 125 m/min are possible.
This diagram is valid for a finishing operation with the following tool- and cutting data:
PM-HSS end mill, diameter =12 mm, coated with TiCN/TiN, number of teeth = 4,
rpm= 660  3.300, radial depth of cut = 2 mm, axial depth of cut = 12 mm, feed per
tooth= 0,1 mm. Material: 42 CrMo 4 with a tensile strength of 800 N/mm2.
Figure 10 shows an example for dry roughing and dry finishing the material 42CrMo4.
The cutting data of the finishing operation are described above, but the cutting speed
is 100 m/min.
The cutting data of the roughing operation are: rpm= 960, cutting speed = 36 m/min,
feed per tooth= 0,08 mm, cutting feed= 310 mm/min, axial depth of cut= 12 mm,
radial depth of cut= 12 mm (slot milling).
Figure 10: Application example for dry roughing and dry finishing
3.5 Machining of plastic models
The design departments of the automotive industry have to develop new car models.
For their design studies they often use a ductile type of plastic moulding material,
called  Plastelin . After the first design step with the CAD-system they machine the
model true to the original on a milling machine tool or on a measuring machine with a
special milling drive. Therefore they use preferably a coated PM-HSS end mill with a
ball nose geometry and with big flutes and small rake angles. The coating is
TiCN/TiN.
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Figure 11 shows the milling cutter and the application example. The range of cutting
data is: Rev.= 4.000 - 6.000 /min, cutting speed= 200 - 300 m/min, feed = 5.000 
10.000 mm/min, feed per tooth = 0,6  0,8 mm, axial depth of cut = 5  45 mm,
radial depth of cut = 5 mm. The milling cutter diameter was 16 mm in this case.
Figure 11: Machining a ductile type of plastic moulding material with a special PM-
HSS end mill
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