1

GDT II

"....Because of its benefits,

GDTP Certification… is now a

long term requirement for Intel

Design Group members."

0.3

Variable gage

Datum D

12.0
11.4

0.3

D

D

CAD/Fab GD&T II

2

Housekeeping

• Quiz 1 next week in lab

– Multiple choice on WebCT

– Focus on Dimensioning & Tolerancing (Lecture 7)

• Individual proposals due next week (W, Th)

– Model something with at least three separate parts

– Document with appropriate drawings

• Assembly drawing

• Detail drawings of three most important / interesting parts

• Team proposals due following week (M, T)

– Team members (2-3 per team, from within lab)

– Proposed design concept (short!)

CAD/Fab GD&T II

3

Indicating Datum Features - Axes

A

0.1 M A B C

D

12.00-12.07

In each case, the datum is the axis of

the hole or cylindrical

datum feature

datum feature

CAD/Fab GD&T II

4

Datum Center Plane

(Internal)

Simulated datum A –

parallel planes at

maximum separation

Datum feature

simulator

Datum A

feature

(slot width)

Datum A

center plane

2

CAD/Fab GD&T II

5

More Datums

Part

Datum A

A. Place part on surface plate (3 d.o.f.)

B. Center part on center hole (1 d.o.f.)

Datum B

Axis of largest

inscribed cylinder

to A

Datum axis B

C. Fix part from rotating by slot (0 d.o.f.)

Datum C: // planes at max.

separation to datum A

Center plane aligned w/ axis B

Datum center

plane C

CAD/Fab GD&T II

6

Size vs. Geometric Form

• Size is specified by coordinate dimensions
• Limits of size (i.e. allowable variation) are

specified by tolerances

• All dimensions (w/ tolerances) have built-in

(natural) geometric controls

• Add GD&T symbols only to refine natural

controls

– Put precision where you need it
– Relax precision elsewhere

CAD/Fab GD&T II

7

Limits of Size Rule

(Rule 1)

When only a tolerance of size* is specified, the

limits of size of an individual feature

prescribe the extent to which variations in its

geometric form as well as size are allowed

i.e. A tolerance of size also controls

form – but it does so indirectly

* Tolerance of size = ± XX

CAD/Fab GD&T II

8

“Envelope Principle”

• Rule 1 establishes the envelope of the

“perfect” part

• Any deviation in form is acceptable, as long

as it remains within the limits of size

Actual part outline

Upper limit of size

Lower limit of size

3

CAD/Fab GD&T II

9

Shaft Example

Form controlled by dimensions (limits of size):

• Circularity
• Straightness of line elements of cylinder
• Parallelism between any opposite lines
• Flatness of ends
• Other geometry?

CAD/Fab GD&T II

10

3.22 - 3.24

“Envelope Principle”

Part must be
perfect at MMC

3.24

3.22

Limits of size

When not at MMC, any

form is acceptable, as long

as it fits within the

tolerance zone

CAD/Fab GD&T II

11

Tolerance Zones

• Circular

– Specified by diameter symbol -

φ

• Non-circular

– Bounded by parallel lines or planes

CAD/Fab GD&T II

12

GD&T Summary Chart (see web site)

4

CAD/Fab GD&T II

13

Envelope Principle

CAD/Fab GD&T II

14

Flatness

Tolerance zone can
have any orientation –
because it’s not linked to
any datum

CAD/Fab GD&T II

15

Perpendicularity

CAD/Fab GD&T II

16

Lego Block Lab

Tolerance zone

perpendicular to datum plane

Tolerance zone
width

5

CAD/Fab GD&T II

17

Rule of Thumb

• GD&T tolerance values are typically no

more than half the size limits.

Size limits

GD&T tolerance

CAD/Fab GD&T II

18

Perpendicularity of Axis

Circular tolerance zone

Material condition:
- Zone as specified at MMC
- Bonus tolerance as part deviates from MMC

CAD/Fab GD&T II

19

Bonus Tolerance

LMC

Bonus tolerance

Datum A simulator

MMC

Tolerance zone

perpendicular to

Datum A

Larger tolerance

zone

CAD/Fab GD&T II

20

Functional Gaging

Simple way to check
part acceptability

zone

tol

MMC

φ

φ

φ

+

=

• Good parts will fit correctly
• Bad parts will not