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