Basic gd&t datums

1. Data Analysis Lab
ME 288
L7 abc

2. GD & T
•Is a method (systems of symbols) for defining
a part’s geometry
–it goes beyond the form description
–based simply on tolerance dimensions
–as directed under rule #1
•A basic dimensionis a theoretical
perfect size, location
or orientation dimension.
12
Varies
Basic Dimension
2h

3. These symbols are used when dimensioning.
1.5h
12
Varies
Basic Dimension
Ø
Diameter
1.5h
0.3h
0.3 h
2h
( )
(
ST
R SR SØ CR
X
2h
h h
90°
h
h
0.6h
60°
h 2h
0.5h
0.5h
30°
h
h
h
15°
2.5h
1.5h0.8h
60°
Counterbore Countersink Depth (or deep)
Dimension OriginConical Taper Square
Arc Length Reference Slope
Radius
Places
or By
Statistical Tolerance
(dimensional)
Spherical
Radius
Spherical
Diameter
Controlled
Radius

4. These symbols are used when dimensioning.
2h
h
h
90°
h
h
0.6h 60°
h 2h
0.5h
0.5h
30°
h
h
Counterbore Countersink Depth (or deep)
Dimension Origin Conical Taper Square
h = height of lettering

5. These symbols are used when dimensioning
1.5h
12
Varies
Basic Dimension
Ø
Diameter
1.5h
.3h
1.6
2h
( )(
ST
R SR SØ CR
X
h
15°
2.5h
1.5h0.8h
60°
Arc Length Reference Slope
Radius
Places
or By
Statistical Tolerance
(dimensional)
Spherical
Radius
Spherical
Diameter
Controlled
Radius

6. A feature control frame (the backbone of GD & T) modifies a
part’s geometry.
0.2AB C
Modifier
Numeric tolerance
(feature tolerance)
Geometric tolerance(characteristic)
symbol (position)
Tertiary datum
Secondary datum with
modifier
Primary datum
2h
Diameter symbol

7. Two additional examples of feature control frames modifying a
part’s geometry.
0.08 0.05 A
Geometric tolerance
(flatness)
Numeric tolerance
Geometric tolerance
(perpendicularity)
Diameter symbol
Numeric tolerance
Modifier
Datum
(A) (B)

8. Geometric characteristic(tolerance) symbols are categorized in
two ways.
*
*
Perpendicularity
Angularity
Runout Circular
Runout Total
Profile surface
Profile line
Symmetry
Cylindricity
Position
Concentricity
Circularity
Straightness
Parallelism
Flatness
Either filled or unfilled
SymbolDescription
Form
Profile
Orientation
Location
Runout
Type of
tolerance
Individual
features
Individual or
related
features
Related
features
Geometric Characteristic Symbols
*
No datum or datumsneeded
A datum or datums
are required
Depending on the situation

9. GD&T geometric characteristic symbols illustrated.
1.5hM1.5hh 0.8h
2h
1.5h
0.6h
h
1.5h
ConcentricityCircularity Modifier
Straightness Parallelism Flatness
1.5h

10. 1.5hh
2h
h h
60°
1.5h
h
h
2h
2h
1.2h
0.5h
Cylindricity Position
All round Profile
surface
Profile
line
Symmetry
GD&T geometric characteristic symbols illustrated.

11. GD&T geometric characteristic symbols illustrated.
h
0.8h
0.8h
3h
0.6h
1.5h
2h 1.5h
30°
45°
0.6h
1.5h
1.1h
**
PerpendicularityAngularity
Runout Circular Runout Total
Between
*
Filled or unfilled
*

12. Table lists GD&T geometric characteristic symbol modifiers.
M
L
P
F
T
Maximum Material Condition
Least Material Condition
Projected Tolerance Zone
Free State Variation
Tangent Plane
Modifiers
STStatistical Tolerance (geometric)
Between
Maximum Material Condition
Least Material Condition
Projected Tolerance Zone
Free State Variation
Tangent Plane
Modifiers
Between
Maximum Material Condition
Least Material Condition
Projected Tolerance Zone
Free State Variation
Tangent Plane
Modifiers
*
Between
*
Filled or unfilled

13. Modifiers
Maximum material condition, MMC
(M)
•It should be taken literally.
•The geometric feature or
size is as large as it can be.
•In the case of a hole, as
small as it can be.
Least material condition, LMC
(L)
•It should be taken literally.
•The geometric feature or
size is as small as it can be.
•In the case of a hole, as
large as it can be.

14. 7B

15. Rule #1:
“Where only a tolerance of size is
specified, the limits of size of an
individual feature prescribe the extent of
which variations in its geometric form, as
well as size, are allowed.”
ANSI Y14.5 –1994

16. When only size tolerance is specified the object’s form can
vary within the stated size limits.
Ø11.8
(B)
(C)
Ø12.2
Ø11.8
Ø12.2
Ø11.8
Ø12.2
Ø11.8
Ø12.2
Ø11.8for entire length Ø11.8 for entire length
External dowel plug Internal hole
(A)
at Ø12.2 MMC must be
perfect form
at Ø11.8 MMC must be
perfect form
Ø
12.2
11.8
Ø
12.2
11.8

17. A cylinder can have a variety of shapes yet stay within the
limits of size.
(A) (B) (C)
Min Min Min
Max Max Max

18. The rectangular prism can vary in shape as long as it stays
inside the volume of the limits of size.
16
14
11
9
9
7
22
20
8
7(A) (B)
18
16

19. 16.0
15.8
Ø8.0 MMC
Ø7.8 LMC along
entire length
of dowel
Ø6.8
MMC
Ø7.0
LMC
Ø
7.0
6.8
Ø
8.0
7.8
Ring gage
Plug gage
(A) Checking geometric form with ring gage
(B) Checking geometric form with plug gage
Checking the size limits envelope:
A ring gage and plug gage are used to check the geometric
form of a pin and hole.

20. Standard Stock Item
•Items whose geometry are already controlled
by established industrial or government
standards
–bars, sheet stock, tubing or structural shapes

21. Datums
•A datumis an exact surface, line, point ,axis or
cylinder from which measurements are taken.
•Ex. A surface plate or a polished slab of granite
(simulated datum).
•3 points define a plane.
•Datum feature–is the surface of the part in contact
with the simulated datum.

22. A height gage measures the height of an object from the
simulated datum surface of a surface plate.
Datum
feature
Simulated
datum
(surface plate)
Measured
height
Dial face
Probe
Up
Down
Note:the datum feature rests on the simulated datum.
the height is measured from the simulated datum and not from the datum feature.

23. Degrees of freedom (12) allow movement in two directions
along each axis and rotation about each axis (clockwise and
counterclockwise).
+Y
-X
+Z
-Y
+X
-Z
In order to measure geometric features
•part motion must be restricted.

24. A datum reference frameconsists of three intersecting planes at
90°to each other.
The part can still move in the positive X, Y and Z directions
“part motion must be restricted”
What if we temporarily clamped the part? → Measurements can be taken
from the simulated datums.

25. Datum surfaces must be indicated on the drawing
2H for single lettering
4H for double lettering
A2HH
H
60°
16
G
H
E
F
Filled or
unfilled
(A) H = height of lettering
(B) Applications
Varies

26. 7C

27. The datum symbol is applied to solid cylinders.
KJ
J
Ø14
Ø8
Ø14
J
-or-

28. FGHØ0.1
Ø12
N
P
R
M
Ø6
Ø12
Ø6
M
The datum symbol is applied to holes.
Note: letters “I”, “O” and “Q” are not used to indicate datumsbecause they may
be confused with numbers one(1) and zero(0).
Double letters can be used e.g. AA, BB etc.

29. Order of Datums:
Primary datum S has 3 pts of contact, secondary datum T has
2 pts and tertiary datum U 1pt of contact.
(A)This drawing
symbology
(B) Means this
S
U
T
3 pts of contact
1 pt of contact
2 pts of
contact
STUØ0.1
T
S
12
2X Ø6 ±0.2
6
M
8
U
10

30. The order of the datumsis critical!
(1 pt)
(2 pts)
(3 pts)
VWXØ0.1
2X Ø6 ±0.08
M
X
W
V
(1 pt) (3
pts)
(2 pts)
True
height
VX
W
Correct inspection procedure
True width
(1 pt)
(3 pts)(2
pts)
VX
W
False
height
Incorrectinspection procedure
False
width

31. The datum axisis formed by two intersecting planes.
G
H
(A) This drawing
8
8
8 8
Ø24
0.5GH
4X Ø4±0.5
Intersecting planes
perpendicular to G
Primary
datum G
Datum axis
(B) Means this

32. The plug gage establishes the datum axis.
(A) This drawing
(B) Means this
C
Ø10.8±0.1
Simulated datum
cylinder C -largest
Ø that fits into hole
Datum axis
Datum feature C
Plug gage

33. The smallest circumscribed cylinder establishes the simulated
datum and the datum axis.
(A) This drawing
(B) Means this
D
Datum feature D
Simulated datum D
Datum axis
19±0.5

34. The simulated central datum planeis established by the center plane of
the largest block that fits into the groove.
B) Means this
A) This drawing
24.2
23.8
12.2
11.8
0.4YZ
Z
Y
Central datum plane
Simulated
datums at
maximum
separation
Datum features

35. The simulated central datum planeis established by the center plane
located by the two blocks at minimum separation.
B) Means this
A) This drawing
18.2
Simulated
datum
planes at
minimum
separation
Central
datum plane
0.6 AB
B
A
17.8