This document provides information about geometric dimensioning and tolerancing (GD&T). It defines GD&T as a method for defining the geometry of a part beyond simple tolerance dimensions. Feature control frames modify a part's geometry and include the geometric tolerance symbol, datum, and modifiers. Geometric characteristic symbols indicate the type of tolerance such as flatness, circularity, or perpendicularity. Datums establish the reference frame for measurements and include primary, secondary, and tertiary datums indicated on drawings.

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 dimension is a theoretical
perfect size, location
or orientation dimension.
2h
12
Varies
Basic Dimension

3. These symbols are used when dimensioning.
h
2h
90°
h
h
h
0.6h
Counterbore
60°
Depth (or deep)
Countersink
0.5h
h
h
0.5h
h
2h
30°
Square
Conical Taper
Dimension Origin
0.3h
1.5h
(
( )
0.3 h
Arc Length
15°
h
1.5h
Slope
Reference
2h
12
X
Places
or By
R
Radius
2.5h
0.8h
Varies
Basic Dimension
Ø
Diameter
SR
Spherical
Radius
ST
1.5h
60°
Statistical Tolerance
(dimensional)
SØ
Spherical
Diameter
CR
Controlled
Radius

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

5. These symbols are used when dimensioning
.3h
1.5h
(
( )
1.6
Arc Length
15°
h
1.5h
Slope
Reference
2h
2.5h
X
12
Places
or By
Varies
0.8h
ST
1.5h
60°
Basic Dimension
Statistical Tolerance
(dimensional)
R
Ø
SR
SØ
Radius
Diameter
Spherical
Radius
Spherical
Diameter
CR
Controlled
Radius

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

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

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

9. GD&T geometric characteristic symbols illustrated.
0.6h
2h
1.5h
Straightness
1.5h
h
Concentricity
1.5h
h
Parallelism
1.5h
Circularity
Flatness
1.5h
M
0.8h
Modifier

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

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

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

13. Modifiers
Maximum material condition, MMC
(M)
Least material condition, LMC
(L)
• It should be taken literally.
• 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.
• 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.
Ø12.2
Ø12.2
(B)
12.2
Ø 11.8
12.2
Ø 11.8
Ø11.8
(A)
at Ø12.2 MMC must be
perfect form
Ø11.8
Ø12.2
Ø12.2
at Ø11.8 MMC must be
perfect form
(C)
Ø11.8
Ø11.8 for entire length
External dowel plug
Ø11.8
Ø11.8 for entire length
Internal hole

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

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

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

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 datum is 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.
Dial face
Probe
Up
Down
Measured
height
Simulated
datum
(surface plate)
Datum
feature
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).
+Z
-X
-Y
+Y
+X
In order to measure geometric features
• part motion must be restricted.
-Z

24. A datum reference frame consists 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
Varies
H
2H
H
A
60°
Filled or
unfilled
2H for single lettering
4H for double lettering
(A) H = height of lettering
E
G
F
16
H
(B) Applications

26. 7C

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

28. The datum symbol is applied to holes.
Ø12
Ø0.1
M
F
G H
N
Ø12
M
R
Ø6
P
Ø6
Note: letters “I”, “O” and “Q” are not used to indicate datums because 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.
2X Ø6 ± 0.2
Ø0.1 M S T U
8
(A)This drawing
symbology
U
6
12
10
U
1 pt of contact
3 pts of contact
(B) Means this
2 pts of
contact
S
T
T
S

30. The order of the datums is critical!
2X Ø6 ± 0.08
Ø0.1 M V W X
X
(1 pt)
V
(3 pts)
W (2 pts)
X
(1 pt)
True
height
True width
W (2 pts)
Correct inspection procedure
V
(3
pts)
X
(2
pts)
False
height
False
width
V
(3 pts)
W (1 pt)
Incorrect inspection procedure

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

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

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

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

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