Position control is the most powerful and versatile GD&T control. It can be used to locate patterns of holes, locate the center of a feature, and control perpendicularity. Position control ensures proper assembly fit and can be easily checked with functional gages. It capitalizes on maximum material condition bonus tolerances and guarantees features are located within their tolerance zones.

1. GD&T 3 - Position Position Control
• Most powerful and versatile control in GD&T
• Capitalizes on MMC bonus tolerances
• Can be used to :
– Locate patterns of holes
– Locate center of feature (e.g. slot)
– Control perpendicularity of holes, etc.
• Easy to check with functional gages
• Insures proper fit of assemblies
CAD/Fab GD&T - Position 2
Coordinate Dimensioning Coordinate Tolerancing vs. GD&T
X 0.005
57% more area
These points would be
rejected using coordinate
dimensions – but they would
still function correctly
1.505
1.495
.014 Y 0.005
Allowable location for hole center
.010
1.995
2.005
CAD/Fab GD&T - Position 3 CAD/Fab GD&T - Position 4
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2. GD&T Position Dimensioning BASIC Dimension
GD&T A numerical value used to describe the theoretically
control
frame exact size, profile, orientation or location of a
feature or datum target
The associated tolerance for a BASIC dimension is
given in the GD&T control frame
Hole size
parameters
Tolerance of location
Basic Datums Exact (theoretical) value
dimensions Box indicates BASIC dimension
Implied tolerances do not apply!
CAD/Fab GD&T - Position 5 CAD/Fab GD&T - Position 6
BASIC Dimensions and Datums Orthogonal Datum Planes
Secondary Datum Plane
2 points of contact
BASIC dimensions must relate to datums 1 d.o.f. Tertiary Datum Plane
1 point of contact
0 d.o.f.
(i.e. part is fixed)
Primary Datum Plane
Why Datum D?
3 points of contact “3-2-1” Rule
3 degrees of freedom
CAD/Fab GD&T - Position 7 CAD/Fab GD&T - Position 8
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3. How to Measure Holes GO – NO GO Gages
• Diameter
– Specified by diameter and tolerance (MMC/LMC)
– Measure accurately with micrometer/caliper Allowable variation
in hole diameter
– Checked with GO – NO GO gages
• Position
– Specified by GD&T frame & basic dimensions
– Measure accurately with optical comparator Hole Hole
LMC MMC
– Checked with functional gages
GO NO GO Handle
CAD/Fab GD&T - Position 9 CAD/Fab GD&T - Position 10
GO – NO GO Gages Checking Gear Hole Size
Gage can enter “good” hole
– but not past “NO GO step
CAD/Fab GD&T - Position 11 CAD/Fab GD&T - Position 12
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4. GO – NO GO Gages Gaging True Position Holes
Go No Go
Go No Go
Holes at MMC (12.70)
26.18 max at max distance
Centers of holes must lie
within 0.18 tolerance zone
25.82 min
Gage tolerance – Gage pins
0.0002” – 0.00002” Holes at MMC (12.70)
at min distance
dpin = 12.70 – 0.18 = 12.52
$36 $57
When two holes are located at true position at MMC (min. dia.), they
may be gaged with pins 12.52 mm in dia. located 26.00 mm apart
CAD/Fab GD&T - Position 13 CAD/Fab GD&T - Position 14
Pin Gage Dimensions Datum B
Pin Gage
Part Drawing
Datum C
Gage Drawing
Pin diameter = hole diameter @ MMC –
GD&T position tolerance Datum A
i.e. dia = 12.70 – 0.18 = 12.52 mm
Pin location specified by
BASIC dimensions
CAD/Fab GD&T - Position 15 CAD/Fab GD&T - Position 16
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5. Position Lab Combined Datums
Snowboard Binding A gear will be
pressed onto
a gear shaft
which runs in
ball bearings
as shown.
Identify
datums on
the drawing
of the shaft to
Perfect part ensure
proper
operation of
the gear shaft C
assembly.
Combined
datum –
centerline
joining A & B
Reject! A B
CAD/Fab GD&T - Position 17 CAD/Fab GD&T - Position 18
Checking Shaft Runout Combined Datums
2. Shaft rotated in
3. Dial indicator measures
V-blocks
tip movement accurately B
D C
A
1. V-blocks center shaft at each end
CAD/Fab GD&T - Position 19 CAD/Fab GD&T - Position 20
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