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Basic Geometric Dimensioning &
Tolerancing (GD&T)
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 Reading
 Skim CH 9, 11
 Review pp. 380 – 381 (pp. 314 – 318 in old text)
(Dimensioning Do’s & Don’ts)
 Assignment:
 Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text):
 Sketch dimensioned views for b and d , only
 Scale drawing so that the parts are roughly double size on your paper (smallest hole diameter is 2 squares in width)
 Draw orthographic, multi-view with straight edge
(not CAD)
 Use English units – 1 square is .20 inches / side
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 Reading:
 Finish CH 11
 Read CH 10 and start CH 12
 Assignment:
 CH 10, p. 429 (Project p. 390 in old text):
 Exercise 10.2 (Fig. 11.49 in old text) using GD&T
 Exercise 10.3 (Fig. 11.50 in old text) using GD&T
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 Clearance
 Used to allow motion between parts
 Running
 Sliding
 Interference
 Used to mechanically join parts
 Force
 Shrink
 Locational
 Used to constrain the position between parts
 Locational Clearance Fits
 Locational Transition Fits
 Locational Interference Fits
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 Clearance Fit
 the internal member always has a space between it and the external member
 Interference Fit
 the internal member is always larger than the gap in the external member
 Transition Fit
 may result in either a clearance or interference condition
 Line Fit
 limits specified so that either a clearance or exact surface contact condition results
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 Tolerance
 The total amount the feature is allowed to vary
(upper limit - lower limit)
 Basic Size (Basic Dimension – GD&T)
 the theoretical exact value that deviations are applied to, and tolerances are computed from, in order to achieve the desired fit
 Deviation
 The amount that a feature may vary from the basic size in one direction (limit – basic size)
 Allowance

 the minimum space between mating parts the difference between the largest allowable shaft size and the smallest allowable hole size
 Clearance Fit has a positive allowance
 Interference Fit has a negative allowance
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 Basic Hole System
 Used to set tolerances when it is easier to change size of the shaft than the size of the hole
 Minimum hole is taken as the basic size
 Most common system
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 Basic Shaft System
 Used to set tolerances when it is easier to change the size of the hole than the size of the shaft
 Maximum shaft is taken as the basic size
 Least common system
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 Maximum Material Condition (MMC)
 Prevailing conditions when the most material is contained in both features
 Occurs when you have the smallest hole and the largest shaft, simultaneously
 Think of it as when the part weighs the most, and still fits all constraints (perfect form)
 Least Material Condition (LMC)
 Prevailing conditions when the minimum material is contained in both features
 Occurs when you have the largest hole and the smallest shaft, simultaneously - or when the part has perfect form and weighs the least
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 General Tolerances
 Specified by notes in the title block, and apply to all feature sizes unless otherwise specified
 Dimensional (Parametric) Tolerances
 Specified for a specific feature size


Limit Dimensioning
 Both upper & lower limit dimensions are specified
Plus-or-Minus Dimensioning
 Bilateral - a positive and a negative deviation
 Plus AND Minus - symmetric, bilateral deviation
 Unilateral - only a positive or only a negative deviation


Single Limit Dimensioning
 MIN or MAX is placed after the dimension if the other feature size deviation is controlled by another element
Angular Tolerancing
 Bilateral Plus-or-Minus in degrees, minutes, seconds
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General Tolerance Development
 Problem:
 Develop a tolerance for a pneumatic cylinder guide
(slide). The guide is a mating feature consisting of a pin and a hole. The pin will run back and forth within the hole as the cylinder extends/retracts. The hole will be produced with a drill and the shaft will be turned on a lathe. The nominal size is 13/16”, and the allowance is
0.002”. The tolerances will be specified to the thousandths of an inch.
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 Q: Is the hole or the shaft the basis for this application?
 A: It is a basic hole system. The hole will be produced with a standard size drill bit, which is difficult to vary in fine increments. The shaft diameter can be easily varied on a lathe.
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General Tolerance Development
 Q: What kind of fit is required?
 A: Since the nominal allowance is positive, a clearance fit will result. Common sense also tells you that a clearance fit is required to allow the running motion. An interference or transition fit would/could cause binding.
 Q: What is the feature size?
 A: The nominal hole size is 13/16”; converted to decimal inches it is 0.81250. This value is rounded to
.812, using the dimensional rounding rules.
 Q: What is a reasonable tolerance for the hole?
 A: From Table 10.2 (Fig 11.13 old text), for a drilling operation with a nominal feature size between .600” and
.999” the middle of the range of tolerances is .004”.
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General Tolerance Development
 Q: What material condition should the tolerance be based upon?
 A: Since the specified fit is a clearance fit, the worst case condition is when the hole is smallest and the shaft is largest. This is the Maximum Material Condition, as it will constrain the maximum material in either part.
 Q: What is the minimum hole dimension?
 A: For a Basic Hole System, the basic size is the minimum acceptable hole size, or 0.812”.
 Q: What is the maximum hole dimension?
 A: The hole tolerance is the difference between the largest and smallest hole. The reasonable tolerance from Table 10.2 (Fig. 11.13, old text) was 0.004”. Adding it to the minimum hole gives an upper limit of 0.816”.
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General Tolerance Development
 Q: How can we show the hole tolerance?
 A: Using limit dimensioning, and standard English unit practices (no leading zeros) the following would work:
.816
.812
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General Tolerance Development
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 Q: What is the maximum dimension for the shaft?
 A: The smallest hole size is 0.812”. For a clearance fit, subtracting the allowance (0.002”) gives the shaft size at
MMC, or 0.810”.
 Q: What is a reasonable tolerance for the shaft?
 A: From Table 10.2 (Fig. 11.13), for a turning operation with a nominal feature size between .600” and .999” the middle of the range of tolerances is .0025”.
 Q: What is the lower limit for the shaft dimension?
 A: Subtract the tolerance from the maximum dimension to get 0.80750”, then round the dimension to 0.808”.
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General Tolerance Development
 Q: How can we show the shaft tolerance?
 A: Using limit dimensioning, and standard English unit practices (no leading zeros) the following would work:
.810
.808
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Std. Tolerance Development
 Problem:
 Develop a tolerance for an enhanced pneumatic cylinder guide. The hole will still be produced with a drill and the shaft will be turned on a lathe. The nominal size is still
13/16”, but the tolerances will be specified to tenthousandths of an inch.
 Q: How can I specify a tolerance when an allowance is not given?
 A: Empirical design. Look for standard tables or
(previous practices) that help. Start with Table 10.1
(11.1) and note that an RC fit is what is needed for a running clearance. Then Appendix 7 (5) shows that a
Close Running Fit (RC 4) is most appropriate.
Appropriate clearances would run from 0.8 to 2.8 thousandths of an inch for a nominal feature size between 0.71” and 1.19”.
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Std. Tolerance Development
 Q: What are the standard limits to be applied to the basic hole size?
 A: The nominal size was 0.8125”, falling between 0.71 and 1.19 in the left-most column of Appendix 7(5). The standard hole limits for an RC 4 Class fit are + 1.2 and
0 thousandths of an inch, or + 0.0012” and - 0.0000”.
 Q: What are the standard limits to be applied to the basic shaft size?
 A: The nominal size is also 0.8125”, falling between 0.71 and 1.19 in the left-most column of Appendix 7(5). The standard shaft limits for an RC 4 Class fit are - 0.8 and
- 1.6 thousandths of an inch, or 0.0008” and - 0.0016”.
This gives an allowance of + 0.0008” between the shaft and hole at MMC, and a clearance of + 0.0028 at LMC.
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Std. Tolerance Development
 Q: How can we show the mating tolerances?
 A: Using dimensions with a unilateral tolerance and
English unit practices (no leading zeros) the following would work (use computations similar to those for the hole example):
.8125
+ .0012
- .0000
.8117
+ .0000
- .0008
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Std. Tolerance Development
 Q: How can we show the mating tolerances?
 A: Using dimensions with a symmetric bilateral tolerance, the following would work (center the dimensional value and adjust the unilateral deviations):
.8131
+ .0006
- .0006
.8113
+ .0004
- .0004
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Std. Tolerance Development
 Q: How can we show the mating tolerances?
 A: Using the basic size, ISO notation standards for fits, and English unit practices (no leading zeros) the following would work. H8 and f7 are the fit designations from the column headings in Appendix 7(5):
.8125 H8 .8125 f7
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Lecture 07B
GD&T & Examples
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 Geometric Dimensioning & Tolerancing
 Abbreviated GD&T
 Controls feature form / location variations,
NOT feature size variations (width, height,
depth); examples include:
 how cylindrical
 how flat
 how straight
 how symmetric
 how parallel
 Specified using internationally recognized graphic symbols for geometric characteristics
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 Straightness
 Flatness
 Circularity
 Cylindricity
 Perpendicularity
 Parallelism
 Position
 Concentricity
 Material Conditions
 etc. ...
M L
See Table 11.4 p. 373
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 Datums
 specify their capital letter label in a frame
(to distinguish them from section labels) and connect them to the feature by:
 a leader, terminated with a triangle, or
 an extension line, immediately adjacent to the frame
(in this case, there are dashes bracketing the letter)
 Basic Dimensions
 specify basic dimensions between controlled features (just as with size dimensions), but distinguish them with frames
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 Feature Control
 Construct Feature Control Frames by:
 specifying the symbol for the geometric characteristic to be controlled (i.e. position) in a box
 specifying the tolerance zone shape and the tolerance (i.e. diameter of the tolerance zone) in an adjacent box, modifying for material condition at tolerance specification
 specifying the relevant datum(s) in adjacent boxes, modifying for the material condition at measurement
 Append notes as necessary to clarify
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 GD&T allows us to:
 control more of the important aspects of the feature - the geometry as well as the size
 avoid tolerance stacking
 have a cleaner, clearer drawing
 specify tolerance zones in a manner more similar to the way they will be verified – it identifies the datum surfaces from which a feature is to be dimensioned
 helps specify how the part is to be inspected and manufactured – implies how the part is to be fixtured
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 Flatness is a characteristic of a single surface:
 If a surface is sufficiently flat, then all points on the surface will lay in-between two parallel planes separated by the tolerance distance
 Tolerance Zone Depiction:
.002
 Feature Control Frame & Leader :
.002
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 Datums are theoretically perfect:
 The datum is assumed to be exact for the purposes of manufacture and inspection.
 For practical purposes, they need to be 10X more accurately produced than any measurement that will be derived from them.
 For manufacturing purposes, these are the first features to produce, since they control the remaining characteristics of the part.
 Identification :
A
1.02
B
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 Parallelism is a characteristic of two surfaces:
 If a surface is parallel, then it will lay inbetween two planes parallel to the datum and to each other, offset by the tolerance distance
 Tolerance Zone Depiction:
.003
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 Feature Control Frame :
.003 A
2.62
A
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 True Position is a relationship between at least three surfaces:
 If the centerline of the feature is positioned accurately, then it will lay within a tolerance zone envelope sized by the tolerance value
 True Position is a tolerance of location:
 Location is specified by BASIC DIMENSIONS
 The basic dimensions originate at DATUM surfaces
 It may be affected by the size of the produced feature, so design intent should be indicated by the MATERIAL CONDITION modifier
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 To place a hole in the part, we need to locate the center of the hole in the coordinate plane relative to the axis of the hole, and then size the hole (allowing a hole size tolerance)

11.200 ± .002
10.000
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15.500
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 Then we add the GD&T information to control the location of the hole center
C

11.200 ± .002

.001 M A B C
10.000
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15.500
B
A
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Note: Datum A forms the bottom surface of the hole, and so the tolerance zone is a perfect, right cylinder
– resting on Datum A and located from
Datum B and Datum C.
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 This tells us the DATUMS that we will measure from to locate or inspect the hole
C

11.200 ± .002

.001 M A B C
10.000
15.500
B
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A
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 It tells us the BASIC DIMENSIONS that control where the hole is located
C

11.200 ± .002

.001 M A B C
10.000
15.500
B
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A
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 It tells us the size and shape of the tolerance zone for the hole center
C

11.200 ± .002

.001
M A B C
10.000
15.500
B
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A
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 And it tells us the worst case material condition used to inspect the hole center
C

11.200 ± .002

.001 M A B C
10.000
15.500
B
In this case, when the hole is at its’ smallest permissible size , the feature location is in its’ most critical state.
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A
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 Example inspection gage for the hole
C 
11.200 ± .002

.001 M A B C
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B
A
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In this case, when the hole is at its’ smallest permissible size and perfectly located , the feature is in its’ most critical state, and just fits about the red gage pin.
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Effect of Material Condition Modifiers with Feature
Size and Geometric Tolerances
Effect of Feature Size with M Tolerance Modifier:
C
10.000

11.200 ± .002

.001 M A B C
15.500
B
M
Produced
Hole Size
11.197
11.198
11.199
11.200
11.201
11.202
11.203
Geometric
Tolerance
.001
.001
.001
Feature Size
‘Bonus’
.001
.002
Total Effective
Tolerance out of size tolerance
0 .001
.002
.003
.001
.001
.003
.004
out of size tolerance
.004
.005
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GD&T Tolerance Development
 Problem:
 Develop a geometric tolerance to control the location of the hole for the enhanced pneumatic cylinder guide.
The hole is still drilled, the nominal size is still 13/16”, and the limits for an RC 4 class fit will still be used; but the positional tolerance will be specified to be within a
.0001” circular diameter of the true position of the hole, located 2.250” and 5.500” from the bottom left corner.
 Q: How do I specify a tolerance like that?
 A: Use GD&T. The locational parameters become basic dimensions (so put a frame around them). These dimensions should originate with a datum, so label the each datum on the drawing. The feature size is called out with a leader, and a symmetric size tolerance can be developed, just as before.
A feature control frame is needed for the positional tolerance. Fill it out with the symbol for true position, specify the shape and size of the tolerance zone at
MMC, and call out each datum needed to locate the hole.
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Effect of Material Condition Modifiers with Feature
Size and Geometric Tolerances
Effect of Feature Size with M Tolerance Modifier:
C
2.250

.8131 ± .0006

.0001 M A B C
5.500
B
M
Produced
Hole Size
.8122
.8125
.8128
.8131
.8134
.8137
.8140
Geometric
Tolerance
.0001
.0001
.0001
Feature Size
‘Bonus’
.0003
.0006
Total Effective
Tolerance out of size tolerance
0 .0001
.0004
.0007
.0001
.0001
.0009
.0012
out of size tolerance
.0010
.0013
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