1. Basic Geometric Dimensioning & Tolerancing (GD&T)
Lecture 06
Basic Geometric Dimensioning & Tolerancing (GD&T)
4/22/2017
Engineering Graphics & 3-D Modeling

2. Assignment: HW 06 Due Today
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
4/22/2017
Engineering Graphics & 3-D Modeling

3. Engineering Graphics & 3-D Modeling
Last HW Assignment: HW 07
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 in old text) using GD&T
Exercise 10.3 (Fig in old text) using GD&T
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Engineering Graphics & 3-D Modeling

4. Engineering Graphics & 3-D Modeling
Fit Purposes
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
4/22/2017
Engineering Graphics & 3-D Modeling

5. Tolerancing Definitions
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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Engineering Graphics & 3-D Modeling

6. Engineering Graphics & 3-D Modeling
Definitions
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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Engineering Graphics & 3-D Modeling

7. Engineering Graphics & 3-D Modeling
Hands-On 11.1
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Engineering Graphics & 3-D Modeling

8. Engineering Graphics & 3-D Modeling
Tolerance Systems
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
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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Engineering Graphics & 3-D Modeling

9. Engineering Graphics & 3-D Modeling
Extreme Conditions
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
4/22/2017
Engineering Graphics & 3-D Modeling

10. Specifying Tolerances
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
4/22/2017
Engineering Graphics & 3-D Modeling

11. 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.
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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Engineering Graphics & 3-D Modeling

12. 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 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 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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Engineering Graphics & 3-D Modeling

13. 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 , old text) was 0.004”. Adding it to the minimum hole gives an upper limit of 0.816”.
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Engineering Graphics & 3-D Modeling

14. 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:
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Engineering Graphics & 3-D Modeling

15. General Tolerance Development
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 ), 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 ”, then round the dimension to 0.808”.
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Engineering Graphics & 3-D Modeling

16. 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:
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Engineering Graphics & 3-D Modeling

17. 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 ten-thousandths 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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Engineering Graphics & 3-D Modeling

18. Std. Tolerance Development
Q: What are the standard limits to be applied to the basic hole size?
A: The nominal size was ”, 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 and thousandths of an inch, or ” and ”.
Q: What are the standard limits to be applied to the basic shaft size?
A: The nominal size is also ”, 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 and thousandths of an inch, or ” and ”. This gives an allowance of ” between the shaft and hole at MMC, and a clearance of at LMC.
4/22/2017
Engineering Graphics & 3-D Modeling

19. 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
.8117
4/22/2017
Engineering Graphics & 3-D Modeling

20. 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
.8113
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Engineering Graphics & 3-D Modeling

21. 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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Engineering Graphics & 3-D Modeling

22. Engineering Graphics & 3-D Modeling
Lecture 07B
GD&T & Examples
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Engineering Graphics & 3-D Modeling

23. Engineering Graphics & 3-D Modeling
Geometric Tolerances
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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Engineering Graphics & 3-D Modeling

24. Engineering Graphics & 3-D Modeling
Symbols
Straightness
Flatness
Circularity
Cylindricity
Perpendicularity
Parallelism
Position
Concentricity
Material Conditions
etc. ...
M L
See Table 11.4 p. 373
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Engineering Graphics & 3-D Modeling

25. GD&T Symbol Construction
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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Engineering Graphics & 3-D Modeling

26. GD&T Symbol Construction
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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Engineering Graphics & 3-D Modeling

27. Engineering Graphics & 3-D Modeling
Why GD&T?
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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Engineering Graphics & 3-D Modeling

28. Example: Flatness (No Datum)
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:
Feature Control Frame & Leader:
.002
.002
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Engineering Graphics & 3-D Modeling

29. Example: Identifying Datums
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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Engineering Graphics & 3-D Modeling

30. Example: Parallelism (One Datum)
Parallelism is a characteristic of two surfaces:
If a surface is parallel, then it will lay in-between two planes parallel to the datum and to each other, offset by the tolerance distance
Tolerance Zone Depiction:
Feature Control Frame:
.003 A
2.62 A
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Engineering Graphics & 3-D Modeling

31. Example: True Position (Multi-Datum)
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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Engineering Graphics & 3-D Modeling

32. Two-Dimensional Tolerance View
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)
 ± .002
10.000
15.500
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Engineering Graphics & 3-D Modeling

33. Three-Dimensional Tolerance View
Then we add the GD&T information to control the location of the hole center C
 ± .002
 .001 M A B C
10.000
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. B
15.500 A
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Engineering Graphics & 3-D Modeling

34. Three-Dimensional Tolerance View
This tells us the DATUMS that we will measure from to locate or inspect the hole C
 ± .002
 .001 M A B C
10.000 B
15.500 A
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Engineering Graphics & 3-D Modeling

35. Three-Dimensional Tolerance View
It tells us the BASIC DIMENSIONS that control where the hole is located C
 ± .002
 .001 M A B C
10.000 B
15.500 A
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Engineering Graphics & 3-D Modeling

36. Three-Dimensional Tolerance View
It tells us the size and shape of the tolerance zone for the hole center C
 ± .002
 .001 M A B C
10.000 B
15.500 A
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Engineering Graphics & 3-D Modeling

37. Three-Dimensional Tolerance View
And it tells us the worst case material condition used to inspect the hole center C
 ± .002
 .001 M A B C
10.000
In this case, when the hole is at its’ smallest permissible size, the feature location is in its’ most critical state. B
15.500 A
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Engineering Graphics & 3-D Modeling

38. Inspection of the Tolerance
Example inspection gage for the hole C
 ± .002
 .001 M A B C
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. B A
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Engineering Graphics & 3-D Modeling

39. Total Effective Tolerance Engineering Graphics & 3-D Modeling
Effect of Material Condition Modifiers with Feature Size and Geometric Tolerances
Effect of Feature Size with M Tolerance Modifier: C
10.000
 ± .002
 .001 M A B C B
15.500
Produced Hole Size
Geometric Tolerance
Feature Size ‘Bonus’
Total Effective Tolerance
11.197
out of size tolerance
11.198
.001
11.199
.002
11.200
.003
11.201
.004
11.202
.005
11.203 M
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Engineering Graphics & 3-D Modeling

40. 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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Engineering Graphics & 3-D Modeling

41. Total Effective Tolerance Engineering Graphics & 3-D Modeling
Effect of Material Condition Modifiers with Feature Size and Geometric Tolerances
Effect of Feature Size with M Tolerance Modifier: C
2.250
.8131 ± .0006
 M A B C B
5.500
Produced Hole Size
Geometric Tolerance
Feature Size ‘Bonus’
Total Effective Tolerance
.8122
out of size tolerance
.8125
.0001
.8128
.0003
.0004
.8131
.0006
.0007
.8134
.0009
.0010
.8137
.0012
.0013
.8140 M
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Engineering Graphics & 3-D Modeling