1. Geometric Dimensioning and Tolerancing
Unit 1 – Introduction, Symbols, and Terms

2. History and Background
Eli Whitney, is the inventor of the cotton gin and a pioneer in the use of mass production methods.
Around 1798, he won a contract to supply muskets to the United States government. The firearms manufacture were based on the concept of interchangeable parts.
He made a presentation to congress by building 10 guns and assembling and disassembling them claiming the same exact parts and mechanisms.
Eli Whitney

3. Limit Tolerancing

4. Limit Tolerancing

5. Limit Tolerancing
Is the hole horizontal position measured from a true vertical plane or from the as built face?
A .005” tolerance on the horizontal and vertical position of the hole means that the position could be off by as much as .007”.
max allowed error
for hole center
+.005
.007
perfect location
for hole center

6. Limit Tolerancing
Limit tolerances don’t have an origin or any orientation or location relative to datums.
The datums are usually implied.
The drawings are subject to different interpretations.
Plus/minus tolerancing works well for individual features of size (ex. diameter of a shaft), but does not control the relationship between individual features very well.

7. Geometric Tolerancing Workbook
Everyone should have a workbook to follow along.
It contains necessary reference information along with class exercises.
Page numbers in yellow on the slides match pages in the workbook.
The workbook for the class. The students should follow along with the workbook. There will be exercises at the end of each chapter for the student to complete. The answers are in the Leaders Guide which is available from Technical Consultants Inc. at the Longboat Key office.

8. History of Dimensioning and Tolerancing Standards in the USA
Mil Std ’s
Mil Std 8A
Mil Std 8B
Mil Std 8C-1963
ASA-Y
USASI Y
ANSI Y
ANSI Y14.5M-1982
ASME Y14.5M-1994
ASME Y
The first dimensioning and tolerancing standards in the USA were military standards in the 50’s. There were revisions A,B and C. The standards have undergone name changes in the past, American Standards Association (ASA).
The current standard is the American Society of Mechanical Engineers (ASME Y ) The M in the title, which meant “metric compatible” was dropped because it was deemed no longer necessary. The standard can be used with both inch and metric values. It is a ASME standard with American National Standards Institute approval. Standards are always improved and work is taking place on the next revision. Additional information can be found at
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9. Lightning bolts show new symbols
Common Symbols
This is a list of all the common symbols used in geometric tolerancing. Notice that most of the symbols are the same in the ISO standard. The symbols with an lightning bolt are new in the 2009 standard.
Lightning bolts show new symbols
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10. 1.10 Geometric Characteristic Symbols
These are the 14 geometric characteristic symbols. This is what the course is all about. Notice that features are organized into categories such as form, orientation, location and runout.
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11. 1.11 Common Tolerance Zone Shapes
Geometric tolerancing defines tolerances zones in which the features must lie. These are the common types of zones. Notice that some of the symbols are 3D tolerances and others are 2D tolerances.
Most parts are 3D and require 3D tolerances. 2D tolerances often refine 3D tolerances. Extrusions might be an example where 2D tolerances are used.
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12. 1.12 Geometric Tolerance Categories “Happy Features”
These are features, happy features. Notice the form tolerances do not require datums. The orientation tolerances always require datums. The location tolerances also usually require datums.
There is a hierarchy in the symbols. The location tolerances form the base. All features have to be located to the datum reference frame. The orientation tolerances are next and serve to refine the orientation of the feature, if necessary. The form tolerances are next and refine the form of the feaures.
Position tolerance locates features with size and profile tolerance locates features without size.
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13. 1.9 Features of Size and Features without Size
The graphic shows that a part is composed of features. There are two types of features, features without size and features with size. We will see later that features of size can be modified at MMC or LMC. Features without size can not be modified at MMC or LMC.
We will also see later that to locate features of size we generally use the position symbol. To locate features without size we generally use the profile symbol.
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14. Tolerances All dimensions require a tolerance.
A tolerance should be as large as possible without interfering with the function of the part to minimize production costs.
Consider how your part will be checked to see if it meets the tolerances.

15. Tolerance Notes

16. Geometric Dimensioning and Tolerancing (GDT)
establish a reference coordinate system by defining datums
provide basic dimensions (perfect dimensions) relative to the datums
specify allowable tolerances

18. ±1° 1.8 units are mm Common Symbols applied to a Drawing 25 10 18
This is a drawing with a lot of common symbols. The instructor should go over each of the symbols and show their application. Additional symbols will be introduced later in the course as they come up. Notice the old datum symbol from the 1982 standard. The reason for the change was to make the U.S, more compatible with ISO. Might see some older drawings like that.
It is a negotiation process where all parties give and take. ISO meetings generally take place twice a year in various parts of the world. The American standards meets also take place twice during the year. They are open public meetings and anyone can attend. Contact ASME for more info.
±1°
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24. C A B

25. Specify flatness.
Document tolerances for the center hole.

26. Document tolerances for the outer holes.

27. Document tolerances for the square hole.

28. Document tolerances for the three slots.

29. Dimension the thickness.

30. Indexer Assembly
This is an indexer assembly. The drawing in the next slide iw the blue indexer plate in this assembly. Notice how the part fits in the assembly. The datums and the tolerancing scheme represent this functional assembly.
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31. Indexer Plate
This is a drawing with geometric tolerancing applied. The part shown has many features. Feature control frames are applied to the various features and give the instructions for that feature. Every feature control frame starts with the words, “This feature” flat, “This feature”, datum A. etc. The tolerance in every feature control frame is total, plus/minus is not allowed in the feature control frame.
Notice that each feature has a feature control frame. It starts out with the left face defined as flat and referenced as a datum. Without the flatness spec it is not clear as to the flatness. The pilot pin is controlled perpendicular to datum A and referenced as datum B. Without the perp spec it is not clear as to the perp. The size dimension on the pilot does not control perp. The bottom flat surface is located to the pilot pin with a profile tolerance and referenced as datum C. The .900 basic dimension defines the distance of the bottom flat relative to the pilot pin. The 3 holes are located to the A,B, C datum reference frame by the basic bolt circle.
General discussion should take place as to possible reasons the designer selected the datums and the various specifications based on the assemble shown on the preceding slide. General discussion should also take place as how the part is to be verified and manufactured considering the specifications.
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32. Indexer Assembly
ASME Y , Digital Product Definition Data Practices
This is a 3 D version of applying geometric tolerancing to the solid model per the ASME Y standard. This is a screen shot. All math data is considered basic.
The Y standard defines the tolerancing concepts. The Y14.41 shows how to apply it to models.
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33. Workshop Exercise 1.1 1.23 12 Student Exercise
Have the students work the problems in the GeoTol Pro and afterwards, click thru having the students help with the answers.
We usually let them have open book so they become familiar with the book and can find information.
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