Geometric Dimensioning and Tolerancing GDT

1. Geometric Dimensioning and Tolerancing (GD&T) 2/18/2012 1

2. Introduction GD&T is a method of defining parts by how they function. The method has been developed over the last forty years. It allows a designer to define the features of a part without increasing tolerances. The current standard is ASME Y14.5M-1994. 2/18/2012 2

3. The Traditional Approach Suppose we want to manufacture tables with a flat top. With a traditional approach, how do we achieve this goal? 2/18/2012 3 With this approach all we can do is to make the tolerances tighter. If we go to say 30With this approach all we can do is to make the tolerances tighter. If we go to say 30

4. The Traditional Approach Making the tolerance tighter is not always good enough. In this case, making the tolerance more restrictive may lead to a good table being rejected. 2/18/2012 4 With this approach all we can do is to make the tolerances tighter. If we go to say 30With this approach all we can do is to make the tolerances tighter. If we go to say 30

5. The GD&T Approach With this approach, we directly control the desired feature. This allows the tolerance on the size to be relaxed. 2/18/2012 5

6. The Traditional Approach The holes in the plate must be located with high precision. How do we achieve this goal using a traditional approach? 2/18/2012 6 With this approach all we can do is to make the tolerances tighter. If we go to say 30With this approach all we can do is to make the tolerances tighter. If we go to say 30

7. The GD&T Approach The same plate dimensioned using GDT. Why is this superior? 2/18/2012 7

8. Datums and GD&T Application of GDT principles begins with the use of theoretical reference datums (planes, circles, lines and points). By using datums, there is no ambiguity as to the reference for the feature because: The feature is designed with respect to the datum(s). The feature is manufactured with respect to the same datum(s). The feature is inspected using the same datum(s). 2/18/2012 8

9. Control Symbol Symbols defining the GD&T control have been standardized. Here is a typical control symbol: 2/18/2012 9 The envelope principle is used in the US and extensively elsewhere. It is hotly debated. For a hole, the diameter of the hole is larger than the envelope which is at MMC.The envelope principle is used in the US and extensively elsewhere. It is hotly debated. For a hole, the diameter of the hole is larger than the envelope which is at MMC.

10. The Envelope Principle Hole with MMC condition. 2/18/2012 10 The envelope principle is used in the US and extensively elsewhere. It is hotly debated. For a hole, the diameter of the hole is larger than the envelope which is at MMC.The envelope principle is used in the US and extensively elsewhere. It is hotly debated. For a hole, the diameter of the hole is larger than the envelope which is at MMC.

11. Geometric Size Controls Controls related to the size of a feature may be used in conjunction with MMC. These controls are: 2/18/2012 11

12. Geometric Controls 2/18/2012 12

13. Virtual and Resultant Condition With GD&T, more than one control may be applied to a feature. The overall effect is determined by the RESULTANT CONDITION, a variable. The VIRTUAL CONDITION is a constant value, which takes into account the maximum allowed variation. Consider: 2/18/2012 13 Resultant is size control in combination with the deviation from the condition. Resultant condition cannot violate the design envelope. Resultant is size control in combination with the deviation from the condition. Resultant condition cannot violate the design envelope.

14. Virtual and Resultant Condition Hole Plate: 2/18/2012 14 Resultant is size control in combination with the deviation from the condition. Resultant is size control in combination with the deviation from the condition.

15. Virtual Condition MMC is .5312-.0156=.5156 Virtual condition is .5156-.0156=.5000 2/18/2012 15 Resultant is size control in combination with the deviation from the condition. Resultant is size control in combination with the deviation from the condition.

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18. Summary GDT allows the designer to directly control a feature. Ambiguities between the designer and manufacturer are avoided through the use of reference datums. The condition of the part is taken into account when a tolerance is applied. Use of the Envelope Principle leads to high precision parts at low cost. 2/18/2012 18