1. ADVANCED MECHANICAL DRAFTING LECTURE #8

2. Challenge of Mechanical Design Only discipline where tolerances of 100ths, 1000ths, or 10,000ths of an inch affect part function

3. Limitations of Standard Dimensioning Practices Difficult to control form (shape), orientation, location, etc. by measuring from other manufactured features

4. GEOMETRIC DIMENSIONING & TOLERANCING Implemented by General Motors in the 1940’s to augment standard dimensioning practices A system that defines zones of tolerance within which features must lie Dimensions that control zone locations and sizes are different than standard dimensions –BASIC dimensions control locations of tolerance zones No tolerance – considered perfect –FEATURE CONTROL FRAMES control shapes and sizes of zones –DATUM FEATURE SYMBOLS identify reference features All three items show in frames (boxes) on the drawing .006 A BA8.500

5. EXAMPLE 4.00 2.00 THIS on the drawing COULD LOOK LIKE 4.00 2.00 THIS when machined DIMENSIONS ALONE CANNOT ADEQUATELY CONTROL THE SHAPE OF THE PART

6. EXAMPLE 1.503 1.498 1.503 1.498.005 SQUARE ZONE

7. + + + + + + + + + Square position tolerance zones vs round position zones “+” = actual hole center Tolerance zone should be round with a diameter equal to the diagonal distance across the rectangular zone THE ROUND ZONE IS 57% LARGER IN AREA THAN THE SQUARE ZONE !

8. EXAMPLE     Ø1.500 (1-1/2) THRU      Choose reference features Locate tolerance zone Specify shape and size of zone Note that the part of the dimension outside the frame applies to the feature and the part inside the frame applies to the tolerance zone

9. DRAWING BASIC DIMENSIONS 1.562

10. GEOMETRIC SYMBOLS LOCATION CONTROL FORM CONTROL RUNOUT CONTROL SEE HANDOUTS FOR SYMBOL CONSTRUCTION

11. GEOMETRIC SYMBOLS DATUM REFERENCE REQUIRED NO DATUM REFERENCE

12. FEATURE CONTROL FRAMES   geometric characteristictolerance zone shapetolerance zone sizematerial condition modifierprimary datumsecondary datumtertiary datum REQUIRED IN EVERY FRAME DEPENDENT ON ZONE TYPE

13. DRAWING FEATURE CONTROL FRAMES Click on tolerance button in dimension toolbar Select elements required in frame Place frame in drawing

14. FEATURE CONTROL FRAME EXAMPLES .005 A

15. FEATURE CONTROL FRAME EXAMPLES  Ø.005  A B C

16. FEATURE CONTROL FRAME EXAMPLES  Ø.005  A B C .011 A

17. DATUM THEORY Any feature (point, line, plane, axis, surface, hole, etc.) can be a datum (measuring reference) Considered perfect Datum features specified on the drawing are simulated in the inspection process

18. SIMULATED DATUM   SIMULATED DATUM SURFACE SPECIFIED DATUM FEATURE SPECIFIED MEASUREMENT ACTUAL MEASUREMENT

19. DATUM PRECEDENCE Follows order listed in feature control frame As many as three datums may be needed to adequately control the location of a tolerance zone Levels of precedence: –P–Primary datum – SUPPORTS THE PART – 3 points of contact with simulated datum –S–Secondary datum – ALIGNS THE PART – 2 points of contact with simulated datum –T–Tertiary datum – STOPS THE PART - 1 point of contact with simulated datum

20. Object with 3 datums    

21.    THREE POINTS OF CONTACT

22.    TWO POINTS OF CONTACT

23.    THREE POINTS OF CONTACT TWO POINTS OF CONTACT ONE POINT OF CONTACT REQUIRED ON REAR SURFACE

24. DATUM SELECTION CRITERIA Function – how do features affect the way the entire part works Relationship to mating part or feature - how do features mate with other part Accessibility – can you measure from the feature ?

25. GSK DESIGN