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Form Tolerances

5. Form Tolerances. Apply straightness tolerances to control surface elements or a derived median line (commonly known as axis straightness) and show the interpretation of those tolerances. Apply flatness to control a surface and show an interpretation of the flatness tolerance zone.

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Form Tolerances

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  1. 5 Form Tolerances

  2. Apply straightness tolerances to control surface elements or a derived median line (commonly known as axis straightness) and show the interpretation of those tolerances.

  3. Apply flatness to control a surface and show an interpretation of the flatness tolerance zone. • Apply a flatness tolerance for a median plane and show an interpretation of the flatness tolerance zone.

  4. Apply circularity tolerances and show an interpretation of a circularity tolerance zone. • Apply a cylindricity tolerance and show an interpretation of the cylindricity tolerance zone.

  5. Form Tolerance Symbols

  6. Form Feature Control Frame • Has only two compartments • Tolerance symbol • Tolerance value • No datum feature references

  7. Material Condition Modifiers • RFS assumed • MMC symbol must be shown when applicable • MMC Applied when omission of MMC could increase cost

  8. Form Requirements from Size Limits • Size limit controls form • Maximum material condition (MMC), small hole or large shaft • Least material condition (LMC), small shaft or large hole

  9. Rule #1 • Perfect form boundary at MMC • As size departs from MMC, form may vary • Perfect form not required at LMC

  10. Perfect Form Boundary Effect on Flat Surfaces

  11. Perfect Form Boundary Effect On Cylinders

  12. Straightness • Control surface elements • Control derived median line (Axis) • Surface line element may be called a line element or surface element

  13. Straightness (Cont.) • Derived median line made from midpoints • Center line is center of feature, no variation • Axis is line at center of actual mating envelope

  14. Straightness Applied to Surface Elements • Form tolerance only applied if the required control is smaller than the size tolerance • Form tolerance larger than size tolerance only permitted with independency symbol • Material condition modifiers not used in form tolerance specifications applied to surfaces

  15. Straightness of Flat Surfaces • Feature control frame with extension line or leader • Direction dictated by view orientation • Straightness does not imply parallelism

  16. Straightness of Cylinder Surface Elements • Variation of surface element straightness • Feature control frame with extension line or leader • Surface straightness does not control axis • Axis straightness does not control surface

  17. Straightness of Cone Elements • Each cone element controlled independently • Angle controlled by other means

  18. Verification of Surface Straightness • Use a dial indicator • Full indicator movement (FIM) • Total indicator reading (TIR) • Use a straight edge and feeler gage

  19. Straightness Applied to Features of Size

  20. Straightness to Control a Shaft Derived Median Line

  21. Bonus Tolerance • Allowable only when MMC is applied • Equals departure of size from MMC • Total allowable form tolerance equals specified form tolerance plus bonus tolerance

  22. Straightness of the Derived Median Line of a Hole

  23. Overall and Unit Length Tolerance • Unit length tolerance makes part easier to produce • Combination of overall and unit length tolerances may be best approach • Combination is specified by one feature control frame with upper and lower segments • Overall limits accumulation of unit length

  24. Gage for Overall Straightness

  25. Gage for Unit Length Straightness

  26. Flatness • No material condition modifier applicable on a surface • RFS or MMC applicable on feature of size

  27. Flatness Applied to a Surface • Only tolerance symbol and value • Surface variations limited by size

  28. Verification of Surface Flatness

  29. Overall and Unit Area Flatness Tolerance • Combination of overall and unit area tolerances may be best approach • Combination is specified by one feature control frame with upper and lower segments • Overall controls accumulation of unit area

  30. Flatness Tolerance Applied to a Derived Median Plane • Feature control frame placed adjacent to dimension or on dimension line • No leader

  31. Verification of Derived Median Plane Flatness

  32. Circularity Applied to a Cylindrical Surface • Only applied as a control of surface form • No material condition modifier • Tolerance zone between two concentric circles • Applies at each cross section

  33. Circularity Applied to a Cone • Only applied as a control of surface form • No material condition modifier • Tolerance zone between two concentric circles • Applies at each cross section • Does not control the angle

  34. Cylindricity • Applicable to shaft or hole • Is combination of circularity and straightness • Two concentric cylinders create boundary

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