Overview of Form Tolerances

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Form Tolerances are tolerances on the shape of a feature.

Important points that apply to Form tolerances:

  • Form Tolerances do not depend on the Orientation or the Location of a feature. Hence all Form Tolerances are expressed without Datums.
  • Form Tolerances always refine the size tolerance. Which means Form Tolerances control the feature variation to a more stringent extent than the size variation.

Combined Envelopes of Form & Size Variation- A special note

The nature of the combined envelope of variation due to Size and Form tolerances is dependent upon the type of features.

  • A form tolerance of Straightness or Flatness can be applied on Axes or Median Planes of features of size. In Cylindrical features the straightness applies to an axis. In case of planar tabbed features the Straightness or Flatness applies on its Median Plane. In such cases the form tolerance enlarges the boundary beyond MMC.
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  • Straightness and Flatness Tolerances applied on surfaces, as well as all cases of Roundness and Cylindricity must contain the form variation within the size envelopes. In such cases a feature reaching MMC must have a zero Form error. An envelope of MMC in such cases is defined as an Envelope of Perfect Form at MMC.
Last modified: Thursday, 30 March 2017, 1:58 AM