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Page 1: Form Tolerances

5Form Tolerances

Page 2: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 3: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

• 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.

Page 4: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

• 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.

Page 5: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Form Tolerance Symbols

Page 6: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Form Feature Control Frame

• Has only two compartments– Tolerance symbol– Tolerance value

• No datum feature references

Page 7: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Material Condition Modifiers

• RFS assumed• MMC symbol must be shown when applicable• MMC Applied when omission of MMC could

increase cost

Page 8: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 9: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Rule #1

• Perfect form boundary at MMC• As size departs from MMC, form may vary• Perfect form not required at LMC

Page 10: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Perfect Form Boundary Effect on Flat Surfaces

Page 11: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Perfect Form Boundary Effect On Cylinders

Page 12: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness

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

element or surface element

Page 13: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 14: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 15: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness of Flat Surfaces

• Feature control frame with extension line or leader

• Direction dictated by view orientation• Straightness does not imply parallelism

Page 16: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 17: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness of Cone Elements

• Each cone element controlled independently• Angle controlled by other means

Page 18: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Verification of Surface Straightness

• Use a dial indicator– Full indicator movement (FIM)– Total indicator reading (TIR)

• Use a straight edge and feeler gage

Page 19: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness Applied to Features of Size

Page 20: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness to Control a Shaft Derived Median Line

Page 21: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 22: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Straightness of the Derived Median Line of a Hole

Page 23: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 24: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Gage for Overall Straightness

Page 25: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Gage for Unit Length Straightness

Page 26: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Flatness

• No material condition modifier applicable on a surface

• RFS or MMC applicable on feature of size

Page 27: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Flatness Applied to a Surface

• Only tolerance symbol and value• Surface variations limited by size

Page 28: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Verification of Surface Flatness

Page 29: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 30: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Flatness Tolerance Applied to a Derived Median Plane

• Feature control frame placed adjacent to dimension or on dimension line

• No leader

Page 31: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Verification of Derived Median Plane Flatness

Page 32: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 33: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

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

Page 34: Form Tolerances

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

Cylindricity

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