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. © 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. © 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. © Goodheart-Willcox Co., Inc.

Form Tolerance Symbols © Goodheart-Willcox Co., Inc.

Form Feature Control Frame Has only two compartments Tolerance symbol Tolerance value No datum feature references © 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 © 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 © 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 © Goodheart-Willcox Co., Inc.

Perfect Form Boundary Effect on Flat Surfaces © Goodheart-Willcox Co., Inc.

Perfect Form Boundary Effect On Cylinders © 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 © 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 © 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 © 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 © 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 © Goodheart-Willcox Co., Inc.

Straightness of Cone Elements Each cone element controlled independently Angle controlled by other means © 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 © Goodheart-Willcox Co., Inc.

Straightness Applied to Features of Size © Goodheart-Willcox Co., Inc.

Straightness to Control a Shaft Derived Median Line © 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 © Goodheart-Willcox Co., Inc.

Straightness of the Derived Median Line of a Hole © 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 © Goodheart-Willcox Co., Inc.

Gage for Overall Straightness © Goodheart-Willcox Co., Inc.

Gage for Unit Length Straightness © Goodheart-Willcox Co., Inc.

Flatness No material condition modifier applicable on a surface RFS or MMC applicable on feature of size © Goodheart-Willcox Co., Inc.

Flatness Applied to a Surface Only tolerance symbol and value Surface variations limited by size © Goodheart-Willcox Co., Inc.

Verification of Surface Flatness © 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 © 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 © Goodheart-Willcox Co., Inc.

Verification of Derived Median Plane Flatness © 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 © 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 © Goodheart-Willcox Co., Inc.

Cylindricity Applicable to shaft or hole Is combination of circularity and straightness Two concentric cylinders create boundary © Goodheart-Willcox Co., Inc.

Actual mating envelope A true geometric counterpart of the feature that is at the maximum size that will fit within the hole or other internal feature or at the minimum size that will encompass a shaft or other external feature. Bonus tolerance Any increase in allowable tolerance that is permitted by the departure of a feature from the material condition specified in the feature control frame. © Goodheart-Willcox Co., Inc.

Center line Circularity A line that represents the center of a feature on a drawing or in a CAD model Circularity Establish the allowable variation on the roundness of any feature that has a circular cross section, including cylinders, cones, and spheres. © Goodheart-Willcox Co., Inc.

Coordinate measuring machine (CMM) A machine that electronically records coordinates for features on a part. A coordinate measuring machine may also calculate feature variations from nominal and report an existing discrepancies. Cylindricity The allowable form variance of a surface bounded by two concentric cylinders. © Goodheart-Willcox Co., Inc.

Derived median line Envelope principle A line that may not be straight that is made of the mid points between two surface elements that are on opposite side of the feature or by center points along the length of the cylindrical shape. Envelope principle The controlled feature must be within a boundary of perfect form at MMC. © Goodheart-Willcox Co., Inc.

Feature control frame Flatness A tolerance specification in a format that shows the tolerance type, tolerance value, any applicable material condition modifiers, and any applicable datum feature references. Flatness The allowable form variance of a surface bounded by two parallel planar surfaces. © Goodheart-Willcox Co., Inc.

Full indicator movement (FIM) Free state variation The condition of a part that permits its dimensions to change after the part is removed from the manufacturing equipment. Full indicator movement (FIM) The difference between the minimum and maximum reading obtained with a dial indicator. Full indicator movement is also referred to as total indicator reading (TIR). © Goodheart-Willcox Co., Inc.

Functional gage A tool that can be used to check dimension limits by placing the gage on or in the workpiece. An example is a plate with four pins that check the locations of four holes. Rule #1 Any dimensional feature of size must have perfect form when the feature is produced at its maximum material condition. © Goodheart-Willcox Co., Inc.

Straightness Surface line element The allowable form variance of surface elements or a derived median line for a regular feature of size bounded by parallel straight lines. Surface line element A line on the surface of a part. © Goodheart-Willcox Co., Inc.

Tolerance value, feature control frame Tolerance symbol A graphical representation of the controls within the form, orientation, profile, location, runout, and miscellaneous categories. Tolerance value, feature control frame The number following the tolerance symbol. This value indicates the permitted amount of variation. © Goodheart-Willcox Co., Inc.

Total indicator reading (TIR) The difference between the minimum and maximum reading obtained with a dial indicator. Total indicator movement is also referred to as full indicator movement (FIM). Virtual condition The combined effect of the maximum material condition and any applicable tolerance. © Goodheart-Willcox Co., Inc.