8Position Tolerancing

Fundamentals

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• Complete feature control frames for position tolerances, properly using the diameter symbol, material condition modifiers, and datum references.

• Sketch the proper location and shape for position tolerance zones.

• Describe the effect of an MMC, LMC, or RFS modifier on a position tolerance.

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• Provide examples that prove the validity of the MMC concept as it applies to position tolerances.

• Calculate position tolerances for simple fixed and floating fastener conditions.

• Calculate the allowable bonus tolerance for a produced part on which a position tolerance is specified at MMC.

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Position Tolerances

• May be used on single or multiple features• Tolerance zone shape determined by functional

requirements for feature• Material condition modifiers are appropriate

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Feature Control Frame

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Material Condition Modifier Application

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Basic Dimensions

• Must be used to define location when position tolerances are applied to features

• Defines true position

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Effect of RFS

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Effect of MMC

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Proof of MMC Concept

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Floating Fastener Condition

• Fastener passes through multiple clearance holes

• Common to use same size clearance holes for all parts, but not required

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Hole = .328 .004Fastener = .312 MMC

Tolerance = Hole (MMC) – Fastener (MMC)

T = .324 – .312T = .012

Floating Fastener Tolerance Calculation

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Tolerance = .012Fastener = .312 MMC

Hole = Tolerance + Fastener (MMC)

H = .012 + .312H = .324 (MMC)

Floating Fastener Hole Calculation

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Fixed Fastener Condition

• Fastener passes through multiple holes, one fixes fastener location

• Fastener fixed by threads, press fit, or taper

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Hole = .296 .003Fastener = .250 MMC

Tolerance = [Hole (MMC) – Fastener (MMC)]/2

T = (.293 – .250)/2

T = .0215

Fixed Fastener Tolerance Calculation

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Tolerance = .0215Fastener = .250 MMC

Hole = 2 Tolerance + Fastener (MMC)

H = (2 .0215) + .250H = .293 (MMC)

Fixed Fastener Hole Calculation

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Advantages of Position Tolerances over Coordinate Tolerances

• Clearer definition of requirements• Is no standard defining coordinate location

tolerances• Increased tolerance zone area• Bonus tolerance is available

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Calculating Diameter of Position Variation

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Paper Gaging Position Variation

• Center of graph is true position • Plot delta coordinate values for feature locations

on graph• Create concentric circles representing tolerance

zones at same scale as the graph grid

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Paper Gaging Example

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Counterbored Holes—Application Options

• One position tolerance to control hole and counterbore

• Separate position tolerances for the hole and counterbore

• Counterbore position tolerance referenced to the hole location

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Threads—Application

• Position tolerance applied to pitch axis• Position tolerance may be applied to minor or

major diameter in rare cases

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Projected Tolerance Zone

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Bidirectional Position Tolerance

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Noncylindrical Features of Size

• Slots, tabs, rails• No diameter symbol in the feature control frame• Do not place the feature control frame on an

extension line• Defines the total width of the tolerance zone

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Bounded Feature