Tolerance of Position (TOP)Part-1

Definitions and ConventionsTrue position is the theoretically exact location of a FOS as defined by basic dimensions.

Tolerance of position (TOP) control is a geometric tolerance that defines the location tolerance of a FOS from its true position.

Implied basic 900 angles: A 900 basic angle applies where centerlines of features in a pattern (or surfaces shown at right angles on a drawing) are located and defined by basic dimensions and no angle is specified.

Implied basic zero dimension: Where a centerline or center plane of a FOS is shown in line with a datum axis or center plane, the distance between the centerlines or center planes is an implied basic zero.

Advantages of TOPProvides larger tolerance zones; cylindrical tolerance zones are 57% larger than square zones.Permits additional tolerances-bonus and datum shift.Prevents tolerance accumulation.Permits the use of functional gages.Protects the part function.Lowers manufacturing costs.

Types of Part Relationships that can be controlled with TOP The distance between features of size, such as holes, bosses, slots, tabs, etc.The location of features of size (or patterns of features of size) such as holes, bosses, slots, tabs, etc. The co axiality between features of size.The symmetrical relationship between features of size

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ModifierCommonly used in these functional applicationsBonus or datum shift permissibleRelative cost to produce and verify Assembly Location of a non-critical FOSYesLowest Minimum wall thickness Minimum part distance Minimum machine stock AlignmentYesGreater than MMC; less than RFSRFS Invoked by showing no modifier To control a symmetrical relationship When the effects of bonus or datum shift will be detrimental to the function of the part To control minimum machine stock Centering AlignmentNoHighest

TOP Theories1. The virtual condition boundary theory: A theoretical boundary limits the location of the surfaces of a FOS.

2. The axis theory: The axis (or center plane) of a FOS must be within the tolerance zone.

Common TOP RFS ApplicationsTOP used on an RFS Basis

Three conditions :The tolerance zone applies to the axis (or centerline) of the FOS.The tolerance value applies regardless of the size of the tolerance feature of size.The requirement must be verified with a variable gauge.

RFS Tolerance ZonesTwo tolerance zones : A fixed diameter cylinder and two parallel planes a fixed distance apart.

The diameter of the tolerance zone cylinder or the distance between the parallel planes is equal to the tolerance value specified in the TOP callout.

The location of the tolerance zone is always centered around the true position of the FOS.

The Location of a Hole Controlled with Tolerance of Position (RFS)

Following conditions apply:The shape of the tolerance zone is a cylinder.The tolerance zone is located by the basic dimensions relative to the datum planes.The tolerance zone appliesRFS.The dimension between the centerline of the hole and datum plane A is an implied basic 90.No datum shift is permissible.The tolerance zone also controls the orientation of the hole relative to the primary datum reference from the TOP callout.Rule 1 applies.The W'CB of the hole is affected.

The Location of a Pattern of Holes Controlled with Tolerance of Position (RFS)

The following conditions apply:

The shape of each tolerance zone is cylindrical.The tolerance zones are located by the basic dimensions.The tolerance zones apply RFS.The tolerance zones also control the orientation of the holes relative to the primary datum reference from the TOP calloutThe tolerance zones are at an implied basic 900 to datum A.Rule 1 applies.

The Location of Coaxial Diameters Controlled with Tolerance of Position (RFS)

The following conditions apply:

The shape of the tolerance zone is cylindrical.The tolerance zone applies RFS.The dimension specifying the location of the diameter relative to the datum feature is an implied basic zero.The tolerance zone also limits the orientation of the tolerance diameter relative to datum axis A.There is no datum shift.Rule 1 applies.

Inspecting TOP Applied at RFSThree separate checks requiredSize of the holeRule 1 boundaryTOP requirements

Necessitates the use of Variable Gauges: a gauge what is capable of providing a numerical reading of a part parameter.E.g.: CMM, Height gauges, Expanding mandrels, and Dial indicators

Steps in InspectionThe location of the hole is established by contacting the datums in the sequence of the TOP calloutA best fit gauge pin is placed in the hole representing the AME.

The location of the center of the gauge pin relative to the datum reference frame is determinedThe center of the gage pin must be within the tolerance zone cylinder that is defined by the TOP callout

Common TOP MMC ApplicationsTOP is specified on MMC basis, when the part function is assembly or when the effects of bonus tolerance and/or datum shift would not have a detrimental effect on the function of the partMMC basis is a more liberal control than an RFS applicationThree conditions applyThe tolerance zone is considered a boundary zone.A bonus tolerance and/or datum shift is permissible.The requirement can be verified with a functional gauge.

The Location of a Hole Controlled with Tolerance of Position (RFS)

The following conditions apply:

The shape of the tolerance zone is a virtual condition cylindrical boundary.The tolerance zone is located by the basic dimensions from the datum planes.The relationship between the centerline of the hole and datum plane A is an implied basic 900 angle.A bonus tolerance is permissible.The tolerance zones also control the orientation of the holes relative to the Primary' datum reference from the TOP callout.Rule 1 applies.

The Location of a Hole Pattern Controlled with Tolerance of Position (MMC)

The following conditions apply:

The shape of the tolerance zone is a virtual condition cylindrical boundary.The tolerance zone is located by the basic dimensions from the datum planes.The relationship between the centerline of the hole and datum plane A is an implied basic 900 angle.A bonus tolerance is permissible.The tolerance zones also control the orientation of the holes relative to the Primary' datum reference from the TOP callout.Rule 1 applies.

Coaxial Diameter Applications

Inspecting TOP Applied at MMCA TOP applied at MMC can be verified in number of ways: Variable gages, Open inspection, CMM, and functional gauging.

Functional Gauge: Functional gages a gage that verifies functional requirements of part features as defined by the geometric MMC tolerances. Also called attribute gauge or a fixed gauge.A functional gauge only provides a "pass" or "fail" assessment of a part feature, i.e. does not provide a numerical reading of a part parameter.The functional gage represents the virtual condition of the tolerance FOS

Benefits of functional gages The gage represents the worst-case mating part.Parts can be verified quickly.A functional gage is economical to produce.No special skills are required to read the gage or interpret the results.In some cases a functional gage can check several part characteristics simultaneously.

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