LOCATION CONTROLS

Location Controls• True Position (Position)*• Concentricity• SymmetryProjected Tolerance ZoneFree State InspectionStatistical Tolerancing

True Position (Position)

True Position (Position) Control:– Is one of the most frequently/commonly used Controls*– Is always applied to a Feature of Size*– Always references datums:*

• Flat/Planar Datumsand/or• Datums of size

– MMC, LMC or RFS can be applied– Can also control orientation (perpendicularity) and form

• Primary reference is generally normal to the feature*• Form can be controlled if functional gaging is used

– Zones:*• Cylinder around axis – Cylinders• Two // Planes – Rectangular Features

Basic Dimensions Specify True Position

Basic Dimensions WithNo Control Frame

Control Frame With NoBasic Dimensions

Dimensional/Coordinate Tolerancing

Callout Square Zone

True Position Tolerancing

Callout Cylindrical ZoneAll Locating Dimensions are made BASIC*

*

*

*

Can also Control Form and Orientation

(Embedded/Indirect Control)*

• All Holes would be rejected without a Position Control (Rectangular Zone)• Holes A, B and D would be accepted at MMC or RFS• Hole C would be accepted with MMC Modifier (at LMC)

Positional Tolerancing— Also Controls Perpendicularity *

The primary reference is generally normal to the feature*

Positional Tolerancing Datums & Gauge Inspection (Full DRF)

Example Callout and Functional Gauge Example

This is an example of how the three planar datums could be called out to set up a DRF for a True Position Control Inspection and where a Functional Gauge can be used.

Positional Tolerancing Datums & Inspection(Hole Pattern and Cylindrical Datum of Size)

Notice the Location of the Datum Callout

To Establish Center Axis

Gage Pins

True Established Datum is Center Axis of Cylinder

Outer FOS is Centered on Datum of Size Aand Perpendicular to Planar Datum B

Hole Pattern is Centered on Datum of Size A and Perpendicular to Planar Datum A

Positional Tolerancing—MMC

Positional Tolerancing— ZT at MMC– Used for Assembly “Pilot” (Locating)

Features– Feature is designed MMC = VC

–In this case .3 smaller–LMC Value stays the same–VC stays the same– Tolerance value is added

to the feature size range– Keeps tight control of Location– Allows greatest variation of size

– More parts pass the Size Check– Increases Bonus Tolerance– Never used with RFS

AddedBonusTolerance

More Parts Pass Size CheckGreater Variation of Size & Bonus Tolerance

= MMC

Positional Tolerancing—RFS

*

No Modifier Symbol in the Control Frame = RFS*

Positional Tolerancing—LMC

THIN WALL*

Positional Tolerancing—Composite Controls for Patterns

Positional Tolerancing Rectangular Feature

•Generally used to “Center” a rectangular tab or cut on a Feature of Size•No Location Dimension Required•Always References a Datum of Size

Tab is centered on Datum of Size B feature within tolerance zone

Smaller “Chunk” is Centered on the Larger “Chunk”

Positional Tolerancing Rectangular Features• Rectangular datum centerplanes can be established with an Adjustable

Gauge• The tolerance zone is two parallel planes about the established

centerplane*• The distance between the two planes is the specified in the related

feature control frame

Chunk is Centered on Cut Cut is Centered on Chunk

No Ø Symbol

Positional Tolerancing Cylinders • Cylindrical datum axes can be established in an expandable gage• The tolerance zone is a 3-D cylinder about the established datum Axis*• The diameter of the 3-D Cylinder is specified in the related feature control

frame

Small Cylinder is Centered Larger CylinderOuter Cylinder isCentered on Hole

The “Go To” Control for Anytime you want to locate any Feature of Size

Concentricity

Concentricity Control:

– Applied to cylinders– Controls Derived Median Points* with respect an Axis Datum– Always applied to a feature of size– Always RFS– Always references a cylinder datum of size* (Established Center

Axis)– Zone is a 3-D cylinder around axis of controlled feature*– Controls the Equal Distribution of mass* (Form). (Not actual

Coaxial Position)• Used to control vibration/wobble in rotating parts*

• True Position often used instead (For Coaxial Position)* • True Position is often used instead but the desired control intent

and inspection methods are different*• Runout often Used Instead (Position, Distribution and Magnitude)

Concentricity Control & Inspection

– Established axis of datum of size is considered separatefrom axis of inspected feature

– Inspection is difficult ($):• Measuring opposite

sides of cylinder at specific locations

• Calculating the actual derived median points

• Those points must be within the cylindrical tolerance zone

Dial Indicators ShownCMM’s Make this Much Faster/Easier

Theoretical Point Cloud

Symmetry

Symmetry Control: – Like Concentricity Applied to a rectangular feature of size – Like concentricity applied to rectangular features– Used when feature must have equal distribution of mass on Both

sides of the feature (Form).*• Not just in the Center

– Used Mostly for Balance between the two sides– Always applied RFS– Always references a datum of size*– Zone is 3-D: two parallel planes*

True Position is often used instead but the desired control intent and inspection methods are different*

• True Position Controls the Center Position of the feature not the equal distribution of mass on either side

Symmetry Control & Inspection

– Center plane of datum of size is established

– Inspection is difficult ($):• Locating a set of

Derived Median Points* of opposing surfaces of the feature

• Calculating the actual center points

• Those points must be within the tolerance zone

Symmetry Control & Inspection

Dial Indicators ShownCMM’s Make this Much Faster/Easier

Projected Tolerance ZoneFree State InspectionStatistical Tolerancing

Projected Tolerance Zone

Projected Tolerance Zones are often used when long fasteners and/or deep clearance holes are involved*

Free State Inspection– For non-rigid parts that

Change Shape after Assembly

• Gaskets*• O Rings*• Sheet Metal*

– Generally inspected pre-assembly state

• Unrestrained• Natural shape

• Datum B is inspected in the free state• The Runout Tolerance is inspected after Datum A is assembled

Statistical Tolerancing

– Tolerance controls and/or values are based upon statistical inspection values

• Many parts are made and inspected

• Data is taken• Variables are identified• Controls and/or

tolerance values (Often Larger) are derived

• Related to Statistical Process Control (SPC)

• “We can learn from scrap parts”

SPC