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