Applying Controls to a Feature of Size (DMP/DML)

• What is a Feature of Size (DML/DMP)?• Rule #1 (Taylor Principle)• Straightness Applied to a Cylindrical Feature of Size (DML)• Virtual Condition• Material Condition Modifiers

– Bonus Tolerances– Regardless of Feature Size (RFS)

• Functional Gauge Inspection• Flatness Applied to a Rectangular Feature of Size (DMP)

Feature of Size• A Cylinder*• Two Parallel Surfaces*

* Something you can Wrap a Caliper Around

Many GD&T Controls which are applied to a Feature of Size are inspected with a Functional Gauge

Feature vs. Feature of Size– In Geometric Tolerancing, “Feature” means that a Surface or,

more specifically, Surface Elements on the Exterior of a feature are being controlled and inspected (Inspecting the Skin)

– “Feature of Size” means that a Center Axis or Centerplane must be be “Established” for control and inspection (Grab the Whole Chunk)

Applying Feature Control Frames to Surface Features Vs. a Features of Size

Control Frame Attached to surface

feature with Leader or Extension Line*

Control Frame Attached To(or closely

related to) the Feature’s Size

Dimension*

**

10.19.9=

“Features Must Have Perfect Form at MMC”*

*

- Example 1

Part Gauge

Allowable Straightness Deviation = 0*

Allowable Straightness Deviation = 0.2*

MMC

LMC

Rule Number 1 – The Envelope Principle – Example 2

The MMC ø.312 Determines theActual Mating Envelope(AME)

At LMC, the Permissible Deviation of Form is the Same as the Size Tolerance (.005)

At MMC, the Feature Can Have no Deviation of Straightness

Problem: Very few parts have Perfect form at MMC

Rule #1 and Orientation•Features Must Also Have Perfect Orientation at MMC•More Deviation of Orientation is Possible at LMC

The Feature must be perfectly perpendicular to the top face of the Part 1 at MMC in order to assemble to Part 2

Gauge

Feature

Rule #1 and Position•Features Must Also Have Perfect Position at MMC•Deviation of Position is Possible at LMC

Straightness Applied to a Cylinder Feature of Size and Introduction to Virtual Condition*

• The Feature Control Frame is Attached to the Dimension•The Control Frame Has a Diameter Symbol for a Cylindrical FOS•The Diameter Symbol Indicates a 3-D Cylindrical Tolerance Zone Around the Axis (DML)*•The Diameter of the Zone Equals the Geometric Tolerance Value*•The Axis of the FOS Must Stay Within the Zone for the Length of the Feature

• This allows for a variation of straightness at MMC• No Rule #1 “Overruled” – Perfect form not required at MMC• The amount of allowable straightness variation at MMC is the stated tolerance value•The tolerance value is also added to the AME to create Virtual Condition

Control Frame Attached to Dimension

Diameter SymbolNot like Applied

to a Feature

Tolerance value added to AME value to create Virtual ConditionMMC

MMC Material Modifier and Bonus Tolerances

– Symbol is Added to the Control Frame to Indicate that the Tolerance Value is Only in Effect at MMC or LMC *

– Produces Bonus Tolerance *– If There is no Symbol, then the Feature is Regardless of

Feature Size (RFS) *– Functional Gaging can only be used with the MMC Modifier

Present

*No Symbol = RFS(Rule #2)

Virtual Condition and Gaging – Example 1•Virtual Condition (VC) Determines the Size of the Functional Gage*•VC Is the Adjusted AME•Rule #1 No Longer Applies •Feature can be out of Straigtness at MMC by the Value Indicated in the Control Frame

*

*

For an Internal Feature:VC = MMC – geometric Tolerance

VC/Gage Diameter = 7 (Gauge Not Shown)

*

*

Virtual Condition and Gaging – Example 2

6.80 MMC6.20 LMCLimits =

“Of 0.2 at MMCAnd at MMCOnly” * **

Gauge Hole

MMC Modifier and Bonus Tolerance – Example 1

6.80 MMC6.20 LMCLimits =

•The Tolerance Value is Applied only at MMC *•The Tolerance Value Increases From MMC to LMC *•More Deviation of Straightness is Allowed at LMC•The Added Tolerance is Called the Bonus Tolerance

Bonus Tolerance

LMC

MMC

MMC Modifier and Bonus Tolerance – Example 2

.615 MMC

.605 LMC

MMC Modifier and Bonus Tolerance – Example 3

Gauge Hole Stays at Virtual ConditionPin at MMC Pin at LMC

Gauge Gauge

Smallest ZoneLargest Zone

Axis/DML StaysWithin Zone MMC - LMC

Allows for Small DeviationOf Straightness at MMC

Allows for MoreDeviation at LMC

MMC Modifier and Bonus Tolerance – Example 3

MMC Modifier and Bonus Tolerance – Example 4

External Feature Internal Feature

Straightness – Regardless of Feature Size (RFS)*

• No Symbol is Shown in the Control Frame

• Tolerance Zone Value Remains the Same From MMC to LMC (No Bonus Tolerance)

• “Regardless of the Feature’s Size” (RFS) *

MMC

LMC

• Often Applied when minimum Wall Thickness needs to be maintained on an internal feature*

• The Tolerance Value is Applied at LMC(Minimum Wall Thickness) and IncreasesToward MMC (Maximum Wall Thickness)

• More Straightness Deviation is Allowed at MMC Because there is More Wall Thickness

LMC Modifier and Bonus Tolerance - Example 1

LMC Modifier and Bonus Tolerance - Example 2

Flatness Applied to a Rectangular Feature Of Size (DMP)

Flatness Applied to a Rectangular Feature of Size (FOS)

• The Feature Control Frame is Attached to the Dimension*

•The Control Frame does not have a Diameter Symbol *•The tolerance zone is two parallel planes about the midplane (DMP)*•The Distance between the planes Equals the Geometric Tolerance Value*•The Midplane (DMP) must Stay Within the Zone for the Length of the Feature

Flatness to a FOS with MMC Functional Gauge and Bonus Tolerance Example

Note: 1) No Diameter SymbolIn the Control Frame *2) No Centerline

Zone is Two Parallel Planes

DMP