Applying Controls to a Feature
of Size (FOS)
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•
•
•
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What is a Feature of Size (FOS)?
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)
- Example 1
“Features Must Have Perfect Form at MMC”
=
10.1
9.9
*
Rule Number 1 – The Envelope Principle – Example 2
Problem: Very few parts
have Perfect form at MMC
At MMC, the Feature Can
Have no Deviation of
Straightness
At LMC, the Permissible
Deviation of Form is the
Same as the Size
Tolerance (.005)
The MMC ø.312
Determines the
Actual Mating Envelope
(AME)
Rule #1 and Orientation
•Features Must Also Have Perfect Orientation at MMC
•More Deviation of Orientation is Possible at LMC
Gauge
The Feature must be
perfectly
perpendicular to the
top face of the Part 1
at MMC in order to
assemble to Part 2
Feature
Rule #1 and Position
•Features Must Also Have Perfect Position at MMC
•Deviation of Position is Possible at LMC
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*
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 – 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
MMC
Not like Applied
to a Feature
Diameter Symbol
Tolerance
value added to
AME value to
create Virtual
Condition
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
*
*
Virtual Condition and Gaging – Example 2
*
For an Internal Feature: *
VC = MMC – geometric Tolerance
VC/Gage Diameter = 7 (Gauge Not Shown)
MMC Modifier and Bonus Tolerance – Example 1
Gauge Hole
“Of 0.2 at MMC
And at MMC
Only”
*
*
MMC Modifier and Bonus Tolerance – Example 2
•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
MMC
Bonus Tolerance
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)*
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•
No Symbol is Shown in the Control
Frame
Tolerance Zone Value Remains the
Same From MMC to LMC
“Regardless of the Feature’s Size”
(RFS)
MMC
LMC
LMC Modifier and Bonus Tolerance
• 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 Increases
Toward MMC (Maximum Wall Thickness)
• More Straightness Deviation is Allowed at MMC
Because there is More Wall Thickness
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
DMP
Zone is Two Parallel Planes
Note: No Diameter Symbol
In the Control Frame