Introduction to GD&T · PDF fileIntroduction to GD&T Session 2: Rules and Concepts of GD&T An...
Transcript of Introduction to GD&T · PDF fileIntroduction to GD&T Session 2: Rules and Concepts of GD&T An...
Introduction to GD&T
Session 2: Rules and Concepts of GD&T
An exploration of the
language known as
Geometric Dimensioning
and Tolerancing
Instructor:
John-Paul Belanger
Review
• Benefits of GD&T
• The GD&T standard: ASME Y14.5-2009
• Features: Surfaces vs. features of size
• Actual local size vs. mating envelope
• Basic dimensions
Today’s Agenda
The “feature control frame”
Maximum Material Condition
Least Material Condition
Regardless of Feature Size
Rule #2
Datums
Datum precedence
Datum targets
Wrap-up
J 0.25
The Feature Control Frame
A Previous Example
Other Examples
Other Examples
Maximum Material Condition (MMC)
–Ext. feature: largest size
–Int. feature: smallest size
15.3
Least Material Condition (LMC)
–Ext. feature: smallest size
–Int. feature: largest size
14.7
Using the “M” Modifier Only a hole made at “MMC” must be within the 0.4…
Still Two Things to Measure
Actual
Size
Stated
Geo Tol
Bonus Tol MMC – act. size
Total Geo Tol (stated geo tol +
bonus tol)
Inner
Boundary (size – total geo
tol)
17.9 0.4 0 0.4 17.5
18.0 0.4 0.1 0.5 17.5
18.1 0.4 0.2 0.6 17.5
18.2 0.4 0.3 0.7 17.5
18.3 0.4 0.4 0.8 17.5
18.4 0.4 0.5 0.9 17.5
18.5 0.4 0.6 1.0 17.5
MMC Modifier Often Helps!
Allows a fixed-size functional gage
Reduces scrap rate
Ensures assembly
Saves $
With the “L” Modifier… Only a hole made at “LMC” must be within the 0.4…
L
If the “L” Modifier Is Used
Actual
Size
Stated
Geo Tol
Bonus Tol LMC – act. size
Total Geo Tol (stated geo tol +
bonus tol)
Outer
Boundary (size + total geo
tol)
18.5 0.4 0 0.4 18.9
18.4 0.4 0.1 0.5 18.9
18.3 0.4 0.2 0.6 18.9
18.2 0.4 0.3 0.7 18.9
18.1 0.4 0.4 0.8 18.9
18.0 0.4 0.5 0.9 18.9
17.9 0.4 0.6 1.0 18.9
Rule #2
• First, a definition: RFS stands for “regardless of feature size”
• Now, the rule: Unless the MMC or LMC modifier is given, assume that a geometric tolerance (and any datum references) are RFS.
Constant tolerance
of 0.1 -- RFS
Datums
• Origin for a measurement From where does a location, orientation, runout, or
profile dimension originate?
85 ± 0.4
Datums
• Definition: A theoretically exact point, axis, or plane that serves as the origin from which a measurement is made.
• Also: A “datum feature” is the physical surface used to create the datum.
Datums
• Datums are the key to consistency:
Product Design
Process Design
Manufacturing
Inspection
Symbol
(Triangle may be filled or unfilled)
(Old datum feature symbol)
Datum vs. Datum Feature
datum plane A datum feature A
Selecting Datums
The most important consideration is…
… part / product ___________
• What features on the detail orient and
locate it in the assembly?
Selecting Datums
• Other concerns: Accessibility for mfg. and inspection
Repeatability
Stability
Cost
Time
Easy: A Single Datum
More Common: Two or Three
• Three mutually perpendicular planes:
1. Primary
2. Secondary
3. Tertiary
P S
T
First Plane
• Primary datum plane:
Requires ____ points of contact (min)
Second Plane
• Secondary datum plane:
Requires ____ points of contact (min)
Third Plane
• Tertiary datum plane:
Requires ____ point of contact (min)
Restricting Degrees of Freedom
• One plane, one hole, one slot:
1. Primary
2. Secondary
3. Tertiary
Restricting Degrees of Freedom
• One plane, one hole, one slot:
1. Primary
2. Secondary
3. Tertiary
Restricting Degrees of Freedom
• One plane, one hole pattern:
1. Primary - 3 degrees of freedom
2. Secondary - 3 degrees of freedom
Polling Question #3
What is the maximum number of datums allowed on a print?
a. 1
b. 2
c. 3
d. 23
e. There is no maximum
Surface vs. FOS Datum
Surface vs. FOS Datum
Surface and FOS
“M” on a Datum Reference
The geometric tolerance
starts at 0.2, but can be
“fudged” by 2 modifiers:
Ø 0.2 stated tolerance
+ Ø 0.8 potential bonus tolerance
+ Ø 0.3 potential shift tolerance
Ø 1.3 max. effective position variation
Shift vs. Bonus
Multiple Datum Features
Multiple Datum Features
Datum Targets
• A “datum target” is a way to identify specific
points
lines
areas
which are then used to establish the true datum.
Good for unusual-shaped parts, or if the function dictates
that only a portion of the surface will be contacted.
Datum Targets
A
A1 Ø15
A2 Ø15
A3 Ø15
Datum Target Simulator
Datum Targets
Datum Targets
Datums -- Summary
Q&A
Please type any questions you have
into the Q&A panel on your screen.
Thank you!
Questions?
Tooling U-SME 866.706.8665 www.toolingu.com/ilt John-Paul Belanger 248.652.1397 [email protected]
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