Advanced course to help understand effect of bonus and shift on the result of a tolerance stack-up (Real effect of using MMC in GD&T)

1. Effect of bonus and shift tolerances on stack-up analysis
Introduction to Tolerance Stack-ups
Purpose of stack-ups:
Study dimensional relationships in an assembly
Establish part tolerances
P1
Calculating stack-ups: P2 P3
SF = P1 + P2 + P3 – N1 – N2 – N3
= P1 + P2 + P3 – ( N1 + N2 + N3 )
S F
SFMax= P1+P2+P3 – (N1+N2+N3)
Max Min
SFMin= P1+P2+P3 – (N1+N2+N3) N1
N2
Min Max N3
Arash Vakily Nov 2005
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2. Effect of bonus and shift tolerances on stack-up analysis
Advantages of using a stack-up study form:
First step towards automation (using excel or other computer programs)
Organized, verifiable, easy to review (especially for bonus and shift calculations)
Arash Vakily Nov 2005
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3. Effect of bonus and shift tolerances on stack-up analysis
Stack-up Study Example Using the Form:
To find Min. distance between points S and F:
1
2 7
S F
5
6 4
3
X- 389
Arash Vakily Nov 2005
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4. Effect of bonus and shift tolerances on stack-up analysis
Virtual Condition
Virtual condition is used to evaluate mating and assembly of the parts
All types of GD&T affect virtual condition as shown in the above example
Arash Vakily Nov 2005
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5. Effect of bonus and shift tolerances on stack-up analysis
Example of Virtual Condition With Positional Tolerance
Note radial versus diametrical values when calculating tolerance stack-ups
/ 10 ± 0.2
VC = 10 + 0.2 + 0.3 VC
+ / 0.3 A
= 10.5
MM
C
R 0.15
VCRad = 5 + 0.1 + 0.15
Nominal
= 5.25
C
LM
0.1 0.1
0.15
Arash Vakily Nov 2005
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6. Effect of bonus and shift tolerances on stack-up analysis
Bonus Tolerance
/ 10 ± 0.2
.)
ad
+ / 0.3 M A
(R
s
nu
VC
Bo
At Bonus Value
MMC
MMC No Bonus 0
LMC MMC – LMC = 0.4
Total size tolerance
Any MMC - D 0 < B < 0.4
Diameter
D
Radial values are half of the values in the
above table +
Arash Vakily Nov 2005
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7. Effect of bonus and shift tolerances on stack-up analysis
Shift Tolerance
MMC = 5 + 0.4 = 5.4 / 10 ± 0.2
+ / 0.3 M
M
LMC = 5 – 0.4 = 4.6 A M
M
C
VC = 5.4 + 0.5 = 5.9
or
LM
C
At Shift Value
MMC MMC – VC 0.5
(Min Shift)
VC
Sh
LMC LMC – VC 1.3
ift
(R
(Max Shift)
ad
.)
Shift for the feature is calculated from
values of the Datum / 5 ± 0.4
Absolute values are used (no sign) + / 0.5 M B C D
A
Radial values are half of the values in the
above table Arash Vakily Nov 2005
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8. Effect of bonus and shift tolerances on stack-up analysis
Example of Bonus and Shift Tolerances
Max. Bonus = 8.6 – 8.3 = 0.3
Max. Radial Bonus = 0.3 / 2 = 0.15
Max. Shift = 12.6 – 12.4 = 0.2
Max. Radial Shift = 0.2 / 2 = 0.1
For calculations using stack-up study form, always Radial bonus and shift are used
Arash Vakily Nov 2005
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9. Effect of bonus and shift tolerances on stack-up analysis
How To Calculate Bonus and Shift Using Stack-up Study Form
Arash Vakily Nov 2005
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10. Effect of bonus and shift tolerances on stack-up analysis
Stack-up Study Example With Bonus and Shift:
To find Min. distance between points S and F:
2
2b 1
7
2s
S F
5
6 6b 6s 4
3
X- 389
Page Arash Vakily Nov 2005
10

11. Effect of bonus and shift tolerances on stack-up analysis
Examples of Special Considerations for Shift Tolerance
Radial Shift 1
2
Max.: 0.25
Min.: 0
3
Radial Shift
Max.: (LMC-VC) / 2 = 0.4
4 Min.: (MMC-VC) / 2 = 0.15
1. Shift (and bonus) calculations are valid for non-circular features that are symmetrical, such as slots and
grooves
2. When there is no MMC for the datum(s) used in stack-up, shift is zero. In this example, datums are planar.
For round datums that are regardless of feature size, same applies.
3. For features that are gauged simultaneously, such as two holes from the same pattern, shift is zero.
4. Example of shift tolerance where datum D itself has a positional tolerance.
Arash Vakily Nov 2005
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