Tolerance Design

1
Tolerance Design
2
Design Specifications and Tolerance

• Develop from quest for production quality and
  efficiency
• Early tolerances support designs basic function
• Mass production brought interchangeability
• Integrate design and mfg tolerances

3
Definition

• The total amount by which a given dimension may
  vary, or the difference between the limits
• - ANSI Y14.5M-1982(R1988) Standard R1.4

Source Tolerance Design, p 10
4
Affected Areas
Engineering Tolerance
Product Design
Quality Control
Manufacturing
5
Questions

• Can customer tolerances be accommodated by
  product?
• Can product tolerances be accommodated by the
  process?

6
Tolerance vs. Manufacturing Process

• Nominal tolerances for steel
• Tighter tolerances gt increase cost

7
Geometric Dimensions

• Accurately communicates the function of part
• Provides uniform clarity in drawing delineation
  and interpretation
• Provides maximum production tolerance

8
Tolerance Types

• Size
• Form
• Location
• Orientation

9
Size Tolerances
10
Form Tolerances
11
Location Tolerances
12
Orientation Tolerances
13
Tolerance Buildup
14
Statistical Principles

• Measurement of central tendency
• Mean
• Median
• mode
• Measurement of variations
• Range
• Variance
• Standard deviation

15
Probability

• Probability
• Likelihood of occurrence
• Capability
• Relate the mean and variability of the process or
  machine to the permissible range of dimensions
  allowed by the specification or tolerance.

16
Tolerance SPC Charting
Figure Source Tolerance Design, p 125
17
Tolerance Analysis Methods

• Worst-Case analysis
• Root Sum of Squares
• Taguchi tolerance design

18
Initial Tolerance Design
Initial Tolerance Design
Figure Source Tolerance Design, p 93
19
References

• Handbook of Product Design for Manufacturing A
  Practical Guide to Low-Cost Production, James C.
  Bralla, Ed. in Chief McGraw-Hill, 1986
• Manufacturing Processes Reference Guide, R.H.
  Todd, D.K. Allen L. Alting Industrial Press
  Inc., 1994
• Standard tolerances for mfg processes
• Machinerys Handbook Industrial Press
• Standard Handbook of Machine Design McGraw-Hill
• Standard Handbook of Mechanical Engineers
  McGraw-Hill
• Design of Machine Elements Spotts, Prentic Hall

Figure Source Tolerance Design, p 92-93
20
Worst-Case Methodology

• Extreme or most liberal condition of tolerance
  buildup
• tolerances must be assigned to the component
  parts of the mechanism in such a manner that the
  probability that a mechanism will not function is
  zero
• - Evans (1974)

21
Worst-Case Analysis

• Ne Te gt Maximum assembly envelope
• Ne - Te gt Minimum assembly envelope

Source Six sigma mechanical design
tolerancing, p 13-14.
22
Assembly gaps
23
Worst Case Scenario Example
Source Tolerance Design, pp 109-111
24
Worst Case Scenario Example
Source Tolerance Design, pp 109-111
25
Worst Case Scenario Example

• Largest gt 0.05 0.093 0.143
• Smallest gt 0.05 - 0.093 -0.043

Source Tolerance Design, pp 109-111
26
Non-Linear Tolerances
Wource Six sigma mechanical design
tolerancing, p 104
27
Root Sum-of-Square

• RSS
• Assumes normal distribution behavior

Wource Six sigma mechanical design
tolerancing, p 16
28
RSS method

• Assembly tolerance stack equation

Wource Six sigma mechanical design
tolerancing, p 128
29
Pool Variance in RSS
Wource Six sigma mechanical design
tolerancing, p 128
30
Probability
Wource Six sigma mechanical design
tolerancing, p 128
31
Probability for Limits
Wource Six sigma mechanical design
tolerancing, p 128
32
Dynamic RSS
Wource Six sigma mechanical design
tolerancing, p 128
33
Nonlinear RSS
Wource Six sigma mechanical design
tolerancing, p 128
34
RSS Example

• Largest gt 0.05 0.051 0.101
• Smallest gt 0.05 - 0.051 -0.001

Wource Six sigma mechanical design
tolerancing, p 128
35
Taguchi Method
Input from the voice of the customer and QFD
processes
Select proper quality-loss function for the design
Determine customer tolerance values for terms in
Quality Loss Function
Determine cost to business to adjust
Calculate Manufacturing Tolerance
Proceed to tolerance design
Wource Six sigma mechanical design
tolerancing, p 21
36
Taguchi

• Voice of customer
• Quality function deployment
• Inputs from parameter design
• Optimum control-factor set points
• Tolerance estimates
• Initial material grades

Wource Six sigma mechanical design
tolerancing, p 22
37
Quality Loss Function

• Identify customer costs for intolerable
  performance
• Quadratic quality loss function

Wource Six sigma mechanical design
tolerancing, p 208
38
Cost of Off Target and Sensitivity

• Cost to business to adjust off target performance
• Sensitivity, b

Wource Six sigma mechanical design
tolerancing, p 226-227
39
Manufacturing Tolerance
40
Summary

• Importance of effective tolerances
• Tolerance Design Approaches
• Worst-Case analysis
• Root Sum of Squares
• Taguchi tolerance method
• Continual process
• Involvement of multi-disciplines

41
Credits

• This module is intended as a supplement to design
  classes in mechanical engineering. It was
  developed at The Ohio State University under the
  NSF sponsored Gateway Coalition (grant
  EEC-9109794). Contributing members include
• Gary Kinzel. Project supervisor
• Phuong Pham.. ... Primary author

Reference Six Sigma Mechanical Design
Tolerancing, Harry, Mikel J. and Reigle
Stewart, Motorola Inc. , 1988. Creveling, C.M.,
Tolerance Design, Addison-Wesley, Reading,
1997. Wade, Oliver R., Tolerance Control in
Design and Manufacturing, Industrial Press
Inc., New York, 1967.
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