Tolerance Design

1. Tolerance Design

2. Design Specifications and Tolerance • Develop from quest for production quality and efficiency • Early tolerances support design’s 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 => 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. USL LSL X 3s tolerance 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 • Machinery’s 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 => Maximum assembly envelope • Ne - Te => 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 => 0.05 + 0.093 = 0.143 • Smallest => 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 => 0.05 + 0.051 = 0.101 • Smallest => 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.

42. Disclaimer This information is provided “as is” for general educational purposes; it can change over time and should be interpreted with regards to this particular circumstance. While much effort is made to provide complete information, Ohio State University and Gateway do not guarantee the accuracy and reliability of any information contained or displayed in the presentation. We disclaim any warranty, expressed or implied, including the warranties of fitness for a particular purpose. We do not assume any legal liability or responsibility for the accuracy, completeness, reliability, timeliness or usefulness of any information, or processes disclosed. Nor will Ohio State University or Gateway be held liable for any improper or incorrect use of the information described and/or contain herein and assumes no responsibility for anyone’s use of the information. Reference to any specific commercial product, process, or service by trade name, trademark, manufacture, or otherwise does not necessarily constitute or imply its endorsement.