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Tolerances

Tolerances. Introduction. tolerancing technique of dimensioning parts within a required range of variation to ensure interchangeability A tolerance should be as large as possible without interfering with the function of the part to minimize production costs. Tolerance Dimensions.

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Tolerances

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  1. Tolerances

  2. Introduction • tolerancing • technique of dimensioning parts within a required range of variation to ensure interchangeability • A tolerance should be as large as possible without interfering with the function of the part to minimize production costs.

  3. Tolerance Dimensions • unilateral tolerance • bilateral tolerance • limit tolerance +.000 -.008 2.250 2.250 ± .004 2.255 2.245

  4. Mating Parts

  5. Cylindrical Fits

  6. Cylindrical Fits – English Units tolerance - difference between the limits of size for a single feature ; shaft .0025 ; hole .0040 limits of tolerance – the max and min sizes of a single feature allowance – the tightest fit between two mating parts ; .0050 nominal size – the general size of a shaft or hole ; 1.5 basic size – the size to which the plus-and minus is applied actual size – the measured size of the finished part fit - the degree of tightness or looseness between two assembled parts

  7. Cylindrical Fits – English Units • clearance fit – gives a clearance between two assembled mated parts ; • min clearance .005 ; • max clearance .0115 • transition fit • interference fit • line fit

  8. Cylindrical Fits – English Units • clearance fit • interference fit – a binding fit that requires the parts to be forced together, much as if they were welded • transition fit – may range from a interference fit to a clearance fit between the mated parts • line fit – results in surface contact or clearance when limits are reached

  9. Cylindrical Fits – English Units • The ANSI B4.1 standard defines a series of fits between cylindrical features in inches for the basic hole system. • The types of fits covered in the standard are: • RC: running or sliding clearance fits • LC: clearance locational fits • LT: transition locational fits • LN: interference locational fits • FN: force and shrink fits

  10. Cylindrical Fits – English Units • RC: running or sliding clearance fits • RC1 to RC9 • LC: clearance locational fits • LC1 to LC11 • LT: transition locational fits • LT1 to LT6 • LN: interference locational fits • LN1 to LN3 • FN: force and shrink fits • FN1 to FN5

  11. Example – RC9 Fit • hole and shaft have a basic diameter of 2.5” • From Tables in Appendix 29 thousands of an inch

  12. Example – LT3 Fit • hole and shaft have a basic diameter of 2.5” • From Tables in Appendix 31 2.5012 2.5000 2.5008 2.5001 thousands of an inch

  13. Example – LN2 Fit • hole and shaft have a basic diameter of 2.5” • From Tables in Appendix 32 2.5012 2.5000 2.5021 2.5014 thousands of an inch

  14. Example Problem

  15. Cylindrical Fits – Metric Units • ANSI B4.2 standard • basic size – the diameter from which limits are calculated • upper and lower deviation – the difference between the hole or shaft size and the basic size • tolerance - the difference between the maximum and minimum sizes

  16. Cylindrical Fits – Metric Units

  17. Cylindrical Fits – Metric Units • fundamental deviation – a letter grade that describes the deviation closest to the basic size • International Tolerance (IT) grade – a series of tolerances that vary with the basic size to provide a uniform level of accuracy within a given grade • there are 18 IT grades: IT01, IT0, IT1, …, IT16

  18. Cylindrical Fits – Metric Units • Hole basis • a system of fits based on the minimum hole size as the basic diameter • the fundamental deviation for a hole-basis system is “H” • Appendices 35 and 36 give hole-basis data for tolerances • Shaft basis • a system of fits based on the maximum shaft size as the basic diameter • the fundamental deviation for a hole-basis system is “h” • Appendices 37 and 38 give shaft-basis data for tolerances

  19. Cylindrical Fits – Metric Units

  20. Cylindrical Fits – Metric Units

  21. Example 1 • determine the shaft and hole limits for: • hole-basis system • a close running fit • a basic diameter of 49 mm

  22. Example 1 • use a preferred basic diameter of 50 mm • use a fit of H8/f7

  23. Example 2 • determine the shaft and hole limits for: • hole-basis system • a location transition fit • a basic diameter of 57 mm

  24. Example 2 • use a preferred basic diameter of 60 mm • use a fit of H7/k6

  25. Example 3 • determine the shaft and hole limits for: • hole-basis system • a medium drive fit • a basic diameter of 96 mm

  26. Example 3 • use a preferred basic diameter of 100 mm • use a fit of H7/s6

  27. Nonstandard Fits, Nonpreferred Sizes • determine the shaft and hole limits for: • hole-basis system • a close running fit • a basic diameter of 45 mm(do not change to a preferred size)

  28. Nonstandard Fits, Nonpreferred Sizes • use a fit of H8/f7

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