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Fits and Tolerances

Fits and Tolerances. Lecture 20. Tolerancing – Control of Variability. Goals Understand the description and control of variability through tolerancing. Use standard tables for tolerancing and control of fit Reference (BTG) P. 312-317 – Dimensioning for Interchangeable Parts

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Fits and Tolerances

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  1. Fits and Tolerances Lecture 20 Autumn Quarter

  2. Tolerancing – Control of Variability • Goals • Understand the description and control of variability through tolerancing. • Use standard tables for tolerancing and control of fit • Reference (BTG) • P. 312-317 – Dimensioning for Interchangeable Parts • P. 349-354 – Standard Tables for Fits • P. 358-369 – Geometric Tolerancing Autumn Quarter

  3. Definition of Tolerance • Tolerance is the total amount a dimension may vary. It is the difference between the maximum and minimum limits. • There is no such thing as an "exact size". • Tolerance is key to interchangeable parts. Autumn Quarter

  4. Ways to Express Tolerance • Direct limits or as tolerance limits applied to a dimension • Geometric tolerances • Notes referring to specific conditions • A general tolerance note in title block Autumn Quarter

  5. +.04 0 +.03 -.01 Direct Limits and Tolerance Values Can be: Limits: Upper limit – 3.53 Lower limit – 3.49 Unilateral – vary in only one direction 3.49 0 -.0X +.0X - 0 Bilateral – vary larger or smaller (may or may not be same amount) 3.50 +.05 -.01, +.10 -.20 +/- 0.05 Autumn Quarter

  6. Geometric dimensioning and tolerancing (GDT) is a method of defining parts based on how they function, using standard ANSI symbols. (More about this in a couple of weeks.) Geometric Tolerance System Feature Control Frame Concentricity Symbol Autumn Quarter

  7. Notes and Title Block ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005" Autumn Quarter

  8. Important Terms – Single Part • Nominal Size – a general size, usually expressed as a common fraction (1/2”) • Basic Size – theoretical size used as starting point (.500”) • Actual Size – measured size (.501”) • Limits – maximum and minimum sizes shown by tolerances • Tolerance – total allowable variance in dimensions (upper limit – lower limit) Autumn Quarter

  9. Important Terms – Multiple Parts • Allowance – the minimum clearance or maximum interference between parts • Fit – degree of tightness between two parts • Clearance Fit – tolerance of mating parts always leave a space • Interference Fit – tolerance of mating parts always interfere • Transition Fit – sometimes interfere, sometimes clear • Tolerance – total allowable variance in dimensions (upper limit – lower limit) Autumn Quarter

  10. Tolerance of B Tolerance: Clearance or Interference Part B Tolerance of A Part A Fitting Two Parts Autumn Quarter

  11. Shaft and Hole Fits ClearanceInterference Autumn Quarter

  12. CLEARANCE FIT+ .003 Shaft and Hole Fits Transition Autumn Quarter

  13. Standard Precision Fits: English Units • Running and sliding fits (RC) • Clearance locational fits (LC) • Transition locational fits (LT) • Interference locational fits (LN) • Force and shrink fits (FN) See Tables in the Appendix (pp. A11-A23) Autumn Quarter

  14. Basic Hole System or Hole Basis • Definition of the "Basic Hole System": • The "minimum size" of the hole is equal to the "basic size" of the fit • Example: If the nominal size of a fit is 1/2", then the minimum size of the hole in the system will be 0.500" Autumn Quarter

  15. Fit Calculations • Clearance = Hole – Shaft • Cmax = Hmax – Smin • Cmin = Hmin – Smax Both Cmax and Cmin >0 – Clearance fit Both Cmax and Cmin <0 – Interference fit Cmax > 0, Cmin < 0 – Transition fit • Allowance = Hmin - Smax (i.e., Cmin) Autumn Quarter

  16. Fit Calculations • System Tolerance = Cmax - Cmin (Sometimes called Clearance Tolerance) • Also, System Tolerance = Σ Ti • So, System Tolerance, or Ts , can be written as: Ts = Cmax - Cmin = Σ Ti • Thus, you always have a check value Autumn Quarter

  17. Example Autumn Quarter

  18. Metric Limits and Fits • Based on Standard Basic Sizes – ISO Standard, see the Appendix material (Appendices 8 - 12) • Note that in the Metric system: Nominal Size = Basic Size • Example: If the nominal size is 8, then the basic size is 8 Autumn Quarter

  19. Metric Preferred Hole Basis System of Fits Autumn Quarter

  20. Metric Tolerance Homework – Example TOL-1B Autumn Quarter

  21. Good Review Material • BTG Chapter 7 • Dimensions and Tolerances • Pages 290-335 • BTG Chapter 8 • Dimensions For Production • Pages 340-375 Autumn Quarter

  22. Assignments • Dwg 39 – G27 – Tolerances – Single Fits • Calculate the missing values for each situation. • Use the tables for preferred limits and fits for cylindrical parts. • Dwg 40 – TOL–1A – Metric Tolerances • Using the given nominal sizes and fit specifications, calculate remaining values. Autumn Quarter

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