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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 Lecture 20 Autumn Quarter
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
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
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
+.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
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
Notes and Title Block ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005" Autumn Quarter
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
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
Tolerance of B Tolerance: Clearance or Interference Part B Tolerance of A Part A Fitting Two Parts Autumn Quarter
Shaft and Hole Fits ClearanceInterference Autumn Quarter
CLEARANCE FIT+ .003 Shaft and Hole Fits Transition Autumn Quarter
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
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
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
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
Example Autumn Quarter
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
Metric Preferred Hole Basis System of Fits Autumn Quarter
Metric Tolerance Homework – Example TOL-1B Autumn Quarter
Good Review Material • BTG Chapter 7 • Dimensions and Tolerances • Pages 290-335 • BTG Chapter 8 • Dimensions For Production • Pages 340-375 Autumn Quarter
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