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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 1Autumn Quarter Fits and Tolerances Lecture 20

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 2Autumn 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

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 3Autumn 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.

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 4Autumn 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

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 5Autumn Quarter 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 +.04 0 +.03 -.01

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 6Autumn Quarter Geometric Tolerance System 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.) Feature Control Frame Concentricity Symbol

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 7Autumn Quarter Notes and Title Block ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 8Autumn 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)

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 9Autumn 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)

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 10Autumn Quarter Fitting Two Parts Part A Tolerance of A Part B Tolerance of BTolerance: Clearance or Interference

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 11Autumn Quarter Shaft and Hole Fits ClearanceInterference

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 12Autumn Quarter Shaft and Hole Fits Transition CLEARANCE FIT +.003

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 13Autumn 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)

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 14Autumn 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"

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 15Autumn 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)

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 16Autumn 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

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 17Autumn Quarter Example

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 18Autumn 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

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 19Autumn Quarter Metric Preferred Hole Basis System of Fits

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 20Autumn Quarter Metric Tolerance Homework – Example TOL-1B

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 21Autumn Quarter Good Review Material BTG Chapter 7 –Dimensions and Tolerances –Pages 290-335 BTG Chapter 8 –Dimensions For Production –Pages 340-375

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Engineering H191 - Drafting / CAD Gateway Engineering Education Coalition Lect 20P. 22Autumn 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.

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