Fits And Tolerances

First-Year Engineering Program

Engineering H191 Engineering Fundamentals and Laboratory I Week 07 Day 01 Tolerancing: Control of Variability

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Objectives • • •

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Understand description and control of variability through tolerancing Understand various classes of fits Introduce multiple part tolerancing

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Tolerance Tolerance is the total amount a dimension may vary. It is the difference between the maximum and minimum limits. Ways to Express: 4. Direct limits or as tolerance limits applied to a dimension 5. Geometric tolerances 6. A general tolerance note in title block 7. Notes referring to specific conditions

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1. Direct Limits and Tolerance Values

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1. Direct Limits and Tolerance Values – Plus and Minus Dimensions

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2. Geometric Tolerance System Geometric Dimensioning and Tolerancing (GD&T) is a method of defining parts based on how they function, using standard ANSI symbols.

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Feature Control Frame Concentricity Symbol

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3. Tolerance Specifications in Title Block

General tolerance note specifies the tolerance for all unspecified toleranced dimensions.

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4. Notes Referring to Specific Conditions General Tolerances could be in the form of a note similar to the one shown below: ALL DECIMAL DIMENSIONS TO BE HELD TO ±.002" Means that a dimension such as .500 would be assigned a tolerance of ±0.002, resulting in a upper limit of .502 and a lower limit of .498 AU 2005

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Important Terms – Single Part

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Nominal Size – general size, usually expressed in common fractions (1/2" for the slot) Basic Size – theoretical size used as starting point (.500" for the slot) Actual Size – measured size of the finished part (.501" for the slot) AU 2005

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First-Year Engineering Program

Important Terms – Single Part

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Limits – maximum and minimum sizes shown by tolerances (.502 and .498 – larger value is the upper limit and the smaller value is the lower limit, for the slot)

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Tolerance – total allowable variance in dimensions (upper limit – lower limit) – object dimension could be as big as the upper limit or as small as the lower limit or anywhere in between AU 2005

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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 AU 2005

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Fitting Multiple Parts

Tolerance of B Part B

Fit Tolerance: Clearance or Interference Tolerance of A Part A

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Fitting Multiple Parts

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Shaft and Hole Fits Clearance

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Interference

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Shaft and Hole Fits Transition

CLEARANCE FIT + .003

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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 Table A.1 in the Appendix (pp. A-3 to A-9)

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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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Basic Hole System • Clearance = Hole – Shaft • Cmax = H____ – S____ • Cmin = H____ – S____ Fill in the subscripts (min, max) in the equations above.

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SMAX SMIN HMAX HMIN

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Basic Hole System • Clearance = Hole – Shaft • Cmax = Hmax – Smin • Cmin = Hmin – Smax Both Cmax and Cmin <0 – _________ fit Both Cmax and Cmin >0 – _________ fit Cmax > 0; Cmin < 0 – ___________ fit

What types of fits are these?

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SMAX SMIN HMAX HMIN

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Basic Hole System • Clearance = Hole – Shaft • Cmax = Hmax – Smin • Cmin = Hmin – Smax Both Cmax and Cmin <0 – Interference fit Both Cmax and Cmin >0 – Clearance fit Cmax > 0; Cmin < 0 – Transition fit

• System Tolerance = Cmax – Cmin • Allowance = Min. Clearance = Cmin

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SMAX SMIN HMAX HMIN

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Basic Hole System Calculate Maximum and Minimum Clearance Clearance = Hole – Shaft Cmax = Hmax – Smin Cmax = .510 – .485 = .025

.490 .485 .510 .505

Cmin = Hmin – Smax Cmin = .505 – .490 = .015 What type of fit is this? Cmax > Cmin > 0 Clearance AU 2005

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Metric Limits and Fits • Based on Standard Basic Sizes – ISO Standard, see the Appendix material (Table A.2 on pp. A10 to A-13 in text) • Note that in the Metric system: Nominal Size = Basic Size • Example: If the nominal size is 8, then the basic size is 8 AU 2005

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Metric Preferred Hole Basis System of Fits

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First-Year Engineering Program

Metric Tolerance Homework – TOL-1B Free Running H9/d9 Basic Size: 8 (1) Nominal Size: 8 ? (2) Shaft Limits: (3) Shaft Tolerance: 0.036 ????

7.960 ???? 7.924 ????

(4) Hole Limits: (5) Hole Tolerance: 0.036 ????

8.036 ???? 8.000 ????

(6) Ts: 0.072 ????

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(7) Minimum Clearance: 0.040 ???? (8) Maximum Clearance: 0.112 ????

CHECK: 0.072 Ts = Cmax = 0.112 – Cmin? – 0.040 = 0.072

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Good Review Material • TG Chapter 6 – Dimensions and Tolerances – Specifically pages 6-18 to 6-22 • TG Chapter 7 – Dimensions For Production

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Today's Assignment • Drawing 33 – G27 • Drawing 34 – TOL-1A

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