Objectives

• Understand description and control of variability through tolerancing

• Understand various classes of fits• Introduce multiple part tolerancing

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ToleranceTolerance is the total amount a dimension may vary.

It is the difference between the maximum and minimum limits.

Ways to Express:1. Direct limits or as tolerance limits applied to a

dimension2. Geometric tolerances3. A general tolerance note in title block4. Notes referring to specific conditions

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ANSI/ASME Standard

ANSI/ASME Standard Y14.5

Each dimension shall have a tolerance, except those dimensions specifically identified as reference, maximum, minimum, or stock. The tolerance may be applied directly to the dimension or indicated by a general note located in the title block of the drawing.

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

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

Feature Control Frame

Concentricity Symbol

3. Tolerance Specifications in Title Block

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

4. Notes Referring to Specific Conditions

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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

Important Terms – Single Part

• 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)

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Important Terms – Single Part

• 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)

• 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

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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

leaves a space– Interference Fit – tolerance of mating parts always

results in interference– Transition Fit – sometimes interferes, sometimes

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Fitting Multiple Parts

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Part A

Tolerance of A Part B

Tolerance of B Fit Tolerance: Clearance or Interference

Fitting Multiple Parts

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Shaft and Hole Fits

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Clearance Interference

Shaft and Hole Fits

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Transition

CLEARANCE FIT

+ .003

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)

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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 – G28-A

• 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

Basic Hole System

• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax

Both Cmax and Cmin <0 – _________ fitBoth Cmax and Cmin >0 – _________ fitCmax > 0; Cmin < 0 – ___________ fit

What types of fits are these?

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SMAX

SMIN

HMAX

HMIN

Basic Hole System

• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax

Both Cmax and Cmin <0 – Interference fitBoth Cmax and Cmin >0 – Clearance fitCmax > 0; Cmin < 0 – Transition fit

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

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SMAX

SMIN

HMAX

HMIN

Basic Hole System

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.490

.485

.510

.505

Calculate Maximum and Minimum Clearance

Clearance = Hole – Shaft

Cmax = Hmax – Smin

Cmax = .510 – .485 = .025

Cmin = .505 – .490 = .015

What type of fit is this? Cmax > Cmin > 0 Clearance

Cmin = Hmin – Smax

Metric Limits and Fits

• Based on Standard Basic Sizes – ISO Standard

• 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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Metric Preferred Hole Basis System of Fits

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Metric Tolerance Homework – TOL-1

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????

????

9.960

9.924

Free Running H9/d9Basic Size: 10

(1) Nominal Size: ?(1) Nominal Size: 10

(2) Shaft Limits:

(7) MinimumClearance: ????

(8) MaximumClearance: ????

(3) Shaft Tolerance: ????(3) Shaft Tolerance: 0.036

(4) Hole Limits: ????

????

10.036

10.000(5) Hole Tolerance: ????(5) Hole Tolerance: 0.036

(6) Ts: ????(6) Ts: 0.072

(7) MinimumClearance: 0.040

(8) MaximumClearance: 0.112

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

Today's Assignment

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