Lecture 20 fits and tolerances

Engineering H191 - Drafting / CAD

Gateway Engineering Education Coalition Lect 20 P. 1Autumn Quarter

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– P. 349-354 – Standard Tables for Fits– P. 358-369 – Geometric Tolerancing



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.



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



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



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



Notes and Title Block

ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"



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)



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)



Fitting Two Parts

Part A

Tolerance of A Part B

Tolerance of B Tolerance: Clearance or Interference



Shaft and Hole Fits

Clearance Interference



Shaft and Hole Fits

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)

See Tables in the Appendix (pp. A11-A23)



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"



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)



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



Example



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



Metric Preferred Hole Basis System of Fits



Metric Tolerance Homework – Example TOL-1B



Good Review Material

• BTG Chapter 7– Dimensions and Tolerances– Pages 290-335

• BTG Chapter 8– Dimensions For Production– Pages 340-375



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.

Lecture 20 fits and tolerances

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