ToleranceTolerance

Geometric Tolerances and Dimensioning

Why Geometric Tolerancing and Why Geometric Tolerancing and

DimensioningDimensioning

To ensure interchangeability of mating To ensure interchangeability of mating parts during assemblyparts during assembly

To eliminate controversy and To eliminate controversy and guesswork when drawing is interpretedguesswork when drawing is interpreted

To ensure the drawing reflects the form To ensure the drawing reflects the form and function requirements of the and function requirements of the manufactured partsmanufactured parts

• Tolerances are used to control the variation that exist on all manufactured parts.

• Tolerance dimensions control the amount of variation on each part of an assembly.

• The more accuracy needed in the machined part, the higher the manufacturing cost

Limit Dimensions• Limits:

The lower and upper allowable sizes are

referred to as the limit dimensions

• Tolerance is :

a. Upper limits: Larger value

b. Lower limits: Smallest value

• Limit Dimensions

Tolerance Representation:Tolerance can be expressed in several ways:1. A general tolerance note in the title block

2. Direct limits, or as tolerance values applied directly to a dimension

3. Geometric tolerance

FIT TYPE• If one part is to be inserted

into or mated with another part, The type of fit can be determined by the following:

• The loosest fit is the difference

between the smallest feature A

and the largest feature B.

• The tightest fit is the difference between the largest feature A and the smallest feature B

Mating Parts• Mating parts are parts that fit together with a

reasonably high degree of accuracy.

Terminology of Tolerancing

• When the shaft and hole are assembled ,the extreme conditions are the tightest and the loosest fit

Methods of noting tolerance• TOLERANCE, UNILATERAL - A tolerance in which variation is

permitted in one direction from the specified dimension.

TOLERANCE, BILA TERAL - A tolerance in which - A tolerance in which variation is permitted in both directions from the variation is permitted in both directions from the specified dimensionspecified dimension

• Fit types

A. Allowance.

B. Clearance fit

C. Interference fit

D. Transition fit

E. Line fit

Clearance fit: Is the fit that gives clearance between

two assembled mating parts.Allowance fit: The allowance is the tightest fit between

two mating part.

Interference fit: Is the fit that leads to interference between two assembled parts.

.

• Transition fit: is a fit between two parts that can result in either interference or clearance.

Line fit: is a fit that can result in contact of surfaces between two parts or clearance.

Chain Dimensions

• Parts are dimensioned to locate surfaces or geometric features by chain of dimensions

• With each measurement based on preceding one, the tolerances accumulate

• Datum Plane The accumulation of tolerances can be reduced by measuring from a single plane called datum plan