Geometric Tolerances and Dimensioning

Tolerance

Why Geometric Tolerancing and Dimensioning

 To ensure interchangeability of mating parts during assembly  To eliminate controversy and guesswork when drawing is interpreted  To ensure the drawing reflects the form and function requirements of the manufactured 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 variation is permitted in both directions from the specified dimension

•

Fit types A.

B.

C.

Allowance

.

Clearance fit Interference fit D.

Transition fit E. Line fit

Clearance fit

: Is the fit that gives clearance between

Allowance fit

: two assembled mating parts.

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