Tolerance Geometric Tolerances and Dimensioning

Why Geometric Tolerancing and Dimensioning To ensure interchangeability of mating parts during assembly To ensure interchangeability of mating parts during assembly To eliminate controversy and guesswork when drawing is interpreted To eliminate controversy and guesswork when drawing is interpreted To ensure the drawing reflects the form and function requirements of the manufactured parts 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.

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