Tolerance analysis

Tolerance Analysis

Dr. P. M. Pandey

http://paniit.iitd.ac.in/~pmpandey

Causes of Workpiece Variation• The machines which perform operations on the

workpiece may have inherent inaccuracies built into them.

• The tools used on the machines are subject to dulling, general wear, chipping, breaking, and differences occurs due to regrinding

• The material used is subjected to variations• The involvement of human elements• Any other cause by Chance.

Term used in dimensioning workpiecedimensions

• Nominal Size: It has no specified limits or accuracy but indicates a close approximation to some standard size. For eg. a half inch nut will fit into a half inch bolt.

• Basic Size: It is the exact theoretical size from which the limits are established through the applications of allowances and tolerances.

• Allowance: an allowance is an intentional difference between maximum material limits of mating parts.

• Tolerance: A tolerance is the total permissible variation from the specified basic size of the part.

• Limit: Limits are extreme permissible dimension of the part

Diagram illustrating basic size deviations and tolerances

Expressing Tolerances

The problem of selective assembly

Hole or Shaft basis

IS:919-1963 (revised): Recommendations for Limits and Fits for Engineering

• Eighteen standard grades of tolerances with designations IT01, IT0, IT01-----IT16.

• Twenty seven fundamental deviations indicated by letters. Capital letters are used for hole and small letter are used for shaft.

• The values of these tolerance grades or fundamental deviations depend on the basic size of the assembly.

• One example of fit may be 60mm H8/f7 Shaft basis.

Fundamental deviations

Commonly used fits for holes

Geometrical Tolerancing

Geometric characteristics symbol

Interpretation of Indication of Geometric Tolerances

Straightness

Flatness

Cylindricity

Tolerance Stacks• Cubes are machined to 1.000±0.005

•If two cubes are stacked the desired height is 2.000±0.005

Design tolerance stack

Process tolerance stack

Effect of tolerance stacking

Control of product limit stack by baseline dimensioning

Tolerance stack control during processing

A: Machine one surface. Locate on side opposite the one being machined. Machine dimension 1.030±0.002

B: Machine slot. Locate on surface accomplished in operation A. Machine dimension 0.530±0.010

C: Machine opposite surface. Locate same as operation A. Machine dimension 1.000±0.002

Depth of slot cannot be maintained within the the limits

A: Same as in method I.

B: same as in method I except tolerance on slot have been tightened to ±0.006.

C: same as in method I.

Depth of the slot is as specified, however tightening toleranceswill result into higher costs.

A: same as in method I.

B: Machine opposite surface. Locate from surface accomplished in operation A. Machine dimension 1.000±0.002

C: Machine slot. Locate on surface accomplished in either operation A or B. Machine dimension 0.500±0.010

This is the best method as no tolerance tightening is done and dimensions are achieved within the specifications.

Cost of Arbitrary tolerance selection