Tolerances

1. Tolerance Analysis
Dr. P. M. Pandey
http://paniit.iitd.ac.in/~pmpandey

2. 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.

3. Term used in dimensioning workpiece
dimensions
• 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

4. Diagram illustrating basic size deviations
and tolerances

5. Expressing Tolerances

6. The problem of selective
assembly

7. Hole or Shaft basis

8. 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.

9. Fundamental
deviations

10. Commonly used fits for holes

11. Geometrical Tolerancing

12. Geometric characteristics symbol

14. Interpretation of Indication of Geometric Tolerances
Straightness
Flatness
Cylindricity

17. 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

18. Effect of tolerance stacking

19. Control of
product limit
stack by
baseline
dimensioning

20. 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

21. 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 tolerances
will result into higher costs.

22. 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.

23. Cost of Arbitrary tolerance selection