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
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.
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
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.
