1. Engineering Graphics Stephen W. Crown Ph.D.
Tolerancing
Engineering Graphics
Stephen W. Crown Ph.D.

2. Objective
To learn how to effectively tolerance parts such that parts function correctly and cost is kept to a minimum

3. Tolerancing
Definition: Allowance for specific variation in the size and geometry of a part
Why is tolerancing necessary?
It is impossible to manufacture a part to an exact size or geometry
Since variation from the drawing is inevitable the acceptable degree of variation must be specified
Large variation may affect the functionality of the part
Small variation will effect the cost of the part
requires precise manufacturing
requires inspection and the rejection of parts

4. Functionality
Assemblies: Parts will often not fit together if their dimensions do not fall with in a certain range of values
Interchangeability: If a replacement part is used it must be a duplicate of of the original part within certain limits of deviation
The relationship between functionality and size or shape of an object varies from part to part
the usefulness of eyeglasses is extremely sensitive to size and shape
the usefulness of glass marbles are not very sensitive to size and shape

5. Cost Cost generally increases with smaller tolerance
There is generally a lower limit to this relationship where larger tolerances do not affect cost (!0.020 Vs !0.010)
Small tolerances cause an exponential increase in cost
Parts with small tolerances often require special methods of manufacturing
Parts with small tolerances often require greater inspection and call for the rejection of parts
Do not specify a smaller tolerance than is necessary!

6. How Is Tolerance Specified?
Size
Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing
Geometry
Geometric Tolerancing
Allows for specification of tolerance for the geometry of a part separate from its size
GDT (Geometric Dimensioning and Tolerancing) uses special symbols to control different geometric features of a part

7. General Tolerances
A note may be placed on the drawing which specifies the tolerance for all dimensions except where individually specified
ALL DECIMAL DIMENSIONS TO BE HELD TO !0.020
Several tolerances may be specified for dimensions with a different number of decimal places or for a different type of dimension such as angles
Specific tolerances given to a dimension on a drawing always supersede general tolerances

8. Specific Tolerances
The tolerance for a single dimension may be specified with the dimension
The tolerance is total variation between the upper and lower limits (tolerance = .020)
Limits
Unilateral tolerance
Bilateral tolerance

9. Tolerancing Holes and Shafts
Terms
Basic size: The size to which tolerances are applied
Nominal size: The general size (0.261 { 1/4)
Allowance
The minimum space between two mating parts
Based on the largest shaft and the smallest hole
A negative number indicates that the parts must be forced together
Max. Clearance
The maximum space between mating parts
Based on the smallest shaft and the largest hole

10. Tolerancing Holes and Shafts
Types of Fit
Clearance fit
The parts are toleranced such that the largest shaft is smaller than the smallest hole
The allowance is positive and greater than zero
Transition fit
The parts are toleranced such that the allowance is negative and the max. clearance is positive
The parts may be loose or forced together
Interference fit
The max. clearance is always negative
The parts must always be forced together

11. Tolerancing Holes and Shafts
Preferred fits: A specified system of fits for holes and shafts for SI units
Hole basis
The minimum hole size equals the basic hole size
Uses the symbol “H” in the tolerance specification
Shaft basis
The maximum shaft size equals the basic shaft size
Uses the symbol “h” in the tolerance specification

12. Tolerancing Holes and Shafts
Preferred precision fits: A specified system of fits for holes and shafts for english units
Based on hole basis
Classes of fit specified
RC: Running and sliding
(Allowance >0, Max Clearance >0)
LC: Clearance and locational
(Allowance =0, Max Clearance >0)
LT: Transition locational
(Allowance <0, Max Clearance >0)
LN: Interference locational
(Allowance <0, Max Clearance =0)
FN: Force and shrink
(Allowance <0, Max Clearance <0)

13. Examples: Holes and Shafts
Metric
Fit 6 H7/n6
Metric: Preferred Hole Basis (H)
Allowance:
Max. Clearance: 0.004
Hole Limits: / 6.000
Shaft Limits: / 6.008
Hole Tolerance: 0.012
Shaft Tolerance: 0.008
Type of fit: Transition

14. Examples: Holes and Shafts
Metric
Fit 6 C11/h11
Metric: Preferred Shaft Basis (h)
Allowance: 0.070
Max. Clearance: 0.220
Hole Limits: / 6.070
Shaft Limits: / 5.925
Hole Tolerance: 0.075
Shaft Tolerance: 0.075
Type of fit: Clearance

15. Examples: Holes and Shafts
English
Fit 0.25 FN 1
English: Preferred Precision Fit, Hole Basis
Allowance:
Max. Clearance:
Hole Limits: /
Shaft Limits: /
Hole Tolerance:
Shaft Tolerance:
Type of fit: Force

16. Example Part A fits into part B
All dimensions for part A are held !0.010
Specify the dimensions and tolerance for B with an allowance of 0.010

17. Example
Solution with allowance of .010
Part A
Part B

18. Example Allowance equals 0.010
Specify dimensions and tolerance for part B

19. Example Allowance equals 0.010
Specify dimensions and tolerance for part B