1. Tolerancing
Chapter 11

2. Objectives
Describe nominal size, tolerance, limits, and allowance of two mating parts
Identify a clearance fit, interference fit, and transition fit
Describe the basic hole and basic shaft systems

3. Objectives (cont.)
Dimension mating parts using limit dimensions, unilateral tolerances, and bilateral tolerances
Describe the classes of fit and give examples of each
Draw geometric tolerancing symbols
Specify geometric tolerances

4. Understanding Tolerance
Tolerancing is an extension of dimensioning
It allows you to specify a range of accuracy for every feature of a product so the parts will fit together and function properly when assembled

5. Understanding Tolerance
To provide tolerances in CAD, you must:
Understand the fit required between mating parts
Have a clear picture of how inspection measurements are performed
Be able to apply tolerance symbols to a drawing

6. Tolerance
Tolerance is the total amount a specific dimension is permitted to vary
Use generous tolerances when possible because increased precision makes parts more expensive to manufacture

7. Quality Control
Large batches of parts may use statistical methods to control quality where a sample of parts are inspected
Specific tolerances are based on the part’s function and fit

8. Variations in Form
Acceptable parts must not extend beyond boundary limits

9. Implied 90 Degree Angles
When lines intersect on a drawing at angles of 90 degrees, it is customary not to dimension the angle
Implied 90 degree angles have the same general tolerances applied to them as any other angles covered by a general note

10. Implied 90 Degree Angles

11. Fits Between Mating Parts
Fit is the term for the range of tightness or looseness resulting from the allowances and tolerances in mating parts

12. Clearance Fit
Where the internal part fits into an external part with space between the parts.

13. Interference Fit
Where the internal part will have to be forced into an external part.
Produces a negative clearance or allowance.

14. Transition Fit
Where there the smallest shaft will fit into the largest hole, but the largest shaft will have to be forced into the smallest hole. Refers to either a tight clearance or interference.
May or may not produce a negative clearance or allowance.

15. Definitions for Size Designation
Nominal size – used for general identification and usually expressed in decimals
Basic size (basic dimension) – the theoretically exact size from which limits of size are determined

16. Definitions for Size Designation
Actual size – the measured size of a finished part
Allowance – the minimum clearance or maximum interference specified to achieve a fit between two mating parts

17. Basic Hole System
Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size

18. Basic Shaft System
In this system, the maximum shaft is taken as the basic size and is used only in specific circumstances

19. Specifying Tolerances
The primary ways to indicate tolerances in a drawing are:
A general tolerance note
A note providing a tolerance for a specific dimension
A reference on the drawing to another document that specifies the required tolerances

20. Specifying Tolerances
(cont.)
Adding limit tolerances to dimensions
Adding direct plus/minus tolerances to dimensions
Geometric tolerances

21. GENERAL TOLERANCE NOTES
“ALL TOLERANCES ±1 mm
UNLESS OTHERWISE NOTED.
ANGLES 1 DEGREE.”
General notes are usually located in the lower right corner of the drawing sheet near the title block. Often, general tolerance notes are included in the title block itself.

22. LIMIT TOLERANCES
Limit tolerances state the upper and lower limits for the dimension range in
place of the dimension values.
… is the preferred method of stating tolerances.
Method of Stating Limits
Note: The upper value is always placed above
the lower value.

23. Plus-or-Minus Tolerances
Allows variations in only one direction from the nominal size.
Allows variations in both directions from the nominal.

24. Tolerance Stacking
The variations produced by tolerances are stacked in chain dimensioning

25. Fits between Mating Parts
Fit is the range of tightness or looseness resulting from the allowances and tolerances in mating parts. The loosest fit, or maximum clearance, occurs when the smallest internal part (shaft) is in the largest external part (hole),

26. Fits between Mating Parts continued…
Clearance Fit A clearance fit occurs when an internal
part fits into an external part with space (or clearance) between the parts.
Interference Fit An interference fit occurs when the internal part is larger than the external part, so the parts must be forced together.

27. USING AMERICAN NATIONAL STANDARD LIMITS AND FIT TABLES
The American National Standards Institute has issued ANSI B4.1, Preferred Limits and Fits for Cylindrical Parts, defining terms and recommending preferred standard sizes, allowances, tolerances, and fits in terms of the decimal inch. This standard gives a series of standard classes of fits on a unilateral-hole basis
so that the fit produced by mating parts of a class of fit will produce approximately similar performance throughout the
range of sizes.
Portion of RC8 Fit Table. The International
Organization for Standardization (ISO) publishes a similar series of fit tables for metric values.

28. TOLERANCES AND MACHINING PROCESSES
Tolerances Related to Machining Processes

29. Metric System of Tolerances and Fits
The preceding material on limits and fits applies to both systems of measurement
The ISO has a system of preferred metric limits and fits
The system is specified for holes, cylinders, and shafts and has similar definitions of terms
Hole
Tolerance
Shaft
Tolerance
50H8/f7
Basic Size
Fit

30. Geometric Tolerancing
Geometric tolerances state the maximum allowable variations of a form or its position from the perfect geometry implied in the drawing
The term “geometric” refers to forms such as planes, cylinders, squares, etc.

31. Geometric Characteristics and Modifying Symbols
Tolerances of form and position (or location) control such characteristics as:

32. Symbols for Tolerances of Position and Form