TABLE OF CONTENTS A tolerance is the amount of variation permitted from a nominal dimension. It is the amount of material that can be added to, or subtracted from, a nominal dimension. Manufactured parts may vary according to tolerances shown on prints. Tolerances are necessary because it is not practical to manufacture parts without some variations. This is especially true when making large quantities of similar parts. Mass production would not be practical without the aid of tolerances. They also allow parts to be interchangeable which is a very important factor in mass production. Tolerances are based on several factors such as, the end use of the part, cost, how it is manufactured, and experience. The purpose of this chapter is to review tolerances; show how they are displayed on prints, and stress their importance when reading prints.
There are three types of tolerances:
Plus or minus.
Maximum and minimum limits.
Only maximum or minimum dimensions.
A plus or minus tolerance is shown with a nominal dimension and indicates the amount of material that can be added to, or subtracted from, the dimension, Fig. 6-1. The example in Fig. 6-1, has a nominal dimension of 1.000 and a tolerance of plus or minus .002. This makes the maximum dimension 1.002 and the minimum dimension .998. Parts that measure 1.002 and .998 or anywhere in between are acceptable. Parts larger than 1.002 or smaller than .998 are not acceptable. Figure 6-2 shows these figures in the form...
