Introduction

It is not possible to make a part of exact dimensions due to inaccuracy of machines. Therefore, some error is to be tolerated within the acceptable limits. The variation in acceptable size is called limit. The allowable deviations in size are called tolerances. These limits have to be narrow to allow interchangeability of parts. Narrow limits increase the cost of production; hence, experience is needed to put tolerances on the drawing.

Terminology

Standard size Any handbook on machine design gives sizes of standard items. Some parts, like bolts, nuts, etc., are standardized for interchangeability of parts from one machine to the other. Some components are standardized for the purposes of the specifications, for example, wire sizes in Standard Wire Gauge (SWG), sheet metals in gauge number, structural sections, etc.

Limits Maximum and minimum allowable sizes to a basic size of a component are called limits.

Basic size This size is calculated by the designer to withstand the stresses in a component. Limits of tolerances are assigned to the basic size. For example, shaft diameter 50 mm is the basic size. This is represented by zero line in Figure 3.1. An instrument like a vernier caliper or a micrometre (shown on right side) is used to measure the size accurately.

Actual size It is the size that is obtained after manufacturing the product. It may not be as per the standard/basic required dimensions.