Specifications and Tolerances

Specifications

Specification is the definition of design. The design remains a concept in the mind of the designer until he defines it through verbal description, sample, drawing, writing etc. It defines in advance what the manufacturer expects to make. It defines what the consumer can expect to get. The specification serves as an agreement between manufacturer and consumer on the nature of characteristics of the product.

It is helpful to recognize the distinction between a design specification and on inspection specification. The design specification deals with what is desired in a manufactured article i.e. It deals with the specification function. In contrast, the inspection specification deals with the means of judging whether what is desired is actually attained, in other words it deals with inspection function(quality of conformance)

Tolerances

It is practically impossible to manufacture one article exactly like another or one batch like another. Variability is one of the fundamental concepts of modern quality control. Therefore, the ranges of permissible difference in dimensions have been standardized under the working limits. The limits of size for a dimension or a part are two extreme permissible sizes for that dimension (high limit and low limit). 

Hence

"The difference between the high limit and the low limit which is the margin allowed for variation in workmanship is called tolerance."

Tolerance can also be defined as 

"The amount by which the job is allowed to go away from accuracy and perfectness without causing any functional trouble, when assembled with the mating part and put into actual service."

Tolerances are set not only on dimensions, but also on other quality characteristics as well, such as temperature, pressure and volume.

The selection of tolerances is very important. A common complaint among production personnel is that designers do not understand production problems. Inspection personnel often complain not only about the poor quality of manufactured product but also about the unreasonableness of specified tolerances.

Therefore, the designers may specify one tolerance; inspection gauge may allow another usually wider tolerance and the foreman may be even more liberal. Each of these three parties views the operation from a different view points.