36.1 MEASUREMENTS AND QUALITY CONTROL
The metric and English measuring systems are the two measuring systems commonly used throughout
the world. The metric system is universally used in most scientific applications, but, for manufacturing
in the United States, has been limited to a few specialties, mostly items that are related in some way
to products manufactured abroad.
36.2 DIMENSION AND TOLERANCE
In dimensioning a drawing, the numbers placed in the dimension lines are only approximate and do
not represent any degree of accuracy unless so stated by the designer. To specify the degree of
accuracy, it is necessary to add tolerance figures to the dimension. Tolerance is the amount of variation
permitted in the part or the total variation allowed in a given dimension.
Dimensions given close tolerances mean that the part must fit properly with some other part. Both
must be given tolerances in keeping with the allowance desired, the manufacturing processes avail-
able, and the minimum cost of production and assembly that will maximize profit. Generally speaking,
the cost of a part goes up as the tolerance is decreased.
Allowance, which is sometimes confused with tolerance, has an altogether different meaning. It
is the minimum clearance space intended between mating parts and represents the condition of tightest
36.3 QUALITY CONTROL
When parts must be inspected in large numbers, 100% inspection of each part is not only slow and
costly, but does not eliminate all of the defective pieces. Mass inspection tends to be careless;
operators become fatigued; and inspection gages become worn or out of adjustment more frequently.
The risk of passing defective parts is variable and of unknown magnitude, whereas, in a planned
sampling procedure, the risk can be calculated. Many products, such as bulbs, cannot be 100%
Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.
ISBN 0-471-13007-9 1998 John Wiley & Sons, Inc.
STATISTICAL QUALITY CONTROL
Magd E. Zohdi
Department of Industrial Engineer