Analysis of Tolerances and Inspection Standards
The specification of tolerances or the standards of quality, accuracy, finish, and so on, that the operation must satisfy play an important part in the methods used to produce the part. In fact, in many cases, the requirements fix the method. The accuracy with which the diameter of a small shaft must be machined and the finish which the machined surface must possess will determine the machines that must be used, the number of cuts taken, and the feeds and speeds.
Hence, at the outset of any methods study, it is important, first, that the tolerance requirements of the operation be known and, second, that these requirements be reviewed for correctness. The assumptions are made that the operator is doing a job which will pass inspection and that the requirements as specified by the designer or the chief inspector are correct. Undoubtedly these assumptions are true in the majority of industrial operations, but enough important exceptions are encountered to make an analysis of tolerance and inspection requirements a point of primary importance.
The following questions should be raised and, as always, answered only after careful consideration:
1. What are the inspection requirements of this operation?
2. What are the requirements of the preceding operation?
3. What are the requirements of the following operation?
4. Will changing the requirements of a previous operation make this operation easier to perform?
5. Will changing the requirement of this operation make a subsequent operation easier to perform?
6. Are tolerance, allowance, finish, and other requirements necessary?
7. Are they suitable for the purpose the part has to play in the finished product ?
8. Can the requirements be raised to improve quality without increasing cost?
9. Will lowering the requirements materially reduce costs?
10. Can the quality of the finished product be improved in any way even beyond present requirements?
Relation of Methods Efficiency Study to Quality.
Methods efficiency studies are made primarily for the purpose of eliminating waste and reducing costs. In so doing, however, it goes without saying that nothing should be done to impair the quality of the finished product or its salability. Because the methods efficiency engineer is interested in enhancing the competitive position of his company's products, he quite naturally must take a keen interest in the factor of quality. Products of superior quality outsell products of inferior quality, other things being equal; hence, an improvement in quality is always desirable, provided, of course, that it is necessary and useful quality. Any improvement that betters the functioning, appearance, or salability of the product should be constantly sought. Unnecessary quality, however, refinements that add to the cost of the product without in any way improving it, should be eliminated.
Sometimes it is difficult to decide whether a certain requirement is an unnecessary refinement or a desirable improver of quality. Such questions can be answered only after a thorough discussion of all of the factors involved. In general, however, because of the competitive condition existing in industry, any suggested improvement in quality that can be made without taking the product out of its price class should be adopted.
The methods efficiency engineer is in a good position to make suggestions that will improve quality. Because he studies a product in detail and considers thoroughly every factor connected with it, he is quite likely to discover ways of making the product better. In addition, because he eventually sets up working methods that are easy, efficient methods, and because he trains all operators to follow those methods, a higher and more uniform quality of workmanship results than where each operator is left to develop methods for himself. As a result, therefore, methods study tends to raise the quality of the finished product.
Results of Analyzing Inspection Requirements.
For machine work, the limits of accuracy within which the part must be machined are customarily specified on the drawing of the part. These allowances are worked out by the design engineers and are based upon the function the part is to play in the finished product and the relation of the dimensions of the part to the dimensions of the other parts with which it is used. Theoretically, the allowances established by the design engineers should be correct; but because the human element enters in here as elsewhere, they should be carefully checked by the analyst.
Close tolerances raise the cost of a machining operation by making it necessary for the operator to work accurately, checking his work frequently. More cuts are necessary if dimensions must be held accurately, and perhaps even additional operations on other machines. There is a tendency for designers to specify increasingly close tolerances, a tendency that many shopmen deplore. However, the performance requirements of many products are becoming daily more exacting, and as a result accuracy requirements are likely to become increasingly severe. Machine shops, therefore, must face this problem and learn how to work more and more accurately. That this objective can be attained is evidenced by the remarkable advances being made almost daily in the automotive and aviation industries.
When tolerances are carefully reviewed, some may be found that appear to be unnecessarily close for the function of the part hi the finished apparatus. Such cases should be presented to the engineers with a statement of the amount that may be saved by allowing greater leeway. If the tolerance really is too close and a worth-while saving will be made by increasing it, the change will in all probability be made.
It will aid materially in getting such changes made if charts showing tolerance and related cost are available for different classes of operations. Such charts serve to emphasize clearly how much costs are increased as tolerances are decreased. They can also be of value to design engineers, for reference purposes.
Occasionally, tolerances are not close enough. Sometimes, by tightening the requirements on a machining operation, the assembly is made easier, and the amount spent on the extra machine work is offset or more than offset by the saving made on the assembly floor. In standardized manufacture, fitting during assembly has been practically eliminated. Parts are machined so that they go together without filing, bending, or adjusting. The same condition is desirable in small-quantity production where much fitting is commonly done, and it can often be approached by tightening the accuracy requirements on the principal parts.
When a product is made to sell for a price, as, for example, a certain grade of shoe, the matter of allowed quality becomes extremely important. It is possible to add operations almost indefinitely that will improve quality, but the added cost will take the finished shoe out of its price range. Hence, it becomes necessary to determine what can be done for the amount of money available. In a situation of this kind, labor effectiveness is of paramount importance. The more effectively operations are performed, the more operations can be done. The more operations, the better the quality, and, hence, the better the competitive position of the shoe.
Source: Operation Analysis, Maynard
Full Knol Book - Method Study: Methods Efficiency Engineering - Knol Book
Updated 4 July 2015
First published 23 Nov 2013