The development of interchangeable manufacture has made necessary a complete revolution in the gaging and inspection systems employed in the mechanical industries. The introduction of the limit system in manufacturing has changed many of the methods previously in vogue. The intensive manufacture of arms and ammunition during the past few years has also more than ever indicated the necessity of accurate and reliable means of gaging and inspection. No book has been published in the past dealing exclusively with this subject, nor dealing comprehensively enough with it to meet the requirements at the present time, and for this reason the present volume has been prepared with a view to cover the principles and the practical application of the limit system of interchangeable manufacturing, and to describe the principal tools and gages that are employed in this work in leading manufacturing establishments in the country.

The principles of the limit system are first dealt with, and then the different kinds of measuring machines and reference and working gages are illustrated and described. Gages of many different types are included - plug and ring gages, profile gages, indicating gages, thread gages, and devices for measuring gears of different kinds. The book should, therefore, appeal to those responsible for the output of shops manufacturing on the interchangeable plan, inspectors, foremen, designers of special measuring tools, toolmakers, and others whose duties are in one way or another connected with the problems of interchangeable manufacture, the gages used, and the inspection methods required.

Previous to the adoption of gages and inspection fixtures, the component parts of mechanisms were made by fitting one to another. As an illustration, take a gasoline engine and assume that the cylinders were finished by reaming. The pistons would be made approximately to size by caliper measurements and then fitted to the bore of the cylinders, the cylinders acting as a gage. In this way, of course, interchangeability is difficult, if not impossible. The question of having each part alike within given limits, however, has not always been considered practicable or necessary, and the first application of the interchangeable system was adopted in the production of rifles. In the past few years, the subject of gaging and inspection methods has received considerable attention. This has been due as much to the desire to reduce manufacturing costs as to the necessity for having all parts interchangeable.

It has been dearly demonstrated in plants where the interchangeable system has been properly applied that a less skilled grade of mechanics can be employed and still turn out interchangeable parts. It requires the services of a first-class mechanic to make a number of parts exactly alike, without a gaging system, but a second-rate mechanic can make a large number of parts exactly the same - within the required limits - and much cheaper, with a proper gaging and inspection system. Hence, the advantage gained by interchangeable manufacture is not only that all parts are made alike, but that the production costs are greatly reduced at the same time.

In the present chapter, the fundamental principles governing the setting of limits and tolerances for various classes of work will be dealt with in detail.

The Limit System - The limit system which forms the basis of interchangeable manufacture is applied in a variety of ways in various manufacturing plants. In some cases, the limit system, as generally understood, is not applied in its entirety. For instance, parts which must fit together are made with certain allowances, but no tolerances are given to take care of unavoidable errors in the manufacture of parts. This cannot be called a limit system, because no limits are given. Consequently, a great deal more time and money is spent in making the parts than would be the case if the complete system were adopted. In cases where the limit system is not adopted, the parts are made to direct measurement, using standard measuring tools, and unless great care is taken and experienced workmen employed, considerable fitting and assembling is necessary. With a complete system of gaging and inspection, this fitting and assembling is almost, if not entirely, done away with, and manufacturing costs are thereby greatly reduced.

The advantages of the limit system are not always thoroughly appreciated, and in the following an endeavor will be made to explain some of the chief reasons why this system should be adopted where it is desired to produce parts cheaply as well as interchangeably.

Advantages of the Limit System. - The adoption of a limit system and the practice of working to limit gages has many advantages. In the first place, it makes an interchangeable product possible and eliminates the necessity of depending upon the judgment of the workman; and probably what is of still more importance, it reduces the amount of spoiled work to a minimum. The initial cost required to install the system in some cases is heavy, but when it is considered that a properly organized limit system greatly reduces inspection and manufacturing costs, the advantages to be gained will, generally, more than offset the cost involved. In adopting a limit system for ordinary work, it is necessary to take either the hole or the shaft as a standard. When holes are finished by grinding, it makes little difference which is decided upon. When the hole in the work is finished by reaming or similar tools, and can be duplicated in size with reasonable commercial accuracy, it is advisable to adopt the size of the hole as a standard. There are, of course, exceptions to all rules, and in some cases manufacturers using cold-rolled steel shafting find that it is preferable to use the shaft as a basis instead of the hole. As a general rule, however, the hole basis should be adopted.

Limit, Allowance and Tolerance - The expression "limit" as employed in machine shop work refers to the permissible tolerance in machine work. A shaft, for example, is required to be one inch in diameter; but it is generally unnecessary, in commercial work, to finish the shaft to a diameter of exactly one inch. It is, therefore, common practice to specify the "limit" - that is, the deviation from the true or nominal size which is permissible. The limit is generally stated by giving the amount that the dimension may be larger or smaller than the nominal size. The diameter of the shaft, for example, may be given as "one inch plus or minus 0.001 inch," which means that the shaft will pass inspection if it is not more than 0.001 inch larger or 0.001 inch smaller than one inch. A common method of expressing this briefly on a drawing is to state the diameter as "1 ±0.001."

There are a number of terms closely allied to the term "limit " used in the machine shop. Among these are "tolerance," "working tolerance," "permissible tolerance," "necessary tolerance," . "clearance," " working clearance," " allowance," "working allowance," and "finish." Of these terms, all those containing the word "tolerance" signify the difference between the maximum and minimum limits; the terms containing the word "clearance" are equivalent to those containing the term "allowance"; and the term "finish" implies the final touch or elaboration that is given to the work, irrespective of the accuracy of the dimensions required. Therefore, as regards the dimensions, there are only three expressions necessary - "limit," "tolerance," and "allowance." The meaning of "limit" has been explained. "Tolerance" is the total difference between the maximum and minimum dimensions of the work. "Allowance," often termed "clearance," signifies the difference between the working parts to produce a certain fit and to provide for lubrication. The amount of the allowance depends upon the class of fit required, whether a running fit, sliding fit, push fit, drive fit, etc. The practice of making drawings which specify the exact amount of allowance between different parts, as well as the limit, is recommended. When this practice is followed, there can be no controversy when work is inspected, because everything is plainly stated on the drawing, and there is no opportunity for misunderstanding as to the accuracy required. The expression "a limit of o.ooi inch" should never be used, as it is indefinite, giving no idea of whether this limit is above or below standard size. It may mean either 0.001 inch above; 0.001 inch below; or possibly 0.0005 above and below standard size. When the limit is expressed as ±0.001 or ±0.0005 inch, there can be no mistake.

Setting Manufacturing Limits on Interchangeable Parts - The setting of limits on work which must be made interchangeable is of vital importance. It is a subject which, gen- erally, does not receive the consideration that it deserves. Many manufacturers set the limits much closer than is necessary to obtain interchangeability. When a part does not need to be made to more accurate limits than  0.003 inch, this limit should be tolerated. Where greater accuracy is necessary, the limits should be closer.

The practice of specifying limits saves much needless expense. For instance, many manufacturers specify the length of a shaft in fractions of an inch and do not give any limits, with the result that the toolmaker or mechanic endeavors to bring the parts exactly to the size mentioned. This is a short-sighted policy, especially where - inch more or less than the size given on the drawings would make no practical difference in the efficiency of the machine. Take also, for instance, gages, jigs, fixtures or other special tools used in the production of interchangeable parts. It is the practice of some concerns not to give any manufacturing limits on these tools, specifying that all dimensions should be exact. Hence, much time is consumed in making the tools, which even under the most favorable conditions cannot be made exactly alike. Some go so far as to state that a plug or ring gage should be made exact, and have no limit of error at all; but this is impracticable, as it is almost impossible to make two parts exactly alike; a slight change in temperature will easily change a plug gage diameter 0.000025 inch. Several large manufacturing concerns have set limits on work produced in the tool-room with satisfactory results.