Drill jigs

The reasons for the use of jigs may be summed up under three heads, the order in which they are stated representing fairly well the frequency of occurrence, though not necessarily the importance, of these reasons: First, reduction of cost; second, duplication; third, accuracy.

Let us first consider the question of cost. As no article can, as a rule, be sold in open competition with similar articles unless its cost is somewhat proportionate to the quality of its competitors, commercial considerations demand that the cost be kept as low as possible, while the quality be kept as high as possible; and jigs are one of the chief agents of this in metal work. When a jig is considered, the first thing to be settled is whether it can be made to pay, and if so, how much. The answer to this often involves very many other questions, but can generally, if not always, be resolved into computations based upon the number of pieces to be made, and the probable cost of labor per piece when made with and without a jig, and the cost of the jig, including maintenance. Also the fact that often a much less valuable machine, or one less busy, can be used with the jig, may be an important consideration. If no other factor than cost of production is Involved, and it is found that the total cost of the jigged work will come very near that of the lot of articles when made without a jig, and no further order is in sight, it is pretty safe to abandon the jig idea; for it is apt to partake very much of the nature of an experiment, and the odds should be decidedly favorable to warrant the risk.

The second reason the duplication of pieces has a somewhat different foundation though cost enters here also, as will be seen later. Suppose the part to be made is subject to wear or breakage, as in agricultural and textile machinery, guns, bicycles, etc. We know, for instance, the strong disinclination anyone has for buying a wheel, the makers of which have gone out of business. It is at once recognized that repair parts cannot be bought from stock dealers, but must be made at excessive cost and delay. So we have before us the importance to manufacturers that the buying public shall have confidence in the interchangeability of parts in order that sales may be made at all upon the open market. It is a fact that where this reason holds good, there is also the reason that costs will be lessened, because production of large numbers of parts is taken for granted. And in considering whether or not a jig shall be made, this combination of reasons militates strongly for the jig. There is also another equally important reason for jigs, based on costs and interchangeability it is that, in fitting and assembling, those parts which are exactly alike require a minimum amount of labor when putting in place. This, perhaps, one may, without danger of exaggeration, say is in most cases in the machine building business the chief consideration.

In the third place, accuracy is often attained only by the use of jigs. There are certain classes of work which could not be finished at all within the limits of accuracy demanded, if jigs of some sort were not used.

It will therefore be seen that the determination of whether a jig shall be made may rest upon a number of questions which often demand great care and practical experience to solve in the way best meeting the requirements of the case.


Drill Jigs

Drill jigs are used for drilling holes which must be accurately located, both in relation to each other and to certain working surfaces and points; the location of the holes is governed by holes in the jig through which the drill passes. The drill must fit the hole in the jig to insure accuracy of location. When the jig is to be used in drilling many holes, the steel around the holes is hardened to prevent wear. If extreme accuracy is essential, or if the jig is to be used as a permanent equipment, bushings, made of steel and hardened, are used to guide the drills.


General Considerations in Designing - Jigs

The design of a jig should depend altogether on the character of the work to be done, the number of pieces to be drilled, and the degree of accuracy necessary in order that pieces drilled may answer the purpose for which they are intended. When jigs are to be turned over and moved around on the drill press table they should be designed to insure ease and comfort to the operator when handling, and should be made as light as is consistent with the strength and stiffness necessary. Yet, we should never attempt to save a few ounces of iron, and thereby render the jig unfit for the purpose we intend to use it for. The designer should see that the jig is planned so that work may be easily and quickly placed in and taken out, and that it can be easily and accurately located in order to prevent eventual mistakes. As it is necessary to fasten work in the jig in order that it may maintain its correct position, fastening devices are used; these should allow rapid manipulation, and yet hold the work securely to prevent a change of location. Yet, while it is necessary to hold work securely, we should not use fastening devices which spring the work, or the holes will be not only improperly located, but they will not be true with the working surfaces or with each other. When finishing the surfaces of drill jigs and similar devices used in machine shops, the character of the finish depends entirely on the custom in the shop, for while in some shops it is customary to finish these tools very nicely, removing every scratch, and producing highly finished surfaces, in other shops it is not required, neither is it allowed, as it is considered a waste of time and an unnecessary item of cost.


Limits of Accuracy

When making drill jigs we must discriminate between measurements that must be exact, and those not requiring extreme accuracy; it is not considered good practice, and it shows poor judgment, to spend the amount of time necessary to locate a hole within a limit of variation of 0.001 inch or even closer, if a variation of 1/64 inch is insignificant. But if the holes must be located exact as to measurements, it is necessary to work as accurately as possible, and time cannot be considered a factor, provided a man improves every minute. Yet the fact that extreme accuracy must be observed does not warrant a jig maker wasting time.

Before starting to work on tools of this character, the workman should first carefully look over his drawing, making himself thoroughly familiar with the construction, and making sure that the measurements given are, seemingly, correct; if in doubt about anything, consult the foreman, or the draftsman according to the custom in the shop in order that every detail may be thoroughly understood, or that any mistake made in the drawing may be rectified.

Many times .one draftsman is puzzled to understand a drawing made by an equally good man, especially so if the work is foreign to him; and a shop man, who may not be very well versed in reading drawings y e t be an excellent workman may easily get puzzled when he attempts to read a drawing of work he is not familiar with. Inquiries and proper explanations are therefore in place, and there should be no hesitation about asking questions, nor any reluctance about replying to them.


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