Popular Science Monthly
��475
���ability of parts is also essential in modern manufacturing. When a machine is standard, any part may be replaced immediately without either filing or fitting.
This is of great significance, where broken parts of the modern industrial, standard machines have to be renewed. Often these machine parts have to be shipped to remote parts of the world, demanding the necessity of an accurate fitting. Inter- changeability is best ob- tained through the employ- ment of jigs and fixtures, because all parts are held in like manner and dis- tances, thus ensuring mechanical accuracy, which eliminates the unreliability of personal judgment.
Let us consider the location of parts. When a finished surface on a piece of work is used as a means of location, it is best to use steel plates or stool-pins (Fig. i) for support in the jig. If the locating surfaces are large, they may rest against finished bosses on the jig casting, and then the wear will not be appreciable. A rough casting or forging should rest on three pins as shown in Fig. 2.
If the part is thin or weak, and the tendency is to spring under the thrust of the drill, a spring- pin with a locking- screw may be added for additional sup- port, as shown in Fig. 3. For the location of cylindri- cal surfaces such as hubs, locating pins set at 45 degrees may be used as shown in Fig. 4. The stops shown in Fig. 4 must be fitted tightly to the jig proper. Sometimes, an adjustable stop ma3' be used as in Fig. 5, thus giving _ allowance for variation in the casting. These, howexer, are generally to be avoided as unskilled operators are liable
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��to tamper with them, resulting in an inefficient jig.
In locating rough castings or drop forgings, care must be taken to avoid resting them on the pins which occur where the molds or dies are parted. Lack of space sometimes prevents the designer from using the spring-pin support, shown in Fig. 3. In place of it, a small jack-screw may be used, as shown in Fig. 6. A locking-screw with a brass plug must be put in, in order to prevent the jack from working loose because of jarring the jig. Bushings are used to guide drills, reamers, coun- ter-bores, etc. They are generally of three types, as shown in Figs. 7 and 8. Bushings must be accurately located in the jig to insure exact duplication of each part. The bushing in Fig. 7 fits tightly while in Fig. 8 are shown both tight and loose bushings.
Clamping is done by means of a standard bolt and nuts, according to the conditions. In Fig. 9 is shown the application of a clamping device. In most cases the location points, them- selves, will serve in firmly securing the work, without additional aid.
What was said with regard to loca- tion and clamping of jigs holds true with fix- t u r e s , though the fix t u r e must be built much heavier, to make al- lowance for the weight of the cut which is usually heavier. Bushings to be used in the drill jigs are of stan- dard type and can be found in any standard machinist's handbook. The few principles of design touched on here by no means exhaust the subject.
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