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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 Fig. 8 



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. 



Fig. 9 



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