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SCEEW 



[Encyclopedia Britannica, Ninth Edition, Volume XXI \ 



The screw is the simplest instrument for converting a uniform motion 

 of rotation into a uniform motion of translation (see ' Mechanics/ vol. 

 xv, p. 754). Metal screws requiring no special accuracy are generally cut 

 by taps and dies. A tap is a cylindrical piece of steel having a screw 

 on its exterior with sharp cutting edges; by forcing this with a revolv- 

 ing motion into a hole of the proper size, a screw is cut on its interior 

 forming what is known as a nut or female screw. The die is a nut with 

 sharp cutting edges used to screw upon the outside of round pieces of 

 metal and thus produce male screws. More accurate screws are cut in 

 a lathe by causing the carriage carrying the tool to move uniformly for- 

 ward, thus a continuous spiral line is cut on the uniformly revolving 

 cylinder fixed between the lathe centres. The cutting tool may be an 

 ordinary form of lathe tool or a revolving saw-like disk (see ' Machine 

 Tools/ vol. xv, p. 153). 



Errors of Screws. For scientific purposes the screw must be so regu- 

 lar that it moves forward in its nut exactly the same distance for each 

 given angular rotation around its axis. As the mountings of a screw 

 introduce many errors, the final and exact test of its accuracy can only 

 be made when it is finished and set up for use. A large screw can, how- 

 ever, be roughly examined in the following manner: (1) See whether 

 the surface of the threads has a perfect polish. The more it departs 

 from this, and approaches the rough, torn surface as cut by the lathe 

 tool, the worse it is. A perfect screw has a perfect polish. (2) Mount 

 upon it between the centres of a lathe and the slip a short nut which 

 fits perfectly. If the nut moves from end to end with equal friction, 

 the screw is uniform in diameter. If the nut is long, unequal resist- 

 ance may be due to either an error of run or a bend in the screw. 

 (3) Fix a microscope on the lathe carriage and focus its single cross- 

 hair on the edge of the screw and parallel to its axis. If the screw runs 

 true at every point, its axis is straight. (4) Observe whether the short 

 nut runs from end to end of the screw without a wabbling motion when 

 the screw is turned and the nut kept from revolving. If it wabbles the 



