SCREW 507 



screw is said to be drunk. One can see this error better by fixing a 

 long pointer to the nut, or by attaching to it a mirror and observing an 

 image in it with a telescope. The following experiment will also detect 

 this error: (5) Put upon the screw two well-fitting and rather short 

 nuts, which are kept from revolving by arms bearing against a straight 

 edge parallel to the axis of the screw. Let one nut carry an arm which 

 supports a microscope focused on a line ruled on the other nut. Screw 

 this combination to different parts of the screw. If during one revolu- 

 tion the microscope remains in focus, the screw is not drunk; and if 

 the cross-hairs bisect the lines in every position, there is no error of 

 run. 



Making Accurate Screws. To .produce a screw of a foot or even a 

 yard long with errors not exceeding -nnn^h of an inch is not difficult. 

 Prof. Wm. A. Eogers, of Harvard Observatory, has invented a process 

 in which the tool of the lathe while cutting the screw is moved so as to 

 counteract the errors of the lathe screw. The screw is then partly 

 ground to get rid of local errors. But, where the highest accuracy is 

 needed, we must resort in the case of screws, as in all other cases, to 

 grinding. A long, solid nut, tightly fitting the screw in one position, 

 cannot be moved freely to another position unless the screw is very accu- 

 rate. If grinding material is applied and the nut is constantly tight- 

 ened, it will grind out all errors of run, drunkenness, crookedness, and 

 irregularity of size. The condition is that the nut must be long, rigid 

 and capable of being tightened as the grinding proceeds ; also the screw 

 must be ground longer than it will finally be needed so that the imper- 

 fect ends may be removed. 



The following process will produce a screw suitable for ruling grat- 

 ings for optical purposes. Suppose it is our purpose to produce a screw 

 which is finally to be 9 inches long, not including bearings, and 1-| in. 

 in diameter. Select a bar of soft Bessemer steel, which has not the 

 hard spots usually found in cast steel, and about If inches in diameter 

 and 30 long. Put it between lathe centres and turn it down to one 

 inch diameter everywhere, except about 12 inches in the centre, where 

 it is left a little over 1 inches in diameter for cutting the screw. Now 

 cut the screw with a triangular thread a little sharper than 60. Above 

 all, avoid a fine screw, using about 20 threads to the inch. 



The grinding nut, about 11 inches long, has now to be made. Fig. 1 

 represents a section of the nut, which is made of brass, or better, of 

 Bessemer steel. It consists of four segments, a, a, which can be drawn 

 about the screw by two collars, &, &, and the screw c. Wedges between 



