SCREW 509 



can never be made to run true on its bearings, and hence the device of 

 resting one end of the carriage on the nut must be rejected. Also all 

 rigid connection between the nut and the carriage must be avoided, as 

 the screw can never be adjusted parallel to the ways on which the car- 

 riage rests. For many purposes, such as ruling optical gratings, the 

 carriage must move accurately forward in a straight line as far as the 

 horizontal plane is concerned, while a little curvature in the vertical 

 plane produces very little effect. These conditions can be satisfied 

 by making the ways Y-shaped and grinding with a grinder some- 

 what shorter than the ways. By constant reversals and by lengthen- 

 ing or shortening the stroke, they, will finally become nearly per- 

 fect. The vertical curvature can be sufficiently tested by a short car- 

 riage carrying a delicate spirit level. Another and very efficient form 

 of ways is V-shaped with a flat top and nearly vertical sides. The 

 carriage rests on the flat top and is held by springs against one of the 

 nearly vertical sides. To determine with accuracy whether the ways 

 are straight, fix a flat piece of glass on the carriage and rule a line on 

 it by moving it under a diamond ; reverse and rule another line near the 

 first, and measure the distance apart at the centre and at the two ends 

 by a micrometer. If the centre measurement is equal to the mean of the 

 two end ones, the line is straight. This is better than the method with 

 a mirror mounted on the carriage and a telescope. The screw itself 

 must rest in bearings, and the end motion be prevented by a point bear- 

 ing against its flat end, which is protected by hardened steel or a flat 

 diamond. Collar bearings introduce periodic errors. The secret of 

 success is so to design- the nut and its connections as to eliminate all 

 adjustments of the screw and indeed all imperfect workmanship. The 

 connection must also be such as to give means of correcting any residual 

 periodic errors or errors of run which may be introduced in the mount- 

 ings or by the wear of the machine. 



The nut is shown in Fig 2. It is made in two halves, of wrought iron 

 filled with boxwood or lignum vitae plugs, on which the screw is cut. 

 To each half a long piece of sheet steel is fixed which bears against a 

 guiding edge, to be described presently. The two halves are held to the 

 screw by springs, so that each moves forward almost independently of 

 the other. To join the nut to the carriage, a ring is attached to the 

 latter, whose plane is vertical and which can turn round a vertical axis. 

 The bars fixed midway on the two halves of the nut bear against this 

 ring at points 90 distant from its axis. Hence each half does its share 

 independently of the other in moving the carriage forward. Any want 



