282 Mr Wilson's Improvement on the Common Vice, S^c, 



of the nut and screw cannot bear always in the same way on 

 either side. Thus the strain, instead of being transferred 

 along the axis of the screw, is sometimes transferred from the 

 upper, sometimes from the under edges of the bearings, and, 

 but rarely, from face to face. 



In both the bench and hand-vice, then, the flexibility of the 

 screw allows the oblique pressure to bend it, and the articles 

 are held in the vice by this elasticity ; and this is one reason, 

 that although the cheeks of the vice have met upon a hard ob- 

 ject, as a piece of iron, the screw can be turned considerably 

 farther. 



But the elastic or insecure holding is not the only fault of 

 the common vice, — the parts obliquely strained wear out very 

 soon ! 



To remedy these evils in the chuck and hand- 

 vice, I fix a straight screw on one of the arms 

 as firmly as possible, and work it by. means of a 

 nut accurately fitted ; or in the table or bench- 

 vice, I fix the box, instead of the screw, to the 

 stationary arm, and work it by means of a 

 straight screw, — or allow the nut to be loose, as at present, which 

 gives it the same advantages as the screw. 



The next thing is to cause the nut, or shoulder of the screw, 

 to work square upon its bearings, so as to transmit the pres- 

 sure directly along the axis of the screw. This I accomplish 

 by making an elongated hole in the moveable arm for the 

 screw to pass through, and by forming the sides of that hole to 

 a particular curve. The nature of the curve is this : — in any 

 position of the vice, if the plane where the axis of the screw 

 cut the curve be noticed, a plane touching the curved surface 

 must be perpendicular to the axis of the screw ; by this means, 

 as will be seen on inspecting the instrument, the screw and 

 nut always act directly, giving a full, steady, and dead pres- 

 sure to the article grasped, — while all the parts are saved from 

 undue wearing. The peculiar form of the curve gives it other 

 advantages, especially it increases the strength. 

 Edinbubgh, 19/^ January 1837. 



