State of the Useful Arts. 273 



do'ivn and blunted, so that the last plugs, supposing the screw of 

 the slide-rest always brought to the same mark, will be larger 

 than the first one. To palliate this evil a very simple plan is 

 followed. The plugs, or other articles, are all made roughly to 

 about one size, very nearly what they are to be when finished ; 

 by this means the rough exterior, which generally occasions the 

 greatest wear of the tool is removed. The prepared plugs are 

 then submitted to the finishing process, the cutter for which 

 having no great extent of metal to go through, is worn but very 

 slowly. This>is a very obvious source of inaccuracy ; but there 

 is another, less apparent but more constantly annoying. It is 

 the flexibility of all the materials, that on which we are operat- 

 ing, as well as that of which the parts of the lathe are composed. 



Considerable pressure is needed to cause the cutting edge to 

 enter the substance operated on, and to force it through that 

 substance. This pressure must be resisted on the one hand by 

 the slide rest and shear of the lathe, on the other by the mate- 

 rial cut. The part of the slide-rest, lathe-bed, tool-holder, and 

 cutter, being subjected to pressure, are bent, and the extent of 

 this bending is proportional to the pressure employed ; so that 

 the tool, instead of describing the right line, which it ought to de- 

 scribe, moves over a line somewhat removed from that, and not 

 even straight, excepting in the rare case of perfectly homoge- 

 neous turning material. The extent of this flexure can only be 

 reduced by giving greater strength to the parts of the turning 

 apparatus ; and as this flexure is more seriously felt in small 

 objects, it follows, that to make even minute work accurately, 

 we must have a lathe of great weight and strength. 



This deviation of the cutting edge from its proper place, may 

 alwaysbe resolved into twodeviations, one horizontal, that is, from 

 or towards the axis, and the other vertical. The first of these 

 deviations goes- directly to affect the radius of the work, and in 

 that which is therefore most to be guarded against. This de- 

 viation is not always from the axis, but is frequently towards 

 it ; indeed, if the tool be properly formed, it ought with most 

 substances to be inwards, as the peculiar form which gives clean 

 and easy cutting also induces a tendency to drag the tool still 

 deeper. The horizontal deviation of the cutting edge is pro- 



