274) Mr Sang's Annual Report oil the 



diiced by the twisting of the shear, by the bending of the up- 

 right part of the rest, and by inaccuracies in the fitting of the 

 slides ; the methods of palliating these evils (for they can never 

 be entirely removed) are apparent and simple. But, if the 

 lathe be constructed, and if all these evils exist, the skill of the 

 •workman is then called into active exercise, in devising a tool 

 which may give occasion to as little inward or outward pressure 

 as possible ; this problem is not one of very easy solution. 



The pressure is resisted on the other hand by the material 

 under operation ; and here we have a source of error very diffi- 

 cult of removal. 



The pressure on the material, exactly in the same way, re- 

 solves itself into two, one horizontal tending to bend the sub- 

 stance, and thus to affect its diameter ; the other vertical, tend- 

 ino- to twist it. The torsion of the spindle and material is un- 

 avoidable, it is indeed the measure of the force (excluding 

 friction) necessary to drive the lathe. The flexure, however, is 

 extraneous, and may be palliated or removed by attention to the 

 forms and actions of the parts ; the formation of the cutting edge 

 being the most important. 



The influence of the flexure is most felt when the material 

 has an elongated form, as with the piston-rod of a steam-engine, 

 or the cylinder for a screw. In such cases a prop attached to 

 the slide-rest, is brought to support the work almost opposite 

 the cutter, but on the turned portion of the rod ; in this way 

 the flexure is almost completely prevented, and a heavy cut can 

 be taken, however long the rod may be ; but such a contrivance 

 is convenient only in the case of cylinders ; and is barely prac- 

 ticable in the case of a slightly tapered cone. In other cases 

 stationary props must be employed to subdivide the inconve- 

 nient length. 



That part of the pressure which goes to twist the spindle, and 

 which ought to be by far the greater part of if, introduces that 

 peculiar efffect which is called criddhng or waving, a pheno- 

 menon not at all capricious as many would imagine, but governed 

 by very exact laws. The avoiding of criddle is a very import- 

 Ant point with all turners; but it is much more easy to avoid it 

 in appearance than in reality ; for often when the surface pre- 

 sents no traces of it to the eye, the car will have detected the 



