Sir John Robison on Cutting Curved-faced Files. 89 



same temper, the rolls would be continually out of order and require 

 renewing. 



We make these few remarks, desirous of putting you in possession of 

 our mode of proceeding, and the reasons ; and which may probably elicit 

 some improvement from your scientific experience. 



We could easily by various means turn the heated steel into any 

 form of curve, but in adopting the means, the object in view must be to 

 preserve the proper sharpness of the tooth, which in this heated state by 

 undue pressure or by hard metals is so liable to be injured ; we trust, 

 however, our next specimens will prove that these difficulties have been 

 overcome. Johnson, Cammell, & Co. 



James Tod, Esq., Secretary, 

 Royal Scottish Society of Arts. Shepfikld, November 7, 1843. 



Sir, — We are favoured with your esteemed letters of the 2d and 4th 

 instant, with their enclosures, and are obliged by your kind attention. 



We now have pleasure to hand you (as requested) extract from our 

 letter of February 13th last, to the late Sir John Robison, explaining the 

 mode we then adopted in the manufacture of the half-round files from 

 steel of parallel thickness, as suggested by him, viz., by means of a 

 screw-press and swages of copper, and which we see is the plan named 

 in the paper communicated to the Society. 



From the specimens sent you last week, you will perceive we have deviat- 

 ed from the plan first suggested '^ of cutting them from blanks of steel pre- 

 pared as for atliin equalling-file." We, however, did not give up this plan 

 until, from practical experience, we found its working very uncertain and 

 irregular, for the file being of uniform thickness, the edges presented an 

 equal or greater degree of resistance to pressure, than the centre ; and the 

 top swage or boss coming in contact with the centre of the file previous 

 to any other part, caused it to bend more freely than the edges, produc- 

 ing various degrees of curvature in the same file. Again, if our top boss 

 was so shaped as to create an earlier and freer pressure on the edges, 

 to ensure a more uniform curve, we then endangered the sharpness of 

 the teeth on those parts of each side of the file, convex and concave, 

 first receiving such undue pressure. These objections and difficulties are 

 all overcome or lessened bj'' our present mode of cutting and turning the 

 files from steel with slightly tapered edges, on one side, thus, ^ - ^ 



The flat surface is cut with a continuous tooth, and can be turned either 

 convex or concave, and the tapered surface can either be cut in ridges 

 or left safe, or uncut, which, from the following extract, you will perceive 

 was Sir John's first idea. In Sir John Robison's first communication, No- 

 vember 10, 1842, after recounting the difficulty of obtaining a smooth 

 half-round file or one of equal continuous tooth, he says, — 



" I propose to overcome this difficulty, and to cut half-round files as 

 truly smooth as flat ones are now struck, by making the blanks of rolled 

 steel-plate ; by striking them in the flat state, and by afterwards giving 

 them the degree of curvature required by means of a screw-press, and 



