Page:Popular Science Monthly Volume 91.djvu/782

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Some Common Abrasives

By Professor William Noyes

Late Director of the Department of Wood- Working, Teachers' College, Columbia University, and author of "Hand Work in Wood"

���WOOD may be either cut down with an edged tool or worn down by abra- sives. The common abrasives are steel, a hard mineral, or a mineral- like artifi- cial sub- stance.

Steel is used for rubbing in two main forms : as a file or rasp, and in the shape of steel wool. The teeth of a file are cut when the metal is soft and cold. The tool is then highly tem- pered, so that the metal be- comes very hard and brit- tle. At least three thou- sand varieties of files are made, each adapted to its particular pur- pose. They are used chiefly for scraping the sur- face of metal, but are also appli- cable to wood for certain purposes.

Files are classi- fied (i) accord- ing to their out- lines into taper and blunt, (that is, having a uniform cross-section through out) ; (2) according to the shape of their cross-section, into flat, square, three- square or triangular, knife, round or rat-tail, half-round,

��The coarse side of a carpenter's rasp magnified showing the inden- tations cut with a triangular point of a hardened piece of steel

���Steel wool shaved from the edges of thin steel disks

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��etc. ; (3) according to the manner of their ser- rations, into single-cut or "float" (having single, unbroken, parallel chisel cuts across

the surface), double -cut, (having two sets of chisel cuts, crossing each other obliquely), open-cut, (having series of parallel cuts slightly staggered), and safe edge or safe side, having one or more uncut sur- faces ; and (4) accord- ing to the fineness of the cut (rough, bastard, second cut, smooth, and dead smooth). The "mill file," a very common form, is a flat, ta- pered, single cut file. One of the two sides of a car- penter's rasp is shown mag- nified in the il- lustration.

The fine or flat side is composed of a series of edges, made by cutting lines, whereas the rough , half round side is made by cutting indentations with a tri- angular point. Steel wool is shaved from thin disks of steel hela together in a lathe. There

��are various grades of coarse-

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