EDGE TOOLS. A general term which includes cutting and scraping tools, as distinguished from those of percussive type, as hammers, and of lever type, as spanners. Edge tools date from the remote palaeolithic age when rough axes were chipped from flints, to be improved by neolithic men who ground and polished their Celts. The true chisel, thrust by hand, appears to have de veloped in the bronze age. The basic difference between an axe and a chisel is that the one is doubly bevelled, the other singly. The result is that the axe does not possess good guidance on the wood, while the flat face of the chisel acts as a guide, helping to cut true surfaces. An adze is of the chisel class, and is used by the carpenter and shipwright to true baulks and other large areas. The carpenter's plane gives still better control by means of the sole sliding along the wood, while in machine tools for wood and metal the guidance becomes perfect.
The keenness of an edge tool, or its "cutting angle" is nearest to that of the razor in the woodworker's chisels, gouges, carving tools and plane irons, as well as in certain machine knives. Hard or tough wood tends to turn over a fine edge or break it, conse quently the angle of the two faces meeting at the edge must be increased. The same rule applies to cold chisels for metals. In the tools used in metal-working machines, as the lathe, drill, planer, shaper and slotter and the milling and sawing machines, the edges must be ground more keenly for the fibrous metals and alloys, as wrought-iron, mild steel and copper, but less keenly for the crystalline kinds, as hard steel, cast-iron and brass. Chilled cast-iron, for the rolls of rolling-mills, is very difficult to turn, and the tool faces meet at an angle of near 9o°, the action being merely slow scraping. Nevertheless this tool is a wedge, in prin ciple, just as much as the finest chisel for soft wood, penetrating and forcing aside the metal. (See also FILE ; MACHINE KNIVES; MILLING-CUTTERS ; PUNCHING AND SHEARING MACHINES; SAWS;