CUTTING TOOLS Keenness of edge, equivalent to a small degree of angle between the tool faces, would appear at first sight to be the prime element in cutting, as indeed it is in the case of a razor, or in that of a chisel for soft wood. But that is not the prime condition in a tool for cutting iron or steel. Strength is of far greater importance, and to it some keenness of edge must be sacrificed. All cutting tools are wedges; but a razor or a chisel edge, included between angles of 15° or 2o°, would be turned over at once if presented to iron or steel, for which angles of from 60° to 75° are required. Further, much greater rigidity is necessary in the latter, to resist spring and fracture. A workman can operate a turning tool by hand, even on heavy pieces of metal-work. Formerly all turning, no matter how large, was done by hand-operated tools, and after great muscular exertion a few pounds of metal might be removed in an hour. With a similarly formed tool, in a rigid guide or rest, driven by the power of ten or twenty men, it becomes possi ble to remove say a hundredweight of chips in an hour; or with a larger and more durable tool driven by the power of 40 or 6o horses, half a ton of chips may be removed in an hour.
All machine tools of which the chisel is the type operate by cutting; i.e., they act on the same principle and by the same essential method as the knife, razor or chisel, and not by that of the grindstone. A single tool, however, may act as a cutting in strument at one time and as a scrape at another.
Clearly, in order that a tool shall cut, it must possess an in cisive form. In fig. I, A might be thrust over the surface of the plate of metal, but no cutting action could take place. It would simply grind and polish the surface. If it were formed like B, the grinding action would give place to scraping, by which some material would be removed. Many tools are formed thus, but there is still no incisive or knife-like action, and the tool is simply a scrape and not a cutting tool. But C is a cutting tool, possessing penetrative capacity. If now B were tilted backwards as at D, it would at once become a cutting tool. But its bevelled face would rub and grind on the surface of the work, producing friction and heat, and interfering with the penetrative action of the cutting edge. On the other hand, if C were tilted forwards as at E its action would approximate to that of a scrape for the time being.
But the high angle of the hinder bevelled face would not afford adequate support to the cutting edge, and the latter would there fore become worn off almost instantly, precisely as that of a razor or wood-working chisel would crumble away if operated on hard metal. It is obvious therefore that the correct form for a cutting tool must depend upon a due balance being maintained between the angle of the front and of the bottom faces—"front" or "top rake," and "bottom rake" or "clearance"—considered in regard to their method of presentation to the work. Since, too, all tools used in machines are held rigidly in one position, differing in this respect from hand-operated tools, it follows that a con stant angle should be given to instruments which are used for operating on a given kind of metal or alloy. It does not matter whether a tool is driven in a lathe, or a planing machine, or a sharper or a slotter; whether it is cutting on external or internal surfaces, it is always maintained in a direction perpendicularly to the point of application as in fig. 1, F, G, H, planing, turning and boring respectively. It is consistent with reason and with fact that the softer and more fibrous the metal, the keener must be the formation of the tool, and that, conversely, the harder and more crystalline the metal the more obtuse must be the cutting angles, as in the extremes of the razor and the tools for cutting iron and steel already instanced. The three figures 1, K, L show tools suitably formed for wrought iron and mild steel, for cast iron and cast steel, and for brass respectively. Cast iron and cast that turners try to avoid when possible, or at least to minimize. Now the greater the slope of the top rake the more easily will the cuttings come away, with the minimum of break in the crystalline materials and absolutely unbroken over lengths of many feet in the fibrous ones. The breaking up or the continuity of the cuttings, therefore affords an indication of the suitability of the amount of top rake to its work. However, compromise often has to be made between the ideal and the actual. The amount of tcp rake has to be limited in the harder metals and alloys in order steel could not be cut properly with the first, nor wrought iron and fibrous steel with the second, nor either with the third. The angles given are those which accord best with general practice.