* The prismatic cutters admit of the usual variations of shape: sometiines two binding screws are used, and occasionally a tail screw, to receive the direct strain of the cut. When the blades are only used for cutting in the one direction, say from right to left, they may, with advantage, be ground with a double inclination; for as all these pointed tools work laterally, the true inclination of some 60° to the narrow facet or fillet operated upon, is then more strictly attained.
Considerable economy results from this and several other applications, in which the cutter and its shaft are two distinct parts. The small blades of steel admit of being formed with considerable ease and accuracy, and of being hardened in the most perfect manner. And when the cutters are fixed in strong bars or shafts of iron, they receive any required degree of strength, and the one shaft or carriage will serve for any successive number of blades.
The blades are sometimes made flat, or convex in the front, and ground much thinner, to serve for soft wood; the tools for hard wood and ivory, being more easily ground, do not call for this application of detached blades.
Figs. 446 and 447 represent a very excellent finishing tool, introduced by Mr. Clement, for planing east and wrought iron, and steel; it resembles the cranked tools generally, but is slighter, it is made smooth and flat upon the extremity; or rather in a very minute degree rounded. This tool is sharpened keenly upon the oilstone, and is used for extremely thin cuts, generally one quarter of an inch wide, and when the corners just escape touching, the work is left beautifully smooth; the edge should on no account stand in advance of the centre line. But to avoid the chatters so liable to occur in brass works, Mr. Clement prefers for that material the elastic planing tool, figs. 448 and 449, its edge is situated considerably behind the center.
In concluding the notice of the turning tools, it may be necessary to add a few words on those used for lead, tin, zinc, copper, and their ordinary alloys. The softest of these metals, such as lead, tin, and soft pewter, may be turned with the ordinary tools for soft wood; but for the harder metals, such as zinc, and hard alloys containing much antimony, the tools resemble those used fat the hard woods, and they are mostly employed dry.
Copper, which is much harder and tougher, is turned with tools similar to those for wrought-iron, but in general they are sharpened a little more keenly, and water is allowed to drop upon the work to lessen the risk of dragging or tearing up the face of the copper, a metal that neither admits of being turned or filed with the ordinary facility of most others. Silver and gold, having the tenacious character of copper, require similar turning tools, and they are generally lubricated with milk.
538 TOOLS for copper; lubricating fluids, etc.
In the above, and nearly all the metals except iron and those of equal or superior hardness, there seems a disposition to adhere, when by accident, the recently removed shaving gets forcibly pressed against a recently exposed surface, (the metals at the time being chemically clean, see page 432, Vol. I.,) this disposition to unite is nearly prevented when water or other fluid is used.
Water is occasionally resorted to in turning wrought iron and steel; this causes the work to be left somewhat smoother, but it is not generally used, except in heavy work, as it is apt to rust the machinery, oil fulfils the same end, but is too expensive for general purposes. - See Appendix, Note AR., page 983.
Cast iron having a crystalline structure, the shavings soon break, without causing so much friction as the hard ductile metals; cast iron is therefore always worked dry, even when the acute edges of 60 degrees are thickened to those of 80 or 90, either from necessity, as in some of the small boring tools, or from choice on the score of durability, as in the largest boring tools and others. Brass and gun-metal are also worked dry, although the turning tools are nearly rectangular, as the copper becomes so far modified by the zinc or tin, that the alloys, although much less crystalline than cast iron, and less ductile than copper, yield to the turning tools very cleanly without water.
But when tools with rectangular edges are used for wrought iron and steel, on account of the greater cohesion of these materials, they must be lubricated with oil, grease, soap and water, or other matter, to prevent the metals from being torn. And the screw cutting tools, many of which present much surface friction, and also rectangular or still more obtuse edges, almost invariably require oil or other unctuous fluids, for all the metals.
It will be shown in the practice of metal turning, that the diamond point, figs. G4 and 65, page 178, Vol. I., is occasionally used in turning hardened steel and other substances; figs. 72 to 74 are constantly used in engraving by machinery, and in graduating mathematical instruments. - See Appendix, Notes AS. to AV. pages 983 to 1001.