The cone and conical shapes in metal, are used for centers of motion, for fittings, and sometimes only as the line to connect two different diameters in the same solid, when it is desirable to avoid a break, or shoulder. The manipulation of the tools hardly differs from that for the cylinder, but the rest is usually placed nearly parallel with the side of the work.
Lathe and other centers, fig. 481, and the pointed ends of spindles, are turned to shape with a small reversed cone at their apex, for their support by the point of the popit head. This little cone is then broken off, and the work is supported by a hollow center of the same angle, fig. 482; the hollow center is filed down to the diametrical line, and curved away, so as to expose the apex of the solid cone; the point of which is finished by a flat file, guided by lying in contact with the portion previously completed. The corresponding hollow center is commenced by a small cylindrical hole, bored a little deeper than the depth of the required cone, as in the section, fig. 483; the aperture is then turned out to shape with the graver, followed by the triangular, or point tool, sometimes with the assistance of a template, and is finished with a conical bit. The conical boring bit, shown in two views, figs. 484. 485, is filed down to the diametrical line to form the cutting edge, and is provided at the reverse end of its shaft with a center for the point of the popit head. This bit does very little cutting, being principally required to correct the small inaccuracies left by the turning tool. The use and construction of conical fittings or bearings, of which the lathe mandrel is a conspicuous example, have already been described. The external truncated cone, is perhaps most usually turned first, and the corresponding hollow cone turned out or enlarged to it. The two are then fitted to the necessary degree of ac-curacy , by the different methods described at length in the third volume. The very slightly tapering cones, often required in fittings of small diameter, are usually bored cylindrical and then opened taper by a broach; particulars of the varieties of this tool, and of the boring bits, have been given in the second volume.
Fig. 481. Fig. 482. Fig. 483. Fig. 484. Fig. 485.
The conical line very frequently merges into the hollow curve, as in the saw spindle, fig. 486, and in previous examples. Such curved forms in iron or brass are first shaped by separate narrow cuts, with the roughing tools appropriate to the material, and are then reduced to a regular line and turned smooth with the round tools, used to a great extent also in separate cuts, followed by the file and emery stick. The shaft of the round tool is applied radially to the curve, fig. 438, but the tool also obtains some rotation on the center of its own curvature, besides some lateral traverse for the finer and finishing cuts. The junction of the curve with the straight line is turned with a flat tool, and requires a little care to avoid forming an elbow by turning the one to dip below the other; this does not readily happen unless intended with convex terminal curves, as in the taper handles of the diestock, fig. 488. These are turned to shape with the graver and then smooth, with that, or with a flat tool, presented and traversed radially around the curve.
The handles of the diestock, and the round steel lever, fig. 490, are common examples of long flat curves in the direction of the cylinder, turned by hand; the end of the lever is bent to shape after the form of the rod has been completed.
The lever post of an ornamental slide rest, fig. 487, affords a small example that combines cylindrical, taper, and curved portions. Many similar small hollows to the last, used for ornament, are obtained by the application of an appropriate sized round tool, or which is preferable, by using one rather less than their width, that the tool may receive a slight rotation upon its cutting edge to equalize the curve. The tools, except for the different vertical angle at which they are presented to the work, are employed on brass or iron much as in turning a similar curve in hardwood. Narrow convex curves in iron or steel, the reverse of the last, such as the edge of a washer, or a bead in a moulding, after they have been reduced nearly to shape with the graver, may be finished with the flat tool for steel, fig. 444, held almost like the softwood chisel, in the reversed position shown and described page 524, in the second volume. Convex edges in brass or gunmetal, when reduced to shape, are finished with the flat and round tools, swept around them as in turning hardwood, the tool supported on the rest; for similar curves on the surface or upon the edges of apertures, the tools are more conveniently applied upon the armrest.
Bounded forms, and edges in steel, iron and brass, are frequently burnished, the effect of which process is to compress the particles on the surface, rendering the work lustrous with a high degree of polish. The burnisher is of hardened steel, generally from five to seven inches long, tapering in form and of a flat oval section and handled ; those of oblong section, half round and round, used in clock and watchmaking and some others, are mentioned page 1042. Vol. III. The burnishers themselves are highly polished, and for successful results, require to be kept perfectly clean, in good condition and free from rust.
Work of iron or steel having been turned or filed to shape, has the file marks first removed with emery paper, or with a fine emery stick and oil. It is then wiped clean and examined, to observe whether all scratches have disappeared, and also that no particle of emery has become embedded in its surface; after which a finer grain is given to it with a worn emery stick, one upon which the emery has become finely pulverized and has nearly disappeared from use. Again wiped thoroughly clean, the work is then further polished with crocus powder, applied on a stick covered with a strip of buff leather.