The smaller examples of the softwood internal cylinder, are the apertures for the "plain fittings," by which one part of the work is attached to another. This method of attachment is both stronger and more readily made in softwood than the screw, and is usually employed when the latter is not essential. The lesser of these holes, from about one sixteenth, to about one quarter of an inch diameter, are conveniently made with the carpenter's ordinary shell or nose bit, figs. 452, 454, Vol. II., and the larger, up to about half an inch diameter with the spoon bit, fig. 453. Fittings larger than half an inch diameter, after being commenced with the bit, are enlarged by a small hook or side tool, used in the manner described for larger works.
The smaller bored apertures, are generally made of sufficient depth to be left from the pointed bit, as in fig. 355. The security of the joint, depending on the pin or counterpart being turned parallel, of a diameter to fit the aperture, and the external faces shown by the dotted lines, true surfaces ; these latter then also fairly meet, and the joint shows no external gap or interval. In some cases it is more convenient and may give additional strength to the joint, to bore apertures in both the pieces, figs. 355, 356; these are then connected by the insertion of a separate parallel turned pin, which may thus be made from a tougher or harder wood.
A center for the first entry of the bit or tool, is struck in the work with the acute angle of the chisel. The rest is placed rather high, that the blade of the tool, lying nearly flat supported on one corner of its edge, may slope slightly downwards; the shaft of the tool being also at a small horizontal angle. The center produced, the small internal cone, fig. 357, is almost invariably used in all materials for the guidance of all internal turning tools. Turned true or concentric, the center of the cone is identical with the axis of the mandrel, and directs central all pointed tools and drills, such as the spoon bit for softwood. The true sides of the cone arrest in their angle and bring to center all tools with blunt extremities, like the cylinder bit or nose bit; and the turning tool, when placed against the side of the cone, is also true. In striking a true center it is however necessary, that the acute angle of the chisel, or the point of the tool used upon harder materials, should be directed exactly to the center of the work; which point is readily detected by the eye, at the moment of presenting the point of the tool when the work is in rapid revolution. Should the tool miss the exact center, it forms both an internal and external cone, fig. 358; the latter has to be turned away by the point of the chisel, directed to its apex, and obliterated in the internal cone, which thus becomes unnecessarily large.
Fig. 355. Fig. 356. Fig. 357. Fig. 358.
The nose or spoon bit, used in a short handle having a rounded end, is pressed into the wood by the palm of the hand, the forefinger and thumb stretched out along the tool, the handle held by the remaining fingers. The bit is advanced in the line of the mandrel, its shaft and handle serving as a guide to show that it is not inclined sideways and neither above nor below the mandrel axis. More care is given to the direction at the commencement of the cut, when the smaller and more delicate bits from their tendency to bend, have their cutting extremities supported by the two first fingers and thumb of the left hand. As the small hole acquires depth, it becomes both a guide and support for the boring tool as that advances, whence it is the more necessary that its first commencement should be made carefully true. The bits require to be frequently withdrawn from the holes to clear them from the shavings, which collect as a hard core in the hollow of the tools and interfere with their cut by setting up heat and friction; both of which may be reduced, by from time to time greasing the blade of the bit with tallow.
Internal cylinders or apertures of sufficient diameter, but of moderate depth, may be hollowed from the solid with the ordinary gouge, in which process the cutting action of the tool is precisely the same as upon external work. Deeper cylinders and other internal forms, are hollowed with the internal tools for softwood, figs. 359 to 363, those most generally in use; together with some others, shown on pages 514 to 516. Vol. II.
Fig. 359. 360. 361. Fig. 362. Fig. 363.
The cutting edges of the various hook tools or internal gouges, and those of the side tools and broads, or internal chisels, may all be viewed as those of simple gouges and chisels, and their different shaped stems and shanks, only as the means by which they are applied to the work and brought as nearly as circumstances will permit, into the theoretical positions of coincidence with planes and of tangents to circles.
The outer sides of the cutting portions of all the five tools, figs. 359 to 363, represented with their cutting edges uppermost, are nearly upright or square to the face of their stems; they are ground and sharpened with one bevil, upon their inner sides only, the cutting edges being keen like that of the chisel. The hook tools, figs. 359 and 362, are sharpened to cut only around the hook portion, the terminal angles being usually rounded off; fig. 360, is sharpened around the hook and also along the entire straight edge, combining the hook and side tool.
The end and the long side cutting edges of the side tools, figs. 361 and 363, are ground after the same manner; they meet at rather less than a right angle, the internal angle being very keenly sharpened. The length of the side cutting portions varies from about one and a half to three inches. The stems of all the foregoing are strong, usually of rectangular section with the corners removed, some are round, and the internal tools are in most cases used in long handles; figs. 362 and 363, are perhaps somewhat more easily held and directed, the cranked formed stem affording increased surface contact on the rest.