The tool having been completely sharpened upon the oilstone, is next taken to the metallic surfaces to have its edges polished, and which is done in the following manner. Without unfixing the tool, the plate C is moved about 2 degrees higher upon the arc B, and the tool is then applied upon the brass surface, which is supplied with a very small quantity of oilstone powder and oil. The tool is rubbed upon the brass surface in the same manner as upon the oilstone, until the chamfer presents a narrow facet with a dull greyish polish; when both edges of the tool have been thus treated, the tool is very carefully wiped to remove every particle of oilstone powder, and the final polish is given by rubbing the tool upon the iron surface, which is supplied with a little crocus and oil.
The upper surface of the tool, or the flat face, should be kept in very good condition towards the cutting edges, this is effected by removing the tool from the instrument and laying its face flat upon the iron surface, upon which the tool is rubbed with the fingers until the slight burr thrown up in the sharpening is removed.
Small tools, such as the revolving cutters used in the various cutting frames employed for ornamenting the surfaces of turned works, are from necessity made too short to be held in the instrument, fig. 1047; in this case they are first clamped in a tool-holder having a rectangular hole suited to the size of the stem of the tool, which is clamped therein by a square-headed binding screw, as shown in fig. 1049, which represents a tool-holder adapted for revolving cutters of a medium size. The stem of the tool-holder is made to fit the socket G, of the instrument, fig. 1047, in which it is secured exactly as described for the slide rest tool, shown detached in fig. 1048.
Drills, such as fig. 1053, intended to be used in the drilling instrument for ornamental turning, are in like manner fixed in a holder, as shown in fig. 1050; but in this case the binding screw is not required, as the stem of the drill fits the cylindrical hole in the holder, and it is prevented from twisting round by a short projecting piece at the end, which is filed down to the diametrical line, so as to slide into the flat-bottomed recess in the holder, and also to fit the drilling instrument in which it is to be employed, as shown in fig. 489, page 555, Vol. II.
To avoid uncertainty respecting the angles of the tools used for ornamental turning, they are usually stamped with figures denoting the angles at which the tools are ground; but it should be remembered that these numbers are measured from a line at right angles to the center of the tool, or, in other words, it is the angle which is ground away, that is estimated, and not the angle, which the edges of the tool make to each other. Thus, in the instance of the tool just described as being ground at the angle of 30 degrees, each side of the tool is ground at an angle of 30 degrees, or the edges differ to that extent from a flat tool, and the sum of these two angles being 60 degrees, it follows that the edges of the tool meet each other at an angle of 120 degrees, or the complement to the sum of the two angles at which the tool is ground.
This will be more distinctly seen in figs. 1051 to 1053, which represent the plan of three tools of different angles. Fig. 1051 shows a single bevil tool ground at an angle of 15 degrees; and consequently the edge of this tool will meet its side at an angle of 75 degrees, or the difference between 15 degrees and 90 degrees, which latter is of course the angle formed by the edge of a flat tool with its side, when it is ground perfectly square. Fig. 1052 represents the plan of an angular tool ground on both bevils at an angle of 30 degrees, and, as just explained, its edges will meet at 120 degrees, or the difference between 60 degrees, the sum of the two angles ground away, and 180 degrees, or the straight edge of a flat tool. The drill, fig. 1053, is ground at two angles of 45 degrees, and the sum of these being 90 degrees, it follows that its edges form an angle of 90 degrees.
The vertical angle, at which the tools are sharpened, is in like manner estimated by the angle ground away; thus, when the piece C, fig. 1047, is elevated to 0 on the arc B, the tool is ground quite square, or at an angle of 90 degrees, and when placed at division 10 the chamfer of the tool differs 10 degrees from the right angle, or it forms an angle of 80 degrees with the face of the tool, and so on of other numbers.
The instrument for setting angular tools, fig. 1047, is sometimes used with the horizontal grinding machine, fig. 1039; it is then applied in exactly the same manner as the instrument for grinding ordinary turning tools. At other times the lap is screwed upon the mandrel of a lathe, so as to revolve vertically in exactly the same manner as a surface chuck; but in this case it is necessary to provide a support for the two legs of the instrument, and which generally consists of a block of wood mounted on a base piece similar to that of a common turning rest. This arrangement is shown in fig. 1054, in which A represents the lap, B the wooden block, which measures about 4 1/2 inches wide and 1 1/2 inches thick, held by two screws to the iron base C, which is secured to the bearers by the rest bolt, not seen in the drawing, but which allows of the wooden block being adjusted, so that its side may be in a line with the face of the lap, when tested by a straight edge applied to both. The wooden block then serves the same purpose as the platform of the horizontal grinding machine, and the instrument is applied in a similar manner, except that it is held vertically, as shown in the figure, which represents the application of the instrument to setting detached angular blades for cutting the threads of screws, and adapted to tool-holders, such as fig. 608, page 630, Vol. II. For sharpening these blades a different form of socket is adopted, in order that tools of various depths and thicknesses may be securely clamped. This socket consists of a flat bar of steel with two projecting sides at the front extremity, as seen in the drawing; the tool to be sharpened is placed in the channel, and held in its position by the small side screw, s. The instrument is then adjusted to the angle required for the depth of the thread of the screw, and which has been already explained in Section IX. of the Chapter on Screws, Vol. II.