Figs. 252, 253, 254.
[Fig. 255] is arranged for flat tools of various angles, or drills with single joints. A, is the stand of brass, with two turned and hardened steel legs. To this is hinged at G, by a screw joint, the part K, the upper part of which forms a semicircular arc, C. A second arc, B is fixed at one end to the stand A, and passes stiffly through a mortise at the top of K. The latter can be raised, therefore or lowered at pleasure upon this second graduated arc, and clamped at any angle by the screw H. To the lower part of K is pivotted the tool holder, D, the upper part of which is pointed, and screws as an index upon the arc C, showing the angle at which it is placed. This tool holder is clamped by a nut at the back, which fits the end of a screw seen near the point. The figure below shows a tool holder which fits into the projecting parts of D, and serves to hold the small flat tools. Below is a similar holder, used for round-shanked drills. F is one of three flat slabs upon which the tools are to be ground, there being one of iron, one of brass, and one of hard wood with a flat strip of oilstone imbedded in it, flush with its upper surface. The tool and its fittings are generally arranged in a box with three drawers; these contain the slabs of oilstone and metal, with the powders necessary for grinding and polishing. To use this instrument, the point of D is adjusted to the required angle for one side of the point of the tool. (It is shown at 40 deg. in the sketch.) The latter is then placed in the holder, and made to project until, when the angle of the chamfer is adjusted on the arc B, the part A is level, and therefore parallel to the surface of the grinding plate. The whole thus forms a tripod, the third leg of which is formed by the tool itself. The latter is first rubbed on the oilstone with a little oil. It is then finished more perfectly on the brass slab, dressed with oilstone powder and oil. Previous to this the tool is moved one or two degrees more upright by the arc B. A narrow facet is thus ground, having a dull grey polish. The tool is now carefully wiped clean, and polished with crocus and oil upon the slab of iron. If the point of the tool is central, with a chamfer both ways, the point of the tool holder is first adjusted on one limb of the arc and the tool ground, and then the same adjustment made on the opposite limb, so that the other side of the point can be operated upon. Thus tools of any angle and any bevel may be sharpened to a nicety without fear of altering the original form of the point, and this may be done, if necessary, during the process of eccentric turning, although, as before stated, it is better to fix the tools well sharpened at the commencement of the work, and not remove them until at least one complete set of circles or other patterns have been cut.
Fig. 255.
Figs. 246, 247.
The instrument just described is evidently unsuited for the drills and bead tools which present a concave edge like [246] A, B, C, enlarged sketches of tools copied from Holtzapffel's work. For these the latter directs to use large or small cones ([247]) of iron and brass, to be dressed, the first (which is the polisher) with crocus, the second with fine emery and oil, the flat side of the tool being held towards the point of the cone, the bevel towards the thick end. Part of the edge of C must be delicately sharpened by hand, as no guide can be used for the step-like portion of the edge. The cones for sharpening are either mounted in the usual manner, by one or both ends in the mandrel of the lathe, or fitted into the spindle of a small drilling lathe-head, the pulley of which is connected by a catgut band with that of the mandrel of the small lathe-head, being fitted with a tailpiece to fit the rest socket, or otherwise mounted on the lathe-bed. The smaller cones especially require to be driven at a high speed. When larger circles or mouldings are to be cut, these small crank-form drills are no longer available, and are replaced by a very simple, but most effective contrivance called the eccentric cutter, by which any work that is within the scope of the eccentric chuck and fixed tool may be executed with great precision and rapidity. This is represented in [Fig. 248][20]—a small oblong frame of brass, about two or two-and-a-half inches in length, and half an inch or so in breadth is traversed by a fine screw, prevented from moving endwise by a collar, as in the slide rest (of which, indeed, this is a miniature). A slide, C, with a little tool holder at the top of it, is moved along the frame by the leading screw, the head of the latter being graduated, and also the upper surface of one or both sides of the frame. The projection A, fits into the end of the drill holder, and is secured by a screw. Circles of a diameter equal to B, B" may thus be cut, and their effect varied by placing tools of any form of edge in the tool holder. Such a tool as A, will thus no longer cut a minute circle forming a hemispherical raised knob, but will form a circular moulding, such as that shown in part at [Fig. 249], except when the tool holder is on the middle of the frame and the tool concentric with the mandrel. The single point tools, however, with single or double bevel, are more commonly used, in this cutter, as mouldings can be turned as efficiently with hand beading tools, with or without the eccentric chuck, according to their required position. It may here be mentioned that eccentric work should always be cut on wood of one colour, or on ivory, as the veinings of the richer fancy woods, which are so beautiful in other cases, only serve to confuse the tracery made by the eccentric cutter. Of all woods for fancy work with the eccentric chuck or cutter, nothing equals African black wood. It is, however, costly, and only ranges to a diameter of five inches, as great part is unsound. The rind is hard, thick, and white, similar to boxwood. Next to this for such work stands, perhaps, cocus, or cocoa wood, which is not the tree bearing the cocoa nut, the latter being a palm, which is more like cane in texture. One of the most effective patterns to be formed by the eccentric cutter is the shell, [Fig. 250], in which one side, or rather one portion of the circles composing it, is very deeply cut, while the opposite part is shallow. This can be simply effected by throwing the sole of the rest out of the level, by placing a thin piece of wood or metal across the lathe bed, so as to tilt up the rest and place it (with the cutter) in an inclined position. The tool will thus begin to cut at one part before it touches the surface elsewhere, and the desired effect will be readily produced. In using the eccentric cutter great rapidity of motion must be given to it, but the tool must be advanced very carefully, or it will be broken. The lever handle is the best to use for the purpose. Akin to the shell pattern are those in which part only of the circles are cut, leaving an effect shown by the border round [Fig. 250]. This is produced in the same way as the last, being, in fact, a ring of shells in their initiatory stages. This is a very effective snake-like pattern, when fairly and cleanly cut. When the eccentric cutter is used, it must be remembered that the principle of work is not quite the same as with the eccentric chuck. With the latter it was stated that the size of the circles depends on the slide rest and the position of their centres on the chuck. In the present case the eccentric cutter regulates the sizes, and the screw of the slide rest itself the positions of the centres of the circles, since the part A of the cutter will always be in the centres of the same, and this part is attached to the rest. It will be understood that this remark respecting position of centres only relates to circles lying on the diameter of the work, such as [Fig. 251], the distance between a and b will be taken from the division plate on the pulley of the lathe. The way to cut the above, for example, will be as follows:—Place the slide rest so that when the cutter tool is in the centre of the frame it shall be concentric with the mandrel. In this position it will only make a dot in the centre of the work. Turn the screw of the cutter frame until you have a radius sufficient for the centre circle. Set the mandrel pulley with the index in No. 360, put in motion the overhead apparatus and cut the circle, move the screw of the slide rest a few turns (thus fixing the centre of the second circle), until you find that the cutter will form the circle cutting the first, and passing through its centre. (Observe, this being the size of the first, the screw of the cutter frame is not turned.) Cut the circle in question, move the mandrel pulley a quarter round, so that the index is in No. 90, and cut another; repeat the process twice more, and 1, 2, 3, 4, will be cut. The position of the centres of Nos. 5, 6, 7, 8, will now have to be determined as before, by working the main screw of the slide rest; but, as their size is less than the preceding set, the screw of the cutter frame must likewise be turned to diminish them to the required degree. When by these combined movements their position and size have been determined, they must be cut by the aid of the division plate, in the same manner as the last, and so on, till the whole have been cut. With respect to the ratio in which the circles diminish, and the precise sizes of them, no rule can be given, as this must depend on the taste of the operator. The sole object in this place is to show the principles whereby these patterns are to be executed. A good deal of care is requisite in practice, and the memory has to be often rather severely tasked. The best plan is always to try a proposed pattern upon boxwood or paper, before risking it upon more valuable material; and, where it can be done, it is well to write down the numbers to be used on the various division plates. A single false cut, it must be remembered, will spoil the whole work, at a great waste of time, loss of material, and annoyance, only to be appreciated by those to whom such an untoward accident may have happened. The drilling apparatus, without the eccentric cutter, but fitted with a round-headed drill, is used for the production of fluted works, such as that shown in [Fig. 252], A and B. The drill being inserted in the end of the spindle, and its point or end (of any desired form, either round, flat, or pointed) being brought opposite one end of the flute, the lathe is to be put in motion as in ordinary ornamental drilling, the mandrel being, of course, held fast by the index and division plate. At the same time that the drill in rapid motion is brought against the work by the lever handle, the screw of the slide rest is slowly turned, and thus the groove or flute is drilled out by the combination of longitudinal and vertical action. The number of flutes in any given size of cylinder is determined, first by a horizontal sectional plan on paper, and regulated accordingly by help of the division plate and index. In making such an article as [Fig. 252], it will economise material, whether ivory or blackwood, or a combination of the two, to form it of at least three pieces, making the divisions at C, D. Care should be taken to leave below the bowl, which should be as thin as paper if of ivory, the part C on which the beads are to be drilled. The pedestal can then be screwed into this, and will not penetrate the bottom of the bowl. Ivory may be screwed in an ordinary set of stocks and dies if care is taken not to screw up the latter too quickly. Lard may be used as a lubricant in cutting this material, whether for sawing or drilling. The part with raised mouldings between A and D is ornamented with a vertical or universal cutter, and for greater ease and exactness a template may be used in the slide rest by means of which all the curves of the moulding may be accurately followed by drill or cutter. The minute beads round the edges of the small mouldings are made with two sizes of A, [Fig. 246]; a little knob is thus formed rising from a hollow. The small knobs used as feet may be rapidly formed by a hand beading tool of semicircular section, similar also to A, [Fig. 246]. A pin may be left on each, or they may be drilled and attached by small screws of brass wire made on purpose. The following cement will enable the turner to make an ivory bowl for the above ornament so thin as to be transparent; indeed it may be thus made so thin as to bend under the fingers, although such extreme tenuity is not required in the present case.
[20] A newer pattern appears on a later page.
