Fig. 282.
First of all, the frame of the rest must be accurately at right angles to the spindle, which fits into the socket. These should, therefore, be turned together, supposing the amateur not to have a planing machine. The whole may be mounted as [Fig. 282], where A represents the carrier plate or chuck; B, the driver, the tail of which should be as long or nearly so as the frame from c to D; F is the side tool to be fixed in the slide rest for metal. The effect of this arrangement is to plane the face of the slide with transverse strokes instead of lengthwise. It may be afterwards finished and polished with oilstone powder on a flat slab of planed iron. When the face is finished, the whole must be reversed, the pin of the carrier plate will bear against the frame, which thus acts as a driver, and the spindle must be turned. In this way accuracy is ensured if the slide rest used is carefully set. The chamfered sides of the slides are difficult to work with the file, but may be so done with care, and with a template of the desired bevel as a guide. The great secret is to take plenty of time, not to press too much upon the file nor to move it too quickly over the surface; fine even strokes, especially towards the finish, must be given, and a final polish with oilstone powder and oil used on a piece of a stick. In turning the screw a back stay must be fixed opposite to the tool in the slide rest to insure the contact of the cutting edge without bending the work.
Presuming that the screw will be cut with stock and dies, it may be stated as a caution that the latter must not be tightened except at the commencement of cutting the thread deeper. The return of the tool by a backward motion (or unscrewing), should not be used as a cutting action, and therefore, should be carried on with the dies in the same position which they had during their descent.
At the beginning, therefore, of each downward movement the dies must be tightened and oiled, and they must not again be touched till the bottom of the screw has been reached, and the upward movement also has been completed, so that they have arrived again at the starting point. If tightened at any other time the screw will be either conical or of a wavy section, either of which forms would be fatal to its use. The castings for such a rest should be of malleable iron, if possible, as being much more easy to work; the guide bars may be of gun metal, as also the chamfered bars, which work on the main frame. This will give a more finished appearance, and will on the whole be more durable and satisfactory.
[The Eccentric Cutter Frame.]
One of the most useful tools for ornamentation, especially of plain surfaces, such as the top of a box cover, is the eccentric cutter, [Fig. 283]. The shank, A, lies in the receptacle holder of the slide rest, and is drilled throughout to receive a steel spindle, carrying at one end a double pulley, B, to receive the cord from the overhead motion, and at the other frame, E, with its leading screw, of which the movable milled and graduated head is seen at H. This frame has one surface, level with the centre of the main spindle, which is cut away as shown, and, consequently, as the point of the tool is on its flat side, which latter rests upon the frame (the bevel being below), this point can, by the tangent screw, be brought into a line with the centre of the main spindle, so that when the cord from the overhead is passed round B, the spindle revolves with great rapidity, and the point of the tool, K, in the position described, makes a simple dot. By turning round the milled screw head, H, either by the thumb and finger or by a small winch handle, fitted on the square part beyond the head of the screw, the tool holder, D (which is in one piece with the nut of the leading screw), is made to traverse the frame, and the tool will cut a circle small or large according to the eccentricity thus given to it. In [Fig. 285] D, is the tool holder on the front of the frame; C, the end of the spindle; L, a bell-shaped washer, which is acted on by the small square-headed screw, drawing D towards the frame and clamping the tool. The whole is in the figure of full size. The tool holder is in one piece with the nut, through which passes the leading screw, and which is continued as a screw for the action of the bell-shaped washer and tightening nut; hence it is necessary to allow a degree of play between the nut and leading screw, to prevent bending the latter when clamping the tool. This is effected by filing off the threads in the nut at the top and bottom, to render the whole slightly oval. The remaining threads suffice for the action of the leading screw: a very slight degree of play in the required direction will be found sufficient. The powers of the eccentric cutter frame will be found sufficiently extensive to make it a most serviceable, perhaps necessary, piece of lathe apparatus. If it cannot be said absolutely to supply the place of the eccentric chuck, it has nevertheless the advantage of great lightness of construction, lowness of cost, and ease of manipulation. The weight of the eccentric chuck, whether single or double, as of all chucks in which sliding plates are used, is a sad drawback to their value—a drawback unfortunately beyond remedy, and specially felt when the slides are drawn out to a great degree of eccentricity. Combined together, these two form a compound eccentric chuck, and in this way are capable of nearly everything in the way of eccentric ornamentation. Where the chuck is not to be had, it is by all means advisable to procure the cutting frame, for which the writer confesses a great partiality. It appears, indeed, to him a far more rational proceeding, as it is also now of extensive application, to act upon fixed work by revolving or moving tools, instead of proceeding in the contrary way; and all these little tools used with the overhead apparatus are so lightly and elegantly constructed, and so well adapted for the parts they have to perform, that the originator of them (native talent devised them), deserves to be well and lastingly remembered; instead of which it is doubtful whether his name is even known. (Sic transit is a quotation too stale for this work.)
Figs. 283, 285, 285B.