The Lathe & Its Uses / Or, Instruction in the Art of Turning Wood and Metal. Including a Description of the Most Modern Appliances for the Ornamentation of Plane and Curved Surfaces. With an Appendix, in Which is Described an Entirely Novel Form of Lathe for Eccentric and Rose Engine Turning; a Lathe and Planing Machine Combined; and Other Valuable Matter Relating to the Art.

by James Lukin

Having treated of screw cutting so far as we are able without trespassing on that section of the present series which is to be devoted to machine work with slide rest and change wheels, we shall enter on the matter of spirals, or Elizabethan twists, the method of making which was long kept a secret by the trade. These twists are essentially screws of very extended pitch, and generally rounded threads. The latter sometimes embrace the central cylinder or core, and sometimes are detached so as to assume a more open form like a corkscrew, and in the latter case two, three, or more threads can be cut, so that the spirals appear to intertwine. These spirals, too, are frequently ornamented with the aid of the eccentric chuck, and thus the work becomes fit for the adornment of the drawing-rooms of the highest in the land. No turned work can in short exceed in beauty these delicate and elaborate specimens of the turner's art. To commence with a single twist of one thread. A cylinder of the requisite length must first be turned. The number of turns the thread or cord of the spiral is to make in a given space must next be determined. We will suppose the cylinder one foot long, exclusive of any mouldings or tenons at the ends, and that the spiral is to make three turns round it, that is, one turn in four inches of its length. Divide the cylinder into three equal parts by the lines B, C, [Fig. 99]. Next rule equidistant lines D, E, F, along the cylinder in the direction of its length; in the present case let there be four such lines. The three divisions first made must be subdivided each into four (always the number of the longitudinal lines). The angles of the parallelograms thus formed must then be connected diagonally as shown in the figure, which diagonals being continued, will be found to describe a spiral line. A second similarly constructed spiral determines the thickness of the thread or cord of the twist. These spirals may now be cut with a tenon saw, and all the material outside the cord carefully removed with the gouge, so as to form a semicircular hollow between the threads. This cannot be done by putting the treadle and flywheel in motion, but the work itself must be grasped with the left hand, while the right holds the gouge upon the rest and guides its edge. After the work has thus been roughed out, it must be finished by rasps and files, or by a kind of plane with a semicircular cutting edge. While the latter is being used, the flywheel must be brought into action, so that the work may be made to revolve with sufficient rapidity to ensure a clean and smooth cut. A hollow plane may be used to round the cord of the twist, and the whole finished with glass paper and polished.

Fig. 99.

The "kind of plane" named was designed by a regular workman for his own use, and was made thus and grasped in the fist, or rather hollow of the hand. The iron was like c, sharp at one edge, the block of wood being slightly bevelled off in front of the cutting edge. The workman, who made scores of these twisted works for the trade, of all dimensions, would run this along the hollows, while the lathe was in motion, with great speed and accuracy. He marked the spirals as described, and then grasping the work in the left hand, and the gouge in the right, turning the work round with the former (the cord thrown off the wheel), he cut out the wood boldly in large pieces with little apparent care, and perfect ease. Then came the plane described above.

[Fig. 100] shows a simple spiral thus made. If the piece to be turned is dark it is not easy thus to mark out the divisions. In that case the following method will answer equally well. It is the plan used and contrived by the writer,[6] and specially handy when a number of similar twists are to be cut, as in ornamented pieces of furniture. A, [Fig. 101], is a straight edge of hard wood, through which, at any given angle (regulated by the number of turns the cord is desired to make in a given length of cylinder), a knife edge is fixed. If this is held as in the figure, and the blade is pressed down upon the cylinder to be cut, the lathe being put in motion, a very correct spiral will be traced, which can be at once deepened by a tenon saw as before. The thickness of the cord being determined it only remains to place this tool again upon the work so that the second line shall be traced at the required distance from the first. A second, or any number of cords, may be thus traced in succession parallel to each other by this simple method. By a slight modification of this instrument, which allows the knife to be clamped at any desired angle with the straight edge, the inclination of the cords, and, consequently, the pitch of the spiral can be varied at pleasure, and a second blade can be added to trace the second line, determining the thickness of the cord with one movement of the tool along the cylinder. The edge of the knife may be across the rest, the piece of wood just overlapping the T on the side next the workman, if the blade is long enough to reach across the work when thus held. The tool will be steadier and perhaps more easy of management in this position. One hand should then lightly press the back of the knife, while the other retains the wood against the rest as a straight edge. East Norfolk Amateur's design of a screw guide is on the same principle, the only difference being the substitution of a revolving instead of a fixed knife edge. In using either it requires some care to allow the tool free traverse, as the least check would spoil the thread.

[6] A similar plan is noticed in Holtzapffel's mechanical manipulation, which the writer had not seen. It is satisfactory to him to find the method thus authorised.

Figs. 100, 101.

The next class of spirals is that in which no central core exists, but the coils stand separate and distinct, two or more rising from the same base. The coils are sometimes flat, sometimes rounded, and still more frequently, in the best work, exquisitely, and (as a casual spectator would say) impossibly carved. The process is as follows:—Turn a cylinder of ivory or hard wood, forming at the end any required mouldings as a base and capital. Determine the number of coils and the pitch, and by one of the previous methods mark out the same. The cylinder is now to be bored out from end to end, leaving sufficient for the thickness of the required cords. This bore may with advantage be slightly larger at one end than at the other, so that a mandrel of wood may be fitted into it, to be afterwards easily withdrawn. This will certainly be necessary if the ivory cords are to be of light substance, as they require support to enable them to bear the action of the tool. After the cylinder has been bored as above directed, let a mandrel of common wood be inserted, and the lines, marked as shown, be cut quite through. The intermediate parts between the intended threads must then be removed carefully (with a round rasp, if of ivory) with any convenient tool if of wood. The cords must then in a similar way be rounded or otherwise moulded, and afterwards the common and now damaged mandrel removed. In the case of ivory the piece of work will not only be strengthened by the insertion of a polished mandrel of ebony, but its appearance will be improved. Sometimes, however, it may be preferable to line it with red velvet or silk, or it may be left entirely open. The further ornamentation of the cords, depending on the eccentric chuck or eccentric cutters, will not be described in the present paper. These open spirals are worth a vast amount of patience and trouble, which their elegant appearance when finished will amply repay.