A lathe of this kind serves equally well for centre work; therefore if the professed turner is without a mandril lathe, one of these constructed in the simplest and most economical manner, and chiefly of wood, that the artificer may be enabled to make it himself, is shown in the annexed figure.
It is put in motion by a foot-wheel and treadle, so that the turner has both hands at liberty for directing the tools. A is the bed of the lathe, consisting of two beams or cheeks, fixed parallel to each other, and leaving a small space between them, as shown in Fig. 2. The bed is supported by three upright legs, as shown in the figure; one of these projects above the bed a sufficient height to form one of the puppets C, for the support of the extremity of the spindle or mandril e E; the other end is supported in a collar fixed in an iron standard or puppet B, which is screwed down upon the bed, by two bolts marked t t. The back puppet D has a tenon which is received through the bed, by which it can be fastened at any place; f is the back centre pin, fitted through the puppet; and g is a screw situated behind it, to advance and keep it up to his work. The mandril is turned round by a band of cat-gut passing round the pulley c, and also round the large foot-wheel G, which is made of cast iron, and fixed on the end of the axis H; this is bent as in the figure, to form two cranks, united by two iron links to the treadle I, on which the workman presses his foot; this treadle is affixed by two short boards to an axis on which the treadle I moves.
The wheel G is of considerable weight in the rim, and being wedged fast on the axis, turns round with it; it is the momentum of this wheel that continues to turn the work while the crank and treadle are rising, and consequently while the workman exerts no power upon them. When the crank has passed the vertical position, and begins to descend, he presses his foot upon the treadle, to give the wheel a sufficient impetus to continue its motion until it arrives at the same position again.
The length of the iron links, which connect the cranks with the treadle I, must be such that when the cranks are at the lowest, the board I of the treadle, to which the links are hooked, should hang about two or three inches from the floor. The turner gives the wheel a small turn with his hands, till the crank rise to the highest, and pass a little beyond it, then by a quick tread he brings the cranks down again, putting the wheel in motion with a velocity that will carry it several revolutions; he must observe to begin his next tread just when the cranks pass the highest point, and then it will continue running the same way with a tolerably regular motion, if he is punctual in the time of his treads. The rest which supports the tool while it is in the act of turning, is made of iron, as shown in Fig. 2; it is supported on the bed of the lathe by its foot, which is divided by a groove in the manner of a fork, to receive a screw bolt, going down through the lathe-bed, and fastening it at any place along it by a thumb-nut; the groove in the foot is for the purpose of allowing the rest to be moved to and from the centre of the lathe, to adjust it to the diameter of the work which is turning.
The height of the rest is of some importance in turning; and for some work it should be fixed higher than others; therefore the shank of the cross piece, or T, upon which the tool is laid, is received into a socket in the foot of F, and can be held at any height by a screw. As the socket is cylindrical, the edge of the rest can be placed inclined to the axis of the work, when turning cones, or other similar work; though the same purpose may be accomplished by the screw which holds the foot of the rest down to the bed of the lathe, admitting it to stand in an oblique direction.
The mandril or spindle is the most important part of the lathe; it is made of iron, in the manner shown at Fig. 3; but the two extremities are of steel, which are hardened after being turned and finished; the small end has a hole made in it to receive the point of a screw, which, as shown at e, Fig. 1, supports the end of it; the other end of the mandril is made larger, and has a hole within it, cut with a female screw, for the purpose of fixing on the various chucks by which the work is turned; the outside surface of the end is turned extremely true, and is fitted in a brass collar at the top of the standard B; one of the bolts, marked t, which fasten the standard down, goes through a stout iron plate, situated beneath the bed, passing between the two wooden cheeks. In the top of the standard is a square hole, for the reception of two pieces or dies of brass which include the mandril between them; these are kept in their places by a piece of iron i, fastened down by screws l l; and m is a screw tapped through this, which presses the two dies together, and thus adjusts them to receive the neck of the mandril without any shake.
The screw which supports the other extremity of the mandril fits in two iron or brass nuts, which are let into the back and front of the wooden puppet C, and by turning this, the mandril can be adjusted to run very correctly in length; to prevent the screw from turning back when the lathe is in motion, a nut is placed on the screw outside of the puppet, and after the screw is turned by its head to fit and hold up the mandril, the nut is screwed firmly against the nut which is let into the outside of the puppet; this causes such a pressure upon the threads of the screw, that it is in no danger of turning back, as it would otherwise be liable to do with rough work.