The degree of velocity with which the surface of an article being turned, ought to pass the edge of the tool so as to be cut by it, differs materially in relation to different metals. Cast-iron, in consequence of its open grain, and containing as it generally does many impurities, is required to revolve very slowly, so as to pass the edge of the cutting tool only at the rate of about100 feet per minute; wrought iron and steel are usually turned when revolving at a rate of about twice as quick; and brass cuts well when coming in contact with the chisel at the rate of about 300 feet per minute. To produce the requisite velocity according to the material or size of the work, pulleys of different diameters are fastened on the spindle, as already stated; so that the larger the work, the larger the pulley, and vice versa.
Mr. Ibbetson, an amateur turner of great celebrity, and the author of a pretty book on eccentric turning, has made many improvements in the mechanism for ornamental turning; his eccentric chuck is exhibited in the annexed figure.
a a is a plate of brass of sufficient solidity, on which are fixed two slides of steel b b, by means of screws; the holes which admit the screws are made a little oval to enable the slides to move nearer to, or farther from, each other, if necessary; c c c c are four pieces of metal firmly fixed to plate a, and having a screw in each, which presses on the slides b b; d is a plate of metal or brass. sliding between b bin & dovetail, and must be made to fit very accurately when the slides are parallel to each other, and is moved between the slides by means of a screw working in a slot made in the plate a, and which regulates the eccentricity, as it moves the plate d, either nearer to, or farther from, the centre of the chuck. e is a circular plate, whose edge is cut into teeth, and which is capable of being turned round its centre, and is held in any position by the catch g, which falls in between the teeth, and is held in its place by a spring h. On the centre of the wheel e is fixed a screw f, (exhibited by the shadow thrown on the wheel,) whose threads correspond with the screw of the mandril of the lathe, for the purpose of fixing any chuck, on which is fastened the substance to be turned.
To this chuck Mr. Ibbetson has adapted a slide-rest, of a peculiar description, as well as his lathe and other appendages thereto, for an explanation of which we must refer the reader to his work, entitled Specimens in Eceentric Circular Turning, with Practical Instructions for producing corresponding Pieces in that Art, published by Wetton, Fleet-street. This work is illustrated by upwards of sixty copper-plate engravings, and imitations of wood-cuts of a superior description; and it is due to the ingenious author not to omit noticing, that they were all produced in the lathe by himself. In the following page we give a copy of two of the figures of the simplest combinations of circles, to which, as well as to all others, the author has annexed plain practical instructions, so that the novice may proceed step by step to produce the same figures by turning the screws of the slide-rest and the eccentric chuck through prescribed spaces; and from these simple figures, by similar successive operations, he may proceed to the most elaborate and beautiful designs.
We shall now add a description of Mr. W. E. Wightman's (of Maldon, in Yorkshire) excellent lathe, as explained by himself in a periodical journal, feeling assured that it may prove of the most eminent service to mechanics, as the arrangements are extremely simple and easily understood, and the construction such as any tolerable workman can accomplish, and avail himself of the advantages it offers, at a moderate cost.
"Fig. 1 represents the lathe, with the cutter-frame fixed in the compound sliding rest, ready for use. A, the triangular bar on which the machine is mounted. B B, two pillars, which support the bar; the parts b b fix it to the lathe-frame. C, the left-hand head; D, the pulley; E, the mandril; e, the screw on which the chucks are fixed; F, the cutter-frame; f, the cutter; G, two wheels, which give a slow motion to the cutter-frame; H, the rod and handle of the slow motion; I I, two heads or puppets, in which is fixed the spindle K, of the cutter-frame; L, a bar of steel, on which the puppets are fixed, and which also fastens the cutter-frame to the compound sliding rest, by passing it through a hole in the tool-frame, as will be seen, on reference to the figure, the part y removing for the purpose; M, a groove turned on the edge of the cutter-frame, for the string N to work on; O represents the frame for double stringing the lathe; P, a movable pulley, whereby it may be fixed per pendicularly over the cutter-frame; Q, a weight attached to a pulley behind the bar, for keeping the string N tight; RRR, the pulley's string and weight connected with the frame O, for double stringing the lathe; T, the index to the division-plate; S, one of two screws for changing the rectangular position of the compound sliding-rest to an oblique.
S, Figs. 2 and 3, represents the slide of the cutter-frame; and k, Fig. 3, the screw, whereby the slide is moved; g, the screw for fastening the cutter. Fig. 2, is an enlarged view of the cutter-frame, when removed from the rest. Fig. 3, represents the face of the cutter-frame. Fig. 4, the back of it, with the wheels of the hand-motion. The letters refer to the same parts in all the figures.