Works exceptional in their dimensions, either of large diameter or of extreme length, are occasionally required in plain turning. The former are usually of the disc character, and are used for bases, plates, platforms, to which to attach other parts, or wheels, all of which are thin compared with their diameter. Sometimes they consist of pieces having long projections or arms, such as the pedestal of the hand rest, fig. 102, and other examples to he found in the practical sections; which require additional height for their revolution while some central or other portion is being bored or turned. Many works of large diameter beyond the capacity of the lathe, may be readily accomplished on increasing the height of center, by the use of temporary lifting blocks, placed beneath the lathe heads. Works of exceptional length are of exactly the opposite character, being usually of inconsiderable diameter, analogous to long rods; for these, the ordinary foot lathe is extended by the lengthening bearers.

A small increase in the height of center, may be obtained by separate parallel slips of metal or wood, placed beneath the lathe head and popit head, on either side of their tenons; the stem of the tee of the hand rest, being raised in its pedestal and fixed at the increased height. Separate lifting pieces, seldom exceed about one inch in thickness and they should always be sufficiently less than the depth of the tenons, that some portion of the latter, may still be contained between the bearers, to center and give security to the lathe heads. The lifting blocks employed to give a more considerable increase in height, are made as short lengths of shallow lathe bearers, figs. 182, 183; their upper and under surfaces parallel, with the addition of tenons beneath fitting the interval of the lathe bearers, pierced with holes for the passage of the fixing bolts. The block fig. 182, is employed for the lathe head, and a similar but shorter length mounts the popit head, when that is required to be raised to the same height, to advance the drill in boring, or for the support of the work.

Fig. 182.

Section V Lifting Blocks And Lengthening Bearers 400155

184.

Section V Lifting Blocks And Lengthening Bearers 400156

183.

Section V Lifting Blocks And Lengthening Bearers 400157

The lifting block for the hand rest, or slide rest, fig. 183, is lengthened out behind and has two tenons, that it may be used to carry the rest, either directly above the lathe bearers, or overhanging them in front. The former is more convenient in turning the center of the surface, and is the more solid position for the tool for internal turning; the latter is required when working towards the circumference of the surface, or upon its periphery. Both the base of the pedestal and the tee of the hand rest, are placed upon fig. 183, parallel with the work, for turning the surface. The pedestal remains in the same position, but the tee is placed parallel with the mandrel, when turning cylindrical edges; the length of the tee, which usually extends beyond the width of the edge being turned, also permitting the tool to be applied to the margins of the upper and under surfaces of the work. Either of the slide rests figs. 145 or 146, may be mounted on fig. 183, upon which they are used in the manner already described.

A longer fixing bolt is required for the lathe head. The bolt of the popit head, usually permanently fixed to it, and the bolts of the different rests, which, like it terminate below in screws, are lengthened by sockets fig. 184, to which their fly nuts and washers are transferred. The lifting blocks used upon five-inch center lathes are about three inches in height, giving the power of turning the light plate-like works described, up to sixteen inches diameter; for which the five inch center lathe is generally abundantly strong. For work falling within their range, lifting blocks are preferable to the gap lathe bearers previously referred to, among other reasons, as they leave the face of the bearers intact, of which the advantages are sufficiently obvious.