Fig. 487.
When a piece of work is rotated between the lathe centres its axis of rotation may be represented by an imaginary straight line and the lower slides must, to obtain parallel work, be set parallel to this straight line, while for taper work the slide rest must be set at an angle to it. Now, in the form of slide rest shown in figure the cross slide is carried by the lower or feed traverse slide, hence setting the lower slide out of parallel with the work axis sets the cross slide out of a right angle to the work axis, with the result that when a taper piece of work is turned that has a collar or flange on it, the face of that collar or flange will be turned not at a right angle to the work axis as it should be, but at a right angle to the surface of the cone. Thus in [Fig. 486] a represents the axis of a piece of work, and the slide nut having been set parallel to the work axis, the face c will be at a right angle to the surface b or axis a, but with the slide nut set at an angle to turn the cone d, the cross slide will be at an angle to a, hence the face e will be undercut as shown, and at a right angle to the surface d instead of to a a. This may be obviated by letting the cross slide be the lower one as in the English form of slide rest shown in [Fig. 487], in which the upper slide is pivoted at its centre to the cross slide and may be swung at an angle thereto and secured in its adjusted position by the bolt at f. The projection at the bottom of the lower slider fits between the shears of the lathe and holds the lower slider parallel with the line of lathe centres, which causes the slide rest to cut all faces at a sight angle to the work axis whether the feed traverse slide be set to turn parallel or taper. In either case, however, there is nothing to serve as a guide to set the feed traverse slide parallel to the work axis, and this must, therefore, be done as near as may be by the eye and by taking a cut and testing its parallelism.
Fig. 488.
The rest may be set approximately true by bringing the operator’s eye into such a position that the edge a a, [Fig. 488], of the slide rest come into line with the edge b b of the lathe shears, because that edge is parallel to the line of lathe centres, and therefore to the work axis.
Fig. 489.
Slide rests which have a slide for traversing the tool along the work to its cut are but little used in the United States, being confined to very small lathes, and then (except in the case of watchmakers’ lathes whose forms of slide rest will be shown hereafter), mainly as an expedient to save expense in the cost of the lathe, it being preferred to feed the tool for the feed traverse (as the motion of the cutting tool along the work is termed) by mechanism operated from the live spindle and to be hereafter described. In England, however, slide rests are much used, a specimen construction being shown in [Fig. 489]. The end face a of the rest comes flush so that the tool shall be carried firmly when taking facing cuts in which solidity in the rest is of most importance. The tool is held by two clamps instead of by single tool posts, because the slide rest is employed to take heavy cuts, and when this is the case with boring tools whose cutting edges stand far out from the slide rest, a single tool post will not hold the tool sufficiently firm.