The gib e, [Fig. 485], is sometimes placed on the front side of the slider, as in the figure, and at others on the back; when it is placed in the front the strain of the cut causes it to be compressed against the slide, and there is a strain placed upon the screws f which lifts them up, whereas if placed on the other side the screws are relieved of strain, save such as is caused by the setting of the gib up.
Fig. 490.
On the other hand, the screws are easier to get at for adjustment if placed in front. When the screws b of the upper gib c, [Fig. 485], are on the right-hand side, as in that figure, there is considerable strain on the screws when a boring tool is used to stand far out, as for boring deep holes. On the other hand, however, the screws can be readily got at in this position, and may therefore be screwed up tightly to lock the upper slider firmly to the cross slide, which will be a great advantage in boring and also in facing operations. But the screws must not in this case have simple saw slot heads, such as shown on a larger scale in [Fig. 490], but should have square heads to receive a wrench, and if these four screws are used, the two end ones may be set to adjust the slicing fit of the slider, while the two middle ones may be used to set the slider form on its slide when either facing or boring. The corners of the gibs as well as those of the slider and slide may with advantage be rounded so that they may not become bruised or burred, and, furthermore, the slider is strengthened, and hence less liable to spring under the pressure of a heavy cut.
Fig. 491.
A slide rest for turning spherical work is shown in [Fig. 491]. a is the lower slide way on which is traversed the slide b, upon which is fitted the piece c, pivoted by the bolt d; there is provided upon c a half-circle rack, shown at e, and into this rack gears a worm-wheel having journal bearing on b, and operated by the handle f. As f is rotated c would rotate on d as a centre of motion, hence the tool point would move in an arc of a circle whose radius would depend upon the distance of the tool point from d as denoted by j, which should be coincident with the line of centres of the lathe.
The slide g is constructed in the ordinary manner, but the way on which it slides should be short, so as not to come into contact with the work. If the base slide way a be capable of being traversed along the lathe shears s s by a separate motion, then the upper slide way and slide may be omitted, g and c being in one piece. It is to be noted in a rest of this kind, however, that the tool must be for the roughing cut set too far from d to an amount equal to about the depth of cut allowed to finish with, and for the finishing cut to the radius of the finished sphere in order to obtain a true sphere, because if b be operated so that d does not stand directly coincident with the line of lathe centres, the centre of motion, or of the circle described by the tool point, will not be coincident with the centre on which the work rotates, hence the work though running true would not be a true sphere but an oval. This oval would be longest in the direction parallel with the line of centres whenever the pivot d was past the line of centres, and an oval of largest diameter at the middle or largest diameter turned by the tool whenever the pivot d was on the handle h side of the line of centres. To steady c it may be provided with a circular dovetail, as shown at the end i, provision being made (by set screw or otherwise) for locking c in a fixed position when using the rest for other than spherical work.