On these rests, therefore, a single washer is more frequently used, which answers very well when the tool post is in a slot, so that it can be moved from side to side of the rest as occasion may require. When, however, the position of the tool post is fixed it has the disadvantage that the point p, [Fig. 598], where the tool takes its leverage, is too far removed, and the tool is therefore liable to bend or spring from the pressure of the cut.
In [Fig. 599] is an elevating device sometimes used on the compound rests of large lathes. The top of the rest is provided with a hub h, threaded externally to receive a ring nut r, around whose edge there are numerous holes to receive a pin for operating the nut. The tool-post is situated central in the hub. When the tool is loose the ring nut can be operated by hand or the tool may be gripped lightly and the ring nut operated by a pin. The level of the tool is here maintained; it is supported to about the edge of the rest on account of the large diameter of the ring nut, and a very delicate adjustment for height can be made, but such a device is only suitable for large lathes on account of the depth of the ring nut and hub.
On small slide-rests the device shown in [Fig. 600] is often found. It consists of a holder h, in which is cut a seat for the tool, this seat being inclined to give the piece of steel used as a tool a certain constant degree of angle, and at the same time to permit of the tool being moved endwise in the holder to set it for height; but, as the tool requires to be pushed farther and farther through the holder to raise it, it is not so well supported as is desirable when slight tools are used, unless the holder is made long, so as to pass through the tool post with the tool. Again, it does not support the tool sideways unless the tool steel is dressed up and closely fits the groove in the holder.
In [Fig. 601] w w are two inverted wedges which afford an accurate adjustment, but the range is limited, because if the wedges have much taper they are apt to move endways when the tool is fastened.
A convenient device for the compound rests of small lathes is shown in [Fig. 602]. It consists of a holder pivoted upon a central post and carrying two tool-binding screws, hence it can be revolved to set the tool in any required position. A similar device is shown in [Fig. 603], in which the central post is slotted at a to receive the tool, and also carries a plate c, held by the nut n, and provided with tool-holding screws b and b′, which abut against the top of the rest, a top view of the device being shown in [Fig. 604]. Plate c may thus be swung around to set the tool in any required position on either side of the rest.
In Maudslay’s slide rest, the tool clamp shown in [Fig. 605] is employed. Screws a are employed to grip the tool moderately firm, and a turn of screws b (whose ends abut against the top of the slide rest) very firmly secures the tool, since it moves the clamp c as a lever, whose fulcrum is the screw a.
Fig. 607.
[Figs. 606] and [607] represent the Whitworth tool clamp, the clamping plates of which change about upon the four studs, and are supported at their inner ends by a block equal in height to the height of the tool steel.