Fig. 644.
In the design shown in [Fig. 644], the end b of the tailblock is threaded and is provided with a handled cap nut a a. In the end of the tailblock where the spindle emerges, is provided a cone, and into this cone fits a wedge-shaped ring, as shown. This ring is split quite through on one side, while there are two other slots nearly but not quite splitting the wedge-ring. When the handle c is pulled towards the operator it screws a up on the end b, and forces the wedge-ring up in the conical bore in b. From the split the ring closes upon the spindle s, and grips it. Now, as the ring is weakened by slots in two places besides the split, it closes more nearly cylindrically true than if it had only a split, there being three points where the ring can spring when closing upon s; and from the cone being axially true with the live spindle of the lathe, s is held axially true, notwithstanding any wear of the spindle, because the locking device, being at the extreme end of b, is as near to the dead centre as it is possible to get it; and, furthermore, when c is operated for the release, the wedge-ring opens clear of s, so that s does not touch it when moved laterally. The wear of the bore of b has, therefore, no effect to throw s out of line, nor has the gripping device any tendency to bend or spring s, while the latter is held as close to the work as possible; hence the weight of the work has less influence in bending it. The pitch of the thread and the degree of cone are so proportioned that less than one-quarter rotation of a will suffice to grip or release s, the handle c being so placed on a as to be about vertical when the split ring binds s; hence c is always in a convenient position for the hand to grasp.
In this case, however, the spindle being locked at the extreme end of the hole, there is more liability of the other end moving from the pressure of the cut, or from the weight of the work; hence it would seem desirable that a tail spindle should be locked in two places; one at the dead centre end of the hole, and the other as near the actuating wheel, or handle, as possible, and also that each device should either hold it central to the original bore, notwithstanding the wear, an end that is attained in the Sellers lathes already described.
Slide rests for self-acting or engine lathes are divided into seven kinds, termed respectively as follows: simple, or single, elevating, weighted, gibbed, compound, duplex, and duplex compound. A simple, or single, slide rest contains a carriage and one cross slide, as in [Fig. 621]. An elevating slide rest is one capable of elevation at one end to adjust the cutting tool height, as in [Fig. 499]. A weighted slide rest is one held to the shears by a weight, as in [Fig. 577]. A gibbed slide rest is held to the shears by gibs, as in [Fig. 621]. A compound slide rest has above the cross slide, a second slide carrying the tool holder, this second slide pivoting to stand at any required angle, as in [Fig. 505]. A duplex slide rest has two rests on the same cross slide, and in a compound duplex both these two rests are compound, as in [Fig. 511]. The rest shown on the Putnam lathe in [Figs. 492] and [499], is thus an elevating gibbed single rest.
Testing a Lathe.—To test a lathe to find if its live and dead spindles are axially in line one with the other and with the guides on the lathe bed, the following methods may be employed in addition to those referred to under the heading of [Erecting].
Fig. 645.
To test if the live spindle is true with the bed or shear guides, a piece such as in [Fig. 645] may be turned up between the lathe centres, the end a fitting into the live spindle in place of the live centre, and the collars b c being turned to an equal diameter, and the end face d squared off true. The end a must then be placed in the lathe in place of the live centre, the dead centre being removed from contact with the work; with the lathe at rest a tool point may be set to just touch collar c, and if when the carriage is moved to feed the tool past collar b, the tool draws a line along it of equal depth to that it drew along c, the live head is true; the dead centre may then be moved up to engage the work end d, and the lathe must be revolved so that (the tool not having been moved at all by the cross-feed screw) the tool may be traversed back to draw another line along c, and if all three lines are of equal depth the lathe is true. The tool should be fine pointed and set so as to mark as fine a line as possible.
| Any View. | Side View. | Side View. | Top View. |
| Fig. 646. | Fig. 647. | Fig. 648. | Fig. 649. |