The methods usually employed to effect this locking are as follows:—In [Fig. 640], s is the tail spindle, b part of the tailblock in section, r a ring-bolt, and h a handled nut. Screwing up the nut h causes r to clamp s to the upper part of the bore of b; while releasing h leaves s free to slide. There are three objections to this plan. The ring r tends to spring or bend s. The weight of r tends to produce wear upon the top of the spindle, and the spindle is not gripped so near to its dead centre end as it might be. If s is a close fit in b the pressure of r could not spring or bend s; but, so soon as wear has taken place, s becomes simply suspended at r, having the pressure of r, and the weight of the work tending to bend it. Another locking device is shown in [Fig. 641]. It consists of a shoe placed beneath s, and a wedge-bolt beneath it, operated by the handled nut c. Here the pressure is again in a direction to lift s, as denoted by the arrow; but when the wedge w is released the shoe falls away from s, hence the locking device produces no wear upon s. This device may be placed nearer to the end of b, since the wedge may pass through the front leg of the tailstock instead of to the right of it, as in [Fig. 640]. But s is still suspended from the point of contact of the shoe, and the weight of the work still bends it as much as its play in b will permit.
Fig. 642.
Fig. 643.
Another clamping device is shown in [Fig. 642]. In this the cylindrical part b of the tailblock is split on one side, and is provided with two lugs. A handled screw passes through the upper lug, and is threaded into the lower one, so that by operating the handle c, the bore may be closed, so as to grip s, or opened to relieve it. This possesses the advantages: First, that it will cause s to be gripped most firmly at the end of b, and give a longer length of bearing of b upon s; and, secondly, that it will grip s top and bottom, and, therefore, prevent its springing from the weight of the work. But, on the other hand, b will close mainly on the side of the split, as denoted by the dotted half-circle, and therefore tend to throw s somewhat in the direction of the arrow, which it will do to an amount answerable to the amount of looseness of s in b. In the Pratt and Whitney lathes this device is somewhat modified, as is shown in [Fig. 643]. A stud e screws into the lower lug d, having a collar at e let into the upper lug, with a square extending above the upper lug so that the stud may be screwed into d, exerting sufficient pressure to close the bore of b to a neat working fit to the spindle. The handled nut, when screwed up, causes b to grip the spindle firmly; but when released, leaves the spindle a neat working fit and not loose to the amount of the play; hence, the locking device may be released, and the centre adjusted to take up the wear in the work centres while the cut is proceeding, without any movement of the spindle in b, because there is no play between the spindle and b.
