Fig. 863. Fig. 864.
Fig. 865. Fig. 866.
In [Fig. 862], for example, a represents a chuck plate hollow across the face, and b a link requiring to be bored through its double eye c, the centre line of the lathe being line e e, and the centre line of the hole in the hub d of the link being denoted by f, and as e and f are not parallel one to the other it is obvious that the holes will not be parallel. Suppose, now, that the chuck face was rounding, and the centre line of d would stand at g g, and the holes in c and d would be out of true in the opposite direction. In this case the error would be equal, but suppose we have a ring or disc such as b in [Fig. 863] to chuck by bolts and plates c, d and it will be chucked true, notwithstanding that the face of the plate is hollow. But were the face of the plate rounding the disc may be chucked as in [Fig. 864], the face f of the work not being held at a right angle to the line of centres e as it is in [Fig. 863]. The truth of the chucking in [Fig. 864] depends upon whether the clamps c were screwed up with equal force upon the work. A hollow chuck plate will lose this advantage in proportion as the work covers more of one side of the chuck plate than it does of the other, but in any event it will chuck more true than a rounding one. Suppose we have, for example, a ring chucked eccentrically as in [Figs. 865] and [866], the chuck being as much hollow in the one case as it is rounding in the other, and that shown in [Fig. 866] will stand out of true to an amount greater than the chuck is in an equal amount of its radius. While that shown in [Fig. 865] would be nearer true than the chuck is in an equal length of its radius, both amounts being in proportion to the length of the line a to that of line b.
If the chuck plate is known to be either rounding or hollow, pieces of paper of sufficient thickness to remedy the error may be placed at c and d respectively. It is better, however, to true up the faces of plates so that the surface of the work bolted against it will be true and stand at a right angle to the line of lathe centres.
In truing up a face plate, the bearings of the live spindle should be adjusted so that there is no play on them, and the screw or other device used to prevent end motion to the live spindle should be properly adjusted.