Works in metal, require both centering and setting, before they are turned; that they may at once run moderately truly in the square hole chuck. One end of the rod or forging is filed square, nearly corresponding to the square in the chuck, the opposite end fig. 198, is then filed flat, and while still in the vice, a steel center punch is driven into the flat end, to indent a slight hollow center for the point of the popit head. The rod is then placed in the lathe, and if its chuck end does not run fairly true, the piece is tried successively in all four positions of the square, that the best may be selected: after which, the side of the work, opposite to the marked side of the chuck, is notched with a file, that it may always be replaced in the same relative position. Should the end of the work not run fairly true in any of the four positions, two of the neighbouring square sides are filed, until the truth of the chuck extremity is sufficiently exact. Subsequently, the opposite end of the work is corrected, or made to run nearly true; a piece of chalk is held upon the rest as the work revolves, to mark the most prominent part of its circumference. After which the rod is refixed in the vice, the chalk mark towards the operator, and the center punch is placed in the hole, but inclined a little backwards, so that the blow may drive the center slightly towards the chalk mark or high side; this is followed by another slight blow, given with the center punch held quite vertically over the new center, and the work is then returned to the lathe for examination. The process is repeated as may be necessary, until the chalk mark extends nearly round the circumference.
The two ends being sufficiently true, the work while in revolution, is marked with the chalk at three or four inter-mediate parts of its length, to show any places at which it may be bent; it is then removed from the lathe, and these curvatures are set or corrected by the blows of a hammer upon an anvil or other solid substance. Work containing much departure from the truth, has to be corrected by blows when lying across a hollow, such as the open chops of a vice; but, as so large a correction would disturb the truth of the centering, this should be done previously, and the rod rendered nearly straight before it is placed in the lathe. Several screws or other small objects, can be turned successively from off the same piece of metal when carried by the square hole chuck; as each is finished, it is deeply nicked in with the turning tool, that it may be easily broken off, and also to present a smaller spot upon the end of the piece, by which the center is more readily found when it is again filed flat.
The friction of the point of the popit head, gradually enlarges the diameter of the center hole it forms in the work, and this enlargement sometimes takes place more to the one side than to the other, so as thus more or less to interfere with the truth of the axis upon which the work revolves. Want of truth arising from this source, is not materially felt as an eviL in the majority of turned works of wood, nor in many small works in metal; but for the latter it is nevertheless desirable, that the center punch used should be a little more acute than the center of the popit head. When, as in most metal work, the accuracy of the centers is of importance, these are prepared with increasing care. For small works, a fine hole is first drilled a short distance up the axis of the work, in order that the extreme point of the popit head, may not arrive at the bottom of the hollow center; the small deep hole then retains the work fairly well in the axial line, notwithstanding the enlargement that may take place in the upper part of the hollow center.
A fine drill is mounted in a drill chuck and the work, previously centered at both ends, is carried towards it by the point of the popit head; the screw of the latter being turned very gently, to avoid breaking the slender drill. When one end is square as with such pieces as fig. 198, it is a frequent practice to place the work in the chuck, and to use a hand drill, which at the same time pierces and supports the work, with less risk of accident to the drill. The hand rest is fixed at right angles to the bearers, close to the end of the work, and sufficiently high to place the drill at the height of center of the lathe; the drill supported close to its extremity by the fingers, occupying the position of the center punch, fig. 199.
The centers of larger works often require to be prepared with considerable care, while they also fully repay all that may be bestowed upon their accurate formation. Subsequently to boring the axial hole and hollowing the center with the turning tool, the center is finished to the exact form by a taper half round bit fig. 485, or by a countersink of the appropriate angle; the end of the work being generally supported by the boring collar, fig. 128, during these operations.
Centering and setting the work in the manner described, is easily performed after a little practice, but the former process, may be assisted or accomplished by mechanical aids, the simplest and roughest being a pair of spring dividers. The round piece of work is fixed upright in the vice, and the points of the dividers are opened and fastened at a little more than its radius; one point being held against the edge of the work, a line is scratched on the surface with the other; the dividers are then transferred to the opposite side, to scratch a line by the side of the first, after which they are held at right angles to their former positions, to make two other scratches; all of which are quickly made, and the centre is then struck in the interstice formed by the four.