If the two ends of the work are properly squared at the time of forging, the extremites are easily squared with filing, but if rugged, unsound portions exist at the extremities, it is necessary to either cut them off with chiseling, or to turn them off with a lathe by means of a preliminary centring. When this first centring is adopted, the recesses made should be shallow, and it is not necessary to accurately centre the work for the entire turning, if the object is merely to cut off the uneven portions at the extremities; but if the object is to take a thin cut off the entire surface, previous to a preliminary hardening, the tap must be tolerably centred. The centring is done by first chalking and scribing a few lines on the two ends while the tap is supported with two vee-blocks on a lining-table. The work is next coned, and the extremities squared with a left-hand corner tool, similar to Fig. 433, and after turning a thin cut off the taper part, and also off the stem, the uneven centre-lumps are chiseled or filed entirely off, if much aslant; if not, they are merely filed to make them parallel to the turned surfaces. When this is done, the tap is hardened and again softened, and the work is again centred if necessary, or the original recesses may be drilled deeper and made broader, if they happen to be made in the proper places at the first centring. The recess in the tap's head is to be always made in the central line along the length of the head, because the head is, or should be, that which is forged nearest to its finished size ; but the recess in the taper end may be made either in the centre of the flat extremity of the tap, or central with any other portion of the taper or parallel part which is forged very near to the finished thickness.

In order to exactly mark the places for the recesses, the tap is put upon two vee-blocks that rest on a lining-table or surface-plate, and one vee-block is put into contact with that part of the taper or parallel portion which is to rotate truly on the lathe-pivots, the other vee-block being put beneath one flat side of the square head. When a tap is to be thus marked, so that one recess shall be in the middle of the flat end of the taper part, the vee-block is put beneath that end, and the work appears as in Fig. 553; but if the recess of the taper end is not to be in its middle, but true with the middle of the cone, the block is put beneath the middle, as shown by Fig. 554. While the tap is situated in either of these situations, a short line is scribed with a scriber-block near the middle of both the tap's extremities ; after which the square head is lifted up with one hand and the tap is moved partly round while still in the gap of the other block; the head is then again put upon the block, with one side of the tap's head at right angles to its first position ; while in this position two more short lines are scribed, one on each end of the work; the tap is next shifted round twice more to put the two other sides of the head upon the block, and mark four more lines, so that four are marked on each end of the work ; all these four short lines intersect and enclose a small four-cornered space shown in the Figures, in the midst of which are the centres desired. The places of the centres being thus found, a dot is put into each, and the work is ready for drilling and coning. The centring of hobs is very similar to that of long taper taps, only more care is necessary with taps, because of their comparative great length.

Drilling and coning of taps or hobs is effected in a lathe. Taps of only a few pounds in weight each are drilled by holding each one separately to a small drill which rotates in a drill-holder that belongs to the lathe. Small taps are thus held in one hand with one centre recess in contact with the drill-point, and the other recess in contact with the poppet-pivot point; while thus held, the other hand is employed to work the poppet-wheel, and thereby to advance the poppet-pin or cylinder, which pushes the work on to the drill while it rotates; when the drilling is done, the drill is taken from its chuck, and a cone driller is put into its place, and with this the drilled holes of the tap or taps are enlarged to the desired width and depth. During the coning of a tap or hob, it is necessary to occasionally move the work partly round, to cause the drill to cut equally around the small drill-hole first made; and if the tap is a large one, of twenty or thirty pounds weight, it is supported on packing blocks, or suspended with a small pulley-block hanging over the lathe. Taps about an inch or an inch and a quarter thick require the mouths of the centre recesses to be about a quarter of an inch wide, and taps of two or three inches in thickness require recesses that are three-eights or half an inch wide.

The angle between two opposite sides of a tap recess should be about sixty-seven degrees, if the recess is to be without oil-channels but with oil-channels, the recess may be seventy degrees. The angle subtended by two opposite sides of the conical pivots of the lathe should never exceed seventy degrees for ordinary lathes and ordinary work; and the pivots should therefore be sharpened as soon as they are become worn enough to make the angle between their sides more than seventy degrees, which renders it advisable to make the centre recesses of all work less than seventy degrees, to cause the edge of the recess to bear on the thick part of the pivot rather than make the pivot's point touch the bottom of the cone. By a proper arrangement, a small space always exists between the inner part of the recess and the point of the pivot, this space being that which is necessary to contain oil for lubricating the work while it rotates. Another advantage also results from making the recess-angle less than the pivot-angle, and consists in the pivot-point being protected from the wear attending an improper adaptation of the bearing surfaces to each other. Consequently, it is not proper to make the pivot touch the recess along its entire length unless oil-channels are provided to admit the oil; if such channels are intended they are made with a narrow gouge or other pointed chisel, and three grooves are cut at equal distances around the recess, after which each one is smoothly filed with the point of a file, and all burs are finally rubbed off with emery cloth. The drilling and coning of taps, hobs, and other work, is usually done in a simple sort of small lathe distinct from the lathes for turning; and when taps are to be turned, lathes of proper sizes are selected for small taps and large ones. Fig. 550 represents a poppet-pivot whose angle is seventy degrees, and on the pivot is a tap's end having a recess of sixty-six or sixty-seven degrees.