turn a 24-in. pulley is a 24-in. lathe, and is said to have a 24-in. swing. In Great Britain, the swing of a lathe is the radius of the largest piece that can be turned in it; hence, a lathe that could turn a piece 48 in. in diameter would be said to have a 24-in. swing.

Center Angle.-The angle of the conical end of the lathe center, Fig. 1, is usually 60°. For very heavy work an angle of 75° is sometimes used. The body of the center is tapered to fit the spindle taper, which is usually in. per foot.

Forms of Centers.-In Fig. 1 (a) is shown the simplest form of lathe center; (b) and (c) are centers for a blind spindle; (d) has a projecting end for use on work where the arbor does not extend through; (e) is a half center used for end-finishing work; (f) shows a pipe center; (g) is a revolving center; (h) is used with work that has pointed ends; (i) is a tapered bushing with a tapered hole; (j) is a tapered bushing with a straight hole; (k) is a pad center, used in the tail-stock when drilling

thin work; (1) is a square center; (m) is a tail-stock center to support round work when drilling; (n) is a flat center for truing centers in work; and (o) is a threaded center for holding nuts or other parts which are internally threaded.

LOCATING CENTERS FOR TURNING

By Dividers.-In Fig. 1 is shown the method of locating the center of a shaft by the use of dividers. A prick-punch mark made in the center of the square locates the trial center.

By Surface Gauge.-In Fig. 2 is shown how a surface gauge a may be used for locating centers on a bolt b.

By Hermaphrodites.-The hermaphrodites are set so that the pointed leg comes near the center of the work, as in Fig. 3. With the bent leg at the respective points a, b, c, and d, four arcs

are scribed, intersecting as shown, and forming a small, foursided central figure. The center e of this central figure is the approximate center of the end of the work.

By Cup Center. The cup center, Fig. 4, can be used to advantage when many pieces of the same kind are to be centered, as in (a). It will not give accurate results if the end is not square, as in (b), or if the cup is not held true, as in (c).

The centering device in Fig. 5 insures that the punch and the work will be held in line, but does not overcome errors due to untrue ends.

Centering Work of Irregular Outline.-Hand forgings are usually not straight, and trial centers must be located, and the piece swung in the lathe centers. A tool is run up close to the work, and the piece is revolved by hand. The trial centers are then moved over, one way or the other, until the inaccuracy is equally divided between the two ends, after which the permanent centers can be drilled.

Changing Center Marks.-Center marks may be changed in case the trial center does not prove to be the true center, by using a prick punch and slanting it in the direction in which it is desired to move the mark, as in Fig. 6 (a); or, the prick punch

may be held at one side of the center, as in (b). In the latter case, the point of the punch will move toward the old center when struck, but will draw the center to one side, as desired.

CENTER HOLES 4

Drilling and Reaming Center Holes.-In Fig. 1 (a) is shown a short twist drill used in drilling centers and (b) and (c) are reamers or countersinks used to give the center the correct

(b)

FIG. 1

taper; (d) and (e) are combination center drills and reamers. The type shown in (e) has a cutting edge at a which recesses the end of the piece for a short distance as shown at (f), to give clearance.

Making Center Holes.-Much time and expense can be saved by a special centering machine, when a great deal of centering work is to be done. When no such machine is at hand, the work can be done in a drill press or a lathe. The center holes should be made to fit the lathe centers. A center, or 60°, gauge is shown in Fig. 2. The point a is used for center

FIG. 2

holes, and the V-shaped opening b gives the correct angle for centers. Point a and openings c and b are also used for grind. ing and setting threading tools correctly.

The method of applying a holder and a wrench to a drill that is centered on the tail center of the lathe is shown in Fig. 1. Fig. 2 shows the method of using a centering tool to prepare the center of the work to receive a twist drill. A support for a

twist drill is illustrated in Fig. 3. The use of a bar tool a to prevent a twist drill from wabbling, is illustrated in Fig. 4. Care must be taken not to overpress the drill, or it will bind.

DRIVING THE WORK

Driving on Centers.-Three types of lathe dogs, carriers, or drivers, are shown in Fig. 1. These fit over the end of the work or mandrel and are clamped to it by the setscrews a. The best form of mandrel is shown in Fig. 2. It is made of tool steel, hardened and tempered. The taper should be from .005 to .01 in. per foot.

1