Centering Machine.—Many shops have a special machine for forming centers which enables the operation to be performed quickly. One type of centering machine is shown in [Fig. 28]. The work is gripped in a chuck C that automatically locates it in a central position so that it is not necessary to lay out the end before drilling. There are two spindles s, one of which holds the drill and the other the countersink, and these are rotated by a belt passing over pulley P. Each of these spindles is advanced by lever L and either of them can be moved to a position central with the work, as they are mounted in a swiveling frame. In operating this machine, a small straight hole is first made by a twist drill held in one of the spindles; the other spindle is then moved over to the center and the hole is reamed tapering. The arrangement is such that neither spindle can be advanced by the feeding lever except when in a central position. The amount that each spindle can be advanced is limited by a fixed collar inside the head, and there is also a swinging adjustable stop against which the end of the work should be placed before tightening the chuck. These two features make it possible to ream center holes of the same size or depth in any number of pieces.
Fig. 29. The Imperfect Center Bearing is the Result of Centering before Straightening
Different Forms of Centers.—In some poorly equipped shops it is necessary to form centers by the use of a center-punch only, as there is no better tool. If the end of the punch has a sixty-degree taper, a fair center can be formed in this way, but it is not a method to be recommended, especially when accurate work is required. Sometimes centers are made with punches that are too blunt, producing a shallow center, such as the one shown in the upper left-hand view, [Fig. 27]. In this case all the bearing is on the point of the lathe center, which is the worst possible place for it. Another way is to simply drill a straight hole as in the upper view to the right; this is also bad practice in more than one respect. The lower view to the right shows a form of center which is often found in the ends of lathe arbors, the mouth of the center being rounded, at r, and the arbor end recessed as shown. The rounded corner prevents the point of the lathe center from catching when it is moved rapidly towards work which is not being held quite centrally (as shown by the illustration), and the end is recessed to protect the center against bruises. Stock that is bent should always be straightened before the centers are drilled and reamed. If the work is first centered and then straightened the bearing on the lathe center would be as shown in [Fig. 29]. The center will then wear unevenly with the result that the surfaces last turned will not be concentric with those which were finished first.
Fig. 30. Tool Steel should be centered Concentric, in order to remove the Decarbonized Outer Surface