Fig. 26. Drilling Centers in the Lathe
One method of forming these center-holes is indicated in [Fig. 26]. A chuck C is screwed onto the spindle in place of the faceplate, and a combination center drill and reamer R is gripped by the chuck jaws and set to run true. The center is then drilled and reamed at one end by pressing the work against the revolving drill with the tailstock spindle, which is fed out by turning handle n. The piece is then reversed for drilling the opposite end. The work may be kept from revolving while the centers are being drilled and reamed, by attaching a dog to it close to the tailstock end and then adjusting the cross-slide until the dog rests upon the slide. Many parts can be held by simply gripping them with one hand. From the foregoing it will be seen that the small centers made by punch C, [Fig. 25], serve as a starting point for the drill and also as a support for the outer end of the work while the first hole is being drilled.
Fig. 27. Centers of Incorrect and Correct Form
The form of center-hole produced by a combination drill and reamer is shown by the lower left-hand view in [Fig. 27]. A small straight hole a in the bottom prevents the point of the lathe center from coming in contact with the work and insures a good bearing on the conical surface c. The standard angle for lathe centers is sixty degrees, as the illustration shows, and the tapering part of all center-holes should be made to this angle.
Fig. 28. Special Machine for Centering Parts to be Turned