Figure 9-21.—Drilling a center hole. 9-20 show the correct overhang for the tool bit and the holder. The point of the tool must be correctly positioned on the work Place the cutting edge. slightly above the center  for  straight  turning  of  steel  and  cast  iron  and exactly on the center for all other work To set the tool at the height desired, raise or lower the point of the tool by moving the wedge in or out of the tool post ring. By placing the point opposite the tailstock center point, you can adjust the setting accurately. HOLDING THE WORK You  cannot  perform  accurate  work  if  the  workpiece is  improperly  mounted.  The  requirements  for  proper mounting are as follows: 1. 2. 3. 4. The  work  center  line  must  be  accurately centered along the axis of the lathe spindle. The  work  must  be  held  rigidly  while  being turned. The work must NOT be sprung out of shape by the holding device. The work must be adequately supported against any   sagging   caused   by   its   own   weight   and against  springing  caused  by  the  action  of  the cutting  tool. There are four general methods of holding work in the lathe: (1) between centers, (2) on a mandrel, (3) in a chuck, and (4) on a faceplate. Work may also be clamped to the carriage for boring and milling, in which case the boring bar or milling cutter is held and driven by the headstock   spindle. Other  methods  of  holding  work  to  suit  special conditions are (1) one end on the live center or in a chuck and the other end supported in a center rest, and (2) one end in a chuck and the other end on the dead center. Holding Work Between Centers To  machine  a  workpiece  between  centers,  drill center  holes  in  each  end  to  receive  the  lathe  centers. Secure  a  lathe  dog  to  the  workpiece.  Then  mount the  work  between  the  live  and  dead  centers  of  the lathe. CENTERING  THE  WORK.—To center round stock  where  the  ends  are  to  be  turned  and  must  be concentric with the unturned body, mount the work on the head spindle in a universal chuck or a draw-in collet chuck If the work is long and too large to pass through the spindle, use a center rest to support one end. Mount a center drill in a drill chuck in the tailstock spindle and feed it to the work by turning the tailstock handwheel (fig. 9-21). For   center   drilling   a   workpiece,   the   combined drill   and   countersink   is   the   most   practical   tool. These   combined   drills   and   countersinks   vary   in size  and  the  drill  points  also  vary.  Sometimes  a  drill point  on  one  end  will  be  1/8  inch  in  diameter,  and the  drill  point  on  the  opposite  end  will  be  3/16  inch in  diameter.  The  angle  of  the  center  drill  must always  be  60°  so  that  the  countersunk  hole  will  fit the  angle  of  the  lathe  center  point. If  a  center  drill  is  not  available,  center  the  work with  a  small  twist  drill.  Let  the  drill  enter  the  work a  sufficient  distance  on  each  end;  then  follow  with a  60°  countersink. In center drilling, use a drop or two of oil on the drill.  Feed  the  drill  slowly  and  carefully  to  prevent breaking the tip. Take extreme care when the work is heavy,  because  you  will  be  less  able  to  “feel”  the proper feed of the work on the center drill. If the center drill breaks during countersinking and part of the broken drill remains in the work, you must remove this part. Sometimes you can drive the broken piece out by a chisel or by jarring it loose, but it may stick so hard that you cannot remove it this way. Then you must anneal the broken part of the drill and drill it out. We   cannot   overemphasize   the   importance   of proper  center  holes  in  the  work  and  a  correct  angle on  the  point  of  the  lathe  centers.  To  do  an  accurate job  between  centers  on  the  lathe,  you  must  ensure that  the  center-drilled  holes  are  the  proper  size  and depth  and  that  the  points  of  the  lathe  centers  are true   and   accurate. 9-13