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Engineman 2 - Intermediate engine mechanics training manual
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Figure 9-4.—Compound rest. The rotating feed rod drives gears in the apron; these gears in turn drive the longitudinal feed and crossfeed mechanisms through friction clutches. Some lathes do not have a separate feed rod, but use a spline in the lead screw for the same purpose. Lead Screw The  lead  screw  is  used  for  thread  cutting.  It  has accurately cut Acme threads along its length that engage the threads of half-nuts in the apron when the half-nuts are  clamped  over  it.  The  lead  screw  is  driven  by  the spindle through a gear train. Therefore, the rotation of the lead screw bears a direct relation to the rotation of the  spindle.  When  the  half-nuts  are  engaged,  the longitudinal  movement  of  the  carriage  is  controlled directly   by   the   spindle   rotation.   Consequently,   the cutting tool is moved a definite distance along the work for each revolution that the spindle makes. Crossfeed  Slide The  crossfeed  slide  is  mounted  to  the  top  of  the carriage in a dovetail and moves on the carriage at a right angle to the axis of the lathe. A crossfeed screw allows the slide to be moved toward or away from the work in accurate   increments. Compound Rest The  compound  rest  (fig.  9-4),  mounted  on  the compound slide, provides a rigid adjustable mounting for the cutting tool. The compound rest assembly has the following  principal  parts: 1.  The  compound  rest  SWIVEL,  which  can  be swung  around  to  any  desired  angle  and  clamped  in position. It is graduated over an arc of 90° on each side of  its  center  position  for  easier  setting  to  the  angle selected.  This  feature  is  used  for  machining  short,  steep tapers, such as the angle on bevel gears, valve disks, and lathe  centers. 2.  The  compound  rest,  or  TOP SLIDE,  which is mounted on the swivel section on a dovetailed slide. It is moved by the compound rest feed screw. Figure 9-5.—Common types of toolholders. 9-4







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