Figure 9-4.—Compound rest.The rotating feed rod drives gears in the apron; thesegears in turn drive the longitudinal feed and crossfeedmechanisms through friction clutches.Some lathes do not have a separate feed rod, but usea spline in the lead screw for the same purpose.Lead ScrewThe lead screw is used for thread cutting. It hasaccurately cut Acme threads along its length that engagethe threads of half-nuts in the apron when the half-nutsare clamped over it. The lead screw is driven by thespindle through a gear train. Therefore, the rotation ofthe lead screw bears a direct relation to the rotation ofthe spindle. When the half-nuts are engaged, thelongitudinal movement of the carriage is controlleddirectly by the spindle rotation. Consequently, thecutting tool is moved a definite distance along the workfor each revolution that the spindle makes.Crossfeed SlideThe crossfeed slide is mounted to the top of thecarriage in a dovetail and moves on the carriage at a rightangle to the axis of the lathe. A crossfeed screw allowsthe slide to be moved toward or away from the work inaccurate increments.Compound RestThe compound rest (fig. 9-4), mounted on thecompound slide, provides a rigid adjustable mountingfor the cutting tool. The compound rest assembly has thefollowing principal parts:1. The compound rest SWIVEL, which can beswung around to any desired angle and clamped inposition. It is graduated over an arc of 90° on each sideof its center position for easier setting to the angleselected. This feature is used for machining short, steeptapers, such as the angle on bevel gears, valve disks, andlathe centers.2. The compound rest, or TOP SLIDE, which ismounted on the swivel section on a dovetailed slide. Itis moved by the compound rest feed screw.Figure 9-5.—Common types of toolholders.9-4
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