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Anti-backlash preload mechanism


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This article explains the workings of the anti-backlash mechanism on my CNC lathe.  Such a mechanism is necessary when cutting profiles and tapers, as the slide moves toward the front of the machine while cutting.  Any tendency for the motion of the slide to lag behind the turning action typically results in a scrapped part.

  Manual lathes such as this one have ACME feed screws, and being unwilling to replace the screw and nut assembly to get rid of the backlash, I built this device to preload the slide toward the front of the lathe instead.

   Above is the "big picture" of the machine.  Real piece of junk, eh?  The original cross slide is visible just left of the center of this photograph.  The tools and some protective panels for the spindle drive have been removed for scheduled maintenance.

  The anti-backlash spring can be seen just up and right of the cross slide.

  Here is a top view of the mechanism, showing clearly its general principle of operation.  A tension spring pulls on one side of a lever, and my mighty thumb pulls on the other via a short cable.  The force on the cable side varies from 15-20 pounds depending on the position.  The cross-slide dovetails are visible in the lower left, and the disc with the holes in the background is the coolant drain.

  The lever is necessary to make the force more consistent across the slide's travel than it would be if a simply connected spring were used.  The fulcrum is a plain steel pin turned to run freely in the aluminum plate that composes the "L" shaped bracket affixed to the rear of the carriage.  Every now and then I give it a shot of oil to keep things moving.

  A side view of the mechanism.  Note the sandwich construction of the bracket.  On the left edge of the photograph one of the bolts holding the other side of the bracket onto the carriage is visible.

  This view shows the underside of the tooling plate bolted onto the original cross-slide to hold both front and rear tools while the lathe is in action.  A steel pin is welded to the bottom near the rear edge to engage the cable on the anti-backlash mechanism.

  For it's simplicity, it works quite well.  However, when using a preload mechanism like this one, boring operations should be done on the rear side of the work, not the front, as the tool has a tendency to chatter otherwise.

 

 

 

 

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