Re: [CAD_CAM_EDM_DRO] Knee or Quill Drive?

Bill Darby wrote:

> Hi
> I have seen various people go to a great deal of work to devise
> adjustments to eliminate the play between the rack and pinion.
>
> However, if you are of a mind to, you can totally eliminate backlash
> between the rack and pinion: Make an anchor point at the bottom of the
> spindle. Connect a small but very strong spring to it. Then connect a
> light aircraft cable to the spring. Lead the cable up to an idler (and
> tensioning) pulley located at the top of the spindle housing and then
> down to a take up spool on the pinion shaft. Run the quill all the way
> down and connect the cable to the take up spool. Ensure the cable is
> tight enough to really load the spring. (The diameter of the take up
> spool should be very close to the diameter of the pinion gear.)
>
> This arrangement allows you to apply as much pressure as you wish
> between the rack and pinion with no up or down loading I think Charles
> Atlas called it "dynamic tension". (And the younger guys say " who's
> Charles Atlas")

Well, I'm sure that you can eliminate the backlash, but how accurate is
a .200 pitch or whatever straight rack? I can't believe you get very
precise movement. I am almost certain there would be a significant
cyclical error when the motor moves the pinion at a steady speed.

I have a ground ballscrew directly driving the quill at the point where the
stop ring usually mounts on a Bridgeport J head. I get about .0015"
of backlash, due to flex in the coupling, mostly. But, the motion is
very predictable, so that the position of the quill is within .0015" or
so of commanded position, over the full 5" travel. I'm sure you
couldn't do that with a rack and pinion drive. I have checked the
accuracy of movement with a dial indicator and also with gauge blocks.

Jon