Re: [CAD_CAM_EDM_DRO] Knee or Quill Drive?

Jon Elson wrote:

>
> 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.
>

I totally agree with every single word you've said.

The information I offered, as I said when I started, "if you are of a
mind to". I sold off a good knee mill and bought a bed mill for that
very same reason. (the quill was spongy) The Z axis ball screw was
located 2.5 to 3" forward of the quill and had to wrestle the quill up
and down through the cheesy little coupling which was attached to the
quill by only two 1/4" bolts about 3/4" apart. I am not sure that the
bed mill was such a great idea either because if you have to do things
like peck drilling, there's an awful lot of weight in the head (even
though it's counter balanced) to haul up and down. IMHO the ball screw
on the quill is the best way to go, but, the engineers should have kept
the ball screw tight to the side of the spindle housing in order to keep
the coupling as short as possible.

With regard to the accuracy of this rack and worm arrangement. I doubt
that it would ever win any awards but, it would be very consistent. By
that I mean that you would have to do a trial part and measure to see,
(What you got!) then make allowance for the errors. But from then on I
think it could hold it's own. Not a very classy system but if you cannot
Champagne.

Bill D