Re: [CAD_CAM_EDM_DRO] Updated DRO using screw threads not pulleys

Doug Fortune wrote:

> Ian Wright wrote Fri, 7 Jul 2000:
>
> > I have been considering a similar set up to yours for a DRO but I was going
> > to use another pulley outboard of the encoder as well. By doing this you can
> > take a complete turn around the encoder shaft (yes, I know it will screw but
> > I thought that the shaft would probably be polished enough for the line to
> > maintain a reasonable position) and it also takes all the strain off the
> > encoder shaft.
>
> How about this improvement? What we'd like is to do is wrap the wire
> several times around the encoder shaft to:
> - increase holding friction (so the wire doesn't slip)
> - take bending strain off the shaft (so the encoder shaft is in the middle,
> not the end), and say the wire comes from left, around the shaft a few times,
> and unspools right.
>
> We can't do this with a bare shaft, because the wire wants to 'walk' down
> the shaft and wear by rubbing against itself. Also presumably there are pulleys
> at the ends which don't walk, causing misalignments.
>
> Answer:
>
> Instead of using pulleys and bare encoder shafts, substitute screw threads
> everywhere. The wire sits in the grooves of the screw threads.
>
> In the center is an encoder shaft with screw threads, so the wire
> (for encoder or even direct power drive) winds around the screw threads numerous
> times, and also at the ends, the wire "walks" down the screw threads at the
> same rate! Everything stays in alignment.
>
> Doug Fortune

This was pretty standard technology for early x-y plotters, plotters for IR spectrophotometers. There should still be a lot of this
old technology around to scrounge parts off of. Look for names like Beckman, Perkin-Elmer. etc. Actually they were drums of
reasonable size at each end with one end connected to the servo, comb on the spectorphotometer( for null, etc ). If I remember
correctly they took about three turns (arond the drum) of stainless steel wire or teflon-coated SS wire....check sporting goods
for teflon coated stainless steel leader.

Dave

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