Re: [CAD_CAM_EDM_DRO] Building A Cheap DRO

Yes, but then the cable will "walk" along the shaft changing the angular
relationship with the bed. This is bad as you want the cable parallel to
the axis at all times. IF (and that is a big IF) you wind the cable around
the idler axle such that the walk ALWAYS keeps the cable parallel to the
axis, then you could avoid the sine/cosine error that will be introduced by
angling the cable. As the saying goes, (been there tried that and was not
happy with the results).

With the ball bearings, I use a large screwdriver as a pry bar to get
sufficient tension on the assembly (with the mechanical doubler) and so far
have had no problems. It has been in use for over a year now (still have to
put the swarf guards on) with flood coolant and no real problems other than
introduced by the operator. I re-calibrated it when I went to version 5.x
of the program to check it out during beta testing and devised an even
better way (I think) to measure an axis.
At 05:45 PM 3/19/02 +0000, you wrote:

>Art Eckstein wrote:
>
> > I "was" worried about the tension issue until I designed the housings that
> > I used that include ball bearings. Not only will this allow greater
> > tension, it lowers the friction required to turn the shaft.
> >
> > I thought about using the multi-turn idea but quickly discarded it for the
> > very reasons you mentioned. The result was to build the system to maximize
> > the amount of cable around the shaft (and keep a simple design) so the
> > contact angle is 180 degrees. I am always trying to think of better ways
> > and one that I recently thought of was to add a pressure roller (again with
> > a ball bearing) similar to what is done on a tape recorder capstan. This
> > should increase the pressure on the encoder axle without increasing the
> > cable tension further. However, right now there are other projects on my
> > plate and the darn thing works as is!
>
>I would think the best way to do it would be to have the wire wrap one
>full turn
>around the drum. This would make it possible to have a lot of tension
>on the wire without putting a lot of side force on the encoder shaft.
>
>Jon
>
>

Bubba

OLDER THAN DIRT
Country Bubba
(Actually the inventor of Country and Bubba)

axtein@...
LaGrange, GA

http://members.dicomm.net/~axtein


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