Re: [CAD_CAM_EDM_DRO] Rotary to linear translation - an interesting idea.

ptengin@... wrote:

> I gotta agree with Bertho. The thicker the ribbon, the larger the
> change. How much it is pressed into the ribbon does not matter, once
> the ribbon is no longer straight, the error does come into play. Only
> a zero thickness ribbon would have no error.

Maybe I'm missing something here and someone can explain why I'm seeing
this wrong. The measured distance a wheel travels for 360 degrees of
rotation is a function of it's circumference. Given a fixed number of
pulses from an encoder, the only way to change the effective measured
travel is to change the diameter, thus the circumference.
Whether the ribbon makes theoretical point contact at a tangent point
or wraps halfway around the periphery does not change the circumference
of the measuring wheel one bit. The only benefit to wrapping around the
wheel I can see is maybe reducing the chance of slipping.
As for printers and such that wrap the cable around a drum, I believe
that mechanically speaking, this is a capstan drive. Capstan drives can
be calibrated for accuracy, taking into account both any stretch in the
cable/ribbon, and any compression of cable as it wraps around the drum.

Some years ago an article in HSM covered a setup a naval machinist
developed for turning long tapers with extreme repeatability between
setups. He used a capstan drum on the cross feed screw with a cable
fixed somewhere on the lathe. A fudge factor was figured in here for the
slight compression of the cable around the drum under tension, which
would... change the effective working diameter and thus the distance
traveled. Though in this case, the linear distance was fixed and a
certain ratio of rotary motion to linear was needed.

Now, can someone tell me how bending a ribbon around a wheel changes the
wheel's diameter? I'm well aware of the effects of stretch/compression
in bending, having worked in sheetmetal. Remember though, the wheel
interfaces with the surface of the ribbon, not it's centerline.

I'm not trying to tick anyone off here, but I see a big flaw in this
idea and unless I can be proven wrong, I'm just trying to keep some
folks from wasting time on the wrong path. Prove me wrong and I'll eat
crow.

Jon