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

Jon,
Never mind the crow. As long as we are disagreeing, one of us is wrong and I like to
learn if it is me (which it very well might be).

I believe there are two ways of thinking about it.
Since we are talking about a ribbon that is not zero thickness we can call its thickness
center line its average length. If the wheel just touches a flat surface, wheel diameter
is fine but if the ribbon has been bent, the wheel surface is a distance away from that
center line: It looks like we added a layer on the wheel to reach the centerline. The
wheel appears to have a larger diameter and turns slower.

The other way is to think about what happens to the ribbon when it is curved. One
side will stretch and the other side will compress. The wheel is measuring the
compressed side of the ribbon and the resultant distance will be slightly shorter than
the average centerline and again, we will measure a too short distance.
Bertho
========================

Jon Anderson wrote:

> 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.
> ---- >snip
> 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