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

Right you are !
This is known as the neutral plane and lies aprox. 1/3 mtl. thickness in from the inside rad.
Scott Acorn
Bertho Boman wrote:

> 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
>
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