Re: [CAD_CAM_EDM_DRO] Re: low cost linear encoders

An article in "Projects in Metal" described a home brewed DRO, based
on making new PC boards for a capacitive caliper mechanisim. I don't
know which issue, they reprinted it in one of the "Metalworking"
article collections. (either 1 or 3, don't know which, its not handy)


There is an other option in our case basically using the same principle:
Make a simple fixture where the strip can slide sideways under the milling machine head or a router. Set the strip at an angle
to get the desired diagonal cut.

Clamp the strip and make one cut and clean (sand-deburr-polish?) the slot.
Slide it over the desired spacing against an accurately repeatable stop (spring finger).
Mill the next slot.
Repeat as needed.

We really do not need an accurate distance.
We just need the same repeatable distance and then we can calibrate it in software.

Bertho Boman
==================================================

> From: "Ian Wright" <Ian@...>
>
> Hi,
>
> Thanks to everyone for your observations.
>
> I haven't seen one of the chinese ones but this Mitutoyo definitely has the
> copper pads cut right across in a diagonal fashion producing a series of
> parallelograms such that the bottom point of one coincides with the top
> point of the next (I'll try a bit of ASII art)
> ............ ............ ............
> / / / / / /
> / / / / / /
> ------------ ------------ ------------
>
> I think perhaps the angle of the cuts is a little more but I couldn't think
> how to show it - anyway, something coupling to the top of the pads would
> obviously have a phase shift in respect to something coupling to the bottom
> of them. Presumably this is what allows it to read to a fine resolution. The
> thing I haven't quite figured out is how the thing works without any
> electrical connection to these pads and, if two or more sensors are
> involved, how they work without interacting - if one sensor feeds a signal /
> voltage? into the pad, surely another sensor over the same pad would pick it
> up so can't see how the phase shift would work. Perhaps its strobed onto
> different sensors.
>
> As to making accurate strip I don't think that would be much of a problem.
> The way I would tackle it would be to make a jig which would have to be very
> accurate but which could be made in separate parts that could be accurately
> lapped to size. This would have a baseplate onto which would be fitted a
> bridge section of 3 parts, the central of these being the most important and
> requiring the greatest accuracy. There would also be two more items required
> which I would make, one being a detent - just a flat piece of steel of an
> accurately known thickness and with one edge formed as a dead central knife
> edge. The final part would be a file, exactly like the detent but with file
> teeth cut along the knife edge. I would cut these teeth with an 'etching
> bar' as is used to make needle files but they could be made with an ordinary
> file. The central part of the bridge would have to be machined and lapped to
> give two exactly parallel diagonal edges the precise distance apart to set
> the detent and file to the exact width of the required pads and at the right
> angle. The two outer parts of the bridge would be then set to keep the
> detent and file pressed against the central part. I would use a bridge
> design as this would guide the PCB strip and would allow me to equip the
> file with a depth stop, so ensuring that all the gaps between the pads were
> the same width. Then it would simply be a matter of filing one notch into
> the PCB, feeding it along until the detent dropped fully into the notch and
> repeating until I had a long enough strip. This is just an adaptation of a
> very old technique but one which is of proven accuracy if the tools are
> carefully made. The fact that the strips used on the Mitutoyo verniers have
> pads in excess of 3/8" long would make this quite a practical proposition.
>
> Ian

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