Re: [CAD_CAM_EDM_DRO] Re: Poor Mans DRO

jim_stoll wrote:

>Why is it that almost all DROs seem to use rotary encoders vs linear
>encoders? (at least that I've heard of - bearing in mind I'm new to
>this and haven't actually ever _seen_ a DRO... :-) Is it a matter of
>it being easier to match the rotational step of a motor directly to a
>a rotary encoder - vs trying to get a rotational step, once driven
>through a lead/ball/etc screw arrangement, to match up to a linear
>increment?
>
>

Nearly ALL commercial/industrial grade DROs use linear encoders or
measuring units of some sort. Sony uses either glass or magnetic scales.
Accu-Rite, Mitutoyo and many others use glass optical scales. Newall uses a
unique scheme with precision steel balls in a tube. Another outfit times
the flight of mechanical pulses (sound waves) in a steel wire with a
magnetostrictive coupling. I have not seen any commercial DROs with
rotary encoders. The reason is keeping coolant out of the works is
very difficult. These units sell for $700 - $1500 or so for standard
2 and 3-axis systems.

The bottom-of-the-line hobby-grade systems use rotary encoder schemes
to obtain minimum cost. Cheap rotary encoders made to read knobs
on equipment (with sleeve bearings) are pretty cheap, down to about $3.00
in quantity. Add a pulley to the encoder and a box to hold it all together,
and you can probably make the encoders for about $15 each.

Motors, stepping or other, have absolutely nothing to do with this. You
are talking about readouts for manually-operated machines, not CNC.
Most, if not all, DROs read out WAY too slowly for CNC. They
give 2-5 readings a second, while a closed-loop CNC needs perhaps
1000 readings a second to make accurate movements at practical speeds.

Jon