Re: [CAD_CAM_EDM_DRO] Building A Cheap DRO

cutsgems wrote:

> I'd like to add a 3 axis DRO to my mini-mill. I've investigated the
> Calareo/Wholesale tool import digital machine scales & readout box,
> nearly $500 to cover all three axis. So what I had in mind was some
> encoders from US Digital, something like 2000 PPR?? to get 0.0005"
> resolution, use a simple wire under tension wrapped around the axle of
> the encoder to convert the linear motion to rotary. Use an old laptop
> & printer port & available software for the readout.
>
> My questions are the mechanics for the encoder.
> Would a 1000 PPR encoder give me the resolution required if I used
> 0.6369" diameter hub on the encoder as opposed to a .3183 diameter hub
> which would only yeild a .001" resolution. Or am I better off using a
> 2000 PPR encoder with the .3183 hub? (The .3183 is a rough calculation
> required to convert 1" of travel into 1 rev on the encoder).

I think you went the wrong way. You'd need about a 0.159" diameter hub
to double the resolution. The hub needs to be smaller to turn twice as fast
for the same linear movement.

> Secondly, when calculating the hub diameter, how do I account for the
> wire diameter in the calculation? Is it 1/2 the wire diameter +/- the
> hub diameter???? Thanks.

Yes, I would suspect it is very close to 1/2 the diameter. but, it is probably
not exactly that. There will be slip between the wire and hub, stretching
of different 'plies' of the wire, and pinching of the hub. All very small
effects, but they add up with a long movement. You can try different
diameters until it comes out right. It shouldn't be very hard to make the
hub a couple thousandths larger than the calculated diameter, and then
test it for error. Take a little off and try again. When you get real close,
decide when to stop.

Jon