Very cheap spindle encoder idea.

[andypugh]

I am converting a Chinese 9x30 lathe to CNC. (http://www.cnczone.com/forums/showth...t=63621&page=2)
I want to be able to cut threads, so I need a spindle position encoder.
The lathe uses a pair of sealed bearings for the spindle, so the outside of the spindle shaft is in open air in the head casting.
I am thinking of simply laser-printing two black/white tracks (20 pulses per rev + index) onto paper, wrapping this round the spindle with tape, and then using a pair of Optek OPB702 reflective sensors to watch the stripes go round.

Is there any reason this won't work? My first thought was a machined aluminium disc, but I suspect my inkjet printer can print tracks more accurately than I can machine a codewheel, and this approach saves me dismantling the head.

[Geof]

Years ago I did a similar thing but it was lines drawn with a drafting pen; laser printers were not even a gleam in anybody's eyes back then.

If your reflective sensor are not already shrouded make a housing so they are looking through a slit about the same width as your lines. This gives crisper definition to the signal.

Also, surely 20 pulse per rev is a bit coarse? One pulse every 18 degrees.

[Al_The_Man]

Have you considered putting an encoder inside the spindle compartment and using a encoder pressure wheel against the spindle itself? the compartment is a relatively clean and protected environment.
BTW, you only need one reading head, unless you want to indicate direction, or up the resolution by using the multiple edges (80 counts/rev).
Oops, yes you need one for marker.
Al.

[acondit]

Andy,

I cut an encoder wheel on my cnc router with 100 holes plus index. I am using it on my 9x20 spindle. You can see the build up of the spindle encoder here starting at message 48 and following intermittently through the thread.

Alan

[andypugh]

Originally Posted by Geof Also, surely 20 pulse per rev is a bit coarse? One pulse every 18 degrees.

I believe it is possible to thread with only an index pulse, so the extra track is just to track in-cycle speed variations. Certainly it would be insufficient if the software was using the pulses to advance the tool, but EMC2 has a prediction algorithm.
I was trying to minimise the requirement for high-speed switching electronics and parallel port bandwidth. However having just calculated that my computer worst-case latency of 18uS allows me at least a million pulses per rev at 3000rpm, I might go for a finer resolution.

[andypugh]

Originally Posted by acondit I cut an encoder wheel on my cnc router with 100 holes plus index.

I hit a bit of a bootstrapping problem there :-) I can't CNC machine parts until I have finished the conversion.

[andypugh]

Originally Posted by Al_The_Man Have you considered putting an encoder inside the spindle compartment and using a encoder pressure wheel against the spindle itself?

Inside the compartment is where I intend to put the encoder (it is also nice and dark in there). However surely a pressure wheel would be very unlikely to run at exactly the same speed as the spindle, leading to a disasterous cumulative pitch error when threading?

[andypugh]

Looking at that encoder wheel above, it occurs to me that with reflective sensors you could make an encoder wheel very easily indeed out of an inkjet printable CD. In fact I might consider that possibility for making absolute encoders for my ballscrews.

[Al_The_Man]

Originally Posted by andypugh However surely a pressure wheel would be very unlikely to run at exactly the same speed as the spindle, leading to a disasterous cumulative pitch error when threading?

Encoder pressure wheels are often used in industry in flying shears etc, A few of the encoder manuf, make such items for this application.
Al.

[andypugh]

Originally Posted by Al_The_Man Encoder pressure wheels are often used in industry in flying shears etc, A few of the encoder manuf, make such items for this application.

I am sure that they are great for measuring out a fixed length of material, but they don't seem ideal for the spindle encoder of a lathe.
Actually, it might work, as during a thread cutting operation the spindle probably only goes round a few hundred times so if the roller and spindle diameters are within a couple of thou the cumulative error in the thread starting position will be only a few degrees, which is probably OK.

Nevertheless, if this idea doesn't work then Plan B is a machined/drilled aluminium disc machined in two parts to clamp round the spindle.

[Geof]

Slip should not be a worry so long as it was constant; not a worry for threading that is because the spindle does not reverse. It would be a different situation for rigid tapping.

[acondit]

Originally Posted by andypugh I hit a bit of a bootstrapping problem there :-) I can't CNC machine parts until I have finished the conversion.

Maybe, if you asked nice, someone else who has done it before would cut a wheel for you. PM me if interested.

Alan