Lathe Spindle Encoder speedratio?

[MatterHKR42]

Once again back for some help.. you guys and gals are super awesome!

So.. this is an issue I knew i would have to circle back to...
The lathe is a Prolight 3000..
the OE encoder location on this machine is on a midshaft between the motor and spindle. There's a 2:1 speed reduction going from this midshaft to the spindle. To get things going, I simply doubled the line count for the encoder count field in the wizard and its been working for regular turning.
Yesterday, I ran about 6 test parts (3d printed blanks) that had a contour pass to set a diameter, auto tool change, then cut some M25 x 1.5 threads.

5 of the 6 parts were perfect... the threads measured right on size, OD was spot on.. but the threads on the last part had a pass exactly 1/2 rotation off, resulting in a two-start thread delete lol.

I know the problem is the encoder needs to be 1:1 with the spindle... but before I go about making that happen, I thought a post here to get your thoughts would be good form.

Option 1. Move the encoder to the spindle
This is a lot of work but if it has to be done, so be it. best way I can think of would be to add another shaft.. bolt the encoder on one end, timing pulley on the other. drill and tap holes to mount the whole unit to the headstock.. add a timing pulley to the lathe spindle.. and get everything connected via belt.

Option 2. Add an index pulse.
The machine does have an index disc and sensor but I'd have to add an I/O expansion (all 8 inputs are in use) to tie it in.. and thats given the acorn can even use an index sensor

option 3. Do nothing and know that every now and then the lathe will screw up threads.

Option 4. find another work around, like perhaps maybe there is a parameter where threading only starts on odd or even rotations revolutions of the spindle

As always, Thanks for the help!

[cncsnw]

The index pulse needs to come around exactly and only once per revolution of the spindle.

If you cannot easily move the encoder, but you can install a proximity sensor or something similar on the spindle, then with some basic interface electronics (i.e. a line driver chip) then you could generate a simulated 'Z' channel and wire that into the spindle encoder DE9, combined with the 'A' and 'B' channels from the actual encoder.

Here is one way of doing that.

[MatterHKR42]

The index pulse needs to come around exactly and only once per revolution of the spindle.

If you cannot easily move the encoder, but you can install a proximity sensor or something similar on the spindle, then with some basic interface electronics (i.e. a line driver chip) then you could generate a simulated 'Z' channel and wire that into the spindle encoder DE9, combined with the 'A' and 'B' channels from the actual encoder.

Here is one way of doing that.

Very clever!
and I think I might have one of those ICs in my electrical inventory..
At the very least, this should buy me time while working to move the encoder

The machine has a index wheel already on the spindle with an optical sensor.. probably wont be as precise as the encoders Z channel.
Same line of thinking.. what about some sort of circuit that "skips" every other Z pulse from the encoder?

Thanks!

[tblough]

Have a look at a T flip flop, AKA a toggle flip flop. You may have to combine that with a one-shot pulse generator. I'm not sure what Centroid is expecting the index pulse to look like. If it just triggers on one of the edges, then the T-type flip flop should be all you need.

[MatterHKR42]

OK so..

Dumb question..
What if I set Parameter 33 to 2, to match the 2:1 gearing between the motor and my encoder..
Then set parameter 36.. bit4 should be motor side encoder. with rigid tapping off the value should be 16..

I only use 1 gear ratio so I don't think it will affect anything else..

This seems like the best option while I make a kit to move the encoder to be 1:1 with the spindle

Thoughts?

[MatterHKR42]

So..

to document this for anyone that runs into the same problem... At least on CNC12 V5.20... manually editing parameter 33 with the high gear ratio and changing parameter 36 bit 4 is a thing and best yet...

THIS WORKED.. without having to splice in a an index sensor on a line driver chip..
Feed per revolution works fine
Multipass threading works fine..

Moving the encoder to be 1:1 with the spindle is still absolutely the best and most correct fix.. but if you find yourself in a situation where your machine needs to run and you have this issue, it will work well enough.