Question re. lathe spindle encoder

[AdrianW]

Hi,
I'm planning on buying an Acorn to control my CNC lathe (based on previous post) but I have a question about the spec. for the spindle encoder - it currently has a sub-500ppr optical encoder driven by a toothed belt @1:1 ratio and I'm struggling to find an 8000ppr encoder that's within budget. I have two main questions:

1) Is the minimum encoder requirement of 8000ppr also applicable to lathe spindles? I'm not sure what the limitations would be if I used less.

2) Can I use a different gear ratio from the spindle to the encoder to increase the effective ppr?

Here's my logic - I'm using a lot of speculation from a position of relative ignorance so please bear with me!

- On a lathe, the absolute position of the spindle (and I mean absolute in the sense of knowing e.g. where a specific chuck jaw is from one work session is to the next) isn't important. What's important is knowing where the spindle is relative to the first cut that was made on a specific work piece.

- If the above is correct, then the only need for a Z (index) output on the spindle encoder is for pulse-count verification (if indeed the Acorn does this).

- If both the above are correct, then there is no reason you couldn't increase the effective pulse count of the encoder by having a different drive ratio to the encoder e.g. having a 1:4 drive would increase a 2000p/r encoder to be 8000p/r of the spindle.

Does this make sense, or am I missing something? I guess you could run into non-linearity problems if one of the pulleys was too small and the belt had any 'set' in it.

[martyscncgarage]

Hi,
I'm planning on buying an Acorn to control my CNC lathe (based on previous post) but I have a question about the spec. for the spindle encoder - it currently has a sub-500ppr optical encoder driven by a toothed belt @1:1 ratio and I'm struggling to find an 8000ppr encoder that's within budget. I have two main questions:

1) Is the minimum encoder requirement of 8000ppr also applicable to lathe spindles? I'm not sure what the limitations would be if I used less.

2) Can I use a different gear ratio from the spindle to the encoder to increase the effective ppr?

Here's my logic - I'm using a lot of speculation from a position of relative ignorance so please bear with me!

- On a lathe, the absolute position of the spindle (and I mean absolute in the sense of knowing e.g. where a specific chuck jaw is from one work session is to the next) isn't important. What's important is knowing where the spindle is relative to the first cut that was made on a specific work piece.

- If the above is correct, then the only need for a Z (index) output on the spindle encoder is for pulse-count verification (if indeed the Acorn does this).

- If both the above are correct, then there is no reason you couldn't increase the effective pulse count of the encoder by having a different drive ratio to the encoder e.g. having a 1:4 drive would increase a 2000p/r encoder to be 8000p/r of the spindle.

Does this make sense, or am I missing something? I guess you could run into non-linearity problems if one of the pulleys was too small and the belt had any 'set' in it.

No,
Line driver/differential encoder must be belted at 1:1 to the spindle for proper operation and performance. The CNC software also used the encoder's index pulse which comes around once per revolutions, you multiply the ratio you multiply the index pulse. No bueno....
Marty

[AdrianW]

Thanks - I had been assuming that the rotations would be counted using the A/B channels, but as you say the SW is likely to use the index pulse to re-set home each revolution, and having four index pulses per revolution of the spindle could have disastrous consequences!

I'll probably try one of the 2000ppr encoders for starters unless anyone says it's a really bad idea - for $40 or so it's worth trying even if I have to upgrade later. If anyone can say what problems I need to look out for with lower resolution encoders I'd be really grateful!

[martyscncgarage]

Thanks - I had been assuming that the rotations would be counted using the A/B channels, but as you say the SW is likely to use the index pulse to re-set home each revolution, and having four index pulses per revolution of the spindle could have disastrous consequences!

I'll probably try one of the 2000ppr encoders for starters unless anyone says it's a really bad idea - for $40 or so it's worth trying even if I have to upgrade later. If anyone can say what problems I need to look out for with lower resolution encoders I'd be really grateful!

Adrian,
Take a look at automationdirect.com
If you are attaching the encoder to a shaft, and can cover it, you can use the light duty encoder, otherwise look at the medium and heavy duty line.
a 2000 line driver differential encoder get's set to 8000 in the encoder box in the Wizard. So a 2000 line encoder will work fine.
Marty