40:50 index pulse scaling

I've got an AliExpress BT30 spindle with a 50 tooth timing pulley, and a motor running it from a 40 tooth pulley. I can't find a 50 tooth HTD5 pulley, so 40 teeth it is. I want to attach a quadrature encoder somewhere so I can do rigid tapping and (eventually) spindle orient for an ATC.

As I have a cylinder above my spindle for the drawbar, it's more practical to attach the quadrature encoder to the motor shaft. I've read other threads about scaling the index pulse from a quadrature encoder by 2:1, but not this weird 40:50 scaling. I want to eventually put an ATC on this machine, so spindle orient is a requirement. Can LinuxCNC scale an index pulse like this?

It would be better to add a separate index sensor to the spindle or spindle pulley.
What is the spindle pulley made of? You could potentially use that as your encoder.

However, 1:1 gearing a commercial encoder to the spindle (or, in fact, 40:50 gearing the encoder to the motor) would be better.

I see your problem finding a 50-tooth pulley. However I think that HTD is a round-bottomed profile and you should be able to make a pulley good enough to drive an encoder by drilling a ring of holes with your CNC then machining off the excess. (and a Delrin pulley will be more than adequate)

Cheers Andy. Thinking about it, scaling an index pulse won't work; the machine will never know where the spindle is oriented. To that end, a 50 tooth delrin pulley for the encoder sounds like a good idea. Just gotta find some space on the head of the machine now. The spindle pulley is steel, but I'd rather use one encoder for revs and index so it's going to get its own pulley.

If space is tight on the spindle then an encoder driven from the motor with the same ratio as the spindle will work just as well.