Spindle Encoder - Speed limitations?

[cncsnw]

250 lines / 1000 counts will probably work just fine.

As I understand it, this is a milling application, so the spindle encoder is used only to generate Z axis feed during rigid tapping, and for RPM display in the status window.

If there is a problem, it will show up first on low-RPM, coarse-pitch tapping. If the spindle encoder counts are coming in too slowly, and each spindle encoder count prompts too much Z movement, then you would have jittery start/stop motion.

How slow is too slow, and how much Z is too much?

Unless there have been significant changes that I am unaware of, the control reads the encoder and updates the desired Z position 4000 times per second. You would like to have at least one, and preferably several, encoder counts come in during that time. Say you have 1000 counts per rev, and you only require one count per interrupt. That would mean 4 revs/second, or 240 RPM. Any slower than that, and the control will (many times per second) think that motion needs to stop and restart.

Say you are tapping 5/8-11. Each turn of the spindle then requires 0.0909" of feed. Each count of the spindle encoder prompts 0.000091"of movement (or the nearest approximation, given your Z axis encoder resolution). That says that you will probably still get reasonably smooth feed, as long as the spindle encoder counts don't stop coming in altogether for 250us or more (i.e. as long as you don't go below 240RPM).

If you need to thread something much larger and coarser than 5/8-11, try thread milling instead.