The opto will provide feedback to Mach for reading RPM and on a lathe will also provide a means of the index pulse for threading, it will not however allow you to synchronise your spindle to axis as you would need for rigid tapping. As has been said a servo can allow you to do that if you use SwapAxis() and tell Mach the spindle is actually an axis. Doing that will allow you to sync moves between the spindle and any other axis so rigid tapping would be possible. I believe the DSPMC can use a normal motor (with encoder fitted) to do rigid tapping and also I think the Centipede allows that.
Hood

Just a thought , could you not use a servo motor in place of the main motor and set it up as a 4th axis, there's plenty of servo's with enough  power and speed to do the main motors job.

I maybe way off the mark here but is it not what your describing just basically spindle speed feedback? Using a servo to drive a spindle will give you what advantage? So won't the Optical sensor giving feedback back to mach control all other motors, eg feed speed, to compensate while threading, if you get slip or loose rpm.

Just a thought , could you not use a servo motor in place of the main motor and set it up as a 4th axis, there's plenty of servo's with enough  power and speed to do the main motor's job

Yes it is spindle speed feedback but it boils down to 'how much' of it and 'what you do with it'. Mach only accepts a _single_ index pulse per revolution; servos use many, many more pulses (e.g. 2^17). Mach only uses the pulse for one or two things: to sychronise the feedrate when screwcutting/threading; to do PID spindle speed control; to do constant surface speed facing.

The computational gymnastics Art forced Windows to perform simply cannot cope with doing much else.

Most servo motors are constant torque from zero up to full revs., unlike a VFD controlled motor which has less torque at low revs.

You can set the servo up as a fourth axis, but then it is difficult to use as a spindle. There is no G-code for 'rotate indefinitely at x rpm' (irrespective of what other axes are doing). Doing a toolchange on a rotary axis is not a meaningful concept.

If you want to do this kind of stuff, and more, you have to accept that wonderful though it is, Mach is perhaps the wrong tool to use. EMC has been built conceptually to do all this, but making concepts into practical applications is hard work.

You could always use an external device such a Smoothstepper which can utilize the higher encoder counts. Using a servo will give higher rpm and constant torque as compared to a stepper where
you are limited by torque at the higher rpms .....in fact the feedrate will not be very linear with a stepper and will be limited in application. For indexing stepper iuse is fine.

RICH

This has all sort of gone beyond me, but like always I just had another thought, Why can't we run two separate motors to the spindle joint mechanically, so you do either one job or another. You could have your main 3ph vfd controlled motor for general turning and threading with feedback of course (RPM FEEDBACK)  and then have a servo or a stepper to run the the spindle , mechanically changed to do your rigid tapping, you could even theoretically do rifling with that set-up.
One motor is treated as the spindle and the other as a 4th axis.
The only thing you need here is a quick change gearbox, from motor to motor, and there would be no crossover in mach.

As mentioned before, fitting a servo on the spindle as a spindle allows you to do this without messing with extra gears etc associated with two motors. All that is needed is to command a SwapAxis() in Machs VB to change from spindle to axis mode. I have servos on the spindles of all my machines, 12.5KW on the lathe, 8KW on the Beaver mill and 5KW on the Bridgeport..
Hood

Sorry Hood just shows that I am way out of league here, I have to read it 2-3 times before I understand, and now Im learning pretty quick, Just out of interest were would you even look for a  12.5kw servo, and is that AC or DC? what drives it?
The largest servo's I have seen are 60v  DC's on a friends Bridgeport mill, they make my 1000oz steppers look like toys.
"I Don't think I'm in Kansas Any More"
Katoh

I got mine from eBay, paid £250 for the 12.5KW and £102 for the 8Kw, I also got the drives from eBay, two 22KW ones for £180 each and a 10KW one for £50. All were new except the 10KW drive and it only had a few hours on it

They are AC servos and drives and are 415v 3 phase input.

Hood

Would I be better off looking for a servo motor to drive the spindle?
would you connect the motor via timing belts , v-belts or direct drive? This is just opening up a hole new can of worms for me, I really thought the norm was to go a 3phase  motor driven by VFD, but now I know better.
Do you still need the optical input (rpm) for spindle or does the servo drive input the information back into mach.

The norm is VFD as it is the cheap easy option but is far away from the best, VFD/Induction motor will lose torque the slower it goes and on a lathe slow speeds are where you want the torque, AC servos and for that matter DC servos have an almost flat torque curve from 0 RPM to full rated RPM.

The reason as said most dont go the servo route is price, if you know what you are looking for and can find deals like I did then it may actually be cheaper but if I had bought my 12.5KW new from Rockwell it would have been over £4000 likewise the 22KW drives would have been that kind of price as well.

As mentioned deals are out there, I have just bought two old Siemens spindle drives for £10 each and also a 30KW VFD for £57, dont have a use for them at the moment but I pick up the bargains when they are going just in case I find a use


I use the Index pulse from the encoders on my servos but I use the SmoothStepper which is fast enough to read the short index from the encoder, if using the parallel port you would either have to electronically lengthen the pulse or go with the opto.

I drive mine with timing belts, you really need to if you are going to use it as an axis. The belts are amazing, on the lathe the belt is a Gates GT3 and is only 20mm wide and stands up no probs to the torque of my motor (83Nm cont 152.5Nm peak)

Hood