Threading with A B phase only?

So can I get some clarification on the method of LinuxCNC executing my operations please? I want 4 functions : 1) control of the carriage positioning motor, 2) control of the cross slide positioning motor, 3) control of the main spindle motor vfd for constant surface cutting speed which is dependent on the changing position of the cross slide, and 4) optional triggering of the thread cutting tool auto-withdrawal and auto-insertion which is dependant on the carriage position. Do the instructing signals from my pc LinucCNC program each go through the Mesa card 7i76e, and can the card run the drivers of different size motors simultaneously on different channels?
If I get a NEMA 23 for the cross slide leadscrew (little movement required in lathe work and also smaller is less obtrusive in fitting), a NEMA 34 for the carriage leadscrew and possibly a different type of motor altogether for the thread cutter auto-withdrawal mechanism (not quite sure yet exactly what that electrical mechanism is!), is that ok so long as each output channel does not exceed 5v? The threading auto withdrawal trigger is preset for dependency on the carriage position hitting a target, so will that be a separate output from the Mesa card or will it be from the same output channel as the carriage leadscrew driver takes its signal from? Also the spindle motor's vfd constant cutting surface speed operation takes its dependency on the position of the cross feed encoder - will that require an independent output from the card or will it be from the same output channel as the cross feed driver takes its signal from?

You won't need the old control display or the threading tool. LinuxCNC will display your position and with two axis control threading will be automatic. The old thread tool just had a solenoid that moved back and forth. You had to adjust the cross slide manually after each pass. Different size motors will not make a difference to the Mesa card. The card only sends step, direction, and enable signals to the driver.

Fantastic. Thanks. As regards the constant surface cutting speed vfd function, 1) do the control signals from the pc for that function go (via the ethernet cable) to the Mesa card then to the vfd, or should they come out of a different pc port and go direct to the vfd. And 2) does LinuxCNC control have the option of selecting Modbus protocol to communicate with the my vfd?

I can elegantly mount a NEMA 23 onto the left hand side of the carriage to drive the cross slide but only with a body length max around 56mm - this body length limit restricts me to a Stepperonline NEMA 23 178oz/in which is rather less power than one might like but then the cross slide is only making very small movements once operations commence and it is not driving a heavy carriage about like the main leadscrew is. The cross slide leadscrew has a reasonably light and smooth action for a 16 x 40 lathe. What do you think?...If gear it down a bit with a smaller pulley on the drive shaft....

The only connection to your computer is the ethernet cable to the Mesa card. Pretty sure you can do modbus comunications from the card too, however I just used the normal spindle interface 0-10v.

Andy what do you think is your maximum and average total current draw by your setup? My workshop is on a 20 amp fuse (I could fit slightly larger but it already causes problems when my wife is in the laundry room!) - it is on a ring circuit covering workshop + downstairs sockets that has absolute limit of 32 amps. The inverter (240 single phase to 240 three phase) is a heavy consumer of amps and so the one I have is only 2.2kw because input limit for this inverter is 19amps. Hence I never had any machinery of more than 2.2kw until this one arrived. I can happily run the 4kw motor at half power by inputting standard 2.2kw parameters to the inverter and accelerating gently, but I am worrying that the total draw of the system when two 1090 oz NEMA 34's (max draw at 5.6 amp per phase) are running, plus the transformers from 240v ac to 24v dc , plus the 24v motor brake, plus general circuitry and lighting and suds pump is going to be too much. What do you think and what do you think is the lowest power Stepper Motors I could get away with?

Ah thanks for that!

Regarding choice of pulleys for the 1090 oz Wantai stepper motors (which are now on their way!) for this Encomat project, can I assume you reused the existing ones for the main carriage leadscrew, but for the Cross slide leadscrew what do you recommend for the driving pulley and for the driven pulley - number of teeth and pitch? Many thanks, Adam.

Yes I modified the original for the Z axis, and bought a pair of HTD5M Aluminum Timing Belt Pulleys off of ebay. I pulled the graduation dial off the cross slide handle and bored one of the pulleys to lightly press on the handle. Then i drilled and tapped for 3 cap screws to secure it.