So far the signal generator (DDS) is just driving a stepper. I have the AC servo on the bench but need to build some cables for the motor and just don't have time right now. I'll let you know when I get things tested.

Another quick update: FWIW, if dedicated to the task, the Arduino can generate a max frequency of 100khz square wave (wide enough to be recognized by a typical drive). I think 25khz would be adequate, so the board can do the job, but I have a lot of other stuff in mind for the board to be thinking about, so testing and retesting the signal speed after each code addition would be a real time gobbler. Therefor I am going to acquire a separate signal generator and just have the Arduino command the desired frequency.

This method will actually dovetail nicely into one of the features I had in mind; that being to automatically vary the spindle speed to maintain a given SFM during profiling or facing operations. This will significantly improve surface finish as well as reduce production time.

Another feature bouncing around in my head is to have the PLC monitor imbalance thru an accelerometer installed in the 4th axis in order to provide an operator warning or perhaps to restrict spindle speed in the event of a significant imbalance before a serious and potentially dangerous problem arises.

The list goes on, but I wonder if any readers have some ideas for cool features that could be added to either the 4th axis or the mill spindle.

Quick Update: After numerous simulations, the Alpha software (1,800 lines of C code) is running live tests on the 4th axis. The test programs run all night and so far are stable.
Working features (so far) are:
 *Communicating with MACH thru serial Modbus, one MACH Brain and a series of macros.
 *'Wait states' implemented. PLC genrates a 'Speed Reached' signal used to make MACH wait until accell/decel events are completed before MACH continues.
 *Interrupt driven E-stop forces immediate signal to MACH regardles of what the PLC was doing at the time.
 *Accell/Decel is autonomous and completely independent of MACH. Controlled by two parameters configurable 'on the fly' via macro.
     example: Speed set to 450 RPM (spindle RPM) M3 to M4 causes automatic smooth Decel to a stop, dir change, accel back to set speed.
 *RPM max limit is configurable 'on the fly' via macro. User enters spindle RPM (or SFM in AutoSpeed) and back gear is calculated automatically.
      commanded RPM that exceeds  RPM max is held at RPM max and a warning indicator (or LCD message) informs operator of overspeed.
      no error or E-stop need to be generated and when RPM drops back, indicators go off and operation continues without interruption.

*'AutoSpeed Mode': user supplies desired SFM and speed is automatically controlled via position of the Z or Y axis
                      Speedmatch feature allows switching 'on the fly' into or out of AutoSpeed and the spindle will accel/decell to the set speed of the new mode.
                      Z or Y axis coords are fed to the PLC and the PLC feeds back current RPM via MACH Brain. This is another 'machining center' capability brought to the benchtop.
                         One step further =  G95 feedrate results in automatic feedrate changes based on PLC reported RPM.

*'Manual Mode':  full manual control 'uncoupled from Mach via control panel. Accel/Decel, max speed, E-stop fully functional even in manual mode.  There is lots more, but this is the basics so far.

Some notes on Smoothstepper use with 4th axis motor controller:

CNC4PC has a custom BOB for the SS and it is convienient to use, but for some reason they decided to lock port 2 permanently as inputs.

The only solution is to cut the traces on the board and put jumpers between the pins. See photos below. If anyone knows of another custom BOB for SS< please post it.

THanks

Last post to this thread. Project completed and software is in beta and ready for customer testing. If I have time I will add a video to my YouTube account that follows this project. Here is the final control box and screen mods:

Very nicely done. I haven't had time to work on any of this project for ever now. Been off on other things. Glad to see things worked out so well for you however.

Post moved to correct area.

Appologies

Hi I am newly registered but long time follower of this forum.
I am glad to run in this tread as I build a mill based on Mach3 Usb board 100khz and do not like to use big and heavy ac motor.
I have Kollmorgen U12M4 Servo Disc and would like to use as a spindle motor so the plan is to connect Mach3 to Arduino to get M3 M4 commands to work as weal as speed control .Would need to build encoder (or 2 one for Arduino one to Mach3) for the motor and hopefully get Arduino to work as PID or velocity mode.
If that is possible I have to mention my limited coding/electronics skils.
Or if someone have bather idea