I finally milled some steel today with my modified spindle drive.
I made some changes to the original design.
I am now using an Arduino Mega board with 2 additional boards.
1 x Mega protoshield for Arduino[ID:192] = $14.95
1 x Relay Shield for Photon[ID:3051] = $30.00
I soldered a connector onto the protoshield for ground, forward, reverse and 10 milli HZ pulse width modulated speed control (green, orange, red and purple).
The protoshield piggy backs onto the Arduino. I do the logic in the Arduino for outputting signals to 2 relay shield relays which run the VFD in forward or reverse.
I use the Arduino PulseIn function to measure the high pulse of the PWM signal and use that to control a timer to output a 5 volt square wave frequency.
the square wave connects to an input pin of a SN7406 open collector hex inverter chip. The corresponding output pin gets pulled up to 24 volts from the Hitachi VFD.
This 24 volt square wave connects to an input on the VFD which controls its drive frequency and thus the motor RPM.
Also the VFD outputs a 0-10 VDC which can represent spindle load. I divide that voltage by 2 on the protoshield and connect the resulting 0-5 VDC to the meter on the Mikini control.
I forgot to mention that I sharpen up the I/O signals to and from the Arduino using a schmitt trigger hex inverter (probably not necessary.)
The drive motor is a 1 HP, 3 phase Marathon motor. Single phase power to the Hitachi VFD runs through a line reactor and output from the VFD runs through another line reactor connected to the motor. I also have braking resistors connected to the VFD.
The end result is that the Mikini controls work exactly as before. Speed is controlled very exactly and power is adequate for the machine. I am limiting speeds to between 375 and 3000 RPM, so the speed read out on the Mikini display will not be correct from 200 to 375 RPM nor from 3000 to 5000 RPM.
I forgot to mention I am using CNC Linux on a PC with a parallel port for running programs.