Thanks to Carl and Arturo and others, I got the spindle and VFD connected and working today. Mach 3 connects to the CNC4PC C6 board, and then to the VFD. I can use Mach 3 to turn on the spindle and set the spindle speed. I can adjust speed using the on-screen override, and I can use GCode S commands to change speed in a file. The VFD has a stop button that still works while the spindle is controlled by Mach 3.
I needed one tiny bit of info, and Carl provided that for me. I needed to know what the VFD provided or needed to turn the spindle on and choose directions. The manual gives instructions on which terminals to connect to, but doesn't say what those terminals need to operate. I was assumming they needed a voltage from somewhere, but it turns out that they provide voltage that the relays on the C6 switch. When the voltage goes to ground on the Com terminal on the VFD, the spindle turns on.
Speed control is actually pretty easy. You configure pins to output step direction from the spindle panel in Configuration, and connect this step output to the step input on the CNC4PC C6 board. Its a pretty small voltage, but its there. When you look at the C6 variable voltage output, as you increase spindle speed in Mach 3 the voltage output increases. This output goes to the VFD inputs, and varies the speed.
I haven't done exhaustive testing, but I did warm up the spindle and vary the speeds. Its very stable, and runs cool and quiet. A big improvement over my noisy Porter cable.
Next comes wiring an Estop system, and doing a clean install of all these parts. But the big hurdle is finally crossed. I'll post more details so others considering the Chinese spindles and VFDs can have more detail on what's involved.
Thanks for all the help here.
Wayne from White Salmon