https://embeddedtronicsblog.files.wordpress.com/2021/02/bsmceo4u-install-manual.pdf...is a copy of the BoB manual.
https://embeddedtronicsblog.wordpress.com/2021/02/05/4-axis-mach3-controller/...is a review of it. It contains the following paragraph:
"10 Volt spindle output.
There is a seperate available 10 volt PWM that can be used to control spindle speed. The voltage output pins are between AVI and ACM ground screw terminals. For this to work, a 10 volt power supply must be connected to the 10V and GND terminals. This has better linearity than using one of the four output pins as PWM. The output signal is driven by a transistor which gives a cleaner looking square wave. If your spindle requires a 5 volt PWM, you can apply 5 volts to the 10V terminal instead. Many diode laser controllers are 5 volt PWM."
There are photos of the waveform at the output AVI which show that it is a PWM pulse train. It looks like the Kress spindle needs a DC signal so this would need a smoothing circuit. The pulse frequency looks like 1kHz so a time constant of say 100ms should do the job. For this a series resistor from AVI to the top of a 100uF capacitor, with the other end connected to Gnd, and the 10v taken from the top of the capacitor, should do the job. I can never understand why BoB manufacturers don't build this into the board in the first place.