Printer to CNC

Hi I am convrting an old home built Printer to a CNC, the mechanics wiring etc are not a problem, What I am not sure of is where to connect the spindle to.
Any help would be highly appreciated.

OOPs forgot to say what board I am going to use, it's a MKs Gen 1.4.
I suppose I could just keep the spindle as a manual operated item..

I use my cnc with a manually controlled (rpm wise) spindle and works well for me. I could connect the 10V control signal to the cnc controller board but it's not really a priority. It's nice to control rpm from gcode but not a must.
Normal 3d printer controllers don't have 10 control signals, it may be pisdible with an pwm based signal though, but it depends on how the spindle is driven, with a vfd or some other device.

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Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Most CNC builds use GRBL rather than Marlin which is a smaller firmware allowing a lot of configuration to be done through the G-code rather than firmware. I run Marlin on a 3D printer and GRBL on a CNC machine connected to the default GRBL pins. I certainly wouldn't run a CNC machine on Marlin.

Why is this in "Prusa i3 and variants"

will you be converting i3?

Printing and milling Two extremes

......Printing -- little forces on mechanics all axes

......Milling --- extreme forces on the mechanics all axes ---- important

Thanks for the input, I see your point, But I have already gone off of that idea, and am building a fixed gantry style with two Nema23 to drive the Z axis with led rods, and single drive for the X and Y axis also with lead rods.