Hey Nathan, hey @Echd,
you wrote earlier you own an Axiom Iconic-8. With its welded base and all-axes ball screws this must be a nice machine. What kind of milling motor is inside the case on the gantry? Is this a spindle on a VFD, or a milling motor?
I still believe the best thing for you would be what I consider the best milling motor solution for hobbyists and semiprofessionals, since it requires no VFD, but is explicitly made for CNC machines. You would need to spend approx $500 once and you would not need anything else (no VFD, no custom wiring, just an analog line for the motor’s speed control interface to the CNC controller). What is still missing is a possibility to attach such a milling motor to the Onefinity CNC. A simple reduction ring to 43 mm would not be enough, as explained in this thread, a mount option for the Onefinity CNC would have to look rather like this (it is a requested feature).
If you look at this AMB milling motor video, you can see how it performs in a CNC machine. Unlike the Makita hand trim router, which is not allowed to be used in a CNC by its manufacturer and looses warranty if you do, these motors are explicitly made for use in a CNC machine. They have an all-steel motor flange with double bearing for high performance and longevity. And they have ER collets.
A similar milling motor was used to mill the entire Jazz Bass in this video. Looks like a rebranded Mafell milling motor.
In principle, you are right, it is a motor that rotates a milling bit. It’s more the performance and the behaviour in different situations that makes the difference. With a spindle, you can get to a very high performance limit without problems. When you increase the mechanical load (more feedrate, more depth of cut, more stepover, harder wood), the spindle will not be slowed down like the hand router, and when you go beyond the limit, the VFD will simply protect the spindle and the workpiece and stop the spindle and the g-code program. A hand trim router is completely unable to do that. If you give it a higher load, it will be slowed down, get hotter, and then fail, possibly with severe damage, in the range from broken bit, ruined workpiece, to shop burned down. A spindle is not only much more efficient, but also much more smart and will prevent damage to your machine and your workpiece.
This also means with a hand trim router as a CNC milling motor, you have to always stand beside the machine. With a spindle and a VFD wired to a safety relay that monitors everything, you can leave the machine alone. If something goes wrong, the spindle and the machine will be stopped. However this are additional costs to the bare spindle/VFD kit.
Here is a video which shows when you get near the limit of the spindle/VFD:
https://youtu.be/h9q6j7POgOI?t=488
It shows how their 220 V 2.5 kW spindle is driven to draw up to 10 A current from the output of the VFD (spindle current drawn in Ampères shown on the lower right). The test consists of driving a 10 mm 3-flute aluminium end mill through an 6061 aluminium block with 1000 mm/min feedrate, 10 mm depth of cut and 8 mm width of cut (US customary units: The test consists of driving a 13/32" 3-flute end mill through an 6061 aluminium block with 40 IPM, 13/32" DOC, 5/16" WOC).
What they don’t show, is what would happen if you go beyond the limit that is set in a VFD parameter: The VFD would trip and the safety circuit would stop the spindle and the g-code program on the CNC controller.