I am guessing something similar is already implemented.
Here we go.
A bit messy, not sure if 100% accurate, but the overall principle is clear.
Opamp 3 is a triangle generator, 420 Hz.
OP 1 is a comparator, generating a PWM based on the velocity pot.
Power FET has a 82 mR shunt for current sensing.
OP2 compares the current sense value and triggers the SCR, which displays "overload" and clamps the PWM setpoint to 0.
The OFF switch also sets the PWM setpoint to 0 and resets the SCR
And now it comes, Opamp 4 feeds the amplified current signal back to the PWM setpoint (via the 100 k resistor between output 4 and + input 1).
If you hold the shaft (which is pretty difficult without mill attached to it), you can provoke a slight current rise.
But the problem is the poor low speed performance. The motor starts to rotate at 5% PWM DC, but torque is pretty low and you can easily stall it by hand. PWM then rises with in a few seconds to 6% where you need a firmer grasp, but would still not advance the mill table. Only when you advance the speed knob further the motor turns with reasonable torque, but then it is far too fast for milling.
Not sure if this can be tuned to acceptable performance. Obvious start would be the 100 k current compensating resistor, I'll try to reduce that and see how this goes. What worries me a bit is that this circuit is peppered with filter caps. Obviously they are fitghting PWM noise, but still it looks a bit excessive.
EDIT changing this resistor did not help much...