Last night I took apart the controller - and took lots of photos.
In terms of basic parts - there is a power/spindle board, the switches, the spindle speed variable resistor, a big power transformer (10cm+ toroidial thing), and then the stepper controller board.
First here you see on the left the basic power input - with 2 fuses - not sure yet why. There is also a fuse at the inlet socket for the box, and a fuse in the plug. This thing is pretty well protected. There are two heatsink devices. One is a 12v regulator (7812) going to the stepper board, and the other a FET (an IRF740) controlling the spindle. The cable labelled SP-SW is the spindle switch.
There is a small cluster of DIP IC’s on the board:
These include a 555 - controlling PWM for the spindle, a 4n25 optocoupler, and the c222 - another kind of optocoupler. These all seem to be related to enabling, and controlling the spindle speed.
Immediately obvious in this image are the 3 stepper controllers - Toshiba TB6560AHQ. This whole board is resting upon a big heatsink attached to these. Close to these are big electrolytic caps - presumably to decouple motor noise from logic, and flyback diodes.
There are 2 Dc/Dc converters - presumably one for the logic components, and one for the stepper power. The big MORNSUN B0505LS-1W is 5v, and the other converter is the 1501-50.
There are then blocks of the 6n137 Optocouplers - 6 of these. Perhaps this is 2 per stepper channel? Each is capable of 1 logic line - perhaps bringing pulse and direction to the stepper controllers.
These look to be driven from some 74hc14d schmitt trigger buffers - which bring the logic from the parallel input to the Opto couplers.
There are some small opto-couplers going in the other direction - these are maybe for limit switches and the emergency stop signals.