This was always going to create a challenge, mainly because of the limited space. In fact, this one took a day to work out.
If I ever (and I reckon "ever" might be sooner than I wish) have to put a proper small CNC motor in there..I simply replace the 25mm sq section with a 25x50mm rectangle section, cut a new piece of acrylic to accommodate the extra width, and I can bolt it straight on (he says...casually!)
I also made an acrylic nut to hold the MDF insert in place. I drilled out a larger piece of 6mm acrylic with a 10mm bit, heated the inside edge of the hole, and basically tapped a thread using the insert. Then trim it to the right size and you are away. You can excerpt a fair bit of pressure on it without cracking.
The steel bracket was then used to mount the insert to the centre section.
The lower end is where I mounted the stepper, and I made up a couple of small proto boards to hold the ribbon cable to power the stepper and take the wiring from a small momentary switch. The switch is activated by a small tab (a suitably cut bit of 25mm angle) on the bottom of the screw rod mount, and acts as the Z endstop.
I soldered 4 lots of 3 wires from the ribbon cable for power to the stepper (see the next part for the other end), and laid it out so it would roll around as the Y axis moved, very much like the original print head ribbon cable. Hey..it worked for the printer..so why not make it the same.
I also dropped in a balun retrieved from the printer just in case I got line noise. Not sure if it needs it, but with the stepper being capable of drawing 2.5A, seems like a good idea.
I used the trusty MDF inserts to clamp the dremel handle inside the centre tube. I drilled 2 x 10mm holes on one side, and directly opposite each hole, drilled 8mm holes. That way I can screw in an insert from the inside out, using the 8mm holes as a place to feed the 6mm hex key needed to screw the inserts through. Then I cut a saddle out of 6mm acrylic for the thin end of the handle, and a custom bent (hit it with a hammer) saddle from the 40mm aluminium angle.
The inserts give the 6mm bolts something to push against when screwed up tight against the dremel handle.
Nearly there....the electronics!