I’m planning on changing the 80mm diameter spindle in my 6040 CNC mill to a much smaller 43mm spindle.
Obviously I’ll need a new motor mount (mounting holes 100mm apart.).
There are standard parts for that.
However…since surface quality is my primary metric…
does anyone know a spindle-holder design that has adjustment screws to perfectly tram your spindle?
(Alignit perfectly vertical to the work surface, reducing tool marks in finish passes.)
Why don’t you try something similar to the tilt adjustable heads on some manual milling machines. You would need to mount the spindle on a plate that has circular slots, and use some blots and nuts to fix it in place. You could take advantage of stuff like the Pro Tram thingies.
I was going to use something like 2 of these:
http://www.sorotec.de/shop/CNC-Mechanik/Fraesmotoraufnahmen/43er-Eurohalsaufnahme-.html
They are attached to a solid aluminium block with the Z-ballscrew-nut. As there is no nut on the other side, I don’t think a circular hole would work because I’d have to cut a thread into a circular hole.
A wider spindle holder that has space for an additional screw next to the to mounting screws (to push aganst the Z axis block and tram around Y) and wide mounting holes in the upper spindle holder with a bolt from the side pushing against the mounting bolts…something like that would work and allow for fine adjustment without resorting to aluminium foil and completely removing the spindle for each adjustment.
This is what the current 80mm spindle holder looks like:
Would POM be strong anough to make my own spindle mount or does it have to be harder?
Wait a second…these 4 M5x13 holes…do they push against the spindle from below?
http://cnc-plus.de/de/Maschinen-Zubehoer/Motorhalter--Eurohals-Motoraufnahme--Fraesspindelhalter-165x88x20-.html
They would allow easy tramming adjustment.
The Sorotec spindle mounts should work, no need to thread a circular hole. You would need to mount them onto a custom made plate instead of the ballnut holder block. The custom made plate would have the circular slots that allow for tilt adjustment using the threaded holes on the ballnut block and some hexagonal head botls (to allow the use of a thin spanner when everything is assembled). The same plate could have a pair of tapped holes that allow a bolt to push agaist the Z axis block to adjust the spindle in another direction, You wouldn’t have to use shims on the machine or remove the spindle for every adjustment.
I don’t think POM strenght is gonna be a problem, as long as you can use thick pieces to compensate for the loss of rigidity, compared to metals, I would be more concerned with its dimensional stability. I really don’t know what to expect, better talk to a plastics guy 
On the second spindle mount, I don’t think the M5 holes go through the part, but you could drill the remaining material to get the result you want anyway. Not sure if it is a good idea to push against the spindle housing with such small area of contact.