I have been helping a friend build a machine for the same purpose. We are doing tests on it and I will probably hang a video on Youtube. Our experience so far:
-Z axis does not need to travel more than 4 inches, but extra travel will be of help when replacing bits.
-Z axis does not need to have high accuracy and backlash is not a problem if you use a "floating head".
-A floating head is needed in order to precisely control depth of cut, independent of the PCB clad conditions. We are allowing 0.1" float on the software, setting zero to the top side of the PCB, but our floating head will allow to float a few inches if required for other purposes.
-For X and Y look for the best lead-screws/nuts with NO backlash, we used grounded ball-screws and zero backlash nuts (they came with the donor machine).
- Spindle: 30000 rpm DeWalt DW660.
-Steppers are 425 Oz-in driven by Mardus-Kreutz micro-stepper boards.
-Tested cutting speeds are 120 ipm X and Y, 25 imp Z. We haven't tested beyond that, rapids are 330 ipm.
-Depth of cut or drill adjusted by a 0-2 inches micrometer.
-For the floating shoe (the part that slides on top of the PCB) we used a turned PVC irrigation pipe, that also serves as a vacuum cleaner tip. The floating head "floats" on a short linear rail with 4 inches of travel, mounted on a Z axis with 9 inches of travel. There is a weight compensation spring on the floating head used to control the drilling force.
We used two 1/8" pins for accurate registration of the two sides, each pin about 0.5 inches outside the board contour at exactly half of the X dimension, Bottom side is mirrored on the X axis. The two pins are flush with the top of the PCB, in order to avoid collision with the floating shoe.
In our tests we have been able to control depth of cut to 0.001" and draw 0.002" ( 0.002" deep) annular rings around all 0.032" drill holes in a test board 2.5" x 3.0" approx. using a 60 degrees V carbide bit. Bottom side registration is also perfect.
My friend is working now on a vacuum clamp fixture in order to avoid taping the PCB to the MDF board we use as a disposable back.
His router is also used for aluminum, and wood milling. He has a Porter Cable 2.5 HP (10000-23000 rpm) router mounted on the Z axis for those jobs (non-floating) and the linear rails for Y and Y axes are mounted on granite slabs. The whole structure weights probably around 700- 800 lbs and the working area is about 24" x 36". Such a beast is not necessary for PCB routing only, but a solid structure is required to avoid vibration when doing fine SMD pads milling.
By the way, we are open to new alternatives for the routing bits, the ones that we are using now (Bits&bits part number 815-HF60 with 0.003" tip) don't last more than a few pcbs.