Comments sought on new build: A CNC Router for RC Gliders and Planes

[JAZZCNC]

Ball screws. I was thinking of 1605 (from Chai) driving them with belts at 2:1 ratio on the X & Y axis and 1204 for the Z being direct drive from the 3Nm stepper. Is the 1204 ball screw OK for the Z or is it better using the 1605? I’d be happier using the 1204 to use the smaller ball nut and therefore getting an extra little bit of length on the X axis.

No wouldn't do that go with 10mm pitch and if around 1500mm then I'd go with 20mm screw, chai now sells these.
Yes 1204 will work ok for Z axis, my machine uses that size. That said I always tend go with 1605 on machines I build has they are more readily available and bit more meat for pulleys etc, they also allow slightly higher tuning.

For the Stepper motors I was thinking of 4 of the 60BYG301B 3.1Nm motors from CNC4YOU? Any others to consider, the SY60STH86-3008B from Zapp? Any difference (I can’t see any in the specs particularly) in the two apart from price?

Either there's very little difference, I've used both.

I’m thinking of going with either the CW-885 Drivers from CNC4YOU or the AM882 Leadshine from Zapp. I’m leaning towards the AM882 but what are your thoughts?

Not used the CW-885 but they will have to be very very good top beat the Am882 so for me it would be the tried and known AM882.

I take it that I can drive the 2 X axis steppers from the same driver so will only need 3 drivers? Is the performance of these drivers much better than the 50V drivers? I think I’d rather spend a bit more and have better “future proofing” even though for cutting balsa and thin ply the 50V drivers will be fine I’m sure I’ll be cutting Ali and all sorts after a while!

NO Bad idea Each motor needs it's own drive. You could in theory run 2 drives from one signal output but don't run 2 motors from 1 drive. I wouldn't recommend using one output has it leads to timing issues and inaccuracy's etc and not worth the hassle.

Couple of reasons for not running 2 motors from 1 drive. The drive ratings are split between both motors so 80V 8A would only allow 80V and 4A for each motor. Most 3-4nm motors are around 4-5A so not enough and plus it's working the drive very hard shortening it's life quite a bit.
BUT bigger reason is the Resonance handling of the drive can not work properly and doesn't know which motor to apply resonance compensation too and gets very confused with result being the other motor gets pushed into resonance.
Believe me you don't want resonance issues they are a night mare, resonance can/does cripple the performance of a machine.
By the way the AM882 are fantastic at handling resonance, you can hear and certainly see the smoothness compared to Analogue and some of the cheaper digital drives.

Another thing to consider when using slaved motors on a Axis is that you will want some way to square the gantry and for this you need 2 Inputs for home switches because each screw needs to be turned individually while homing.

Which BOB would you recommend?

No contest PMDX 126 hands down the best. . . Cheap NO.!!

Power supply. Is it best to buy one readymade, i.e. the PS806-5 from Zapp, or have a go at making my own? Is there a significant saving to be made?

Again no contest build your own this way you can have the exact voltage and Amps required plus it works out cheaper because you'll need 2 of the PS806-5. Will build your own for less than one PS806.