Quote Originally Posted by Wal View Post
X Axis driven by 2x 20mm ball-screws with a lead of 5mm driven by a NEMA34 motor connected by cam-belt/pulleys.

Y Axis driven by a 16mm ball-screw with a lead of 5mm driven directly by a NEMA23 motor mounted to gantry side-plate.

Z Axis driven by a 12mm ball-screw with a lead of 2mm driven directly by a NEMA23 motor.

1. The first plate will act as a mount for the linear bearings riding on 20mm supported rails (TBR20 from Zapp) mounted to 80mm aluminium extrusion and will also act as a brace
From your selection of ballscrews and bearings it appears that you are planning on ordering them from England? If you want to save money on them then linearmotionbearings2008 on eBay offers any size you need if you send him a message, along with fast service. I'm not sure if he has TBR bearings still, so you may have to use SBR. TBR are stronger than SBR, but you could probably compensate for that by just using one size bigger, i.e. SBR-25mm.

Using 20mm ballscrews is only beneficial on the X-axis, of a router, if they are 10mm pitch. You can get 10mm pitch, 20mm diameter, ballscrews from China, but not on eBay. If you used RM1610 the critical speed will limit your rapid feedrate to about 6-7m/min, but that's still respectable for this size machine. To get any more is expensive. See this post.

Most people use RM1605 on Z, but if you want to use something smaller (e.g. RM1204) that would work just fine but there isn't really much point unless you're very short for space.

20mm is plenty of thread engagement for M8 in aluminium.