Y-Axis Designs

[Jonathan]

Option A is best - the problem with the other one is that the rails (obviously) offer no support parallel to the direction of travel, so when a force is applied parallel to the Y-axis, the lower bearings can move slightly along the rails without the ballnut moving. It wont move very much, but that still means you have a lower stiffness. It's therefore not as strong as having the ballnut directly between the rails.

You should find this document useful:
http://www.thk.com/sites/default/fil...l/a/ee_A15.pdf

Also, depending on the design of the rest of the machine, you may be better off putting the Z-axis rails on the Z-axis plate (which holds the spindle) and the bearings on the Y-axis carraige. It tends to reduce the overhang. I'm not keen on mounting the rails directly to the aluminium extrusion either...

[JAZZCNC]

Jonathan is correct that "A" is technicly the better option but I've built machines with ballscrews in all locations, on front, up high and at rear and I've yet to see any significan't differance that makes me say don't use any of them. The only time I'd shy away from any of the others is if machine is going to cut hard and aggresively in hard materials.

I also don't agree that mounting profiled linear rails directly to extrusion is a bad thing provided the rail is wide enough and obviously profile is straight good HD quality. If done correctly then it can be very accurate and easy to use. I do this all the time and assure you it's accurate enough for any router type machine. If your wanting milling machine levels of strength and accuracy then your using the wrong material any way.!!