I know others have chimed in and given very good information, my two cents is as follows:

Quote Originally Posted by JoeHarris View Post
Further to my previous post I have some questions:

1) to drive the two ball screws on the x axis I am using a belt driven by one stepper and driver. I am doing this because of fear of missed steps. If I use digital drivers (not closed loop) does this issue go away?

No digital driver do not remove the risk of missed steps. I am running twin screws with twin drives (one slaved to the other via Mach) and using digital drives (the very AM882's that Jazz refers to). I had been running them at 8.5m/min velocity and 1.75m/sec^2 acceleration for a good while, without any stalling what so ever at all. That was for the X axis as each one I tune a bit different. I am using 1605 ballscrews and direct drive. Currently I am running them at 8m/min V and 1.5m/sec^2 A and the little bit of jerkiness that was present has gone. Running a good solid voltage for the motors helps a lot. Currently running 60V and will be running 68V when I finish building the new power supply (almost have al the parts and slowly getting comfortable with how to d o it correctly).

2) if I stick with the belt drive arrangement shown does it matter where the motor is placed in the belt loop. I have shown it off to one side currently, I had it in the middle previously? I figure it shouldn't make a difference? Probably need to add in another idler though.

I will leave this to those who have experience with belt drive systems.

3) the four sides of the base of the machine is comprised of two aluminium plates fixed to the ends of two aluminium profiles. These sit onto a series of aluminium box sections. I have no cross bracing in this frame. Would it be prudent to add some in??

Yes, rigidity is your friend as long as it does NOT cause binding or increase enertia unduly.

I intend to build the timber table next so I have a base to work up from - any comment would be greatly appreciated before it is too late!!

Yes when building your table do the best you can to make sure the surface is truly flat and set up points with which to bolt the machine to it so that once built the timber frame can be secured and act as additional part of the rigid structure to help absorb vibration from the machine.
Good luck and keep asking, will help as able.

Michael