Timing Belts and Pulleys

I posted this in the DIY-CNC group and it was suggested this might
be a better group to ask....


I'm still in the process of cobbling together my gantry CNC mill. It
is made up of mainly linear actuator assemblies gathered on ebay.
They are all currently powered by servos directly connected to the
ballscrews of the actuators.

Seeing there is no way I'm going to be running the motors at speed
to get the rated torque, I want to gear them down using timing belts
and pulleys. I went to the www.sdp-si.com site and used their
calculator. It came up with 50 tooth pulley and a 25 tooth pulley, a
belt spaced 60.21mm apart.

My questions...

1) With a 50 and 25 tooth pulley, will this give me an exact 2:1
ratio or will there be an error factor? Will I need to calibrate my
mill by counting steps and measuring the distance?

2) Can I place the pulleys at the specified distance, 60.21mm? Or do
I need to add some to allow for belt tensioning? In this case would
I be better off making one side adjustable? I will probably use two
stages to give me 4:1, so do I need to make the middle pulleys
adjustable?

3) What is backlash like with timing belt pulleys? I've read some
other posts, and it seems the elliptical tooth profile is a good
choice.

4) How will timing belts effect the required servo stiffness? I
currently have the gain quite high to get a rigid feel (still
haven't milled anything though). Do the timing belts allow for a
softer (and hence quieter) gain ie will the stiffness be multiplied
like the torgue? Or, do the timing belts add a springiness to the
drive?

Thanks,
Frank