While waiting for the SX2 mill that I want to convert to be back in stock at ArcEuro I've been giving some thought to the pro's and con's of converting to ballscrews for the x and y axes. Here are my thoughts (perhaps someone will correct any false assumptions):

1. Backlash:
Cheap (well, cheapish) chinese ballscrews (C7 spec) typically have about 0.003" backlash. I have seen reports of people acheiving 0.001" to 0.002" with an acme screw and a well made Delrin anti-backlash nut so, even if they are a bit optomistic, there doesn't seem to be much difference.

2. Driving torque requirement:
Ballscrews have about 90% mechanical efficiency while acme screws have about 50% so, on the surface, it would seem that a much smaller stepper motor could be used. However, the original leadscrew is 2 mm pitch while the ballscrews are 5 mm pitch so the ballscrews will require actually require about 40% more torque.
Of course we also have to consider the required resolution. I would be happy with the 0.0002" that the acme screw will give at 1/2 step. To get nearly the same resolution (0.00025") with the ballscrew will require 1/4 stepping. Now a stepper at 1/2 stepping produces about 70% of the full step torque. If 1/4 stepping it only produces about 40% of full step torque.
So, for the same resolution, I will require steppers with rated torque over twice as high for ballscrews as for acme!
I find this surprising! Am I right?

3. Fast traverse speed:
Clearly, for the same speed from the electronics, the ball screw is 2.5 times as fast. Or is it? Does the microstepping affect this?

4. Wear rate:
I can't find any figures but I guess the ballscrew wins here. The acme anti-backlash nut will require an easy adjustment.

For my hobby use 3. and 4. are not very important so, also considering cost, keeping the acme screw would seem to be the best alternative - or do you know different?

I look forward to comments.

Russell.