The upgrades on my mini mill so far have been fairly small, and I promised "big" changes. Well, here's the first big change: I decided up upgrade to ball screws. When I did my original conversion, I made some low backlash Delrin nuts for the lead screws. They worked great, but the screws themselves leave a bit to be desired. It takes a lot of energy to turn a standard lead screw, and the original screws were very fine so it took a lot of turns so get anywhere. Since stepper motors like mine make their max torque at low RPM, the high RPM needed to move my mill table quickly meant I was risking skipping steps.
For this application, ball screws are better in almost every respect. They take much less energy to drive because the friction is so much lower. Lead screws are generally about 20% efficient, while ball screws are around 90% efficient. That means that much more of my motors' energy can go into moving the mill instead of being wasted just turning the screw. Ball screws have a much more course pitch too, so it takes fewer turns to move the table, lowering the RPM the motor has to turn so it operates closer to it's maximum torque. The only real down side is that you can backdrive ball screws, meaning if you shove the table hard enough, it will turn the screw. Lead screws can't do that, and that's why you don't see ball screws on manual machines.
I'm using 1605 ball screws that I got on e-bay. They are not the world's highest quality screws, but they were inexpensive and are much better than the original screws.
The Y screw was basically a direct replacement for the original. The X screw took a bit more work. There is very little room for the X ball nut on this mill. The first thing I had to do was whack the top off the ball nut flange. In what will probably be a shock to everyone, this cheap low end ball nut is through hardened and was very hard to machine. Not realizing how hard it was when I started, I chewed up one of my carbide endmills in the process. You can also see here the sleeve going through the middle that I 3D printed allowing me to take the screw out of the nut without all the balls falling out(it's tedious to get them all back in the right way, assuming you don't lose any).
I also had to clearance the bottom of the cross slide. I could have ground down the nut, but I was more comfortable clearancing the cross slide. Cast iron is much easier to remove than hardened steel.
Here's how the nut and screw sit in the cross slide:
I goofed and ordered my X screw just a little bit too short. I should have gone one size longer. Oops... To make up for that without losing any table travel, I decided to make a longer end bushing instead of a bearing like I was planning. I used the same aluminum bronze that I made my gibs from because this is exactly the application this material is made for. Here's my new longer bushing on the right with the original on the left:
Here's how the bushing fits in the table:
