MR1 ballscrew upgrade

MR-1
1

Hello Everyone,

I’m just starting my MR1 build… considering a few modifications and thought I’d share for feedback.

The first mod would be a ballscrew upgrade with double ball nuts to help eliminate any backlash… Initially I was only considering this for the X and Y axis, but then after disassembling the X/Z carriage I couldn’t help but notice the contrast between the heft of the impressive cast iron carriage and the tiny Z axis acme leadscrew. A larger ballscrew would seem to be a better match for rest of the machine (though I understand the reasoning for the acme leadscrew with the challenge of shaving costs everywhere possible to keep the price of the MR1 as low as possible, while maintaining impressive performance).

A couple questions:

  1. How much improvement in chatter reduction and surface finish do you think could result from reducing backlash compared to the stock setup?

  2. In order to use the stock electronics and CutControl, the upgrade ballscrew pitch would have to be a multiple of the stock pitch (for adjusting the pulses/revolution setting on the stepper driver). That’s not a problem for the X and Y axis (5mm pitch), but the stock Z axis pitch is 3mm – which is common for ACME leadscrews but not for larger diameter double ball nut ballscrews, as far as I understand. I was thinking of using a readily available 5mm pitch ballscrew for the Z axis. Are there any options in CutControl for changing the ballscrew pitch the way you can in DIY controllers such as Mach 3? Am I missing something or does this make sense?

Thanks for any feedback.

2

We have a machine in our shop where we modified some ball nuts such that two could be used and counter clocked relative to one another. This yielded zero backlash, but we didn’t see any change at all in performance in terms of chatter or surface finish. You’ll see a slight improvement in overall accuracy, but its relatively minimal because the stock ball nuts have relatively little backlash to begin with.

I would advise against changing out the Z lead screw- one of the reasons its an ACME screw is so that it wont back drive when power to the Z motor is turned off. If you install a ball screw there you need to either install a tension spring (reduces drilling thrust btw), or incorporate a motor brake that engages when it loses power. Secondly, because we were able to control the size of the anti-backlash lead nut, it takes up much less radial space compared to a 12mm ball nut. This means that we were able to place the axis of the spindle closer to the gantry beam, which is critical for stiffness. Even a 1/4" additional offset would be very noticeable in terms of performance at the limit.

5 Likes
3

Thank you Daniel for the feedback, much appreciated!

4

That’s interesting. Never thought to check it because I thought 4mm pitch for 12mm ballscrews was standard.