two ballscrew-nuts or one

Q

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H-M Supporter Gold Member
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Apr 9, 2014
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Hi All
I have a Bridgeport mill that was originally one of those with a hydraulic stylus/copy attachment. I am currently up-grading it to cnc. The original screws had double nuts, but all the information I can get now is that people state one ball-nut because two increases the wear on the screw/ nuts. Any advice please.
thanks
 
2 nuts can decrease the backlash because you adjust them against each other. Stay with 2.

Dave
 
To add to what Dave said, there are a couple reasons for using 2 nuts. One is to increase load capacity (unlikely on a mill), and two is to reduce backlash. In this arrangement there will be a spacer or stack of springs between the two ballnuts to remove the backlash. There are other ways to create anti-backlash nuts, the double nut method is generally considered one of the better methods. Unless there is something wrong with the screws or nuts, I would leave them in place (say a small "thank you" that you lucked out with such excellent setup) and proceed with the rest of the conversion.

Ballscrews and nuts are a big part of the expense of a conversion; if yours are in good shape it will save you about $3k USD (not sure what parts go for in your neck of the woods, but that's about what a "kit" costs here including a new yoke).
 
ok settled going with two nut's . Any idea as to what the pre-load would be and if the z axes(quill ) would be the same?. thanks again
 
Is there a spring between the nuts, or is there a rigid spacer? If there are springs, there is usually an adjustment nut as well. Pre-load is usually adjusted to about 10-20pct of the rated screw capacity. Probably around 100kg. If it is a rigid spacer, then you won't be able to adjust the pre-load (it was set at the factory). Depending on the design, you may be able to shim slightly to take up wear.
 
Thanks DMS

there are springs, or rather, there were springs between the nuts. So I will be sourcing some press tool die springs or I will have them specially made. The interesting thing is that these nuts were only attached via two 1/4" bsw cap screws into the yoke. I think Bridgeport used a custom ball-nut as the clearance in the yoke will only accept a 25mm ball-nut. Not the original 32mm. I have just read DMS's conversion and it seems 25mm ball screws are good, so I will be going with that. The pre-loaded nut was attached with two grub screws that are 2" inches long with lock-nuts to secure them. I doubt if this is original so a more rigid set up will have to worked out. Also the y axes is a left hand thread. Is this a requirement, or is more for ease of use in a conventional form?

thanks again for your input
 
You don't need a left hand threaded screw. Every controller I have seen will allow you to reverse directions in software, so thread orientation is not an issue.
 
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