# two ballscrew-nuts or one



## quickcut (May 23, 2014)

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


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## dave2176 (May 23, 2014)

2 nuts can decrease the backlash because you adjust them against each other.  Stay with 2.

Dave


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## quickcut (May 23, 2014)

Thanks Dave will do


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## DMS (May 23, 2014)

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).


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## quickcut (May 26, 2014)

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


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## DMS (May 26, 2014)

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.


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## quickcut (May 27, 2014)

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


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## DMS (May 27, 2014)

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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