[CNC] Tell Me About Ball Screws

[spumco]

Yea, what he said. Don't reinvent the wheel. Copy someone else's research.

And don't spend huge money on high-dollar ground ball screws. Get some rolled ones and pre-loaded nuts - you'll never notice the difference in positional accuracy between ground ($$$) and rolled ($) with the mill you're converting. Most controllers also have ball screw mapping features which let you calibrate the controller so it adjusts the steps per inch depending on where the axis is in the travel. All you're really looking for here is zero backlash, low friction, and decent accuracy.

On the other hand, check Fleabay for used ground screws; you might score a useful set fairly cheap. Just remember that ball screws are hard as rocks - you can probably turn/machine the ends, but cutting one in half and putting a new end on it takes some serious machinery.

 

[TomS]

I've attached my original ball screw and ball nut specification for your reference. I sent this to Linear Motion Bearing during my build for quoting purposes reference. I'm not sure if you are direct driving your ball screws or using timing belts and pulleys. My ball screws are direct drive so please take that into consideration when determining final length. I also specified single ball nuts but since have installed Linear Motion Bearing double ball nuts. Another consideration when calculating ball screw length is the thickness of your shaft support/motor mount material.

Hope this helps.

 

[cjones6108]

Have you read this thread? It looks like it has some of the answers you are looking for.

http://www.hobby-machinist.com/threads/pm-932-cnc-conversion-build.52659/

I had not seen this yet -- man, this is primo!

 

[cjones6108]

Yea, what he said. Don't reinvent the wheel. Copy someone else's research.

And don't spend huge money on high-dollar ground ball screws. Get some rolled ones and pre-loaded nuts - you'll never notice the difference in positional accuracy between ground ($$$) and rolled ($) with the mill you're converting. Most controllers also have ball screw mapping features which let you calibrate the controller so it adjusts the steps per inch depending on where the axis is in the travel. All you're really looking for here is zero backlash, low friction, and decent accuracy.

On the other hand, check Fleabay for used ground screws; you might score a useful set fairly cheap. Just remember that ball screws are hard as rocks - you can probably turn/machine the ends, but cutting one in half and putting a new end on it takes some serious machinery.

Very true, I have no intention of doing much in the way of inventing here, as this path should be pretty well beaten already. I appreciate all the accumulated knowledge.

 

[cjones6108]

I've attached my original ball screw and ball nut specification for your reference. I sent this to Linear Motion Bearing during my build for quoting purposes reference. I'm not sure if you are direct driving your ball screws or using timing belts and pulleys. My ball screws are direct drive so please take that into consideration when determining final length. I also specified single ball nuts but since have installed Linear Motion Bearing double ball nuts. Another consideration when calculating ball screw length is the thickness of your shaft support/motor mount material.

Hope this helps.

Thanks for this. Other thoughts come to mind on this, so I think I will post them in a new thread.

(an hour later) So, after some additional reading I'm thinking to go ahead with direct drive for this project, although there can definitely be advantages to going with the timing belt/pulley option.

caj

 

[cs900]

On the other hand, check Fleabay for used ground screws; you might score a useful set fairly cheap. Just remember that ball screws are hard as rocks - you can probably turn/machine the ends, but cutting one in half and putting a new end on it takes some serious machinery.

Very good point. I got some really nice thomson (made in the USA!) ground ballscrews for dirt cheap off ebay. I couldn't be happier with them. After mapping my ballscrews I'm holding +/-.0005 pretty consistently with steppers.

And also on this topic, machining new ends it's as terrible as most people make it out to be. The screws are only case hardened which means only the outer most layer is super hard. I've found it you take a hand torch to the ends to be machined you can anneal the case very quickly and it cuts much easier after that.

 

[spumco]

Very good point. I got some really nice thomson (made in the USA!) ground ballscrews for dirt cheap off ebay. I couldn't be happier with them. After mapping my ballscrews I'm holding +/-.0005 pretty consistently with steppers.

And also on this topic, machining new ends it's as terrible as most people make it out to be. The screws are only case hardened which means only the outer most layer is super hard. I've found it you take a hand torch to the ends to be machined you can anneal the case very quickly and it cuts much easier after that.

Very, very good to know. Opens up some otherwise unusable ebay finds. MAPP gas work ok, or oxy-fuel?

 

[cs900]

mapp works great.

 

[jbolt]

On my PM-932 conversion I went with pulleys on the x,y axis to keep the machine envelope as small as possible due to space limitations. If I had more shop space I probably would have gone with direct drive for simplicity. That being said I like the compact format and am glad I went the way I did.

I have a 4200 stepper on the z and it works great. With the large stepper the z does not move when off like a servo will.

Sent from my SM-G955U using Tapatalk

 

[cjones6108]

Very good point. I got some really nice thomson (made in the USA!) ground ballscrews for dirt cheap off ebay. I couldn't be happier with them. After mapping my ballscrews I'm holding +/-.0005 pretty consistently with steppers.

And also on this topic, machining new ends it's as terrible as most people make it out to be. The screws are only case hardened which means only the outer most layer is super hard. I've found it you take a hand torch to the ends to be machined you can anneal the case very quickly and it cuts much easier after that.

After you anneal and machine it, do you then need to re-harden it? I recently made a small custom carving gouge (for wood) where I had to soften, shape and then re-harden. For the re-hardening, I just heated it again and quenched it in a little hydraulic oil. Does that sound about right or is there a better method?

caj