[ironDigit]

would there be any good reason to use a dual ballscrew setup on a metal mill except for the fact that they are available and will most likely rustaway for a long time if i don´t.

the advantages i figured would be : better load dividing resulting in hopefully "less wear and smoother operation cause the leverage effect is neutralized though i could mount the screw in the middle its not perferred.

maybe i could even squeeze out the last microns of backlash if i preload one to pull on the load and the other to push.(i hope that makes sense)

And most of all i got 20mm lead per turn and a 15mm dia. and as my Z might get quite heavy (50kg) i also want to be sure the screws can cope.Hence the lesser wear they will be put to the test in the setup i'm planning for and a screw slipping or binding while idle should be ruled out with 2screws.

but all i want to know for now is if the combo of 20mm lead on 15dia. ballscrew and a 50kg Z-axis is healthy.

By the way when powerfull motors are used i guess one stepper/servo should be enough if a belt and pulley's are used to drive both leadscrews solving the synchronizing problems you get with dualdriving single axis'.

I hope some of the illuminated could chime in on this and we'll burry the dualghost forever.

THX in ADVANCE.

[ironDigit]

would it help to know these are allegedly ground ballscrews by nsk but i haven't been able to confirm though the pictures on the nsk downloadwebsite are simalir to mine i can't make out if the pictured ones have an ?oilgroove? running in the middle/bottom of the screwthread.i guess this could have been a custom option.

i've been true the ballscrew manual that seemed most applicable to me from nsk and the dynamic load rating for hopefully my screws was a couple-o-thou KiloNewton but i guess this does not give an indication of how much the ballscrew backdrives and thats what i'm concirned of with 50kg's+ and making a heavy cut or is it totally up to the servo to keep position and should i worry about wear only.

any1 know how to identify a NSK ground ballscrew except by looks cause it looks ground to me.

comment on reloading the balls in a secondhand ballscrew would also be appreciated since i know mine came from a production environment ,as in when and why would you want to do so ,except when you dropped and lost some balls ofcourse.

cheers guys !

[juzwuz]

Do you have the model number of your NSK ballscrews? I think if the code starts with a W then it is a ground ballscrew and if it starts with an R then it is a rolled ballscrew. I haven't looked at the NSK catalog for a long time so I can't quite remember what the exceptions are.

[ironDigit]

hmm

I can find any kind of indication on neither the nut or screw.
Not a single stamp anywhere.

THX for trying.

[ironDigit]

how come no replies at all ?

[juzwuz]

Can you upload some pictures of the nut and ballscrew? Are you sure that they are from NSK?

There's good info about identifying ballscrews in the following link:

http://www.cnczone.com/forums/showthread.php?t=8813

[Zumba]

Dual ballscrews are for situations where the carriage could bind, e.g., CNC router where the gantry is three pieces: 2 sides + bridge.

If the router had a rigid gantry with four pieces - 2 sides + bridge + lower bridge - only one screw would be necessary. In fact, two screws would be detrimental. You'd add unnecessary expense, unnecessary friction, and unnecessary overall complication.

You always want the largest and/or highest precision screw that'll fit your application design-wise and budget-wise. Two crappy screws or two small screws will be less accurate than one large, accurate screw. Large screws are also easier to machine.

Backlash should be eliminated by a preloaded 2nd nut and properly preloaded thrust bearings. If you still get backlash, somethings wrong, and I guarantee you that the fix is cheaper than buying an entire 2nd ballscrew assembly.

If multiple ballscrews were actually practical, preloaded nuts, multi-circuit nuts, multi-start screws, etc would not exist. All those contraptions were devised to solve motion problems with a single screw.

[ironDigit]

i'm totally with you there Zumba.

But given the situation that the only ballscrews i'll be getting me hands on for a while are 20mm lead per turn i was not sure if they will or will not cope the 50kg Z-axis.
wich is why i was comtemplating a second screw to help bearing the load wich offcourse is a result of the added friction.

I could offcourse just use two nuts and have spare screws for if i wear m out in the next 30years!

are my worries just for a 20mm lead per turn ballscrew not enjoying a 50kg load swinging from it.Cause i can see that the load capacity is mostly lower for higher pitch screws then the lower ptches a the manufacturers sites.

I'll see if i can post some pics later i've only got me crappy webcam right now.

Thx for digging in yall.

[ironDigit]

would sure be nic if some1 could tell if these are NSk and how to recognize ground vs. rolled without any markings on them.
They do have an oilgroove running along the bottom of the threads.

any thoughts on the dual leadscrew vs. dual nut to ease the friction/wear/maintanace a lil on the Z-axis ?

THX.

[digits]

Dual ballscrews are for situations where the carriage could bind, e.g., CNC router where the gantry is three pieces: 2 sides + bridge.

If the router had a rigid gantry with four pieces - 2 sides + bridge + lower bridge - only one screw would be necessary. In fact, two screws would be detrimental. You'd add unnecessary expense, unnecessary friction, and unnecessary overall complication.

You always want the largest and/or highest precision screw that'll fit your application design-wise and budget-wise. Two crappy screws or two small screws will be less accurate than one large, accurate screw. Large screws are also easier to machine.

Backlash should be eliminated by a preloaded 2nd nut and properly preloaded thrust bearings. If you still get backlash, somethings wrong, and I guarantee you that the fix is cheaper than buying an entire 2nd ballscrew assembly.

If multiple ballscrews were actually practical, preloaded nuts, multi-circuit nuts, multi-start screws, etc would not exist. All those contraptions were devised to solve motion problems with a single screw.

I'd agree that twin screws are more expensive and complicated, but I'm not sure that I'd agree that twin screws have no advantages - if you apply a force to one corner of an object, its natural tendency will be to rotate - if you apply two parallel forces to two corners, it will tend to move in the desired direction.

Mori-Seiki seem to make some pretty good use of twin screws in their machines - and they claim to do it for balance : http://www.moriseiki.com/english/pro...dcg/index.html

[Zumba]

digits, if a ballscrew can be installed down the center of the moving object, a single larger screw is better than two smaller screws. Ignoring complexity, the larger screw has a higher critical speed.

I'm not familiar with the Mori-Seiki machines that use twin screws, but I'm sure there's a logical reason for it. A properly engineered carriage will not bind even if the force is applied on one side, unless it was designed that way.

A good example of a machine designed to absolutely require twin screws (or twin R&P) is the typical CNC router with a "3-piece" gantry, as I stated earlier.

http://www.thermwood.com/twood_site/...53/M53_516.jpg

[Zach_G]

Those are precision ground. Note the shinyness and very smooth, backlash free motion when you rotate the nut on the screw. If they were rolled, you'd see a small groove between ball grooves as a result of the metal being pushed out of the ball grooves. You'd also be able to feel a slight amount of wiggle.

For an example of when two screws are in good use see: http://www.cnczone.com/forums/showthread.php?t=34954.

[ironDigit]

THX guys i was starting to believe my question must've been TOO stupid to answer.

i see some very nice arguments in there some new and some i had already considered thx for that.
The last thing i would love to have a opinion about is if it would be wise to use 2 of these 20mm lead per turn to move the 50kg Z-axis or should one be able to take the load with no more then normal wear and tear.

i'm sure it helps to know that i'll be using lineair guides so the guides will supply for very little friction making the z-axis completely dependent of the ballscrew to support its weight.

Digits sure supplied for some class A reading material in that mori seiki link. Some very good explanations in there accompanied by some very nice porn uhh.. i meant pictures.

THX alot guys.

[digits]

digits, if a ballscrew can be installed down the center of the moving object, a single larger screw is better than two smaller screws. Ignoring complexity, the larger screw has a higher critical speed.

I'm not familiar with the Mori-Seiki machines that use twin screws, but I'm sure there's a logical reason for it. A properly engineered carriage will not bind even if the force is applied on one side, unless it was designed that way.

A good example of a machine designed to absolutely require twin screws (or twin R&P) is the typical CNC router with a "3-piece" gantry, as I stated earlier.

http://www.thermwood.com/twood_site/...53/M53_516.jpg

Zumba, I'd agree if you said putting a single screw through the centre of gravity of the moving object - when you are pushing something like an X-axis with a heavy head somewhere unknown along it, the COG may be nowhere near the centre of the axis. With one screw on either end, the off-centre mass doesn't tend to cause the axis to want to rotate.

Yes, ofcourse, you can build your carriage stiffly enough to constrain out the tendency to rotate - but do you want to waste strength and stiffness fighting unbalanced forces that you could simply balance by pushing on each end? Would you really shunt a railway carriage on one corner?

As for your one bigger screw being better, you'd know more about that than me, but for a CNC machine, wouldn't the reduced rotational inertia of two smaller screws count for anything?

Cheers.

[digits]

For an example of when two screws are in good use see: http://www.cnczone.com/forums/showthread.php?t=34954.

I am flattered that you linked to my build, but if anything it's probably a good lesson in just how tricky twin screws can be to get working! My design racks because the twin Z-axis columns aren't stiff enough to withstand the forces generated while accelerating the 60+kg x-axis from 0-500IPM in about 0.1s. The new design I'm working on will still use twin screws, infact it might use 6 for 3 axes, as I really do believe they offer mechanical advantages if you can eliminate all racking and get it to home perfectly.

Cheers.

[digits]

Digits sure supplied for some class A reading material in that mori seiki link. Some very good explanations in there accompanied by some very nice porn uhh.. i meant pictures.

There is even more porn in the new mill/turn machine on the front page of their website!

Currently, I'd say they have a lot to answer for at the moment - they inspired me to use twin screws, but there is a lot of fiddling required to get all working.

Cheers, and good luck!