dual ballscrew

[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]

Originally Posted by 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

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]

Originally Posted by Zach_G 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]

Originally Posted by ironDigit 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!