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Wobblybootie
27-12-2009, 08:40 PM
Just wondering if anyone has thought about spinning the Ball screw nut, keeping the Screw itself immobile? You wouldn't get the Screw whipping etc. There has to be a downside but I cannot see it ...

irving2008
27-12-2009, 09:30 PM
Just wondering if anyone has thought about spinning the Ball screw nut, keeping the Screw itself immobile? You wouldn't get the Screw whipping etc. There has to be a downside but I cannot see it ...
Yes its been done before and it works well.. and the downsides are:
1/ the motor driving the ballnut is on the moving assembly so adds to the weight needed to be moved
2/ the ballnut has to be driven via a timing belt or similar arrangement which just makes things more complex...

Gary
28-12-2009, 12:06 AM
While it will work, it actually adds a lot of cost to it.
As mentioned you have a larger mass to move due to mtor motor moving with the nut, you also have to design the gear mechanism with a timing belt and pulleys and mounting for the motor.
When you could just buy a thicker crew with a larger lead, or use rack.

Lee Roberts
28-12-2009, 12:43 AM
3/ Wiring would also need to be considered for a moving assembly, making things a little more complex as well (snagging).

Wobblybootie
28-12-2009, 01:47 AM
OK Thanks ... It was just a thought ... I will keep it in mind though, not sure why but just store it away for future reference.

John S
28-12-2009, 01:06 PM
Modern lasers do this, they call it spindle drive.
Large commercial lasers moved away from ballscrews some while ago and went on to precision rack and pinion because of the whipping over long lengths and by precision I mean $$$$$
Usually two large pancake servo motors driving split loaded pinions and complex electronics to stop them crabbing.

Now they have moved to spindle drive where they have two massive end castings / fabrications where a large , typically 70 to 90mm ball screw is held stationary in tension of about 20 tonnes and the ball but is driven by a 7 hp servo motor.

Because laser cutting is non contact they can reach insane speeds.

It's not a new idea, the old Bridgeport BOSS series mills that appear on Ebay with the big finned stepper motors and squarish table did the same on the X axis. Nut is in bearings on the knee driven by belt from the stepper, it's only an inverse Z axis after all.

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ptjw7uk
28-12-2009, 02:27 PM
I would think you could get away with smaller ballscrews as they will be in tension and no whip plusthe ball nut will be smaller as well.

Not sure where the weight part comes in as most of the weight will be on the track bearings.

peter

Wobblybootie
28-12-2009, 02:38 PM
I can see the hangups with over complications but as for the extra weight I would have thought in a way that could be a plus point after all weight = mass and mass is good ... right??

John S
28-12-2009, 04:58 PM
Fixed mass is good, heavy bed and rails etc, moving mass is bad, takes force to stop it.

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irving2008
28-12-2009, 05:00 PM
I can see the hangups with over complications but as for the extra weight I would have thought in a way that could be a plus point after all weight = mass and mass is good ... right??
not when you have to accelerate it... inertia plays a part.. however there are 2 sides to the story...

If this was the X-axis the motor has been added to the gantry mass, but since we are not having to accelerate the inertia of a big screw, just the nut the motor can be smaller. Since on bigger machines the inertia of the large screw rather than the gantry is often the overriding factor in deciding motor torque there may be net benefit in terms of a smaller motor requirement.

Wobblybootie
28-12-2009, 10:50 PM
Indeed it was the X axis I was thinking about ... apologies for not making it clear. The machine I was pondering was the Steele 4 X 8 router ... he (Steele) discussed assorted methods of driving the X.

1. Rack and Pinion
2. Dual lead screw, Single Stepper, Long Belt
3. Dual Lead Screw, Twin Stepper, Slaved in Mach
4. Dual Lead Screw, Single Stepper, Dual Belt

I was wondering about a single screw 'underslung' down the centre of the X axis but was worried about the Screw sagging and whipping.

I have digested the points raised here and consider all to be valid and helpful. I will return to my planing and thinking with yet more knowledge.

Thank you all for your input.

irving2008
28-12-2009, 11:03 PM
I am not sure of the one you mean... have you seen the BlackToe 8x4 (http://buildyourcnc.com/blackToe4x8.aspx)design - he uses a chain drive...

Wobblybootie
29-12-2009, 12:26 AM
Oops sorry :redface: I gave bum info it's a 48 X 24 inch machine. http://www.solsylva.com But when I obtained the plans it was called something else .. I was confusing it with the MechMate which I have also been looking at. Too many thoughts at once.

Smiler, If you read this it may clear up the confusion in my PM.

Once again Sorry folks.

ptjw7uk
29-12-2009, 12:02 PM
Did a search last night and skf are selling a rotating ball screw arrangement, not cheap I bet
http://www.skf.com/portal/skf_us/home/news?contentId=290063
I also recall that someone did a mill conversion to the z-axis where they rotated the ball nut, although I am not sure what the advantage would be in that case.

Peter

John S
29-12-2009, 12:12 PM
Very similar to what the lasers are using today but the Bystronics have the nut inside the servo motor rotor making it a very compact arrangement gives it's a large ball screw and a 7HP motor.

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