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anti-whipping ballscrew idea

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Black Forest
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Joined: Thu Jan 31, 2013 11:40 am

anti-whipping ballscrew idea

Postby Black Forest » Wed Nov 02, 2016 6:14 pm

Is there any reason why this won't work? On a long ballscrew turning at high rpm's we all know there is a problem with whipping of the screw. A rotating nut system is good but complicated. Why not a piggyback type of arrangement with a Delrin block riding on a linear wagon on the linear profile rail that hits a stop just shy of halfway and therefor stops the screw from whipping as the nut travels to the extreme end of travel. There would be such a setup on each side of the ballnut. In the video I only modeled one support but you get the idea hopefully.

https://www.youtube.com/watch?v=kkcVWIsXGjk

BTA Plasma
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Re: anti-whipping ballscrew idea

Postby BTA Plasma » Thu Nov 03, 2016 10:56 am

Sure if you doubled up on them...BUT...really any vibration in the balls crew will lower the life of your thrust bearing(s) and motor. So if your guide is not holding the screw relatively tight you may get vibration any way. Mostly hard on the bearings. Mazak used to make a huge traveling column machine that used a spring operated V groove stand for the long ball screw. It would jump up when the nut traveled past it. Ball screws have a huge mechanical advantage but a mechanical disadvantage in length. If you are serious about spending thousands on good (non rolled) ball screws you should think about rotating the nut. It is the classic way of dealing with whip. You may need 2 thrust bearings even if your not going to rotate the nut or you will cook your bearings near the motor.

Even a pair of deep groove bearings will eventually give up the ghost on a ball screw,.,,,maybe not for a while on a non contact cutting machine. Is there a reason you want ball screws on your plasma? You could do it on the cheap with rolled threads. Another expense would be making sure they are covered enough for the dust but not enough to allow moisture to stagnate and you will need to lubricate the nut.

Black Forest
Posts: 132
Joined: Thu Jan 31, 2013 11:40 am

Re: anti-whipping ballscrew idea

Postby Black Forest » Sat Nov 05, 2016 3:44 am

Thank you for the reply. I wrote that I would use two, one on each side. I just didn't take the time to model the second delrin unit.
Rotating the ballnut I am familiar with and that was my plan but then I thought of this method. After I posted this on here I did find a commercial unit that uses this method I posted here. They have a YouTube video showing exactly what I had in mind.

https://www.youtube.com/watch?v=zO0uONzO3uI

I know rack and pinion is the usual drive mechanism and the easiest to implement. This is not a business with me. To tell the truth building the machine is my big incentive as much as using the machine. If you look at the next video that comes up you will see a machine I built to shear sheep. I have two bad knees and a bad back. I decided to build a machine to take as much of the work as possible out of shearing my sheep.

It is a disease I have to overbuild and over think things. But it is my time, my money and I like it. My wife not so much!!!!!

BTA Plasma
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Re: anti-whipping ballscrew idea

Postby BTA Plasma » Sat Nov 05, 2016 9:40 am

!! The disease is real!

Really its a move forward. I like your thinking. The things that come to mind in an assembly like that is bench testing for longevity and support bearing life. In that last video they should put that assembly under non vented, non water bed plasma table and let the vaporized metal fall on it to see how many cycles they successfully run before the bearings on the supports wear out. Typically in any assembly the more the components the more chances there are for failure and I see this as no exception. The best way to prevent it would be bellows but..even with bellows we will have a service life.

The one striking feature of that video is how little the whip pushes the ball screw out of alignment and how bad the vibration must be. If you can imagine the outer diameter of a ball screw at 40mm or 1.5748" what size would you introduce your supports? You cant go much bigger or you wont influence vibration. Too close and your fighting the screw as it rubs the support. One could generate a formula taking mass and length and other forces into the equation and use that as a basis for getting close to the real world application. Keep in mind if your using a stepper and your spinning average 800rpm usable power with a 5-1 pitch your travel speed will be 160ipm with a resolution of .0001 (at microstep /2000 or .001 at full step!). If you introduce a increase speed pulley set you would increase resolution and increase your usable speed range but drop you usable power dramatically. For this reason the full step lead screw idea for cnc plasma was discontinued by competitor X. A servo at 3500rpm and a 5-1 pitch your travel speed is now 700ipm and the resolution depends on the encoder on your motor. Some other considerations to think about.

What would be nice is if you could find a 5000rpm servo motor and use that. Now your in the ballpark with a fine encoder. We have a bridge mill that has a 200"+ ball screw with a fast travel pitch. It uses a nut that is in excess of 9"s long. The motor us a huge 40 series Nema AC with a large pulley/belt to the screw. But the screw is almost 4" in diameter.

Black Forest
Posts: 132
Joined: Thu Jan 31, 2013 11:40 am

Re: anti-whipping ballscrew idea

Postby Black Forest » Sat Nov 05, 2016 10:55 am

My thoughts were to have the linear rail and ballscrew enclosed with a slot with opposing brushes that a plate that connects to the gantry side plate. With positive air pressure inside the enclosure to inhibit plasma and water from entering. Between the brushes and positive air pressure it "should" keep most of the contaminants out. I have used this type of build on machines used in a BIG sandblasting environment. It worked a treat.

I for sure am going to use servos.

BTA Plasma
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Re: anti-whipping ballscrew idea

Postby BTA Plasma » Sat Nov 05, 2016 10:59 am

Positive air pressure is a good idea. In our CNC lathes the Japanese engineers at Mazak found that positive air pressure inside the turret will eliminate coolant mist from contaminating the area. I have seen it also act like a venturi for dust and dirt depending on the distance from the air outlet to any outlet in the area you want to keep clean.

BTA Plasma
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Re: anti-whipping ballscrew idea

Postby BTA Plasma » Sat Nov 05, 2016 10:59 am

And where is the like button! ;)

Black Forest
Posts: 132
Joined: Thu Jan 31, 2013 11:40 am

Re: anti-whipping ballscrew idea

Postby Black Forest » Sat Nov 05, 2016 11:13 am

linear enclosure.png


This is what I was thinking to do. Not exactly but it give the idea. On each side of the slot would be a brush that will go bristle to bristle and the sliding part will move between the brushes. As how a linear glass scale does with rubber type.

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