Ball Screws-????

[RUJOCO]

O.K, I can't stand it anymore and I just need to know!
( I can already hear the laughing)

Can someone tell me about ball screws and their application? What little info I've seen still isn't clear to me.

I've had a mill for about 2 yrs (rf-31 clone), done only small stuff and pretty much been learning as I go. Mostly things related to my knifemaking. (guards & jigs)
Please chime in when you can stop laughing. Be kind, I'm a new guy. Thanks.

[Swarf&Sparks]

Easiest way to explain it, think of car steering-box.
Recirculating ball, adjustable for minimal play (backlash).
Ball screws on machines simply reverse that scenario.
Low friction, minimal (zero) backlash.

[RUJOCO]

Thanks, thats a start.
But where exactly are they positioned on the mill? Are they an accessory for the table? If so, where? A clear picture would help alot.
Sorry for the ignorance but I'm trying to learn.

[Swarf&Sparks]

They replace your existing X,Y and Z axis feed screws.
Used for CNC machines/conversions to reduce friction and backlash.
If you are using your machine manually, you will never need ballscrews.

[BobWarfield]

If you are using your machine manually, you will never need ballscrews.

'Tis true, however, ballscrews can improve even a manual machine. When suitably designed so they can't be backdriven (i.e. forces on the table move the ballscrew in an unwanted way), they enable climb milling on the manual machine for a nicer finish and less tool wear.

They are absolutely essential to a CNC machine for two reasons. First, they don't know where they are for the most part. They assume that if the motor turns, the table will move exactly the right amount. Even most servo systems don't measure table position, they only measure that the motor shaft moved the right amount. One definition of backlash is lost motion: the shaft moved some amount and the table didn't. So the CNC loses track of where it is unless we eliminate backlash using ballscrews.

The second reason is that the CNC does things that aren't done on manual machines, and that cannot be done with backlash. On the manual machine, if you want to mill in a circle, you use a rotary table or other fixture to spin the workpiece in a circle. A CNC operates as if the operator knew exactly how to move the handwheels in X, Y, and Z to describe any smooth curve. Note that just cutting in a circle involves several changes of direction for each axis. The old machinist trick of turning the handwheels to remove the backlash won't work!

There is probably more than you'd care to know about this sort of thing on my web site in the CNC Dictionary and associated CNC Cookbook articles:

http://www.thewarfields.com/cnccookbook/CCDictionary.htm

Best,

BW

[RUJOCO]

O.K I think I got it now.
They hold the ways in place so the motors for x-y axis move the table freely....right?
Thanks for your patience.

[Swarf&Sparks]

True-nuff Bob.

Hope I didn't dumb it down too much. Was looking to simplify, given the opening question.

Rgds, Lin

[RUJOCO]

Lin,
You couldn't dumb it down enough for me. This machining stuff has grabbed me by the you know whats recently. I thought I knew everything I needed to know to complete my little projects. I now realise I've been muddling through after reading a good portion of the threads the last couple nights.
And I thought I knew it ALL....SHEESH!!

[SGW]

This is a link to a ball screw manufacturer, which might give you some more details:

http://www.rockfordballscrew.com/

[Swarf&Sparks]

Welcome to the asylum Rudy.
The disease is chronic and terminal.
The good news is, you have a great support group here.

[Techtchr]

Essentially a ball screw does the same thing as the lead screws on your mill. It takes rotary motion from the hand wheel or motor and translates it to linear motion. It moves the table's longitudinal X and cross feed Y . In the case of CNC mills you need one on the Z axis as well., or any other extra axis you might have. On a conventional machine tool like a manual mill when you move the table to a certain position with the hand wheels and let go of one of the hand wheels, the forces on the part being machined will not move the hand wheels( unless you are really taking heavy cuts) So if I'm turning the Y hand wheel and my hands are off the X, the X shouldn't turn. On CNC machines this is not the case. Because of the design of ball screws it is possible to put force on the table and move the motor shafts (linear to rotary motion). That is why on a CNC machine when the motors aren't turning, they are holding the shaft (ball screw) so it won't move. While many of us hobbiest have home built CNC machines with acme type lead screws, if you need real precision ball screws are the ticket. You are just going to pay for front row seats so the price will be higher.

You might want to go to McMaster Carr and look at pictures of Ball screws and nuts and compare them to acme. It isn't difficult to see how backlash, lost motion or "slop" is removed in the mechanism when you see a picture.

Matt

[RUJOCO]

I checked out the link that SGW (thanks) posted and finally put it all together. Diagrams help alot. I've got my nose in architectual drawings all day so I need a reference point for the talk.
Thanks to all for not treating me like a dimwit.

[lynnl]

Having never seen the inner works of a ballscrew nut, one question I've always pondered: Are all the balls always in engagement? or is there a reservoir of extra balls that are cycled into and out of engagement as the screw traverses through the nut? Or are there still other options? ...i.e. multiple implementations.

[Mcgyver]

they recycle them, or rather recirculate them. there's a tube going from one end to the other. http://commons.wikimedia.org/wiki/Im...ting-tubes.jpg

Also, the balls are not all the same size, they alternate large small large small etc. this way the large ones, in contact with the screw, are not revolving against each other in opposing directions - don't take the nut off the end of the screw!

edit: this shows it better

http://www.barnesballscrew.com/ball.htm

[QSIMDO]

I was told they were used during the Inquisition.



....I'm merely the messenger.