screws/backlash elimination

[sixpence]

Hello all

I have been lurking for a while now. This site is the most informative of all that I've found. I am not as technically literate as most of you are so your help with my fumbling questions would be appreciated.
1. What is the difference between a ball screw and a lead screw?
2. I think the ball screw from SKF is what I am looking for:diam 8mm :right hand lead 2.5mm:total length 936mm for the x axis

model designation from catalogue SH 8x2.5R

there does not seem to be any backlash elimination involved with the nut. 3. How do I eliminate backlash with this particular screw/nut assembly.
Am I heading in the right direction here .
In South Africa help seems to be on the sparce side.

[HuFlungDung]

Hi Sixpence,

A ball screw and nut assembly has a circuit which is filled with steel balls. As the screw rotates, the balls roll in the "threads" of the screw and nut. This provides the low friction of the unit. Because the balls would just fall out eventually, the ball nut has a return tube ciruit that scrapes the balls out of the groove in the screw and returns them back to the beginning of the ball circuit at the front end of the ball nut. The thrust action of the screw is non-wearing because the balls roll, rather than slide.

A lead screw is a simple bolt and nut, with conventional V threads or Acme threads. The screw threads slide past one another, with considerable friction and subsequent wear. They also require quite a bit of free play to ensure that lubricant can enter, or else there is a danger of the threads seizing up.

You cannot easily adjust the backlash in a low precision ballscrew, if it has a single nut construction. If it has double nut construction, then the two nuts can be shimmed apart and then locked in place, to reduce the backlash.

[Al_The_Man]

Sixpence, Also all ball screw are not created equal, a lot now are rolled rather than precision ground, you can usually tell a rolled ball screw by the groove on top of the threads, rolled ball screws typically have .004" error/ft where precision ground have .0004" to .0001" error/ft or better. if you are looking at a CNC application get a loaded ball screw in either case as this ensures minimum backlash.
Al

[Al_The_Man]

Hi, I should have said PRE-loaded ball screw, sorry.
Al

[chuckknigh]

OK, a suggestion. Ballscrews are expensive, and this is your first machine.

Yes, I said *first* machine. It's human nature to redesign stuff during and after the build...you WILL want to build a second one.

The best suggestion I can give you is to keep reading and learning...but in the meantime just get something moving under computer control. Get an old 486, run TurboCNC on it, and build a machine on which to learn. It'll end up being cheaper, in the long run, to build two...a cheap one for learning, and the one you actually want, than it would be to build your final machine, first. It's counterintuitive, but it's true.

Now...lead screws. It's simple. You fix the position of the screw, and then allow it to rotate. If it's not allowed to rotate, the nut will travel along the screw thread. This is the part you hook to the moving part of your machine...the gantry, or the moving table, depending on your design.

ANY screw thread will work for a lead screw...there are a range of options. In the US, there is an inexpensive threaded rod called "allthread" which is used by a large number of first time builders. I's not overly accurate, but is inexpensive, and allows you to get a table up and running using locally available parts.

The next step up in quality is ACME thread. It's the "squarish" thread type found on vises and clamps in workshops. When it's made well (precision ground) it's actually quite good, and is more efficient in transferring power than the triangular thread on allthread. Practically, this means that there are lower losses in the system...smaller motor is needed.

The highest quality thread is a ballscrew. Basically, it's a "helical ball bearing" that has such low drag that the screw will "fall" down the thread, without YOU turning it.

Anti-backlash nuts are needed on all of these theads... First, backlash is the "wiggle" present between the threaded rod and the nut. It can be seen in moving tables, when you turn the screw as much as half a turn, before the table moves. Seriously...go in your garage, go to your vise, and take a look. Now that you know to look for it, you'll be amazed!

How you take care of it is simple. You use *2* nuts, spaced so that they push outwards against both sides of the threads. A common way to do this is to put a spring between them, and force the nuts apart. Other ways include a fixed "spacer" that goes between the two halves of the nut, and some screws that push the two halves apart. There are lots of ways to minimize/eliminate backlash...and doing it doesn't require ballscrews.

Go to your hardware store, pick up a few nuts and bolts, and experiment. Try some of the designs you've seen on this forum...some of them are quite simple, and very effective.

You may decide that you don't need that $100US ballscrew assembly for your 12" X-axis.

-- Chuck Knight

[Hobbiest]

Check out this link http://www.homecnc.info/ballnuts.htm you still have to buy the ball nuts, but you can eliminate the backlash and pay much less.
I agree with chuck though, use something inexpensive first.

Hey, garage door openers that use a screw, have 5/8-10 acme in them, and can usually be had for free at overhead door stores.

[Cold Fusion]

http://cgi.ebay.com/ws/eBayISAPI.dll...category=55826

Would these be a good set to buy?

[twombo]

936 mm for the X axis? Leads me to believe that you are looking at a medium size router, say, 2 foot by 3 foot or so.

That looks pretty long for an 8mm dia screw. Might be prone to "whipping" at higher rates off feed.

I would think that they might be ok units on a little machine like the Taig or Sherline metal working machines.

I am biased to some degree but I like and am using, these:

http://www.danaherlinear.com/section...IT=Descrip&GI=

Simple, rugged, and resonably resistant to dirty, dusty environments, yet not real pricey (pricey is, of course , relative).

[Cold Fusion]

I just got off the phone with the guy who is selling these. I wouldn't be buying that particular model. I would be buying the 10mm ones that are 39" long. Yeah, 3'x2' is correct. I would like to use feeds around 30-40ipm.

[sixpence]

Could I use the 8mm diam screws for the y axis = 640mm and the z axis and the 10 mm for the x axis. do they have to be matching leads?

[Cold Fusion]

You can use different leads. Just be sure to configure it in your cnc controller software.

[sixpence]

Thank you all so much for taking the time to explain to me in english and not the new language I have discovered"TECHNEES"
You have given me a lot of info and 'leads' .
Thanks for now.