Ballscrews101 was Re: Ball screw adjustment? and Rolled Ballscrews

Hello,

An interesting coupla' threads, which also have input from the "drag on steppers " thread...

First, Yes there are some on the list who have made their own anti-backlash nuts for ballscrews. Most are a duplication of the "two-nut" method, and will use belleville washers or a spring to provide the "anti". This is the "non-stiff" way to do anti-backlash when using ballscrews which have pitch variation(s).

When using ground (more accurate, MUCH more $$$) ballscrews and ballnuts, you may be able to use a SOLID spacer between the nuts. It is usually made so that it can expand in length(threaded or shims) for fitting purposes. I would not suggest its' usage with rolled ballscrews, as the pitch variation inherent in these lower grade screws will make for uneven drag on the system.

Techno-isel sells (used to sell?) ballscrews and anti-backlash ballnuts pretty reasonably, but limited in the sizes/lengths offered as what they were doing was selling the "parts" of their manufactured cnc table line. The pertinent part for us is that their ballnut design used "constriction" to achieve anti-backlash, and therefore eliminated the second nut (in favor of a special nut casting and some additional machining). This technique is often seen in acme form threads and can be ordered from most screw suppliers in several variations; techno is the only place I've seen it used with ballscrews.

Anyway, this single nut "constriction" technique can also be accomplished by using "oversize" balls in a standard nut. I first saw this suggested by Dan Mauch and have successfully used the technique several times. That thread is in our CCED list archives online; I'd suggest a search of "ballscrews" and/or "ballnuts" for a wealth of info on this subject. In that thread, it was mentioned that the balls used in a ballnut may not all be the same size, to aid in the "rolling" (as opposed to "skidding") of the balls (which is also why the form of the threads is NOT semi-circular, but often called a Gothic Arch form, with variations/improvements claimed by mfgs.

Ball bearings .001 over-size have worked for me, and I start by replacing 1/2 the balls in a big/small pattern. Going to ALL over-size if the B/L is not sufficiently removed. Be sure you are sitting in the middle of a clean sheet on a carpeted floor for best results... The job of inserting/replacing the balls is made easier if you use a lathe to turn "centering collars" which hold the ballscrew centered within the ballnut opening, but can be done without such aid.

Now to the screws themselves:

I have written before that it pays to remember that MANY uses of ballscrews DO NOT require great ACCURACY from the screw; they are used in these cases for their mechanical EFFICIENCY. Since this is true, it pays to let the supplier know that you intend to use the part(s) for a POSITIONING system which RELIES on the screw accuracy. It is true that some software (EMC) has compensation tables for thread inaccuracies, but many of the packages used here by list members will not...

It seems that every once in a while I post some variation of "it all has to work together and fit YOUR needs" type of comments. But it has been awhile so here goes...

"The SYSTEM of parts that you select must work well TOGETHER and fit YOUR needs. It is a good idea to REALISTICALLY assess those NEEDS. OFTEN, in the absense of KNOWLEDGE, guesses are made which OVER-STATE the NEEDED reality."

So, having said that, let's look at the remaining question(s) of rolled vs. ground; and belted drive vs. direct.

Rolled screw from MOTION sources (like BS&A, in San Jose, or Rockford) will often have an .003/ft NON-CUMULATIVE stated tolerance. I was surprised by Dougs post in which he stated that he had found periodic error in rolled screws; (but it wouldn't be a surprise if these screws were not intended for LEADSCREW usage. So I'd like to know where thay came from?) this periodic error may also have been in some increment smaller than what would TYPICALLY be needed for many on this list, who I believe are working for +/-.001" type usage. I do not try to get "tenths"(.0001) from cheap parts. I have found no problems in periodic error in rolled ballscrews when used in an ".001 grade" machine. HOWEVER,...
I am NOT using McMaster-Carr screws as I believe their ballscrew product is in the above mentioned "efficiency", rather than "accuracy" category (I have used their ballnuts successfully).

Side note: The ballnuts they have sold me are from Rockford, maybe the screwstock is also? If so, amybe Rockford can specify what M-C is selling?

Next, I do not just use the "calculated" steps per unit to set up the software. The rolling process is generally pretty accurate as to CONSISTENCY of the error (repeatability is high) so the steps per unit may need to be "adjusted" to match the REALITY of the screw/nut combo you are using.

Belt drive vs. direct:
A well designed belt drive will beat a poorly executed direct drive. And vise versa. They BOTH work well, when DONE well...

As before, it really depends on WHY one is chosen over the other, and WHAT "grade of accuracy" is intended for the finished product. Taken to the highest levels of needed accuracy, a direct drive will be EASIER to make than a belt drive. At lower levels of accuracy performance, the belt drive often gets the nod. Which is why you see direct drive on high end/hi-speed tools; and belt drive on "jobbers" mills. But DO remember that most currently mfg. cnc metalworking machines are NOT relying on the screw(or at least not the screw ALONE) for their accuracy! So it can be a mistake to try to "learn" from the "big boys", UNLESS you know enough to do/know what they are doing ALL THE WAY through the drive system...
OFTEN our "homebrew" jobs DO rely on the screw ALONE for accuracy of the final part...

Which brings me to a concluding thought. False economy.

Choose carefully where to put the "holes" in your system. Accept that there WILL be "holes" in your system; at least as long as you are limited in funds or skills. Keep in mind the comments above re: looking at the WHOLE of the SYSTEM, and don't get TOO close to any ONE part. You will find that cnc is a "balancing" act of engineering; part of its charm, IMO.

I choose to use an "appropriate" grade of screw (which means rolled for our typical .001 machines), and I try to save on the NUTS; by either machining an adapter to use two cheap nuts as anti-backlash or by loading "over-size" balls into a single nut.

Hope this helps.

Ballendo


P.S. I do agree with the poster who suggested that plastic nuts for mill/drills and milling machines is inappropriate (even tho I LOVE those Kerk and BS&A Anti-B/L nuts for lighter duty).

P.P.S. It pays to remember that much of what is written by mfrs. about their steppers/screws/drives is often aimed at the "high dollar" users... There is a HUGE difference between our "typical" machining application and a high speed-ed, granite slab-ed, airbearing-ed, ground ballscrew-ed silicon wafer fabbing process-ed machine!

I mention this for those wondering about "ringing"/damping of steppers. In MOST of the uses considered on this list, our machines have enough "damping" inherent. A sherline leadscrew (poorly adjusted) is one case where damping (or a better/Gecko drive) may help. And yes, a belt drive to the screw WILL provide such damping in mid-size machines, at the risk of introducing periodic errors. (Doug, could this be what you were seeing?) A poorly executed direct drive can also cause periodic errors in a leadscrew system.