Leadscrew design questions for mill retrofit

"Pete Bergstrom" wrote in news:41115b05$0$65609$ snipped-for-privacy@newsreader.visi.com:

My suggestion: Use INA combination thrust/radial bearings (both bearings in one package), and pre-load the screw. I would probably exchange the alum endcaps for steel of the same design, the set-up will be more rigid, the alum will deform under the bearings over time or heavy load, leading to increased backlash. How about a split-nut set-up for backlash compensation? You can either have two half-nuts with an adjuster, or to full nuts with one drilled for set- screws to press against the first to push them apart. You could do this with one nut, provided the length it comes in agrees to that method.

Pete, As Anthony said, replacing the aluminum pieces with steel would be best. Bronze nuts for the steel leadscrew will work best. You should be able to buy split nuts. These have a screw that pulls the two halves of the nut together to minimize backlash. Also available, but harder to adjust in the location you must put the nut in, are nuts which screw together. The thread that holds them together is a much finer pitch that the 5 TPI for the leadscrew. Screwing them in or out changes the backlash. If you are really going to put CNC on this mill later consider doing it now. The CNC should use ballscrews. They have much less friction. This lack of friction makes them unsuitable for a manual machine because they self feed easier. So when climb milling the table lock must be set to drag enough to prevent the cutter from dragging the work into it and breaking things. As far as shipping damage goes, if it arrives damaged send it back. And why should the choisest part of a new screw be in the middle? Are you planning to measure the lead error? If so, I guess you might find a three foot area in the six foot length that is best. BTW, I've bought acme screws several times in the last 25 years and they have always been much better than the maximum error allowed. But I've always bought USA made. I have seen leadscrews from Taiwan that were terrible. I made a little stepper driven X-Y stage to put on the Bridgeport for milling small complex curved parts and the screws it came with were visibly out of tolerance. The thread looked like the die was wobbling while cutting the threads. ERS

The Bridgeport design uses a pair of precision angular contact bearings on one end and (to maintain alignment) an inexpensive radial bearing on the other.

If you're really going to undertake this kind of custom work, you might as well do it with ballscrews. The parts cost will not be that much more, and the effort about the same (even simpler in some respects), and the results will be worth it.

Lotsa surplus ballscrews on eBay.

What hardinge does in this case is to fix the handle/dial end of the leadscrew with an effective thrust bearing, at the dial end. The far end of the screw is simply supported in a radial bearing, but the clearances are such that the turned down end of the screw is allowed to float axially inside the bearing.

You can approximate a thrust bearing by loading two radial bearings up against each other.

There are *two* kinds of backlash you need to consider. The first is the sort that happens when you push/pull on the handle itself, and the imperfect thrust bearings allow the entire leadscrew to shift axially with respect to the slide. If you are using radials loaded against each other, or true thrust bearings, this is trivial to get down to around a thousanth or less.

As others have suggested the bronze nut you choose for the leadscrew will have some natural lash which you can reduce by making it a split nut of some sort. I myself happen to be partial to hardinge's two-part nuts that thread together. I would do it that way.

Others have mentioned using ball screws. If you are planning on running this as a manual machine, do NOT use ballscrews. The lack of friction in those means the table will walk away from whatever cut you are taking, or hog in if you are climb milling. Unless you put a lock on the table and on the shaft, it will not work well.

Jim

If you *do* put ballscrews into a manual mill, be sure to also provide a brake for the handwheel so the work can't drive the ballscrew. (It normally can, if you don't have either a stepper or a servo motor loading the handwheel to prevent rotation. A climb milling cut on an axis with ballscrews can get *very* exciting *very* suddenly. :-)

Good Luck, DoN.

Excellent idea and it seems very simple to do.

I'm primarily attracted to thrust bearings because of space issues, but simplicity is important too. I don't know a lot about bearings, but my understanding is that a good thrust bearing will have some control over radial movement while primarily working to control axial movement. It seems like that's exactly what I need.

I'm not sure what you mean by imperfect versus true thrust bearings. Can you say more? I meant the bronze nut and flange (like MSC has on page 3890 online) when I said thrust nut, since that is what I'd attach to the knee casting. No bearing involved there. The ends of the Acme rod would be supported by a radial bearing at one table end as you described above and thrust bearings at the other end.

Are there pictures or drawings of this arrangement available somewhere? For example, if I buy CDs from lathes.co.uk, would it be on there? A manual from Hardinge? I'll probably jump in and get it working with one bronze nut and simple end bearings first, and then refine it with some of the suggestions I've been given.

Oh, and I really do like climb milling results, based on the couple of projects I've milled to date.

As far as I can see, I don't think I have a lock on the X axis for the table (there is a lock handle that might work for either X or Y, but it looks to have been misused since it swings freely and I can't tell which axis it applies to), so I'll eventually have to put one on. Using ball screws would move this up in urgency, but I'm hoping that putting a lock on the shaft will get things going for this initial step.

Thanks!! Pete

I think I've found these now in MSC's catalog. They look like they'll do very nicely!

Thanks!! Pete

Ah, that kind of nut sounds like a really good way to go. Do you have any leads on where to find these? I haven't seen them in MSC or McMaster.

That's kind of a way out, and I'd rather have a working mill than an ongoing project right now. Thanks for the pointers.

Nope, just have come to expect damage in handling, even with something labeled precision. Your points are good.

I've been trying to keep to American-made materials in projects; with this one it's a no-brainer.

Thanks!! Pete