rolled ballscrew - wavey! Does anyone else have this?

[DrStein99]

I bought a rolled 5/8 ballscrew, 5-tpi. After turning down an end, I tested spinning it to notice - around 3/4 the way it was bent; just a bit. Enough to shake the shaft @ 750 rpm.

I didnt know if I did the bend, or something happened since I didnt check it when it was shipped to me. No biggie, I was just going to cut that off and use the length I needed.

I ordered two more, and checked them as soon as I opened the box. The 24" length was REALLY wavey, and the 36" length has a .007 wave it in across a 12" section.

I was wondering - since I see it's a common shaft lots of people use - did I get a bad batch, or are ALL the 5/8 rolled ballscrew wavey? I am expecting the shaft to run straight and true, as the website explains "precision shaft converts radial to linear motion". I know i've bought stainless steel all-thread that has less waves in it, then THIS stuff.

I obviously called the vendor and they are accepting an rma; but I didnt speak with a specialist yet - just the phone operator.

[ViperTX]

Nook specifies the straightness of their ACME screw to be 0.010 inches per foot to a max of 0.030 inches over 6 feet for their rolled screws.

[JRoque]

Both my 6' Thomson rolled ballscrews purchased from Reid Tool came with this defect. One of them is much worse than the other. Either the manufacturer straightness spec is false or the supplier is mishandling them.

JR

[NC Cams]

Rolling of threads or ball paths results in the generation of huge residual stresses - simple fact is that you have to yield the metal to get the shape to "take" so the residual compressive stresses come with the territory. The longer the part sits after the rolling, the more the residual stress will try to relax itself. This results in bowing and bending that you're seeing.

There are ways to minimize this via heat treat processing (special stress relieves, pre and post heat treat, special straightening, etc). However, unless you hot straighten a part prior to the part going thru the transformation zone, you can pretty much expect movement to occur.

The specs posted in a prior post would NOT be unreasonable (0.010 per foot). We've seen cams move that much or more from the residual stresses generated during grinding. Our solution is to implact straighten them prior to finish grind and receck/restraighten billet cams prior to installation. Tool steel cams are a whole another story altogether.

In the case of ball screws, you would have similar recourse (impact or press straighten rolled screws) or else buy ground screws in the first place...

[DrStein99]

If I could afford ground ball screws I would buy them.

I understand there is a tollarance on the rolled ballscrews; which on the 36" piece I have, .007 would be - within acceptable margins (according to what I've read here).

The fact still remains (so far); myself and JRoge makes two people that were sold wavey junk; and one friend who says his rolled-threads were very good and had no problems at all; so that's two out of three.

[NC Cams]

WIth all due respect, if you see 0.007 runout and the spec calls for 0.010 max in 12", you are within specs which is technically NOT junk. Unacceptable, more than you want, too much for your needs, yes,

But not junk as far as the 0.010 spec goes.

There ARE non destructive ways to straighten parts - we do it all the time. We've had cams with 0.009-0.012 tir and got them to be under 0.0005 when properly straightened. On and they left here straight and had runout in them by the time they got to the customer. Truly due to the perverse nature of inanimate objects.

However, our straightening process is material, technique and severity of bend/bow dependant. Thus, we'd NOT be inclined to try to explain how to do it verbally - would be sort of like trying to explain how to ride a bike over the phone.

You can explain it until you're blue in the face but spend a couple hours peddling and all of a sudden, it's real easy to do.

I'd be inclined to contact your vendor's tech dept (not the operator or a secretary) and ask an engineer to fax you a copy of their actual specs. If they meet your needs, you'll either have to live with them or find a way to tune up the screws to be straighter.

We too had problems with shafts being straight/bent until we learned how to deal with and fix the problems. I suspect you'll find that some shafts will be fine and some will be like pretzels - that is just going to be the nature of the beast and/or the quality philosophy of your vendor.

[DrStein99]

The first piece was bent at one spot only; like it was BENT in that direction at the point. I didnt measure it out of the box so I didnt know if it got that way before or after it was delivered to me. The fact that the whole thing is straight, and then bent in a direction at one point; made me suspicious.

The 2nd 24" length was visibly wavey in most every way; I didnt even measure it and it really doesnt take a speciailist to see how terrible it was. \

After I looked at that right out of the box, I went over the 3rd piece with a close eye. At first, it looked and appeared consistantly straight. Then I started by moving feeler gauges on it, to find it was wobbly. The .007 variance was on the BEST piece I got 36" in length; out of three pieces.

Since I am returning the first two pieces, to review by a specialist - I was going to send back all three.

---------------------------------

You seriously dont need to worry about your techniques to straighten a rolled rod; I have not the interest or the facilities to do anything like that.

[JRoque]

hmmm, after studying all possible material equations related to the alloy and manufacturing process, I conclude the proper technique for a solution is: whack it on the hump with a rubber hammer. I'll let you know how it turns out after I try it.

JR

[ViperTX]

What I've noticed is that some companies do a bad job of packaging ballscrews....so, I would suspect that is the problem.....the other choice regards how the ballscrews are handled ......

[DrStein99]

The company replaced the ballscrews; I got the delivery today. The new ones are straight as a laser sight.

[LeeWay]

I ordered the rolled ball screws from Mcmaster Carr. No problems at all with them. They always ship that kind of stuff in a tube.