Re: [CAD_CAM_EDM_DRO] Re: rolled ball screws

> Hi Les:
> I found your comments on this subject very interesting.
> I just dealt with a Hiwin dealer, regarding preloaded ballnuts in rolled
> screws, and was told...NO WAY, CAN'T BE DONE!!!
> The reason given, was that the inconsistencies in the rolled screw would
> make the screw run so rough that most of the benefit of ballscrews would

be

> lost.

Hello Marcus

Hmmm well HIWIN does it (they say)and I have used them for several years.
I was told by other manufacturers that it could not be done, at least by
them.

At the time I was specifying ball screws for a large corporation so we
had application engineers come in from all the major manufacturers.
Just about all recommended ground ballscrews for any cnc use.
But I wanted to see a stiff preloaded rolled screw. I could software
comp inaccuracies and I just didn't see any major technical problems
with someone being able to make them. Neither did any of our staff.
And I didn't buy the ridiculous price of ground screws. We did a lot
of that kind of stuff (large scale thread grinding) and it is not that hard.

Then HIWIN told me about their "precision rolled" product. They were
close by so they invited me to tour their facilities. That is where I saw
them fitting oversize balls.

I was very concerned in that I could not find a preload drag torque
specification for that product line in their literature. They said they
didn't have one published. I thought "uh oh". Not good. But then
we just took one out and I hand rotated the nut and it felt super
smooth. Later I hooked one to a dc motor and monitored current
while driving the nut back and forth. The current (and therefore torque)
varied no more than a few percent!

So I use them. I think HIWIN has great products, but their sales
and engineering staff here are kind kept in the dark. They don't
have the literature and knowledge of their product lines. Often
they don't even know what they have stocked other than it's there
and the part #. So I am not surprised that a rep is confused.

As far as whether this preload can be done _reliably_....
I think it is just a matter of having the right effective spring
constant in the nut housing so it can expand without imposing
huge loads on the balls. Four point contact ball bearings are
commonly sold by many manufacturers and used a lot.
It is also a matter of keeping lead pitch consistent.
The HIWIN screws I have measured are remarkably good
as far as pitch accuracy.

Here is one possibility- perhaps they have been mistakenly selling me
GROUND screws all this time! Maybe I ought to run up there and
get a truck load... ;^)

Les
Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html