Re: [CAD_CAM_EDM_DRO] Bent Nook ballscrews ?????

Kim Lux wrote:

>Hi people.
>
>We've purchased a couple Nook ballscrews, one directly from Nook and the
>rest from McMasterCarr. All the ballscrews were slightly bent. An
>email friend of mine purchased one from McMaster Carr and had the same
>thing.
>
>On Friday I increased the tension on our Z axis zero backlash ball nut
>mounts. It worked OK for the machining we were doing then. However
>today we are machining a different, longer part and we found that the
>ballnuts are jamming on the screw, probably due to the bend. (There is
>no foreign matter on the screw, etc.) Backing the tension off the
>preload cured the problem.
>
>I'm not sure that there was a backlash problem when I tightened the
>preload on Friday. The parts coming off the lathe were spot on for
>tolerance.
>
>

What you need to do is determine whether this is a point bend or a bow.
Use the best straight edge you have, and lay it along the "inside" of the
bend at different points. If the straightedge appears to lay flat along the
screw when you have one end at a particular point, it indicates a kink
at that spot. If you can't find any point where the edge lays flat against
the screw, then it is bowed along its entire length, probably due to
thermal stresses during the hardening process.

Although it can affect accuracy, a kink in the screw can be straightened.
You need to put the kink at the edge of something straight and hard,
like the table or bed of a machine tool, and clamp the screw with wood
blocks to prevent marring the surfaces. Then, apply increasing forces to
the free end in the right direction, until the free end doesn't return to
quite the same height. This is real tricky, and using some kind of force
gauge so you can apply increasing forces accurately would be very helpful.

If the kink is very large, the screw will likely have a slight increase of
pitch at the kink, and this may already be the case, as you note your
preloaded nut binds at that point.

Were these screws shipped in a container that would prevent them from
getting bent? If not, you might have to arrange to pick up your screws
at the McMaster warehouse. Machine tool ballscrews are usually shipped
in wooden crates to prevent this. Cheaper screws are often shipped in
really heavy mailing tubes, where a crumpled tube would at least be
an indication that it could be damaged.

Jon