Re: [CAD_CAM_EDM_DRO] Backlash

Vince Endter wrote:

> I have been trying to chase down a backlash problem I have with my
> Hiwin ballscrews for almost a year. I even sent the ballscrews back
> to Hiwin and they said there is no detectable backlash in them. I
> made a video of the test recommended by Hiwin. In the test I
> attached a ball bearing to the end of the ball screw shaft so I am
> not measuring any out of squareness in the end of the shaft. I am
> using a tenths reading (.0001") indicator on the end of the ballscrew
> and a thousands reading (.001") on the table. I am measuring
> between .0003" and .0004" backlash in the end play of the radial
> contact bearing, and .004" on the ballscrew/table movement. The gibs
> are adjusted to that if I push as hard as I can anywhere on the table
> I can see only .0005" deflection. The mill was recently reground and
> hand scraped by a professional machine tool rebuilder.
>
> There are only two places I can see the backlash coming from. One is
> the yoke, which was supplied by Hiwin (very large cast iron, I can't
> see that flexing), and the other is the ballnut/screw combination.
> The ballscrew has a single nut. Hiwin says they take care of back
> lash in one of two ways, either with a different pitch between the
> nut and screw, or by adjusting the ball size in the nut. They won't
> say which method is used in my installation.
>

I have a fair bit of "lost motion" in my machine, about .001 to
.0015" on every axis. But, none of that is truly "backlash" as
I have good anti-backlash screws/nuts on all axes. What I do
have is "flex" in the whole system. (My X and Y screws are very
high quality ground precision screws, and are accurate to .001"
over the entire length of the axes!) But, they weren't made for
use on a Bridgeport, and are .895" on the major diameter. So,
the screws compress, and probably bow a bit, too. The sxis
drives are mounted out on aluminum plates which allow some
flexing from where they are mounted. The Z also has a good
screw, but the quill is not in good shape, it has a lot of scars
from chips wedging between quill and housing. So, the quill
presents a lot of drag on that axis, causing the link from nut
to quill to flex. I am not shocked by this, and don't plan to
spend a great deal of time trying to get this reduced.

> Here is the video of the test:
> http://www.flyingcritters.com/video/Backlash.wmv
>

All I get is an audio clip with a steady whooshing background.
No video window popped up at all.

Jon