On 1/26/2018 5:14 PM, Jim Wilkins wrote:
"Bob La Londe" wrote in message
news
On 1/26/2018 10:26 AM, wrote:
On Thu, 25 Jan 2018 18:05:06 -0700, Bob La Londe
wrote:

I am sure somebody can tell me why this is a stupid idea, but I
needed
to get this ball nut off the ball screw without dumping the balls
all
over the floor. I don't know if the screw is salvageable or not.
Its got
a bend in it somewhere. If its just in the machining in the end I
might
be able to turn it down and machine some bushings. If the screw is
bent
in the main span its done. I might be able to straighten that, but
it
wouldn't be very accurate anymore. Normally guys screw the nut off
onto
a piece of cardboard tube, but I have bad luck with that. This
hand made
stub of aluminum "ballscrew" mates up with the end of the
ballscrew I
need to check, and I was able to just screw the nut off onto the
stub. I
had to hand grind a radius tool out of HSS in order to thread the
holder. It took me a couple hours, but it worked out perfectly. Of
course that time is wasted except for the learning experience if I
can't
save the screw.

Anyway, now I can roll the screw on my granite surface plate and
determine where its bent.

http://tacklemaker.info/gallery/1_25_01_18_6_00_57.jpeg
Greetings Bob,
If the screw is indeed bent but the bend is a large radius I think
the
screw might still be OK after being straightened. I used to
straighten
shafts used in piston to turbine engine conversions for small
planes.
The shafts would need to be straight within a couple thou over
about
40 inches. After straightening I had to check the shafts pretty
much
completely along the length of the shaft. And I never saw localized
small radius distortions, they were always over long distances. So
maybe the same thing would hold true for your screw. As I see it a
sharp bend would cause the ball groove(s) in the screw to be
compressed in one spot and expanded in another, opposite spot. A
large
radius bend would cause the same effect but it would be much less.
If
the shaft is then straightened I think the distortions in the ball
groove(s) would be lessened to the point that it wouldn't matter.
Eric


The screw doesn't seem to be significantly bent. I got maybe
0.00075 variance from one end to the other by resting the screw on a
granite surface plate and passing an indicator over the top of the
thread. I checked on multiple lines along the screw. Then to
double check myself I grabbed a piece of .002 thin shim stock and
tried jamming it under the screw all along the screw at 4 different
rotational positions of the screw. I know the indicator is better,
but I can get a feel with the shim stock. A sort of confirmation.

Then I ran the indicator over to find the high point of each turned
step/shoulder. The results were about the same. I double checked
myself by using .001 difference gage block stacks as improvised
go-nogo gages to slide under the turned surfaces. I confirmed it's
within .001 all the way around.

I'm at a loss.

The thing is I could see the screw bind and release before it was
removed, and I could feel it bind and release while I was moving the
machine back and forth by hand. I'm totally at a loss.

The only thing left I think is to go ahead and dump the balls out of
the ball nut and check it out. The thing is it felt perfectly fine
once the screw was out of the machine. Maybe it has alternating
ball sizes and at some point in the past somebody packed the balls
without alternating them creating the same affect as a bent screw.
I think that is a shot in the dark, but I don't think it can hurt
anything to check.

I'm just guessing now.

Did you notice a pattern in where it tightened?




Yes. It was anywhere along the travel. I think it was every turn.
That's what made me think bent screw.