On 1/26/2018 6:20 PM, Jim Wilkins wrote:
"Bob La Londe" wrote in message
news

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.

I asked because I think a problem with a ball or bearing might not
repeat at the same frequency as a bent screw.
-jsw



Oh the bearings were definitely bad. I think they were what allowed it
to work at all before. I'm putting it back together, but I drilled out
the bolt holes and pulled the alignment pins for the front cover that
holds the thrust bearings for a little slop. I ran the saddle forward
and snugged down the cover mounting bolts. (Kind of like the machine to
fit approach you have to do when fixing a Chinese mill.) So far its
smooth, but the ball nut mounting bolts are only t-handle tight. I have
not gotten in there with a long handle L key to get them good and tight.
Maybe tomorrow. I'm done for today. I can't turn the screw by hand,
but almost. If I could get inside and get a good grip with both hands
maybe I could. The table and saddle are heavy and the the balls are
still nice and tight in the ball nut. It spun all the way up and down
the screw, but I only get about 1 to 1-1/2 turns on the spin test. Like
I said. Nice and tight. I have the end of the shaft wrapped with
leather and gripped with locking pliers. I can turn the screw with two
fingers, and its not pulsing or binding so far. The cover did snug down
shifted over very slightly. There had to have been a little continuous
bind on the screw before.

It wasn't a super productive day, but I did get a mold finished and
shipped, and I got the CAM done for three more steel embossing dies.
I'm not counting anything on the Hurco as an accomplishment until I'm
sure I don't have to tear it down again. If I can get it going tomorrow
I might reward myself with a day out on the river trying to drown some
rubber worms.

Well, I think its about beer:30. Maybe even beer:45.