Re: Ball screws and nuts

Brad Heuver wrote:

> From: "Brad Heuver" <bheuver@...>
>
> This week I'm in the possession of the Cintimatic's ground ballscrew
> assys! They appear to be in excellent condition; 1.25" dia, with 0.200"
> lead. They've got preloaded tapered roller bearings on one end in a
> substantial steel mount. The ball nut is about 3.25" overall length,
> with 2.115" hubs on either end. In the center is a larger ring of about
> 3.050" dia.

Yee Hah! Assuming they are still in good condition, those are VERY
fine and expensive screws!

> This ring was the mounting location for the nuts, and has 8
> concentric holes around it, as well as a small (1/8") lube hole. (Grease
> was used in these, it that standard practice, or is way lube used?)

I don't know. I do know that my NSK ball screws from the Excellon drilling
machine were lubed with grease. They were also covered by Goretite
bellows - which the grease dissolved. The way lube application to
ball nuts in Bridgeports may just be for convenience of the one-shot
lube system. And, way lube may be entirely the wrong lube. Way lube
is for very modest loads (on a PSI scale) whereas ball nuts and other rolling
bearings have extremely high pressure at the contact points. I'm greasing
my screws.

> Anyway, for those of you have done Bridgeport ballscrew conversions,
> is there any chance the 3" diameter of these nuts will package in the
> yoke area of the B'port?

Yes, I think it will work. There is a bit more than 3" between the X and Y
nuts, if I remember. It might be tricky. You might also need to offset one
of the nuts (like putting the X nut either on the left or right side of the yoke).
The Y nut needs to be in the center or you won't be able to get the yoke
out of the saddle. there are a couple of difficulties with this. Bridgeport
assumes you can just unscrew the screws from the nuts, which you don't
want to do with ballnuts. What I did was mount my nuts (which were in
rectangular blocks with bolt holes) to the top and bottom of a big block
which bolted in place of the original yoke. Now, here are some of the points
where it gets tricky. The left side of the table is pretty open, when you
take off the left side X screw bracket. It needs to be able to slide over
the nut, or you can't get the table on or off. The right side of the table
is not as open, it won't even pass over the original yoke. It is more solid
so it can hold the power feed (or servo axis drive).

The Y nut needs to go on the bottom of the yoke block. I bolted both of
these nuts to my substitute yoke block. The bolts for the Y nut are countersunk
in the block, the X nut bolts to threaded holes in the top of the block
(concealing the Y bolts).

So, the disassembly procedure is to disassemble the X axis drive on the right
side of the table, and leave the screw hanging. Remove left side screw bracket.
Manually slide table to right, until top of yoke is exposed. Remove X nut,
then remove bolts for Y nut. The yoke block can now be removed.

> I don't think there is anything functional in
> that dia, but have hesitated to disassemble the nut. Should I be
> concerned with nut dissassembly? If I run the nut off of the screw, will
> the balls fall out, or are they constrained?

Yes, the balls will fall out, and you will lose a bunch of them. it will look
like a silver waterfall!

> The nuts have no markings
> on them, so I assume the are Cincinatti mfd nuts.

Not necessarily. They may put a housing over the nut which cleverly
conceals who made it. But, at the rate they use these things, it wouldn't
be odd if the did make their own.

> I looked at Roland's
> series on ballscrew conversion in HSM last night to refresh myself on
> the process. Roland, you recommend retaining the existing yoke, which I
> understand. Jon, you made a new yoke from scratch, and commented it was
> too stiff. Would you please expand on how the stiffness affected your
> machine? I've yet to remove the table of the old B'port to evaluate the
> package, so I'm just soliciting opinions...

Oh, no, the yoke wasn't too STIFF, it is just fine. the only problem is
that I left a lot more meat on it, and should have cut a bit more off somewhere.
it is not a stiffness problem, it needs to have a big cutaway in the back, to
prevent hitting the rear edge of the hole in the knee top, I think. If you get
your Bridgport yoke out, you will see a big horizontal V cut in the back
of the thing. I couldn't see why this was needed, so I didn't cut mine there.
Well, it must be needed, becuse it hits something there.

As for retaining the original yoke, I can only see one way of doing this.
If you made an adapter that was the same OD as the Bridgeport bronze
nut inserts and adapted to the Cinci. nuts. Do the Cinci nuts have a bolt flange
face, or are the 8 holes radial? I wasn't too clear on that. The downside to this
placement is that the nuts are off to the side, and may restrict motion.
The X is probably not a problem, but with the nut hanging a minimum of
3.25" either in front of or in back of the yoke, I think there could be an
interference. Putting the nut on the front, and then leaving the big hole in the
front of the knee open, to allow the nut to pass partway through the knee,
might work. The hole in the front of the knee seems to be about 3" dia,
I think. Then, you could mount the Y axis drive on a collar, to space it
out enough to give the nut the necessary range of motion.

Does this make any sense? If not, feel free to ask for clarification. Once
you get it all opened up, it may make more sense.

One other comment, the knee gets a lot of gunk dropped into it, very bad
for ballscrews. At the least, you really want way covers, to keep that
stuff from falling in there.

Jon