Re: [CAD_CAM_EDM_DRO] Re: Bridgeport Ballscrew Backlash

Scott,

If youve checked all the pulleys etc for slip, sounds like the balls are worn out. The higher backlash measured at the extremes is due to tighter ways at the extremes and more force required to overcome it. I'd pull the ballscrews apart and check the balls. If it's like most ballscrew sets the balls are going to be a standard nominal size. I'm guessing .1250" as most of the Bridgeport sets use that size or 3mm .118". I'll bet you'll find that the balls measure less than what they should be. They aren't that difficult to replace. Just need to make a sleeve with an ID of whatever the shoulder size OD that butts up to the screw portion of the ballscrew. It's ID should be bottom of journal to buttom of journal on the screw, then the new balls need to be fed in 1 by 1. Time consuming but very doable. Once all the balls are replaced and with the sleeve in the nut, start threading the ballscrew in and let it push the sleeve out.

If they are .1250, I'd go back with a .1255" ball if they arent too tight. Make sure the ballnut is full too. I've heard accounts that the balls alternate in size, one big, one small, I've never seen one like that.

Ron
Polaraligned <polaraligned@...> wrote:
Thanks Jon, I appreciate any help here.

The ways were not rescraped. They looked to be
in decent condition. Automatic oil pumps helped
save a lot of these machines.
I put a precision level on the table and shimmed
it level at one extreme of travel. At the other
extreme of travel, (x-axis), I needed to shim the
level up .006" in 10" to be level again.

I snugged down on the X and Y gibs again and was able
to rock the table .002", pulling the table towards me
and measuring about 13" out from the centerline of the
table. Bridgeport's spec for this measurement is .0005".
Weird thing was that after the first pull that moved the
table .002", I was not able to move the table more than
the .0005" spec. The gibs seem fairly tight and I am
hesitant to tighten them more.

As for ballscrew alignment, I used the same factory housing
for the motors and pulleys. The leadscrews on these
machines has only one end restrained so I would think
if the screw was not in alignment it would show as extra
backlash only on one end. I would think that the "unrestrained"
end would be the best, and it would get worse as you
approached the restrained end. Of course, I could be
wrong here.

I replaced the original steppers with Baldor 12.5 oz-in
servos with 1000 line encoders. They are reduced 4:1 with
Gates GT2 timing belts. Using Gecko drives and Mach.

I test the backlash by mounting a .0001" graduated dial
test indicator to the spindle and using a 1-2-3 block
mounted to the table for a straight edge to read off of.
I set the MPG for single step mode with a resolution of
.001" and bring the block edge into the test indicator.
I step the block so the test indicator is reading in it's
mid range, then count how many single steps are required
to get the indicator moving again.

I don't even need the indicator to count the .001" increments
needed to overcome the backlash. As I hit the arrow key on
the keyboard I can hear and feel the ballscrew kick in.
The servo makes a slightly different noise and you can hear
when the ballnut "grabs". You can also feel the vibration
from the step pulse when the ballnut "grabs".

I redid the backlash measurements after tightening the gibs
down:
X- Center of travel: .009"
Right extreme: .013"
Left extreme: .011"

Y- Center of travel: .005"
Right extreme: .009:
Left extreme: .009"

I am testing the backlash over the course of less than
.020" of table movement. It is hard for me to comprehend
that the backlash is caused by anything other than the
ballscrew.

Help!!
A few pictures of my many hours of work:

http://smg.photobucket.com/albums/v223/polaraligned/Bridgeport/

Scott

> Something is seriously wrong, here. I can think of a couple of possible
> explanations. Obviously, wear is not likely, unless the machine was

used

> in some very odd way that caused massive wear only at the ends of the
> screws.
>
> Did you scrape or grind the ways? If not, the ways may not be fitting
> well, as they have wear in the middle. You may be able to detect this
> by leaning on the table with a dial indicator showing deflections

against

> the spindle. Generally, the top of the saddle wears convex, and the

bottom

> of the table wears concave, allowing the table to rock when the

direction

> the screw is driving it reverses. This LOOKS like backlash, but is a
> totally
> different phenomonon. If you have a master precision level, you can

lay

> that
> on the table and reverse the travel direction. If it is rocking, you
> will see a
> sudden shift in the level right at the reversal.
>
> Another possibility is the screws are not aligned well with the table
> travel,
> and the bow in the screws is both binding it, and causing a

deflection of

> the movement. This would be expected at the ends of travel. If you
> reground
> the ways, the screw and nut will no longer line up, and some adjustment
> will have to be made.
>
> Jon
>

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