Re: [CAD_CAM_EDM_DRO] Re: Bridgeport Ballscrew Backlash

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.
>
>
>

Unless the machine is rocking due to the change in center of gravity
(quite possible, depending on the floor and how it sits on it) then the
ways are WAY worn, or the table has developed a sag. Manual Bridgeports
really suffer form this, the BOSS machines are supposed to be better because
the table is shorter and better-supported. You can check with the level
sitting across the knee first, run the table end to end, and compare the
change in reading with what you get on the table.

Note that the "chromed ways" are only "half" chromed! The top
of the knee and top of the saddle are chromed, the undersides of
the saddle and table are bare cast iron. Guess where the wear ocurrs?

.006" /10" is a huge tilt, if that is all concave wear on the bottom of
the table. I'm hoping some of that it rocking of the machine.

At the middle of travel, look for a sudden change of the level reading
when the direction is reversed. If you see this, it means the bottom of
the table is an arc wider than the top of the saddle, and applying
force via the leadscrew rocks the table like a teeter-totter.

>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.
>
>
>

You can't tighten them more than snug at the ends, or you will have
binding. This rocking in the X-Y plane could be any one of the slides,
even the knee ways, it is hard to tell without measuring each one.

>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.
>
>
>

That makes sense to me, too. Oh, I'm getting a picture! Here's the
scenario: The top of the saddle is worn slightly convex, the bottom of
the table is worn concave in the middle, but is pretty flat at the ends.
With the table in the middle, the table can rock slightly, until one end
or the other of the curves meet up. At the ends, it is like a board over
a roller, it can rock a lot!

>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.
>
>
>

OK, there is a lever arm of about 3" from the center of the ballscrew to
the plane of the table slide. it is another couple inches from the
slide to the
top of the table. If the table were to rock over the saddle, this would
cause the position of the nut and the table to differ. One obvious way
to test this is to put a vise or other block on the table as tall as you
can set up, and repeat the measurement. If you get much larger indicator
deflections higher up, then rocking is guaranteed to be the mechanism.
If you see the indicator move the WRONG WAY first, before starting back
the expected direction, you have it nailed that the table is rocking.

Second, any looseness in the mounting of the screws or nuts can allow the
nut block or the end of the fixed screw (on the X axis) to move where it
should be rigid. This is most of the problem on my retrofit (not a BOSS
base).

I just find it REALLY hard to believe the screw itself is worse at the ends
of travel. That is totally contrary to a great deal of experience many
rebuilders have had with used machines. The only possible explanation
of this is that the screws were reground on a machine that did not match
the pitch of the original screw. They lined it up in the middle, and the
pitch error widened the groove at the ends. You might have been able to see
this sort of problem visually, if the original numbers you posted had held.

The fact that adjusting the gibs has altered the backlash readings at
the ends
tells me the STRANGE READING problem is NOT in the screws, though.
These new
numbers are not so astonishing as before, just a little odd. Most
likely, you
have worn balls in the nuts, at the least, and also worn ways. You may
be able
to reduce the backlash to a tolerable amount by reballing the nuts.
There is a
guy selling oversize balls on eBay, but I think you are going to need
special
oversize ones, not just the plus .001" ones he lists.

It would be best to find out how much of this error is in which component
first. You may be able to put the dial indicator on the end of the
screw and
check that movement right on the ballscrew's centerline, to eliminate the
rocking problem.

Jon