Re: [CAD_CAM_EDM_DRO] Re: Bridgeport Ballscrew Backlash

Dan Mauch
low cost stepper and servo motors.
cases for Gecko drives
kits and assembled 3-4 axis drives
www.camtronics-cnc.com
www.seanet.com/~dmauch

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, January 24, 2007 6:43 PM
Subject: 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
>
>
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