Re: Bridgeport Ballscrew Backlash

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mark Vaughan" <mark@...> wrote:

>
> Hi Scott
>
> Looks very nice, especially the shiny paint, and polished belt casing.
>

There is far more into it than a pretty makeover. The whole
head is rebuilt. All bearings, belts and worn parts (minus
ballscrew). All oil passages are cleaned out, oil lines replaced,
oil regulators replaced, oil pump cleaned. Pulleys and castings
machined to accept new servos. Wipers installed. Motor wiring
replaced. New controller installed (currently 60% done, but
machine is running on new controller). Just the cleaning is
an amazing experience on one of these. The amount of crud that
comes out of one is incredible.

>
>
> Just a couple of queries re your problem which may not have been

considered.

>
> 1 The bed play if mechanical could also be the bearings at the end

of the

> screws, this may be more probable to have play than the ball nut and

screw

> itself. I have seen many ballscrews fitted that just have a basic

ball race

> without any thrust race which by design needs some play. There are also
> different axial float tolerances between bearings of the same size,

some so

> called higher spec bearings are designed for higher speed use and

have more

> play to account for expansion. It's would be easy to go to the

bearing shop

> for some new ones, and be sold what looks better but in our case

isn't. On

> long ball screws you control all the end play at one end of the

screw and

> leave the other to float again due to thermal expansion, one of the

reason

> more accurate machines use glass slides on the table rather than

encoders on

> the spindle. Before someone buts in, I haven't a clue what's on your
> Bridgeport. You may be able to get a DTI from the bed onto the end

of your

> screws to check this.

I am going to check endplay and post it. Thanks.

>
> 2 Are the servo drives set up correct. When you set a servo up, it's
> response will look nice with P and D parameters, but it will stop

before it

> gets to position or just after, there will always or near always be an
> error, it needs a little more oomph on the last bit. As you add I

overshoot

> will increase and the error will become zero. Then you play with the

figures

> to get a crisp response without any error. The difficulty is knowing

whether

> there is an error or not, easy with a Rutex drive, you have a

computer set

> up graphically showing response position error etc, I'm not sure how

easy it

> is with a gecko drive unless you add a counter to the encoder

signal. I have

> set up many baldor drives, with pots, a pulse generator and a CRO,

we had no

> way to know if there was an error, only the shape of the response.

We would

> manually measure backlash with a handle on a screw, then use a DTI

to set

> the drive so the error after backlash was zero.
>
> If there is a servo error it would play havoc with the manner you use to
> check your back lash.

I did not adjust the servos "officially". I just tweaked
the pots so the Gecko did not trip and the motor sounded
and ran smooth. One major shortcoming of the Gecko's is
there is no software to help with adjustment of error.
The servos move in fine and precise increments as witnessed
with the dial test indicator.

>
>
>
> As I said just an idea, but something easy to miss in the depth of

it all,

> would be nice if it's something simple you can tweak.
>
>
>
> Regs Mark
>
>
>
>
>
> Dr. Mark Vaughan Ph'D. B.Eng. M0VAU
>
> Managing Director
>
> Vaughan Industries Ltd, reg in UK no 2561068
>
> Water Care Technology Ltd, reg in UK no 4129351
>
> Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
> Phone/Fax 44 1872 561288
>
> _____
>
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Polaraligned
> Sent: 24 January 2007 20:08
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: Bridgeport Ballscrew Backlash
>
>
>
> 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.photobuc
> <http://smg.photobucket.com/albums/v223/polaraligned/Bridgeport/>
> ket.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
> >
>
>
>
>
>
> [Non-text portions of this message have been removed]
>