I did to exactly that. With the dial indicator on the ends vs the center, it
moved 10 times more in the center of the gantry than it did on the ends. I
will be the 1st to agree that the servo tuning could probably be better. But
this thing really is(was) a wet noodle. At 12ft long, with the ends
disconnected from the ball screws, +/-1 inch differences between the ends of
the gantry wasn't difficult to achieve. The two servos driving them really
interact very little and behave independently of each other with one not really
affecting the tune of the other. More than likely this has been the true root
cause of the majority of my servo tuning/carving issues with this machine since
day 1. (We bought it about 15yrs ago.) Originally the machine's two ends were
driven by rack and pinions with a 12ft long torque tube connected between the
pinion gears. I had been blaming most of the wobble problems on torsional
twisting of tube, belt squirm on the 7":1/2" sprockets on the belt reduction
pullies for the servo, and a huge inertia imbalance for the servo, being used
with a gear ratio more appropriate for a stepper motor.
While I kind of liked the idea of using cable trussing, my colleges did not.
So we decided to go ahead with the steel tubing we had on hand.
That said, I a cut a piece of the 2x4x1/8" wall steel tubing, drilled holes in
it spaced 1ft apart and mounted it on edge on the back of the beam. It has
made a huge difference, and now the gantry no longer wobbles when homing and
hitting the center behaves approximately the same as hitting it on the ends.
Todd Zuercher
P. Graham Dunn Inc.
630 Henry Street
Dalton, Ohio 44618
Phone: (330)828-2105ext. 2031
-Original Message-
From: Chris Albertson
Sent: Tuesday, May 14, 2024 9:26 PM
To: Enhanced Machine Controller (EMC)
Subject: Re: [Emc-users] Beam Stiffening?
[EXTERNAL EMAIL] Be sure links are safe.
I think I fell for the “beam is bending” idea too. It might be. The way to
find out is to measure the beam center with the dial indicator and then measure
the ends of the beam. It might be that the entire beam is moving
Is the gantry belt driven? Long belts can act like springs. The solution is
wider belts. It is easy to see that a belt that is twice as wide is twice as
stiff.
I’d measure movement at the ends before any more thinking about the beam.
As for modifying the beam, you have to model it. Guessing and “eyeball
engineering” generally does not work well. Any fix is going to be very
expensive. It is best to know it will work.
Everything that you add to that beam also adds mass. Mass is what you want to
get rid of.
> On May 14, 2024, at 1:56 PM, gene heskett wrote:
>
> On 5/14/24 14:34, Eric Keller wrote:
>> Do something cheap because I'm not convinced it's the beam. I've
>> done troubleshooting on things like this, and sometimes it's
>> stiffness and sometimes it's not stiffness. But it really doesn't
>> make sense that it would sit there and ring after a move, so you also
>> may have some tuning to do. Possibly a notch filter?
>> Eric Keller
>> Boalsburg, Pennsylvania
>> On Mon, May 13, 2024 at 4:50 PM Todd Zuercher via Emc-users
>> wrote:
>>>
>>> Anyone have any brilliant ideas to stiffen a woefully inadequate cross beam
>>> on a gantry router without adding too much mass? What is there now is a 4"
>>> x 8" rectangular 3/8" walled extrusion that is 145" long.
>>>
>>> Under normal jogging commands the two servos control the ends of this
>>> gantry reasonably well, but while the axis is homing the thing shakes and
>>> wobbles terribly bad.
>
> This, on 3rd or 4th read, sounds as if the two servo's are not in tune with
> each other. Tuning servo's is not my strong suit, (and the only servo I had
> was destroyed by the new autotune pid in linuxcnc, it found settings that mde
> it ocillate and fried a $125 motor in around a minute. But this would be a
> lot easier to synchronize if stepper/servo's were used. Rigged with a home
> switch, maybe a prox switch since its non contact, with logic rigged so they
> can back away from home and move in sync the rest of the day, getting sync is
> running toward home until the switch trips on that end of the beam, run
> toward home until both ends have tripped, call that home. From then until
> powerdown, both motors getting the same step/dir signals will be in sync till
> the powerdown. No fighting because the two servo's are not in an identical
> state of tune. Hanpose has nema 34 and 42 motors of 12 NM, probably with more
> torque and speeds than your servo's. The best description is that they just
> work. And they use much less power than regular steppers to get the job done.
> A diff you can see in the power bill if replacing burn your hand regular
> steppers.
>
> How fast and how strong are the servo's you are using now? Gear ratio's too.
>
> Also If I put a dial indicator in the center of the bridge and hit the
>
