Manually squaring the gantry's "resting position"

I’m getting down to the tuning details on my 60120 build, and have measured the gantry at being 0.125" off of “square” - i.e. if I home the Y axis to square it (to within a few thou over a few feet, no problem there) then kill power to one of the motors (servos), that side of the gantry moves 0.125" to “relax”.

The typical advice to deal with this is to shim the gantry, but then you need to know which riser(s) is causing the offset, etc.

Is there an alternate set of steps I could to do adjust the gantry to be more square by default? I’m thinking, let the motors hold it square, then loosen and retighten certain bolts, at least to get closer to square so less shimming is needed.
I certainly didn’t do anything special when assembling it so I assume there’s some wiggle room in there.

1 Like

When you built your machine did you check to make sure the table was square? I got a bit ocd during this stage and after many tries I finally got it to a point I was happy. I mean using a tape measure corner to corner isn’t going to give a really high amount of accuracy.

Are your proximity sensors the same distance? Maybe one is accidentally shorter or longer then the other forcing the machine to twist when homing? I used calipers when setting the proximity sensor probe distance to limit the amount of error.

May be something to check.

Let us know what you find.

Regards
Nick.

The table was square corner to corner, and I’ve got the sensors set (and home offsets calculated, I’m using linuxcnc) such that after homing, the gantry is square to the Y rails. Even if the table were wonky, it’d still be square to the rails. It cuts square.

The problem is, without the motors pushing, the gantry is not square to the rails.
Fixing this is why we have two homing sensors :wink:

This means the two Y motors are fighting to hold the gantry square, using power and making noise, and likely causing unneeded friction.

I don’t think this has anything to do with the sensors or table

1 Like

Mine was the same way. The frame was dead on square, but the gantry was off by a little under 0.25"

Like you said, the motors square it to the sensor. So far everything has been accurate, but it stings me right in the OCD knowing it’s not perfect.

If you find a way, I’ll be happy to follow!

Your mileage may vary but I found when trying to get the gantry physically square the order that I tightened the bolts mattered. I’m sure there’s better more accurate ways to do it, but I knew that the prox sensors we’re going to get me the rest of the way there. Worked my way around snugging one at a time then checking if I was will against the bumps. Once I found the magic combination I went back through and tightened the bolts still checking the bumps with each one…some times I had to back one out and try another to keep things squared.

1 Like

Maybe think of it like this: who cares if it’s off when the machine is powered off? I can tell you from experience with a lot of different brands of machines that it’s hard to get a gantry that is powered off totally square. Even if it is square to itself a push on one side will move that side more than the other. So physically square is never really a guarantee.

Where you can really apply, and comfort your OCD is to dial in those sensor flags as close as you can get them. Instead of worrying about the gantry state when off, you can take comfort in knowing that it’s dead square every time you home it!

Our squaring system is pretty cool actually, I’ve run other machines that didn’t have something nearly as easy to use as ours.

If you want to make sure that your flags are perfect drill a hole at four corners as far as you can go on your table and pull a tape diagonally. If you can get those two values perfect your error (if there is any) shrinks the smaller part you make. You’d be surprised how perfect you can get the gantry using this method.

Oh, now I really going to send everybody to their therapist :rofl:

Have any of you check if that 4080 for your gantry is straight? Mine has a bow to it. One side is also larger than the other…

eric cartman help GIF by South Park

1 Like

I’m definitely not getting this across, then. I care about ganty stress when the machine is ON. Any error in the gantry’s squareness has to be taken out by the motors USING FORCE and HELD THERE for the entire job. Steppers already have a death grip on the machine so don’t sound or act differently, but they spend the whole job trying to bend the gantry into square, adding stress and friction to the machine, wear to the pinions, etc.

The problem is worse with servos, since they use the minimum power needed to hold position. A machine sitting still should use zero power. When I find (and hear!) it’s using 80 watts just to sit still, something’s wrong.

The more square the gantry is at rest, the less stress on the machine while running.

Ahhh, I see the concern.

The force needed to keep the gantry in square in really minimal, probably no more than you you’d impart using your index finger to dent an apple. I get what you’re saying: I suppose you’re leaving some power on the table, but in reality it’s so little it doesn’t really make any practical difference. That’s why us, and many other manufacturers use software to square things up. Even a perfectly built (and square) gantry can rack if pushed while unpowered. The software squaring guarantees that even if this happens, it’s accounted for and corrected upon homing.

There’s no reason you can’t fiddle with the gantry to get it squared in a “relaxed” state however. We just don’t publish instructions on how to do that because of what I said above.

When steppers are at rest (stopped) they are using max power, although I think on ours they drop power a bit when they aren’t getting any step signals (not 100% sure on how our drivers are configured though).

With servos (which we don’t offer) you can use a servo brake if you want to power them down when not in use. (and hold the machine in position)

1 Like

9 kg in my case.

That’s a testiment to how tight and stiff the gantry is (yay) but also how not-square it is (boo).

That seems like a lot. I’ve never measured one in weight, but typically I would’t expect a gantry in a relaxed state to be more than 1/4" (6mm) off.

If you’re seeing something more than that you might have some other issues going on. I’d go back through the build instructions and see if you notice anything. You can do some measuring too with a square and see if there’s something odd going on between the gantry beam and the risers.

Also don’t rule out your table being out of level.

These are just somewhat educated guesses though. I’d say if you’ve been though some of the basics definitely get in touch with support.

As I noted, it’s 1/8" off. It takes 9 Kg to move it 1/8".

I was hoping there’d be some easy “just loosen and retighten the following bolts…” to relieve the tension from assembly, but if nobody’s bothered to do this before…

Heh, homework for Avid: take a few machines, home them, and back off spring tension on one of the Y motors. See how much it moves, and how much actual force it takes to push it back. I wonder what percent of the stepper’s grip is being wasted on this?

It might be that, it wouldn’t hurt to try. Maybe support has an internal procedure that I don’ t know about…

If you’re only an 1/8th off that sounds pretty normal to me… However support has seen WAY more machines than me and are the best to answer here.

https://www.avidcnc.com/support/

They are pretty quick at getting back to people, I’d highly recommend getting in touch with them.

I actually just did do a “loosening and readjusting” a few weeks back. I laser cut my “dimensional lumber” out of 1/4" steel plate and adjusted the cross members. I also made sure all of the sensors were straight and square to the sensor flags. You will find that most of the sensor flags are in fact, not square. The knurled ones on my machine are all off centered

My machines just sounds different now. It trammed easier this time too.

But during that exercise I discovered the gap in between the two haves of the frame pieces that make up each side of the PRO60120 has grown. This would be the splice in the frame that holds the linear rails and the rack.

It could be because my 60120 is older and doesn’t have the extra set of legs. Dropping heavy stuff on the table is probably the cause. I do a lot of heavy stuff and constantly mounting and unmounting different spoilboards, jigs and a water table to boot!

DJDelorie I struggle with this same issue. I too was off (a bit less than yours but more than I was satisfied with). Avid suggested I shim. The feedback I received from most was, that I was being OCD. But in my life, I’ve had it drilled into me that you adjust the variable(s) you control to minimize the variable you can’t…so I shimmed. I did this for the exact same concern you mentioned; I didn’t want to live with the wear and tear overcoming that force on my machine would induce. Coincidentally, the amount of force my gantry required to manually square was measure at 21 lbs with a strain gauge. Yes, the gantry motors are strong enough to overcome the force required to square the gantry. But that means added wear on gears, pinions, etc. My beam was off because the mounting bracket was cut oversize on one end and wouldn’t allow the gantry beam to “snug up” nice and tight when bolted down from the back bracket. In my case, shimming allows my gantry to sit square in free state.

Here’s what I ended up doing, and I think I got the results I wanted:

First, I powered everything up and homed the machine, then moved out to Y=10 or so. At this point, the gantry is “square” (or close enough for this purpose).

I loosened the four bolts (on each end) holding the gantry to the top plates on the riser, and the bazillion bolts between the red plates and the gantry/riser. I didn’t remove them, just a quarter turn out from snug or so.

Now, I knew which way the motors were pushing to square the gantry, so I moved one of them a “bit more” (I did it in steps from the controller, might have been 0.050" or so) figuring any stress from the shape of the parts would be compensated for by this.

Now I went around and snug everything back up, first the top plates, going back and forth between the two ends to keep things balanced. Then snugged the red plate bolts. Then went back and tightened everything.

After all this, I re-homed Y (because of the extra steps I put in, which un-homed it) and the motors were using almost no current :slight_smile: When I unpowered one of them, that end didn’t move at all (used a dial indicator here, too :slight_smile:

I did not need to shim it.

So after running the machine for a while, it “relaxed” back to it’s not-square position. I’ll try shimming next.

I’m curious about the conditions for this. I think you said you have servos, not the standard steppers. I know on mine (which has the nema 34 steppers), most of the force required to move the unpowered gantry is in moving the dead motors backwards through the 10 to 1 reduction. If you loosen the rack gear tensioner to disengage the gears then you would have just the force to straighten out the gantry w/o the drag of the motors. Is that how you measured 9kg? Because on mine, I’m pretty sure 9kg of force on one end with the motor disengaged would move it well over a half inch, and mine is only a 4ft wide gantry.

Unpowered servos offer no drag load, whereas unpowered steppers to. Also, it’s a static load, so you’re not trying to turn the motors once you stop pulling.

Er… if your gantry moves that much, perhaps it’s time to check the bolts for tightness? :wink: