Squaring of the trucks

Hello all and thank you in advance.
So, im in the process of building a primo. I’ve got the base to within a 32nd of square at the diagonals. Trying to square the trucks with the measurements to the corners. Im a 1/2" out in the gantry and a 1/4" out on the y.
What have I done wrong?
What are my next steps?
Thank you

I haven’t built the primo. But I did review the docs which talk about squaring the trucks. Did you do this before installing the core? Are you trying to follow those instructions?

I am. I did square the base first and I must admit I did get over zealous and finished putting everything together. I have taken it apart and started squaring. The core is back off and trying to square the trucks.

I’m trying to figure out what I’ve done wrong but when I move the y gantry back and forth to settle it in, when I take the measurements on each end im a 12" off.
X gantry I’m a 1/4" different.
I’ve tried the tension bolts but nothing changes.

Hmmm. We’ll have to wait for someone who has done it to chime in. I might be inclined to let the motors handle the last 1/2". Once they are powered, they will move in lock step (unless you are using dual endstops, in which case, they move in lock step after homing) so you just need to start it square. What size is the build? 1/2" on a smaller build is bigger than 1/2" on a large build.

Thank you for the response.
Size is x-36", y-30".
Laser diode will be going on it.
I do plan on using dual end stops.

Is it bad that I read the subject and immediately thought of Optimus Prime in glasses with a pocket protector?

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It may be possible your prints aren’t square. Take a corner assy and put in two 18" pieces of pipe and snug them in. If they aren’t square, the rest probably isn’t.

So, would it tube the trucks that aren’t square?
If I have my x and y rails within a 32nd on the diagonals, the base should be good. Am I correct in my thinking?

If I am going over the basics, forgive me but this may help others as well.

Start with a square on paper. Upper left corner is labeled “A” and the other corners are labeled “B”, “C” and “D” going clockwise. A square is “right” if the distances AB, BC, CD, and DA are equal. By extension, distances AC and BD have to be the same. This also means that each corner is 90 degrees. If the AC and BD distances are not equal yet the sides are equal, you have a parallelogram / “tilted square”. Now for a rectangle, it can get a bit more complicated. Distances AB and CD are equal to themselves and BC and DA are equal to themselves. The diagonals are equal as well. Yet, with a rectangle, it is possible to have the diagonals equal but the rectangle itself is not square. This is because AB=DA and BC=CD in length. Let’s make the assumption that the corners are square and the measurements are proper.

What I did with mine to check for truck squareness is that I put the cross members together without the core (that has to be square or the game is off). I tied a string to the center of the cross piece and pulled in both directions looking for truck drag. If the tilt stayed the same in both directions, then that is a square issue within the trucks. If the angle changes from one direction to the other, then that is truck friction related.

Mine ended up being a tad off with the truck squareness. My frame is true to 0.5mm optically on the diagonals and sides. I “cheated” here. I am doing it through end stop manipulation and software. I am using for software “https://github.com/johnboiles/grbl-Mega-5X/tree/mpcnc-ramps-updated” and it is set up for the dual end stop operation. By adjusting the endstops, I brought all motion just about as square as I could within itself. It took some time tweaking, measuring, moving endstops but it was worth it. Because of the 3D printed components that may have been printed square but can change as they cool, you are not going to get 1/2 a thou accuracy out of this system (would be nice though - x32 stepping @ 16 t = 0.005mm / 0.0002" per step). The specified steppers are pretty strong and can keep the gantry square while operating.

So, if the frame is square, make sure that the gantry is square first and moving under control square. When that is done, then put in the core. I put mine on with the stepper engaged to hold the truck square as I tightened down the anchor screws.

Good luck.


Thank you sir. Lots to unpack here. I appreciate your explanation.

No problem. Side effect of being in the industrial engineering and automation business for many years.

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I got it very close now. Belts are on, just gotta finish extending the wires from the steppers. Then I get to hook everything up to my Rambo 1.4 and stumble thru that whole process. More questions coming. Lol


I believe that your trucks might be adjusted too tight.

When the tension on the trucks is appropriate, it really does square up and adjust when the bolts are adjusted, but as soon as they’re even the slightest too tight, that stops. At that point further adjustment doesn’t make a difference. The only thing to do is to loosen the bolts right off, and start again. The appropriate amount of tension is very light, and the bearings will barely touch the rails and roll easily with only a slight tilt, or finger pressure. This is harder to get right with the belt in place.

I would suggest that you loosen the truck bearing bolts to have some visible slack, let them “rest” overnight and try re-tightening them and adjusting them after.

In the process of trying to adjust, I went with trying only tightening, and I went too far on my Y axis. Not tothe point of damaging anything, but I had to loosen off all 4 bolts and start over.


The way I adjusted the tension on the trucks was easy. I put the truck onto the pipe that it was going to be riding. If I could tip the pipe to 15 degrees and the truck moved, I deemed it fine to start. If it was too tight, it would not move and too loose and it started to move as soon as the pitpe was twisted - at this point the truck had too much twist capability. The key is to tighten it up just enough to not have any twist but freely move. I have had to visit the tightening bit as the plastics settle in.

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I asked this exact question during my build and the basic response I got was that squaring the trucks doesn’t really matter. The problem is where the gantry rails clamp onto the trucks themselves. That connection is not perfectly rigid so there is room for that pipe to wiggle in there if your pipe isn’t perfectly straight (my EMT conduit was not).

Are you able to push the trucks into square at all with your hands? Essentially, when they are slightly off can you physically push them into square, but then when you let go they wiggle back to being 1/4"-1/2" off? If the answer is yes then it should be fine. Once you get the belts and motors on you should be able to square it up.

I stressed a lot about this, but I got advice of just go forward and don’t sweat it too much and that ended up working out. You can always go back later and adjust if you need.


Are those solid bars for the base?