The broken circle

Sunday I’ve finished to assemble my MPCNC (100x85x15cm). Yesterday finally I found the time to test the machine with a pen, drawing a circle and a square (50mm). Ouch! The lines are fluttering and all the circles are a kind of ellipse but the worst is that the circles are not closed! The frame seems pretty squared but the two cross bars of the gantry look like a “X” rather than a “+” and I was not be able to fix them.
I’ve 3dprinted all the parts (IE version), electronic boards and software are standard. Pulleys 20 teeth, 8 mm threaded rod and I think Marlin configuration is correct. I set steppers drivers for plenty of power and the belts seem to be OK. At least it seems to me…
Next weekend I’ll check carefully all the hardware and the software but if, in the meantime, anyone cares to give me some suggestion or a check list I would be very happy and grateful!

Something is loose. I am willing to bet your pen mount is the weakest link.

Did you change your steps per mm in the firmware for the 20 tooth pulleys? An easy test is to just measure the square against your CAD.

Prusa calculator shows me 160 steps for X and Y and 5120 for Z. The dimensions of the square are 49.9mm along X axis and 47.5 along Y axis (2.5mm of difference). The “vertical” gap between the two edges of the circle is about 2.5 mm again.
The pen mount makes me immediately suspicious, it seems to me steady but tomorrow I will triple check it. I’m more concerned about the bars of the gantry: perhaps my 3d printings are not accurate enough? Or loosing all the nuts and then titghtening again could resolve my problems?

For me I do not see a pen retaining issue. I say this because I see two circles, both of which are very similar.

If the pen or marker was not being held steady the lines drawn would be more jagged, and the finished circles would be very different.

I do see an out of square gantry, as you have indicated with your X and + description. Have a look at both ends of X and Y when they are
presented up too the corner feet. Do you find that one hits the corner and the other ends falls short? If so by how much does it fall short?


It’s probably both. The squares are actually parallelograms=out of square, disconnected circle = loose pen. If it was just out of square the circles would be ovals, and still connected. it is easy to see it wonky because of the thin and thick lines.

You are right. The fix is probably just take the 4 long bolts out of the center assembly while it is still on the rails and then put it back together. Should be good to go

Well, I’ve measured all the fundamental dimensions: conduits lenght and the diagonals were correct. Despite this the bars of the gantry weren’t perpendicular. The problem is in the z-rollers and z-roller locks of Y axis. I dissassembled the gantry and I saw that when I tighten the lock in the z-roller the axis of the Y conduit swivel so it is no longer at 90 degrees with X axis. This leads to a difference of more than one inch between the two edges of the Y conduit, measured on X axis. The z-rollers of X axis are perfect indeed. I don’t know why…
Now I’m furiously grinding the indicted pieces, tomorrow I’ll say to you :slight_smile: . I hope not having to reprint the z-rollers (12 hours of time lost).

After sanding z-rollers and the locks and having X and Y axis perpendicular, here’s the first test. I think it is very good (pen holder needs some adjustment yet)!
Big circle = 100mm

Way better!

I seem to be having the exact same problem. What parts were you sanding to get this squared up? I also noticed that when I try to move on the axis that is having the issue the motors do move but make a pretty unpleasant sound, so I am guess its pinching on one side.

I’ve not only straightened the conduits sanding their housing in the z-rollers and locks. I’ve also put some paper tape on the opposite side of the housing. When you tighten the locks the conduits must be perpendicular otherwise you have still to sand and stick other tape. Empirical but effective!