Help tune newly built delta machine

Hi!

I'm new to this, obviously, and my first build is the assembly of an older "kit" from anycubic called kossel k2 (a delta with 20x20 framing profiles and 180mm max printing diameter). It uses carriages running on the tower profiles (no linear motion rails)

The build feels mechanically sound with several observations:
1. one of the rods is "notchy" in its movement
2. the guiding wheels on the top triangle are not (cannot be) aligned to the motor pulleys and the belt tensioner. That creates a slight angle on the drive belts above the tensioner/carriages, that gets more acute the higher the carriage goes
3. the bed (plate) feels wobbly on the supplied mounts. However, by shuffling the mounts and tightening them against the side of the glass I was able to find a stable bed position (at least from what i can feel)

I've had Z height consistency problems levelling the bed. Turned out to be mostly loose belts, so I've added the springs and was able to get within 0.15mm (isn't it too much still ?)

My first "test cube" printed larger on x and y (was supposed to be 20 by 20 by 10 mm. Measures 10.24mm by 10.55mm by 10.00mm). Should I increase DELTA_DIAGONAL_ROD in firmware? By how much?

Any advice is greatly appreciated!

Photos of my Delta

I'm adding the results of the circle test: A 40mm diameter circle gets printed 40.7mm on Y and 41.04mm on X

Edited 1 time(s). Last edit at 09/27/2019 06:47AM by charliefd.

Your X and Y dimensions are scaled differently. This is often because your towers or carriages are slightly rotated or diagonal rods do not have the same length. Nobody can tell you how much to change diagonal rod length or something else with any good certainty. The dependencies are more complicated that that.

You should try to run some numerical calibration. If you have a DUET board then it is built in in RepRap firmware. Otherwise there was some web page for it. If even that fails then you can check out this: [github.com]

Edited 1 time(s). Last edit at 09/27/2019 02:37PM by hercek.

The rods are glued in at the factory using a jig, i expect them to be fairly equal length. The plastic Carriages have lots of flex and they don't inspire "precision" at all. I've order some rails and carriages and will replace the profile riding carriages with rails. It is disappointing not to be able to print even close to true with the factory supplied components.

Thank you for pointing me so precisely towards the most possible causes for my troubles! I used the web page you are (probably) referring to "least-squares calibration calculator", it doesn't seem to converge towards better values overall, I probably need to fix the source of inconsistencies in positioning first!