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Kossel XL puts sideways jumps in long straight lines

Posted by jsryork 
Kossel XL puts sideways jumps in long straight lines
March 12, 2017 07:43AM

Last September I built a Kossel XL kit from [builda3dprinter.eu]. I have been very pleased with it, especially the dimensional accuracy and repeatability. I am using Marlin firmware, printing PLA, onto a heated glass bed with a dilute PVA solution to avoid corners lifting. I design with OpenSCAD and use Kisslicer Pro.

Then I noticed the upper surface of flat prints (50mm * 80mm * 1.2mm) is not as smooth as I expected. Looking at the 'shiny' side, I can see that the long diagonal stokes to make the flat surface have jumps of about half an extrusion width in various places, and this leads to some doubling up of material. The overall size and shape of the printed parts is fine, but the surface is uneven.

I have looked back through parts I printed at the end of 2016, and I do not see this at all. On these, all the first layer fill lines are straight, with no jumps.

I have re-tensioned the belts, checked the pulleys on the slide motors, recalibrated the offsets, and adjusted the motor supply current trimpots, all to no avail.

Yesterday I hand coded a G-code file to make 3 sets each of 5 linear strokes, 100mm long in the X direction, at different Y positions. The one at Y=0 clearly shows this 'jump' on the line. This print is reproducible, with the jumps on the lines in pretty much the same place. So it is not some electrical noise or random mechanical slippage.

During this investigation, I have twice had a situation where I turned the printer on, did a home, and one slide went down not up, and I had to kill the power. On the first occasion it was the C pillar (I labelled them anticlockwise from the front left, and wanted to avoid confusion with the XYZ axes of the printed parts), and the following morning it had fixed itself. Last night it did it with the A pillar. This morning it was not fixed, but after I adjusted the A motor trimpot from 580mV to 470mV, it was OK.

I am puzzled that the printed results have got worse over a few months, even though I have not changed the hardware, and the firmware only for calibration results.

Any hints or suggestions would be welcome.

Best wishes

open | download - tlong_03_Y50_M50_0.gcode (2.5 KB)
open | download - IMG_2703.jpg (80.6 KB)
open | download - IMG_2701.jpg (122.9 KB)
open | download - IMG_2702.jpg (114.8 KB)
Re: Kossel XL puts sideways jumps in long straight lines
March 13, 2017 12:55AM
This might be either play in the rod ends or stiction "erupting" at the same place.
Do you see anything binding at the effector or carriers? ( flat spot on the wheels/bearing balls )

Edited 1 time(s). Last edit at 03/13/2017 12:56AM by o_lampe.
Re: Kossel XL puts sideways jumps in long straight lines
March 13, 2017 05:16PM
What @o_lampe said, plus check all the screws for tightness, especially the screws that attach the rods to the effector and to the carriages, and the whole structure for rigidity... things tend to work loose over several months of printing.

I'm guessing that one of the carriages will reverse direction at the time that the effector jumps sideways, and that play somewhere in that part of the mechanism causes the jump.

My only guess about why your motors are suddenly driving in the wrong direction is that the limit switches are sticking closed? So the software thinks the axis is already homed, and drives it down so that it can pull it up to find the point where the switch closes.Except that the switch never opens, so the software just keeps driving that motor downward.
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