What an interesting few days I spent with my Kossel. After spending almost a day understanding and performing a very thorough calibration, the first prints in PLA looked pretty good. The pictures are still on my phone, so you just have to trust me on the quality for now.
Here's what I learned during calibration:
- check your tower motors are assigned correctly. When looking from the hotend down on the print bed, the sequence should be XYZ in a counter-clockwise arrangement. If not, you will have mirrored movement on some cartesian axes, i.e. if you swap the X and Y motors (YXZ in CCW sequence) you'll print with an inverted X coordinate axis.
- determine the max z height at 0,0 first
- determine the delta_radius by finding the tower spot that has the least amount of deviation from ZMAX at origin and then tune DELTA_RADIUS with "M665 R" until the tower spot matches ZMAX.
- Then adjust the end stop offsets at the other two tower spots with M666 . They will have to be negative.
- Now I think it should not be the case but I had to reiterate this procedure about 5 times before all the values settled. It appears that there is some cross-talk between those numbers, so doing the M665 and M666 settings then affects what your ZMAX is. Adjusting the ZMAX will then require to go through the radius and endstop settings again.
Once I settled on repeatable numbers, I printed a few parts and found that the FT hotend leaves a lot to be desired. Mine was not very stable in terms of tip position which caused a few prints go sour because of the tip "re-adjusting" itself and latterally shifting some layers. I found a solution to that issue and I have a rock solid tip now.
Then I found that the hotend is leaking. PLA is squeezing out on top of the hotend, probably between the threaded rod and the heatsink it is screwed into. As the rod is not bottoming out when screwed into the coldend, I am not surprised. Actually, you can screw the hotend into the heatsink until it touches the lowest fin of it. Of course, at that point the coldend is not so cold anymore and the hotend will not reach 185 degC as the heatsink wicks the heat away.
Once I added a cork washer in between the two parts, that worked a lot better while stabilizing the tip position as well.
During prints of more than an hour, the little 40mm fan can just barely keep the coldend cool enough at PLA temps and so I refrained from trying ABS yet. Clearing a clogged coldend because of retracting melted plastic into it is not enough fun to do it again.
On large flat areas I can see a variation in the extrusion thickness that follows a pattern. I am not sure where this is coming from but I have to figure this out as it builds up obstacles the tip runs into during the continued print.
Other than that I am pretty happy with the Kossel. The modifications I made to the tower wheel trucks and the build height all worked out well and made the positioning very precise and repeatable. I have an original E3D v6 on order and am looking forward to installing it. Hopefully this will get rid of the hotend issues. Once that's done I might have a pretty decent printer.
The next thing would clearly be fixing the "issue" with the small print bed diameter. :-)
Oh, and I removed the inductive probe. G29 takes a ridiculous amount of time for a somewhat accurate measurement. Emphasis is on "somewhat". If you do your calibration right, there is really no need for the G29 stuff, in my eyes.
Folger Tech Kossel 2020 Rev. B (in calibration), 1st time builder.
Raised the verticals to 1000mm and diagonal rods to 257mm,
Sintron carriages with Delrin wheels and original E3D-v6.
they are a bit of a pain to get dialed in, but it is all worth it in the end.