I was not happy with the first prints out of the delta printer. In spite of upgrading the firmware from Marlin 1.0 to Marlin 1.1.3 and running the auto-calibrate several times, things were not as good as I wanted.
One problem was with one axis end stop, which turned out to be loose on the rail. The square nuts (instead of true “T” nuts) was the problem. After much fussing I was able to tighten it, but I think I’ll be buying some proper “T” nuts soon. With that fixed, things were better, but still not great.
I realized another problem was the “probe offset” might not be correct. The Marlin 1.0 default was 0.2mm, but the Marlin 1.1.3 default was 0.7mm. I was using 0.7 and the first layer height seemed high (it wasn’t printing the first layer at all).
Yesterday (Oct 12) I manually calibrated the printer using the paper strip method, then checked the additional distance from “0” to where the probe end stop triggered. It turns out to have a bit of a dead zone. Using the default test height of 315mm, zero was found at 1.13mm. The probe end stop triggers ON at 0.83 but OFF at 0.93. In the middle is a dead zone (0.83) that is either ON once triggered, or OFF before triggering.
As the auto-calibrate uses the probe trigger point, the distance between zero and trigger on was 1.13 – 0.73 = 0.4mm. This was more than 0.2 and less than 0.7 explaining why the auto-calibrate height number was off.
Using the new depth (315 – 1.13 = 313.87) and probe offset (0.4), The prints should be good.
Except they are not.
Watching closely while it started printing, I saw the problem immediately. The bowden extruder mechanism which decouples the extruder from the hot end was not feeding filament properly. Frequently the extruder gear was turning, but no filament was moving. There’s an adjustment wingnut to set the tension, but I found with experimentation that it is just too finicky to work reliably. It seems there is not setting of the tension that will always work first time, and you can’t be fiddling with tension all the time during a long print.
Examining the bowden extruder mechanism, I think I see the problem and the solution. The mechanism is plastic. The idler is a bearing and washers, but the tensioner is just a bolt and wingnut. What is required is a good quality aluminum housing with spring tension similar to that on my cartesian 3d printer that I built.
A quick search on amazon.ca found several excellent choices in the $13 range, so I ordered an appropriate one. It should arrive next week and I’ll resume the testing. I’m quite certain a good extruder will solve the final problems, and perhaps with a final auto-calibrate I’ll be up and running.
Of course, Marlin is now at version 1.1.6, but I’ll hold off trying that until after I get the current version working.