HI, I have bought the kossel mini 3D printer kits recently. I have now assembled all the parts, following the online instructions and get the printer set. However, I have faced a problem.
I have the extruder head stay one A4 paper thick above the bed surface for three tower positions (A, B and C). However, my D tower (the center) stay around 3mm above the surface. The problem is the printer visually calculate the print plane as convex surface.
People say you have to increment the delta radius (by the diagonal smooth rod value) to lower the extruder head so it move flat. However, no matter how I change the value (increase and decrease) it doesn’t changed…
Can anyone help me?
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Which firmware are you running and on which board? I bet you’re using an 8-bit (Arduino & Co.) board: with my first delta kit - a Kossel indeed - the very same problem bugged me for months! No matter the manual adjustments I applied, either the center was too high or the sides were grinding against the plane.
I eventually solved the matter by installing a Z-Probe and letting the board auto-calibrate the plane, but the results were often mixed in quality. Some edges would be ok and some other wouldn’t.
My final solution was to move to an ARM-based board (a Smoothieboard). It worked fine at the very first try. No curvature problems.
Such boards are waaaay more useful then 8-bit ones, especially for deltas! You should look into them, if my first assumption is valid and you are indeed using an Arduino+RAMPS.
Yes, you’re right. I’m using Arduino Mega 2560 with RAMPS 1.4. According to you, I have to either install a Z-probe or switch to Smoothieboard right?
Yes, those would be the easiest alternatives IMO. They would also bring a good cost-value factor to your machine, as a Z-Probe is useful for letting the machine self-calibrate before each print and a Smoothieboard… is plainly awesome. Its performance makes a delta feel like a robust machine instead of a stuttering mass of mechanical pieces, as an Arduino often makes it behave.
Also forget about flashing firmwares, just change a text file and reboot the board. Embedded networking and web interface, remote upload of gcode files… No way I’m ever going back to an Arduino after having got used to all these commodities.
About the Arduino: Have you tried erasing the EEPROM before flashing the firmware? IIRC on Arduino some values like the arm length are stored in it and no matter how you change them in the fw, they keep getting read as the “old” value in the EEPROM. The sketch needed to clear the EEPROM should be in the Arduino IDE’s examples folder.
Wow, I think I’m gonna try smoothieboard then 
I have followed this guide to solve the problem. Thing starts working when I follow the guide. I keep changing XYZ offset and delta radius and the center starts lowering down. However, I have reached a maximum delta radius ~105mm (with XYZ towers stay ~1mm above the surface when Z = 0), the center still have ~ (negative)2mm to reach the surface… I can’t increase the delta radius anymore or the extruder head will go beyond the boundary of the print bed at Z=0 on each tower… p.s. my print bed diameter is 170mm… Manual Delta Calibration using M665 and M666 - YouTube
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I remember that this one worked quite fine for my Kossel when I was on Arduino+RAMPS with the Repetier firmware (thumbs up for their web configurator!). However it needs a Z-Probe.
Thank you very much. I really appreciate your help ^^ So, I will buy a Z-probe then.
My pleasure to help a fellow delta owner out 
Check out the Micromake delta effector on Thingiverse, if you can get it printed you’ll just need a microswitch and some cable to make the probing work, it uses the nozzle itself as the probe.
Thanks for your suggestion. I will definitely try it out. Btw, I have solved the problem… when the delta radius is increased until the tower points go beyond the print bed boundary, increasing the diagonal rod length can drive them back into the boundary… Now I have a flat print surface but slightly concave (previously convex)… I gotta fine tuning it a bit???
Awesome! Try a calibration cube and measure it, because changing the rod length in the fw, IIRC, makes the printed dimensions go wary.
As your problem is solved already with the delta radius(my idea was the EEPROM stores the values and needs to erased to get new values work)
I still advice use this calibration guide to make sure you have accurate prints.
http://www.thingiverse.com/thing:745523
Also you current electronics can barely handle the delta calculations.
The biggest problem is the LCD screen update, the little 8 bit controller can’t really handle moving the printer calculate the delta coordinates and update LCD.
You can tweak down the delta segments per second to around 120-140 without any loss of accuracy, and also increase the LCD update frequency and remove the X and Y coordinates showed in the LCD it will help a lot.
Smoothie board will be a nice upgrade, but take a look to the Duet wifi controller : https://www.duet3d.com/
This is the best available electronics for a Delta printer, really good stepper drivers, segmentation free delta calculation (no other electronic offer this as far as I know), wifi support with really good web interface so you can use a tablet, phone whatever device with a browser to control the printer.
They also offer an IR sensor to autobed level, but I would use a BLtouch sensor as it doesn’t affected by glass or using mirror as print surface.
BR.,
Tamas
