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?