OpenSCAD ISO metric thread library / functions
I printed M6x12, M10x16, M8x16 bolt and nuts.
Three bolts needed to tap because of tiny blobs. I broke the M6 bolt while tapping it.
Rough sketch of new belt tensioner
The belt tensioner in Mini Kossel is flawed design. To adjust each belt tension, the top three vertex frames need to be loosen first. And different number of turns of the tensioning bolts creates skewed three axis.
Initially I played a tension mechanism that uses three bearing, which overly complicated. So I switched to one bearing (idler) mechanism that slides up and down in the rail or channel. Apparently one night after I found identical tensioner design on Reprap forum.