[EDIT: Bit of research and it's not possible to calculate the torsion constant for arbitrary shapes since they are not symmetrical and will warp, leading to large errors. Only option is to use FEA (Finite Element Analysis) software.
That makes me feel better, I thought I was just being lazy using FEA. :whistling: If you just want to calculate the K value and dont want full blown FEA then I tried this software and seemed very good. About £40 for a licence, if you go for it.

or this is free working demo, cant save or print (think you can screen capture tho) aproximate k value but very quick for calculating composite beams.


Ross, I think you can still contribute. You were working on a Z axis weights sheet - that could be useful to tab at the front of this one because at the moment the Z axis weight (or mass!) is a user estimate.
sure paste it in, the spindle design bit has lead me to start at the cuuting tool so Im working on the feeds and speeds and required forces at the mo. so that should help as well.