I also did my own measurements today. I also used a dial indicator, but instead of a fixed weight I used a “hanging scale” (not sure of the right term in English, I used this thing). I manually pulled the scale, looked at the dial indicator, and then noted the values.
My observations:
- “Belt”: my belts seem to stretch 0.320mm per kg for a one meter long belt. (Determined from a simple regression using 3 different belt lengths and 5 weights per belt length, so a total of 15 measurements.) Quite similar to Jamie’s results in this post, where he finds values of 0.351mm and 0.377mm per kg per meter.
- “Rack”: the XY frame tubes only move 0.005mm when I put 1 kg of weight on the end mill in horizontal direction. (My legs are a lot shorter, the metal part of the leg is only 85mm).
- “Bend”: in another thread I estimated this at 0.040mm to 0.050mm for a one meter 304 stainless steel tube.
My measurement of “twist” is where it gets interesting.
I did not directly measure it (I tried, but didn’t work well). Instead, I put the gantry at a certain position and turned on the steppers. I measured the belt stretch at that position by pulling on the front roller with my scale. At that X position, with a 1 kg load, the belt stretch was 0.220mm.
Then I pulled on the end mill at various Z heights. That same belt stretched 0.133mm when I pulled 25mm below the bottom of the Z axis tubes, 0.128mm at 35mm, and 0.119mm at 45mm.
Around 62% of the force was on this belt, the other 38% on the belt on the other side. If 100% of the force would be on a single belt (the whole 1kg), the stretch would have been 0.214mm at -25mm and 0.192mm at -45mm. This is very close to the 0.220mm belt stretch I measured by pulling on the roller!
That would mean that the “twist” effect is very limited.
So, long story short, my measurements seem to indicate that a lot of the stiffness can be gained by reducing belt stretch. At reasonable stick out, “twist” seems to be a much less important issue. The “bend” is relevant, but also quite a bit less than belt stretch. “Rack” is almost non-existent for reasonably short legs.
I have been working on a “rack and pinion” style system using a rack cast in epoxy. I glued some GT2 belt to the bottom inside of a aluminum C section tube, closed the ends, and poured epoxy in it. It makes a perfectly matching rack for the belt. I’m still working on a roller that moves over this epoxy rack using a closed-loop GT2 belt and looks like a tank tread. I have seen something like it mentioned on the forums before, and I hope it works better than BraunCNC’s glued-on-GT2-belt-rack attempt. Anyway, so many ideas and so little time… 