Oh, this is good into different tubing types:
> Nice essay, Dave, and very helpful. My concern on the thinwall tubing
> starts at the factory -- and may end there. True seamless tubing is made
> by forcing a ram through a billet, and then rolling things out. I've
never
> seen (or looked for) a study on tolerances in wall thickness. With the
> successive rolling exercises, it would be easy for a 0.4 nominal wall to
be
> a 0.3 on one side and a 0.4 on the other. Unless the factory has x-rayed
> (or ??? as equivalent), I'm uncomfortable with a really thin wall. The
> reason I'm uncomfortable is that thinner tubing walls are more likely to
> fail by buckling, not just dimpling. So, I'd love to try something else,
> but for now I stick with good old 531, 4130 in all its permutations, and
> 6061-T6 (aluminum).
>
> harvey sachs
> mclean va
You are absolutely right Harvey. Tubing wall thickness does vary quite a bit. In addition many tubes are not very round either. I use a micrometer on all my tubing and, at time rejected certain tubes I thought were too far out of tolerance.
I am leading to the debate of Seamed tubing VS. Seamless. We have all be lead to believe that seamless tubing is the cats meow and that seamed tubing is only good for making mail box poles. In reality this is not true. Both types of tubing have advantages and disadvantages. Welded tubing is metallurgically superior in many respects to the seamless tube material. Primarily in the area of prior work (hot work, and cold work). Early on in the production of the strip the billets are cross-rolled to distribute nonmetallic discontinuities and essentially erase the effect of these discontinuities. Discontinuities of this nature can be preserved through the processing of seamless tubing since it is worked in only one direction.
As the industry has moved to oversize tubing with thinner walls, seamed tubing can actually be technically superior. The reason is that as the seamless billet progresses from short billet to long thin tube, the mandrels inside the tube can get pushed off center a few thousandths in the course of manufacture. If a mandrel gets off center, there is no reason or mechanism to put it back to concentricity. When you get down to .4 or .5 mm walls, .05 mm (.002") of eccentricity creates a difference in wall thickness of .1 mm. In a seamed tube, the wall is drawn down from only about 1 mm to the final .4 or .5 mm, which is done cold, so there is no opportunity for eccentricity to develop.
Well, there is my rant on seamed (welded) tubing. I have been using quite a bit of True Temper recently and find it to be very accurate and a joy to build with. Over and out.
David Bohm
Bohemian Bicycles