Ideas on how this was fabricated ?

The following pics are of a bicycle crank arm, manufactured in reaonably small quantities around 20 years ago. What makes them different is their shape.

Made of Chromoly steel, hollow with no visible seams. hexaganol cross section, tapering from one end to the other in 2 dimensions (one is obvious, the other is widthwise end to end - wide end at crank spindle, narrow at pedal)

formatting link
Suggestions so far are :

  1. 2 Halves Stamped from sheet steel, welded together and then finished.

  1. A Custom tube - drawn and tapered to exact requirements, with ends formed together in a tool, brazed and finished.

The trouble with 1 is that I can's see any evidence of the weld - the 'seam' between the two halves is absolutely 100% - straight and sharp. The rest of the visible welding though is not so good, so I am not convinced by this.

No. 2 seems possible, particularly given that the ends (where the 'seam' betweenm the two halves are) looks definitely 'manually' finished (not absolutely straight and true). But, I can't see how tubing can be shaped hexagonally and then tapered in this way.

Any light anyone could shed would be very welcome!



Reply to
Loading thread data ...

It could be that that the two halves were brazed. But the plooks like it was plated. If thats the case then it could have been welded, blended, then the other pieces welded in, and finally plated. If you can, buff a little of the plating off and see if there is a brazed joint. ERS

Reply to
Eric R Snow

My thoughts:

- Maybe welded together by an outside company? Then shipped to another which did the flanges crappily, then finally chromed.

- The "HUTCH" stamping I think would definetly have to have access to the inside to back up the stamp. This suggests bent plate welded with at least one seam (as pipe, or two as you suggest).

- Certainly, if it was welded, it was finish ground so you couldn't tell. Except... there are some dings/pits along at least one edge. Possibly undercuts while welding which didn't get filled in? Does their shape look like that might be?

- Can you get any images of the inside, dental mirror on hand maybe? If there's any oxide, flash, whatever on the inside then certainly it was welded at some point.

As for drawing, I would suppose something like this could be swaged to shape. Why you would want to do it is beyond me... :^)


Reply to
Tim Williams (Ben) wrote in news:

Made in 2 halves, welded, finish machined, tis why the edges are so straight, but the ends (radius) are not. The only other option, was it was cast, then machined.

Reply to

It's a lot easier to make and finish a nice outside weld than it is a fillet weld like the gobby-looking ones. The halves may also have been resistance-welded like some tubing is made from rolled flatstock.

Reply to
Don Foreman

Man, that's an interesting one. I suppose that none of those three bosses would let you look inside with a borescope, huh? The small one that apparently fits a drive-pin on the chainwheel probably goes all the way through, too, like the ones on the ends?

If it *does* open to the inside, you could find an inspection shop that has a small, flexible borescope, and you'd probably be able to see where the seam(s) is with that.

Or you could just take a hacksaw and cut it in half for a look.

-- Ed Huntress

Reply to
Ed Huntress

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.