"Don Foreman" wrote in message
...
On 13 Jan 2005 14:55:07 -0800, (Ben) wrote:

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)

http://users.fcpl.com/~ben/aero/crank1.jpg
http://users.fcpl.com/~ben/aero/crank2.jpg
http://users.fcpl.com/~ben/aero/crank3.jpg
http://users.fcpl.com/~ben/aero/crank4.jpg
http://users.fcpl.com/~ben/aero/crank5.jpg
http://users.fcpl.com/~ben/aero/crank6.jpg
http://users.fcpl.com/~ben/aero/crank7.jpg
http://users.fcpl.com/~ben/aero/crank8.jpg

Suggestions so far are :

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

2. 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.

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.

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. g

--
Ed Huntress