(Ben) wrote in
om:

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.

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!

Thanks.

BJS


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.



--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

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