On Thu, 30 Oct 2008 05:32:32 -0700 (PDT), wrote:

Hi!

No grand schemes today, nor political commentary, I am OT (On Topic).

I'm building a seat frame suppoort for my MOEPED. It must be 16 inches
OAL (overall length) and the legs are 3 inches long. The existing part
is neatly bent from stainless, but those broad curves mean I can't
mount the underseat lighting. (OSS: overseat steering, means my hands
and legs don't block the beams)

I've used up THREE one-yard lengths of 3/4 inch OD by 1/16 inch wall
mild steel tubing from Home Depot so far. It's time to ask for help.

I've mitered the mating ends at 45 degrees, ground them flat, added
fixturing holes. and proceed to braze: the alignment screws up. Tie
wire stretches during brazing, and a clamp I made still allowed
slipping.

I've notched the tubing deeply leaving a thin tag. When bent closed,
the tag area is still open; the bronze doesn't fill a gap that wide.

I think if I notch the tubing leaving a wider tag then when bent, the
stretch on the far side will close the gap, but HOW DEEPLY? You see,
when bent, the far side gets stretched and the V opens up laterally.

Imagine a tube bent but not notched: it would widen to half-
circumfrence. So there is behavior related to depth of notch

I don't want to trial a whole lot of different bend depths, but I can.
There's scrap to do this with.

Have any of you here got some experience with this?

Is there a magic number for this configuration? Can I compute it
without FEA (finite element analysis)?

Doug Goncz
Replikon Research
Seven Corners, VA 22044-0394

Design thoughts:

Can you cut the tubing all the way across, get the next size tubing
down that will nest (5/8" OD), and make a filler on the inside of the
joint?

Or blacksmith it? Use some 5/8" solid rod, heat to red hot and bend
it to the angle you need, heat again and beat it to round, clean up
with grinder so it'll slide inside. Then you braze.

And/or one size up, and make a sleeve that your joint slides into?

A fishmouth sleeve - like a U or a taco shell around the bend, flat
side at the inside angle - will reinforce without wrapping around the
back side of the joint.

Or you fit the saddle to the front side and tack it in place, then
take a hammer and a little heat, and wrap the back side around the
back of the joint. Even overlap the two ends.

-- Bruce --