[TheForge] Re: Sheet metal bender

[Mike Spencer]

Dan> What kind of bender would I need to make 12" diameter pipe/tubing
Dan> out of 24 gauge stainless steel?  I want to make it in 2 ft
Dan> sections. which means investing in a crimper a bender...

I don't recall the gauge but when I made mine, I used stainless sheet
thin enough that could give it the required curve by working it back
and forth over a mandrel by hand.  I have a proper tinsmith's mandrel
that sticks out ca. 4' into the air from the bench but a piece of
clean 4" schedule 80 or and old driveshaft lashed or clamped to saw
horses would work as well. IIRC, my flue is 13-1/2" diameter, close to
what you want to do.

To close each section, use a lock seam.  For that, a break that will
turn up 1/4" in opposite directions on opposite edges of the sheet
would be best but you can fake it by clamping the sheet firmly between
two pieces of fairly heavy angle with ca. 1/4" protruding and
hammering it over as evenly as possible.  And then you need a lock
seam tool (which you can make with a little forging, some filing and
perhaps a weld) and the 4" pipe or mandrel to close and lock the
seam. 

Do I need to describe in detail how a lock seam works?  I could scan
some images for the web if anybody wants it.

I have a crimping jenny for ordinary stovepipe crimps but instead of
using that, I connected sections using 12" wide bands of sheet metal
with flanges turned out (i.e., made into a kind of clamp) and closed
with some small bolts.  I made them wide because they're not gasketed
and I wanted to reduce chances of stray sparks somehow sneaking past
as might happen with bands barely wide enough to hold the pipe in
place. I could detail that too, if it's unclear and anybody is
interested.

Dan> ...and what about elbows?  I'll need two 90 degree elbows.  How will
Dan> I make them?

You can cut the ends of two pieces of pipe as if to make a pipe miter [1],
then connect them with a peened seam.  Should I try to post some pics
of how that's done on my web site?

DaveM> If you can get by with 45* in stead of 90* you will have a lot
DaveM> less trouble getting the smoke out.

Yes. Those 90 deg. turns really mess up your draft. Same technique for
45 deg elbows, just miter to 45 deg instead of 90, then make the seam.

Alternatively...

My local hardware store sells (or used to sell?) ordinary galvanized
pipe elbows in 6" dia made in 4 segments joined with peened seams.  If you
could find one where you are, you could take it apart and use it as a
pattern to figure out how to do it.  They also sell 6" stainless flue
liner pipe elbows, also made in 4 segments.  But these use a clever
joint that I think would be hard to duplicate without a proper
tinsmith's jenny and a fair amount of practice.  

Of course, if you have a welder that will do thin stainless, you could
just cut the miters and weld them together, same for the straight
seams.  I didn't have a suitable welder when I made my flue.


- Mike


[1] http://home.tallships.ca/mspencer/winmiter.html or your favorite
    method using other software or graph paper.

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^