[TheForge] Fanning the flames

[Bruce Freeman]

Paul,

I have had similar thoughts about building bellows.  The 3/4" plywood
will probably work fine, though you might want to use hardwood plywood
which, surprisingly, is often no more expensive than softwood plywood.

If I were doing this, I'd shoot for two paddles per piece of plywood.
Draw ~38" circles in opposite corners of the 4x8 sheet, connect them
tangentially, and you've got most of the lay-out done.

If appearance doesn't matter, consider using a non-traditional paddle
shape.  Paddles can be square if you like (though you'll have to fold
the leather like an accordion at the corners), or any shape you like.

The chief problem in constructing the bellows is not the paddles and
leather but the nozzle and it's associated block of wood, which must
allow a good, long-lasting, freely-moving joint with the upper and
lower paddle, and which must attach firmly to the center paddle.  Get
that design right and you've got the problem licked.  Traditionally,
the leather at this point was the hinge, but you might want to use
steel hinges under the leather to ensure longevity and a good joint.

By "ribs" I assume you mean the cross-pieces on the inner or outer
side the paddles to strengthening the paddles cross-wise to the
boards.  If you use plywood of sufficient strength, these would be
ornamental only, and I don't understand why you'd make them of plywood
anyway.  Do I misunderstand you?

Naugahyde might work as the "leather," but I'd be concerned that it
would be too weak.  (Also, you should find out what temperatures it
can stand -- it might crack in the cold.)  I've thought about this a
lot, but since I use a gas forge I've never gone beyond thinking.
What you might do is use a double layer-- cotton "duck" or canvas
combined with Naugahyde.  The cotton fabric is not completely
air-tight, but it is very strong.  The Naugahyde is air-tight but not
strong.  Problem is, the Naugahyde really should go INSIDE to hold the
air -- which won't look right at all (if that matters).

Alternatively, you could glue the two layers firmly together with the
Naugahyde on the outside.  Another possibility is to use three layers
- 4+mil polyethylene inside to hold the air, cotton fabric for
strength, and Naugahyde outside for appearance.  With the plastic
layer holding the air, gluing might be unnecessary --  though I'd be
concerned about leakage between the upper and lower chambers.  Maybe
you could spread the paddles, glue the plastic to the three paddles,
then glue the canvas to the plastic over the three paddles, then nail
the Naugahyde to the three paddles.  Similar problems exist at the
nozzle end, but this will be wrapped with leather for strength anyway.

Plastic capped roofing nails would be excellent for attaching the
"leather" to the paddles.
http://www.homedepot.com/h_d1/N-5yc1vZ1xgf/R-100122817/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
or
http://www.homedepot.com/h_d1/N-5yc1vZ1xgf/R-100210019/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
but they won't look "authentic".  What you want is ~1" circles of
tinned steel to drive the nails through.  Home Depot sells disks of
galvanized steel, maybe 1.25" dia., but I can't find them in the
on-line catalog.  Nailing the leather to the paddles is problematic
when you use plywood, especially softwood plywood.  You might want to
drill a small pilot hole for each nail.  Again, if authenticity
doesn't matter, use construction adhesive instead of nails.  Or use
construction adhesive, then use roofing nails.

Keep us apprised and let us know how it turns out.



On Sun, Dec 19, 2010 at 9:00 AM, Paul N <crosspein at sbcglobal.net> wrote:
> I'm thinking about building a double-lung bellows as a winter project.
> (Actually 2 of them, one for myself, and a second to use as a working
> facsimile at a historic site nearby)
>
> I have two references ("The Blacksmith, Ironworker and Farrier" by
> Aldrean A. Watson, and "How to Make a Blacksmith's Bellow" by Robert M.
> Heath) The designs are very similar, but not identical. These are big
> boys, about 38" across, and probably 5' long.
>
> I'm wondering if anyone on the list has any advice to offer. I was going
> to make the paddles and ribs out of 3/4" plywood instead of the 1"
> planks described in the books. I'm also wondering if I can get by with a
> naugahyde or similar sides to avoid the expense and maintenance of leather.
>
> I'm thinking that if the plywood isn't heavy enough to draw air, I can
> hang some weight on the bottom paddle as needed. Similarly for the top
> paddle. I can pile weight on top of it as necessary.
>
> This sounds workable, but I'm wondering if anyone tried this and found
> it disappointing, or perhaps had a suggestion to offer.
>
> **Paul
>
>
>
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-- 
Bruce
NJ