[TheForge] Fanning the flames

[peter fels & phoebe palmer]

On 12/19/2010 7:23 AM, Bruce Freeman wrote:
> 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?
The ribs probably are to keep the boards together and flat.
> 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).
Composite materials are available for that. The actual # of psi in the 
bellows
should be low enough not strain the nauga's hide, i'd guess
Heard of using sailcloth sealed with rubber cement.
> 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.
>
>
>