[TheForge] Sheet Metal Bender

Bruce Freeman FREEMAB at pt.fdah.com
Tue Sep 19 12:33:12 EDT 2006 

Dan,
 
Use the galvanized.  This is a night-and-day different situation than
that that killed Paw-Paw.  Paw-Paw was deliberately burning off thick
zinc plating from some objects, using a forge at high temperature in a
relatively unventilated shop.  Even so he may have survived the
experience had he not already been suffering from emphasema.  In short,
Paw-Paw got a very large dose of zinc oxide fumes.
 
By contrast, the galvanized pipe you'd be using would not generally get
hot enough for the zinc to burn.  Where it does burn, it burns slowly,
so will not in the remotest stretch of imagination fill your shop with
fumes.  Ordinary ventillation will more than suffice in such a case. 
Ventillation is needed anyway whenever you burn coal.
 
If that STILL doesn't sit right with you, then please recognize that
the only section of flue that may lose zinc would be that closest to the
fire.  Above 3 feet over the fire, it just won't happen.  So do this: 
Pickle off the zinc from one section of galvanized pipe using vinegar or
dilute phosphoric acid (available in the tile dept of Home Depot for
cleaning up after grouting).  Vinegar will result in immediate minor
rusting of the steel, whereas phosphorid allegedly won't.  Then paint
the bare steel, or wipe it with polyunsaturated oil, to retard rusting. 
(The paint will burn off at hot spots.  So will the oil, but oil's
cheaper.)  Use this de-zinced section as the section nearest the fire.

 If you put a cap on your flue so rainwater doesn't get in, the
galvanized will probably last long enough to be worth the effort.  

Of course, stainless WILL last longer, if you want to go to the trouble
and expense.  And a slip roll would be easier than hand-rolling, but
hand-rolling would work if the stainless isn't too springy.
 
Bruce
NJ


>>> dan at irontreeworks.com 9/19/2006 12:06 PM >>>
Bruce,
   I don't want to make it myself.  I want someone to show up at my
shop this
weekend and install my new side draft hood, fix the old Champion forge
I got
for $100, and install a new chimney for free! :)

  However, this does not seem like a reasonable event, so I priced some
stove
pipe.  I can get the 12" dia galvanized 24 gauge round ductwork from
Menard's
pretty cheap.  I can't find anyone that sell/makes the plain black
stove pipe
in 12 inch dia.  After what happened to Paw-PAw, I am alittle scared
of
galvanized pipe close to the forge.  I have students in my shop, and I
don't
want them hurt....soooo.....I thought stainless.  I looked into having
some
made and found out that it would be over $600 for 12 feet.....so....if
you
can't afford it....make it!

I thought stainless would withstand the elements.  I like the plywood
circle
idea.  Seems easy.  I still think that using a slip roller would be the
easiest
solution for the straight tube if I can find one to borrow this week.

Dan


Quoting Bruce Freeman <freemab222 at yahoo.com>:

> Daniel,
>
> First of all, why do you want to make this yourself?
> To have stainless steel instead of galvanized?  Or
> because stainless steel isn't available in 12"
> diameter?
>
> Next, correct me if I'm wrong, but isn't 24 gauge
> pretty light stuff?  Like you can easily flex it by
> hand?  If so, then bending pipe shouldn't be too
> difficult - roll it around a slightly undersized
> mandrel (maybe 10" or 11") and let it flex back to
> 12".  Lock seams are nice for the prefab stuff, but it
> would be a deal easier just to drill and pop-rivet the
> edges and ends.  Or even use self-drilling screws if
> you like them.
>
> Someone else advised 45 degree elbows, and I second
> that.  Whatever you choose, odd shapes can be made
> according to diagrams available in any text on sheet
> metal work. Visit a used_book store and peruse the
> contents.  The Audel's series should suffice, but
> there are text books for metal shops that are fine
> too.
>
> You can also fake it by means of a clever device Clay
> Spencer showed me, and which I've since used:  Imagine
> you want to make a pipe with a 12" diameter end on a
> vertical axis, and goes to a 12" diameter end on a
> 15-degree-off-vertical axis.  (Three of these would
> make a 45-degree bend.)  Construct a device as
> follows:  Cut two (~1/4") plywood circles, 12" in
> diameter.  Drill three holes in each, oversize for
> available threaded rod.  String the two circles on the
> threaded rods:  nut, washer, circle, washer nut, (then
> a lenght of threaded rod), nut, washer, other circle,
> washer, nut.  You'll end up with the two circles
> separated on the three threaded rods by some distance.
>  Now adjust the distance as you prefer, and adjust the
> angle to 15 degrees between the planes of the two
> circles.  You want the closest approach of the circles
> to be some reasonable amount (say 2") because this
> will be the narrowest sheet metal  you'll have.
>
> Now roll this pattern along a piece of paper, and it
> will describe a pattern to be used in cutting the
> sheet metal. When actually cutting the metal, be sure
> to leave tabs to overlap the next pipe section so that
> you can join them with rivets or screws.
>
> I have used this technique to develop a pattern for a
> side-draft hood of unusual shape, and found it worked
> quite well.
>
> Bruce
> NJ
>
> >>> "Daniel Kretchmar" <dan at irontreeworks.com>
> 9/18/2006 11:17 PM >>>
>
> Greetings,
>            I need 12" diameter chimmey pipe for my new
> side draft hood.  I
> looked into menards.  They have galvanized 12"  24
> gauge chimney.  I looked
> into having some custom stuff made but it's too
> costly.....so the next
> question is.....can I make it myself?
>
> What kind of bender would I need to make 12" diameter
> pipe/tubing out of 24
> gauge stainless steel?  I want to make it in 2 ft
> sections. which means
> investing in a crimper a bender.....and what about
> elbows?  I'll need two 90
> degree elbows.  How will I make them?
>
> Advice is appreciated
>
> Dan
>
>
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