Need some help with sheet metal manipulation

[Kozzy]

I was hoping that someone here might be able to give advice or point to resources regarding education on manipulating sheet metal before I start on a project.  I've done enough sheet metal work to know the deal but this is more about finessing an existing bent sheet back into shape without introducing new distortions and that's a bit out of my normal wheelhouse.

This is for the windmill restoration I am involved in at the local museum.  We're restoring an old 20 foot diameter windmill and a few of the fan blades have a secondary bend on some ends from handling long ago.  So basically, the blades have the normal cylindrical curve except at the ends, part of the top is folded over with a secondary bend.

We don't have an english wheel or planishing hammer so we'd have to hand stretch the sheet back into the proper plane.  Also can't heat shrink any areas with a torch because it's galvanized sheet.  Handling is a bit tough because of the size of each panel and the fact that disassembly is not an easy task here due to being riveted.   Manipulating sheet properly takes a bit of skill, knowledge, and lots of voodoo to do well. 

Before we go at this thing, I wanted to "read up" on the subject (if resources are available) and ask for input.  I *think* I know enough about the subject but often, a little knowledge and understanding is more "dangerous" than none.  Since approaching the correction can cause worse damage if we get too aggressive in the wrong directions, I thought I'd fish for more input on the subject.  

[ThomasPowers]

Have you though of talking with old school body men in Auto Repair?   I would make a mount for the hub and rotate each vane over the work surface in turn. I'd also use wooden, rawhide and HDPE hammers on it and not metal ones.

There's a company local to my northern abode that repairs windmills you might contact them and see if they are willing to give any suggestions: williamswindmill.com

[Daswulf]

I wonder if it might be worth making a wood form matching the undamaged shape and coaxing it back to form with clamps and softer mallets then working out any wrinkles. In the process you might be surprised that heating Below any temps that would burn zinc Are helpful. Just never move the metal too much or too fast at one time. 

I don't get any panels that large in my collision repair work. 

 

[Jim Coke]

Greetings Kozzy, 

     I have a 6 ft slip roll that I have used to straighten sheet... You might call around some local heating and cooling shops to see if they have one you could use.Just a thought. 

Forge on and make beautiful things 

Jim

 

[Smoggy]

I've had success working thin sheet materials cold over a bag of sand and taking it a little at a time. The sand supports the parts you don't want to deform but gives where you are hitting it. Same principal as filling a copper pipe with sand to bend it without colapsing. As Thomas stated wood/rawhide/HDPE or even a lead hammer. You may find a batten type tool (don't know the correct name) usefull too as used by plumbers of old for lead work.

[Kenhughes]

Jim Cokes idea is what i would use ive been panelbeating since the days of lead and hammer and file and no bog filler LOL.

[bigb]

I just got this book and I am about 2/3 through it, I can highly recommend it. The author is an accomplished sheet metal worker and walks you through everything and shows how much can be done with just hand tools, although he covers machinery also. Best book I ever read on sheet metal shaping and forming.

 

https://www.amazon.com/gp/product/0760344922/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

[ThomasPowers]

The information BigB is trying to convey is: 

Professional Sheet Metal Fabrication

Ed Barr

Published by Hachette Book Group

ISBN 10: 0760344922 ISBN 13: 9780760344927

[Kozzy]

Thanks, all.  Been tied up a couple of days and didn't get a chance to respond.

I'll take another look at the riveting and get a count of "bad" panels--might actually be easier to simply replace them via shear and slip roll fabrication.  Otherwise, I'm thinking a hybrid of Daswulf's notion of cutting a form and covering it with something akin to a partial sandbag--making something that is close to the needed shape but a bit forgiving under the hammer.

I appreciate the suggestions of soft hammers.  I was originally thinking standard body working hammers because I have those sitting around but will seek alternatives.  

And I'll check out the book--as it was a subject that I wanted to learn more about anyway.  Of course, experience is the best teacher in this stuff.  I do a lot of normal sheet metal work via break and shear but currently don't have a slip roll.  With large radii like this, we usually bump form...but that's always been for one-off parts.

[BIGGUNDOCTOR]

Those bends are not that bad as they are more of a radius then a sharp kink. Have you tried just pulling on he end to see how easily it moves? Sometimes a simple reverse course of how it was originally put in will do the trick. How good will it have to be? If it is close and 40' up in the air no one will probably notice if it is a tad off.

And yes, if you can remove the blades they will be easier to straighten out.

As to sand bags. Remember that you do not want to stretch the metal, only bend it. Sand on the backside is not a solid surface and will allow the metal to stretch. That is why it is used when forming bowls, etc. I would say use a solid surface like a floor as a backer , and a small "sand bag" as the hammer for the majority of the moving. Once it is close you will have to go a little beyond flat to account for spring back. Again, it depends on how good is good enough.

 

[bebeaux]

"The Key to Metal Bumping" is well worth the price. Old school knowledge at its finest, back when turret tops were the latest automobile construction trend. I have a few books by guys named Ron, but this one explains the repair methods better than any I've seen. https://www.eastwood.com/key-to-metal-bumping-book.html

[Kozzy]

6 hours ago, bebeaux said:

"The Key to Metal Bumping" is well worth the price. Old school knowledge at its finest, back when turret tops were the latest automobile construction trend. I have a few books by guys named Ron, but this one explains the repair methods better than any I've seen. https://www.eastwood.com/key-to-metal-bumping-book.html

Thanks---I already picked up the other book mentioned and will look into this one too.  Nothing better than learning new stuff.  Was putting together an Eastwood order anyway so wouldn't hurt to toss this in the pile.