Sheet metal bending question....

The twist is in the angle between the bottom and the side you bent on. Need to reform the angle. Paul

Just bought an inexpensive baking pan hoping it would fit an old toaster oven. Only $1, and it seemed decently made so took the chance.

I found it would fit by bending the flanges down along the short edges. To my surprise, the pan, which had been quite flat on the bottom, developed a sharp bistable twist. The bending was done freehand, using fingers alone.

Would holding the pan flat during the bend have avoided the twist, or is the twisting intrinsic to deforming a drawn shape of this sort? Can a twist be "tuned out" of a drawn shape? More pans are available if it's necessary to start over. It's very easy to make the twist worse, but making it better requires visualization skills I lack.

The pan is about 11 by 7 by 1 inch deep, the flanges I bent down were along the 7 inch edges. The twist that resulted is close to half an inch.

Thanks for reading, and any ideas.

bob prohaska

------------------------

It's really hard to tell without handling the pan, but perhaps the edges of the bottom have been stretched larger than the center. Making many narrow bends sequentially can do that, since the metal stretches to accommodate being bowed out. If the center was larger the edges would be stable and the bottom would bulge and be in/out bistable. Hammering around the center against a solid steel block might stretch it enough to remove the twist, though judging where and how much to hit is tricky, an acquired skill. An auto-body planishing hammer with a smooth face is neater than a ball pein.

Vise-Grip or other flat jaw sheet metal pliers are very handy, I used them to fine-tune the shape of the air cleaner latch I made. When you need a bend longer than your tooling allows, change the angle gradually and uniformly up and down the length to avoid stretching. Clamping the metal between two wood blocks below the bend line and pounding on a third block pressed down against the bend line is simple, neat and effective.

I added 10 years to the life of my car by making and welding in fender-shaped metal rust patches, some of which are barely visible. The lowest body shop quote to fix one quarter-sized hole was $800 and the dealer's body shop wanted $3000, an incentive for me to learn to make the compound curved wheel well lip detail. The first patch paid for the whole (used) MIG setup plus night school cost.

Are you saying the bottom of the pan needs to be stretched, as in given more area, to relieve the stress? I can do that, though it won't be pretty.

I tried simply straightening the flanges I'd bent, and the twist promptly vanished. But, of course, the pan was now too wide. So I cut the offending flanges mostly off with a pair of snips. The twist returned. Looks like the twist is intrinsic to the pan and was somehow restrained by the flanges.

It's really tempting to think that forming what remains of the top flange will flatten the pan bottom, but I can't visualize what it'll take. Is that what you mean by:

Clamping the metal between two wood blocks below the bend line and pounding on a third block pressed down against the bend line is simple, neat and effective.

Thanks for writing!

bob prohaska

>

Are you saying the bottom of the pan needs to be stretched, as in given more area, to relieve the stress? I can do that, though it won't be pretty.

I tried simply straightening the flanges I'd bent, and the twist promptly vanished. But, of course, the pan was now too wide. So I cut the offending flanges mostly off with a pair of snips. The twist returned. Looks like the twist is intrinsic to the pan and was somehow restrained by the flanges.

It's really tempting to think that forming what remains of the top flange will flatten the pan bottom, but I can't visualize what it'll take. Is that what you mean by:

------------------------

My initial guess of what you did may have been wrong, I was thinking of a narrow rolled edge at the top like on my pans instead of a wider flat flange. If the side wall has bent into an arch because the upper edge stretched you may not be able to fix it unless you shorten the edge by crinkling it, perhaps with little twists with strong tapered nose pliers, not fragile little electronics needle nose ones. It's informative to try even if the result is trash. The causes of sheet metal distortion aren't always obvious and it can be easy to fix but expensive to replace, like the housings of lawn and garden equipment. I straighten the corrugated galvy roofing on my wood sheds after fallen branches crumple it, by rubber-hammering it over 1-1/2" water pipe on sawhorses.

This is the tool for more serious crimping, like joining stove pipe or forming a custom round pan or cover.

A steel plate held upright in the vise might help as a form to bent the flange of the next pan down gradually and uniformly. Once you have the bend half completed the vise (with padded jaws) can finish it, again a little at a time across the full width.

I was indicating the ANGLE between the edge you worked on and the bottom is not identical for the entire length. If originally 90 degrees, part is not 88 degrees and part is not 92 degrees. That will translate in to warping of the bottom of the pan. Paul