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beanbag
04-07-2010, 04:42 AM
In this particular case, SS. I squished it in the vise, but there is still about 10-20 thou of curvature that won't come out.

winchman
04-07-2010, 04:50 AM
In order to straighten it, you have to bend it slightly beyond flat. Try putting a shim on each end of the piece on one side with another shim in the middle on the other side.

Evan
04-07-2010, 04:52 AM
SS is famous for having a LOT of spring back and needs to be bent well beyond the desired final shape.

J Tiers
04-07-2010, 08:28 AM
You can "planish" it..... once sheet has a spring in it, you have ea heck of a time getting it out....

You have to stress the material more than the residual stress to get rid of the bend. pressing it flat cannot do it, you can't exceeed the condition you want if you do that, but you HAVE to exceed it to un-bend it.

One way used commercially is to press flat with platens having small dimples in them... these stress teh surface more than the original residual bend, and relieve it . You may see pieces done that way in products, looks like "diamond plate", but the "bumps" go IN.

Another way is stretching....

neither of those is home-shop-friendly

You'll have to reverse-bend just enough... Watch out that you don't stretch spots, or you'll have "cans" (circular bumps like an oil can bottom) that you will never remove, because the metal is stretched relative to the metal around it..

davidfe
04-07-2010, 11:24 AM
I understand than using an English Wheel is one way of
getting a panel straight.

Another is proper use use of a correctly shaped dolly
and hammer.

I have not done either. But have seen it on DVDs and read
on posts on metal shaping type boards.

HTH

davidfe
04-07-2010, 11:25 AM
In this particular case, SS. I squished it in the vise, but there is still about 10-20 thou of curvature that won't come out.

How about a little more informaiton regarding the size, alloy and thickness?

dp
04-07-2010, 03:52 PM
Sheet metal that won't flatten could have had an area of it expanded. If so it will not flatten by bending. If that is the case then this video may show how to use heat to flatten it:

http://www.youtube.com/watch?v=mptiiRLEJs0

Black_Moons
04-07-2010, 04:08 PM
Iv read about methods of using heat to remove expanded sections (outward dents basicly), by heating with a torch and then rapidly cooling with a damp rag.
3M also puts out a type of disk designed for this.. I forget the name however.

Mcgyver
04-07-2010, 04:10 PM
If that is the case then this video may show how to use heat to flatten it:


apparently it'll also harden the steel :)

strokersix
04-07-2010, 06:13 PM
Flat steel plate and a nearly flat body hammer. Difficulty will depend on how big the part is and whether you can tolerate marks. With a smooth plate and hammer, marking will be minimal. I do this all the time with mild steel. Works with stainless too just won't move as easily.

Similar to but not the same as straightening bent nails with a hammer.

Weston Bye
04-07-2010, 06:20 PM
Don't know about flattening, but I found that sand- or bead blasting one side of sheet aluminum will put a bow in the metal, bowing toward the blasted side. Put a nice matte finish on the piece but it wasn't flat anymore.:mad:

boslab
04-07-2010, 07:24 PM
Depends how thick it is, you can flatten thicker stuff of around 1/8 by rolling, less is harder, you need to temper roll then anneal, then flatten.
small bits can be done in a press.
mark

beanbag
04-07-2010, 07:52 PM
one piece is .036 x 1 x .5. It got a bend in it from trying to cut it with a shear.

Some other pieces are .09 x 3 x 4.

Paul Alciatore
04-07-2010, 10:57 PM
If it is that small, you may be able to flatten it by placing it on a somewhat soft surface and running a roller over it. Soft surface = several layers of cloth on top of work bench. Or a thin sheet of rubber on work bench. You may have to experiment with the thickness of the soft material and the diameter of the roller.

dp
04-07-2010, 11:35 PM
apparently it'll also harden the steel :)

That probably won't matter if it's a chip tray he's building. Even a chocolate chip tray :)

beanbag
09-29-2011, 04:08 AM
Bump, because now I have to deal with 1"x13" strips of mild steel that were cut with that big shear thing where you step on a pedal and a big blade comes down. The pieces have a twist in them.

Will heating the piece up with a propane torch while holding it against something flat work? I recall it worked on a piece of 1/8" aluminum.

jugs
09-29-2011, 04:58 AM
Bump, because now I have to deal with 1"x13" strips of mild steel that were cut with that big shear thing where you step on a pedal and a big blade comes down. The pieces have a twist in them.

Will heating the piece up with a propane torch while holding it against something flat work? I recall it worked on a piece of 1/8" aluminum.

No, as stated previously, you have to over bend/twist.

When you do get it flat you'll find it will have developed a sight curve:eek: :mad: , from stress introduced by shearing.

If you want straight & flat sheet metal it needs to be slit (balanced stresses) not sheared

vpt
09-29-2011, 08:26 AM
Your getting into autobody kind of work here. Like mentioned you need to go past flat to get something back strait so putting the piece on something flat and doing anything to it won't make it flat.

Your gonna have to find high and low spots and work them out either by hand if the material is thin enough or with pliers, hammer, vise, etc. If the material is twisted evenly from one end to the other you may be able to grab one end with the vise and use pliers on the other end to twist it back. Nice and easy, do a little, check, do a little, check.

J Tiers
09-29-2011, 08:50 AM
Almost ANY type of stress will be OK to relieve the stress causing the bend... if there is enough.

Maybe you can just bend it back, going slightly over bend to get it to spring back to flat. You have to exceed the yield point to get it to "take a set" the way you want.

Stretching is particularly good, but work-hardens the material. Grab both ends and pull.

pressing in a planishing press (the bumps noted in my other post) stresses teh surface and can be used, but only if you have the press.

Just flattening against something is not workable, since it can only get within the "springback" amount, not all the way flat.

if you heat it enough, it will stress relieve and may lay down flat.

if you have a jeweler's roll set, rolling slightly and pulling as it comes off the rolls might work well.

To hammer it flat needs skill, you have to stretch the "short" areas to match the others, which will flatten it. Think the old-time skilled saw-straighteners, sight the blade, tap it in a few places, and voila, it is straight.

it is a common problem.

BTW, that sandblasting actually made the blasted surface alightly longer than the untouched surface, hence the bend..... perhaps blasting in certain places might bring the twist back flat!

aboard_epsilon
09-29-2011, 09:10 AM
Iv read about methods of using heat to remove expanded sections (outward dents basicly), by heating with a torch and then rapidly cooling with a damp rag.
3M also puts out a type of disk designed for this.. I forget the name however.

Yeah, i was looking at the you tube vids on that the other day ..fascinating

They call it "oil canning" when the metal has stretched and keeps returning to the same shape

couple of vids there somewhere

One with a nine inch grinder with a stainless disk on ..no abrasive ..spins and heats up metal ..that must be the 3M disc

The other, with guys heating up tiny spots with an oxyacetylene totch ..whilst putting water on it immediately afterwards .

there's also a guy using a pin welding gun that can do the heat up without the pins

Another one shows blocks of dry ice used to shrink a panel dent

There is also a new thing, were the guys heat up the dented panel with a heat gun ..then spray computer cleaning co2 rattle can into the centre of the heated area ..to freeze it...looks like it works a treat ..

The last one would have cured VPT's girlfriends car issues..Doesn't involve damaging the paint.

all the best./markj

saltmine
09-29-2011, 11:46 AM
I know I recently had a larger piece of 316L stainless that had an expanded area in the center of it. I hammered around the edges of it for what seemed like hours, and it did little to flatten the piece. Finally, I heated the thing in the center, with a MAPP gas torch and slapped a wet shop towel on the plate.
When I removed the rag, the piece was almost completely flat. I then clamped one end down to the bench, and using a 120grit "flap wheel" on my angle grinder, polished the discoloration out of the metal. Apparently the heat from using the flap wheel was enough to stress relieve the plate because, when I got the whole piece polished, it was nearly "dead flat". Great, now all I had left was the other six plates, and I'm done....

928gene928
09-29-2011, 01:03 PM
Saltmine hit the nail on the head with his "Heat Shrink" approach. Used this many times through the years on various automotive projects. Now a days folks use stainless steel disks in a hand grinder to produce the heat rather than a torch. I have also used the disk method with good results, information can be found on the web about their use and techniques.
Having spent most of my adult life in the metals process industry in a whole range of areas, I can tell you that the reason shape problems whic have been described exist is because the metal is longer in the area of concern. Center buckle, oil can, edge wave etc. are all length issues. The most common industrial way to remove these shape problems is by using a leveler. A leveler is a machine with a bank of nested rollers, top and bottom. The OD size of the rools depends of the gauge of material being worked. A give roll diameter has a range in which it works the best. By pulling the material through the rolls and adjust the pressure applied to various sections of the strip, the short portions of the strip are stretched to the same length as the defect. The industry term is to bend and stretch the material beyond it's elastic modulus.
For the average guy the key is to understand why the shape defect exists, and not to induce more problems by hammering in the wrong area.