OK, I've done it exactly the opposite way....and now I'm intrigued. Why a concave former? What are the advantages?
I made my first stab at this with a 280 radius, and the steel sprang back to around 520 to 540 radius. It's so far out I didn't take too much trouble measuring. I'm thinking I'll cut it down to 200 radius next.
There are several ways to bend a curve, it depends what force you are using, If you can anchor one end and swing a bar around the curve like pipe benders do with copper, then its a convex former.
If youre using a hammer or other blunt force trauma, a convex former will produce a LOT of surface damage. A concave former allows you to put all the force into a very small area (the centre) to give you more movement per whack. It also gives a better finish.
My method would be to bend it cold in a two pin jig bender. As I don't use photo host sites I can't post a picture of mine, but basically get three two 200mm lengths of 25mm bar or tube and a short length of 100mm. Weld the short length between the long ones creating a fork deep enough for your material to fit fully down into the pins of the fork. Then clamp in a vice and having marked your curve on a board start pulling the bar between the pins to bend it. Creep along the length pulling the material and then checking if it fits your curve, If you over bend turn it upside down and pull the overbend out. I have bent dozens of curves using this method.
Your system is simple and it will work.
Do you have a mapp torch? (Knowing you you will have a brass pump up parrafin blowlamp hanging around) :roll:
Once bent into the former, start heating at one end. When it is a dull red slowly move the flame along the metal a few inches at a time, and then allow to completely cool. DULL red, not cherry or orange or white :shock: :shock:
As long as nothing catches fire you will have annealed the metal to that shape and it will barely spring back at all.
If it does need hardening, depending on usage, thats not too difficult to do.
The metal is thin, it might do the job. All you want is to stress relieve it, not turn it into a fighting boomerang :shock:
Just enough that you can see it going red and then let it air cool.
being a traditionalist where tools are concerned i was sure you would have a blowlamp, although I think they are banned now due to people not knowing how to work them. At apprentice college back in the 60's there would be a dozen of us 16 year olds, all competing to see who could turn theirs into the furthest reaching flame throwers across the workshop. More than one had to go visit matron for balm and bandages.
Oh what fun we had =D> =D> =D> =D> =D> =D>
Probably a daft Q MikeG, but here goes anyway. How many times have you put that MS strip through youR jig (shown)?
If "only" once may I suggest that before you do anything else, you do it again to see if the curvature has increased after a 2nd go. If so, try a third time - 3 mm strip in MS is not all that thick but still demands some force to "persuade" it, and not all MS is SO soft!
While softening it with your torch MAY work, if it's only a relatively small one you run some risk of it not heating evenly - therefore "hard" (er) patches along its length.
IF the above 2nd/3rd/another go does get you nearer to the required curvature, then I'd suggest a fairly simple mod to your present (pretty useful-looking IMO) jig, viz:
Replace the block under the long arm with, ideally a ball bearing (or 2, height to suit strip width) mounted on a bolt fixed to a trunnion affair (ply?) on the long arm. The bearing/s will direct the bending force onto a more localised area of the strip to be bent and "should" (maybe!) further increase the curvature of the strip!
Take notice of by how much the extra bend session/s (and the bearing/s?) have tightened the curvature after your 1st go, and if necessary, work out by how much you need to reduce the diameter on the main (circular) former to achieve what you need.
Sorry, not very "scientific" I know, but I THINK that should (maybe!) get you where you need to be - in the end!
(By all means send me a "snottogram" if it doesn't work as above, but I've got a feeling that it just might. ;-)
Thanks AES. Useful stuff. The answers will, I hope, also be interesting. I put the first piece of steel through the larger radius then the altered (smaller radius) former, once each (a day apart), and measured. After the second go, with a 200 radius former, the steel came out at around 350/ 375 radius. I then did it a second time at that radius, and did the best I could with my little plumbers blowtorch, and left it to cool in situ, tight on the former. It came out marginally under 300....around 290. So although the heating made no difference to the colour, it was still enough to relax the steel and reduce the springback. With a nice little clamping jig it will be perfect for welding tomorrow.
The other 3 pieces only had only one go through the former, but the same heat treatment regime, and they all came out the same, and the same as the first one which had had 3 bending sessions altogether. So my most important criteria has been met: they're all the same.
BTW, I meant what I hinted at above - OK, your bending jig does look a bit "agricultural"! BUT who cares? I assume the jig is not for weekly (or even annual!) use, but simply for a none-off job that's just cropped up? If so, with a bit of fiddling it's worked out fine. What more does anyone need for a one off job? Well done Sir =D>
And I look forward to seeing the "pictorial "proof ;-)