More information on dishing techniques has arrived, from Tom Holt, in the U.K.
When I asked if I could quote his description, he answered "Sure; but like I said, I'm a real newbie at this - if you could see the spangenhelm sitting on my desk next to this computer (yup, I'm a wannabe armourer too...), you'd understand why I'm a bit reticent about telling other people how to bash tin...
I'm a novelist by trade; I write historical stuff, and I'm obsessive about detail. So when I started a book about armourers, I decided I'd have to learn how to do it before I could write about it. I plan on writing at least one book about a swordsmith; so a contact point with people who really know about the subject would be really useful."
I complimented him on his description of dishing, and he said "Well, I'm a better writer than I'm a tin-basher... Not that that's saying much.
The SCA hasn't really taken hold here in the UK. We do have two very good armourers, though; one of whom (Roger of Lancaster) was kind enough to teach me the basics.
I've made a full suit of armour, plus a spangenhelm and a great helm (the great helm was much easier...) The most recent bits are OK, but the first attempts look like a plane crash...
I've used pop-rivets on my spangenhelm too; now that I've made up a long sett, i suppose I ought to replace them with proper rivets."
So, here is his article:
I'm pretty much a newbie at sheet metal work myself; but I've found this technique is straightforward enough that even I can do it...To dish a piece of sheet metal (ie create a uniform hollow depression in it), take a tree-stump and (cutting into the grain) scoop out a smooth bowl-shape, about the same size as the hollow/bulge you want to make in the workpiece (this is called a dishing stump). Take a ball-pein hammer or (better) a hide mallet or (best) a lignum vitae or other hardwood mallet with an egg-shaped head. Lay the sheet over the scooped-out depression in the log-end and centre it as best you can, so that the deepest part of the hollow you want to make is directly over the deepest part of the scoop in the stump.(The heavier the hammer or mallet you use, the faster you'll go, but you'll make more marks and run more risk of distorting or even tearing the metal. I use the lightest mallet that'll get the job done)You're tempted to start in the middle and work outwards; but if you do that, you run the risk of beating the metal in the middle too thin, as you squeeze material like toothpaste away from the direction you're working in (imagine the metal is a flock of sheep; if you start in the middle you're shooing it away to the edges; if you start at the edge and work inwards, you're herding the sheep in to the pen...)Working from the edges, with regular taps, not too hard, hammer the sheet against the sides of the scooped-out hollow; your aim is to pinch the metal between the mallet and the stump, so don't bash unsupported metal; work your way round in a tight ever-decreasing circle until you reach the middle, at which point you should have a nice even, rounded dish. Don't hit too hard; each blow should have the same force as the preceding one. Get a rhythm going, it helps consistency. I prefer to hit at the same spot all the time and turn the work; it's easier that way to be sure you haven't missed a bit, and to keep the circle going.Now turn the work over. You should have a uniform convex dome, but there may be marks on the outside, particularly if you didn't get the surface of the dishing stump glassy smooth before you started. To lose the marks, get a steel or hardwood ball mounted on a stake and secure it in a vise. Hold the work over the ball and lightly tap it all over with a hammer with a flat, polished head or (better) a wooden or rubber mallet. Again, don't bash, or you'll do more harm than good; light blows and lots of them will give you a better finish. As before, work from the outside towards the centre. Bear in mind that this operation, called planishing, is compressing the outside of the metal, just as dishing compressed the inside; and the more force you apply, the thinner you'll make the work. Again, use the lightest mallet that'll do the job.Steel hammers, formers and planishing stakes leave more marks and make the metal thinner; I've found that I can dish 16ga steel quite satisfactorily without using any steel implements at all.There are probably better ways of doing this - I believe you can use a sandbag instead of a stump, for example; all I can say is that this method had given me perfectly good results - and if I can do it, any fool can...
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