customizing round bosses for dished shields

[Orlirva]

I checked here a few months ago for advice on modifying a 'standard' round commercial (spun) boss/umbo into one that would fit a curved surface… in my case it was a 6th c, deeply curved (not concave) oval shield, and also byzantine concave-dished shields. I did not find any information ( except that it was hard to do), and a reference to a german firm that would make one custom – I would gladly have gone that route and aquired a nicely made forged (not spun) boss, but they had a steep minimum order requirement to ship to the USA and so it fell outside my budget.

I tried it and did not find it hard although it took a couple of hours of fussing. When I knew how to do it I was able to do another one for a concave shield in about an hour. The main tools required are a sturdy vice and a large ball peen hammer. I found two ways to get 'dish' and a combination seems to work best:
1) lay the boss upside down on vice's top (or other anvil-like surface), hold it with a gloved hand, and beat all around the inside bend with the peen; work the two sides that you want to curve down extra hard.
2) I was also able to wedge the boss into the vice and use the vice power to squeeze out the inside bend… although if you do that too much it will get a big dimple, so its best to give it a light squeeze, move it a centimeter, and squeeze again, working around and back again if necessary.
3) A third technique is needed to finish the job: using the vice to get the down-bend/curve in the flange just right. Basically the jaws are not tight, but just provide a place to wedge the flange and you use muscle power and leverage to bend the flange. Again it took multiple light bends working around, rather than one big bend… and start deep and inside and do the outside edge last. The bends are for fine tuning and don't provide the 'dish', or not much anyway … If there is not enough dish you need another round with the hammer (1) or the vice squeeze trick (2).

Using some scrap leather on the vice helps not mar the surface, although I ended up scouring and polishing the whole surface afterwards, and that was ok too.. THe inside edge looks dimpled, but in my case the dimples are fairly small and even and so not at all bad and the boss certainly looks a lost less like a modern spun product.
Rick

[M. Demetrius]

What you're describing is the same problem people have when taking an umbo with a flat flange, and trying to fit it to a curved scutum. The geometry of the bowl part refuses to allow the flange to make a smooth bend, without some reshaping of the sides of the bowl. And if you had any doubt about the strength of an arch/dome, this project will settle it for you.

The best way, of course, would always be to make the flange curve fit the shield first, then dish out the bowl. That may not always be possible, though. It's not as easy as one might think before actually putting the pieces together in the shop, is it? Good job on figuring it out.

[brennivs - tony drake]

These are what I made to dish bosses out with, as Demetrivs says if you dish your flat plate first, then hammer out the dome. If the plate needs to be dished quite a bit, then make a cradle from hard wood, and if you can line with about 3mm plate to suport the plate as you hammer the dome.


Best of luck
As I dont have a pic of a shield boss I have done, I have borrowed one from the RMRS society Web site, http://www.romanarmy.net/
This pic shows Three of many bosses I made years ago ( ones faceing you rectangular ) and you can see by the curve of the shield, these were all done on one of the flat formers in the pics above.

Regards Brennivs

[M. Demetrius]

That's almost the same tool (in function anyway) that I use. I just sort of combined the two and had the one cut from a 24" diameter oil field some kind of thing that a welder had out in his yard. They make huge fire pits out of the rounded ends of these things, but he cut me a part of the side, and I ended up with this:



With a few clamps, I just put the metal sheet on, bend it down and wham the tar out of the middle of it, making a bowl surrounded by a flange that's already curved. I also am lazy, so use a torch to heat the bowl red while I beat it. Then I have to get the black off the front, but that's not so bad with a wire wheel on the grinder.

[PhilusEstilius]

David. It looks like you and I had the same idea in mind for I created a brass tube with a curved end to it then I braised it onto a plate it's one I use for making eye guards for Horse Chamfrons it is 100mm in diameter which is around 4 inches.
I do have a larger diameter one for doing the umbro of Scutum it's only a tube and nothing like that super job you have, it does make one wonder if indeed this is the way that the Roman craftsmen solved this problem.

Here is a link to it http://www.northumberland-computers.com ... a/tube.jpg

This tube works wonders for getting the curve needed around the faces of horses

Here is a link that shows how http://www.northumberland-computers.com ... roman9.jpg

[M. Demetrius]

I don't know how they did it, to be honest, but it sure makes it easier, doesn't it? We all three came up with the same general solution to the same basic task, so it certainly lends credence to the sensibility of it. Odds are, smiths of old had some tricks they could teach the very best of us today, too.

Here in the US, people are relearning blacksmithing. Some things may never be relearned, since the great-grandfather smiths often died with their techniques committed only to memory, as the new mechanized shops replaced their craft. Pity.

[QVINTVS ARTORIVS CORVINVS]

My boss stake / form is very much like yours mate!

I had a friend toss a piece of 1/2" plate in a hydraulic press and weld a couple of chunks of square stock on it for legs.
Very quick and easy

I also use it to reshape KA bosses (flat surface) to the esact curvature of our scutae

[M. Demetrius]

Well, 4 out of 4 of us saw the same problem and came up with essentially the same solution. Pretty cool, eh?

[jvrjenivs]

Well, 4 out of 4 of us saw the same problem and came up with essentially the same solution. Pretty cool, eh?

Not only you 4 of 4. I know that one of the new Indian companies also make them this way.

[M. Demetrius]

Well, there ya go. I guess, when I thought I'd hit on something original, I just proved "there's nothing new under the sun". Should've known. :wink: