Re: Restoration - Vintage Gretsch COB
It really not that difficult, you just need to be patient and smart about it. When you take the heads of you have to assess what shape the shell is actually in. In most cases, including this one, its in a ellipse - a smooth, elongated circle. Most of the time shells get out of round from pressure, or heat, - something steady - so its not going to be crazy wonky like zig zags and crazy stuff.
I measured the shell along the diameters from lug to lug noting the longest diameter and the shortest (if its an ellipse, the longest diameter will be 90 degrees from the shortest). Then you either compress the long diameter or stretch the short diameter until you have its right all the way around.
The real difficulty comes when one side of the shell is out of phase of the other - the long diameter on the batter side isn't the long diameter on the reso side... and when the batter bearing edge doesn't lie flat on a table top, that is another dimension you need to correct for as well. Getting all that to line up takes some massaging as you can imagine. That's why it took me a few tries.
I also had to do all that with the hoops.
Luckily, brass is a really soft material that you can shape easier than other materials. Making the proper corrections doesn't take too much force, but you can definitely over correct, which is a big problem because it compounds the number of corrections you need to make. When you do too much correcting you run the risk of cold working the metal and weakening it. Its a valid risk, but it all honesty you'd have to really really bend that metal and bend it back. When a shell has a head and hoops on it, its not going to be more than about 1/4" off; that's not much and won't put you at risk of any metal fatigue. If it were something crazy like 2+ inches out, you'd have to be very careful not to correct too fast and possibly need a heat source for major bends.. but at that point you might as well scrap it.
"Lay the backbeat home."