Firewall with wire edge - metalworking knowledge needed

I am in the process of laying out and building a firewall for my Fordoor sedan project http://www.jalopyjournal.com/forum/showthread.php?t=147848 . I have a pattern of the firewall sheetmetal worked up in cardboard. At the bottom of the firewall there are some large notches that clear the frame kickup. Here is a picture of the cardboard pattern:



The plan is to make this piece from 18 Ga. steel. As I look at these rectangular notches I can't help but think it would be nice to have some kind of finished edge rather than just the cut edge of the 18 Ga. Two possiblities come to mind a hemmed edge or wired edge. For a wired edge I believe I would need some large radius in the corner so I can work the metal over without it tearing. Can anyone suggest what size the radius would need to be? Does anyone have any suggestions on how else this edge could be finished?

 

I think you should radius those corners near the top of where it slides over the top of the frame, and roll the lip inwards with about 1/4" radius and go in for about 1/2" maybe to stiffen it up. You can bend it in with a radius, or you can make a separate bent in piece with a radius and butt weld it to the firewall right at the edge of the radius and grind it smooth so it looks like you bent it in with a radius. You could even have a raised beaded lip that rolls back in if you want to get fancy.

Don't make the gap between those side flaps and the frame too narrow or it'll be a real bitch to get the body on the frame without scraping up the paint on the frame. When I lowered my channeled truck cab over the frame, I left about 3/8" gap in there and draped some heavy leather aprons over the frame to make it less risky, but it was still tough because you have to lower it down very carefully straight down.

 

You should probably think about how you're going to attach the floor to the firewall at this point too, and how you're going to mount the front of the floor and/or firewall to the frame there, before you get too far along on the firewall.

The firewall is what gives the body strength up there, especially keeping it from racking side to side, so rolling some lips at the edges will definitely stiffen it up. Without a lip, it'll feel like a big floppy piece of 18 gauge steel and it'll rattle and flop around. Once you start bending lips into the edges, it'll start to feel like a real car part. A rolled in lip would be stronger than a hemmed or beaded edge, especially if it rolls in an inch or so, or if it gets tied into the floor there.

 

I would make a hammerform ot of 3/4" plywood, and roll the edges with a hammer and leather faced slapper. You can trim and shape the plywood firewall plug with a grinder, untill it fits exactly. Leave a 1/2" lip on your sheetmetal blank and with it clamped in place securely with a few C clamps, then bend the lips over the edges of your hammerform. Remember, if you wish to roll any beads in the piece, do so before bending anything...

 

A method I used was to gas weld some 1/2" conduit to the cowl perimeter, trimming it slightly where the hood hinges dimple it on each side. Then I simply sheet metal screwed the plate firewall into it from the passenger compartment side. No fasteners showed, it came out well for a different look. Fudge was used to fill in the weld seams.

 

If you make a hammerform, you can make the rolled over edge on the lower part of the form by using a router with a roundover bit on the wood. Then make the upper part of the form cut at the edge right before the bead rolls over. Keeping the flat area of the piece of sheetmetal clamped between two pieces of wood like that with as many c-clamps as you can find really helps to keep the flat part of the steel flat. If you happen to roll some beads in the sheet metal first, you can use a router to make some grooves in the upper piece of wood that clearance the beads so you can still clamp it flat like a sandwich without distorting your beads.

3/4" partical board or MDF works okay, or if you want it really strong glue two pieces of 3/4" MDF together for the lower form. You can paint the partical board with some kind of shellac or varnish or waterproof yellow glue and the form will last longer -- especially around the edges where you'll be beating against it. Don't get MDF or partical board wet with water or it swells up and falls apart.