Banging Sheet Metal

making bulges

So on the Cheetah side, I am slowly ramping up for replacing all the engine gauges that I currently have on the panel with the CGR-30. I got it at Oshkosh, and finally, the box arrived a couple weeks ago.

Ain't she a beauty? Assembled on the bench for testing and familiarizing myself with it :)

Ain't she a beauty?

Because of it, I will lose a few 2 1/4 inch round gauges, and I decided to make some covers for the panel, not to have unsightly holes in it.

Of course I could've bought them from Spruce or whatever, 5 bucks a pop, but if you know me, I would rather spend hours making something I can make. Besides, this is a nice first sheet metal exercise :). And, I wanted a nice domed bulge on each one of them. Just Flat wasn't enough.

So, I got to it.

First iteration was making a jig that would sandwich the sheet between two boards, with holes over and under the metal. Then, I would press a "piston" thru the holes using my vice as a press. Pictures are better than a thousand words.

Three piece jig + soon-to-be-'piston'. Oak boards sandwich the sheet between them. Ply plate has the "piston" and is used as a backing plate for it.

Jig pieces

That didn't work out. Vice jaws are not perfectly parallel, and the "piston" is not exactly the same diameter as the holes -- so it gets misaligned, even though there are four guiding bolts in the corners... Here's the jig assembled, and one of the first tries.

First try...

I think I tried it two or three times, with the same result.. no matter how much I tried to align the "piston", it would make a bit more pressure on one side of the jig than on the other, either resulting in a tear, or in an uneven "bulge".

Out of desperation, I took a piece of metal, laid it over one of the holes in the oak jig board, and started banging away - and was surprised! I was getting almost what I wanted, just not clean enough.

I tried sandwiching the metal between two plates and banging via the top hole - and made a fairly nice bulge! The edges weren't very clean, but I reasoned that if I opened the top hole up a bit more, that would give me access to the edge, and I will be able to bang on the edge a bit more, defining it better.

Ha! No. Same stuff.. I was able to make a fairly nice bulge, but the edge will be all "torn" and ugly.

Bulge is there, but the edge is god awful

I also tried putting a profile on the bottom, "die" plate of the jig, with a very sharp edge and curvature under it.

Second jig

No luck. I think I spent a couple hours, making over ten "bulges" and trying different hammering techniques...

All those tries.

The Two Hammers

So I gave up, and emailed Bill Rose, the gentleman who gave me all the SolidWorks parts for standard HW, and, in general, helps me a bunch with my silly SWX and other questions, for advice.

Bill suggested using MDF for the jig instead of oak. Apparently, irregularities in wood grain would make it hard to define the edges uniformely. He also suggested using corking tools akin a big "chisel" on the edges to better define them.

..and.. it worked!

The third jig is ugly, but it works. Upper hole is enlarged just to get access to the edges (I didn't bother making the top hole concentric with the bottom or even round; there was no need).

The third jig

I used the two-headed plastic/rubber hammer's rubber head as a corking tool, lightly hitting it with the plastic mallet, and having it slide from the edge into the center of the bulge.

This bulge still has some "orange peel" on it and needs to be smoothed out, but the edges are very well defined!

Check out those edges!

But of course, I screwed up again - the hole in the bottom, "die" plate of the jig, is of a wrong diameter (I used the wrong holesaw). But hey, at least I think I figured that technique out!

Good evening in the shop tonight :).

It is always better to have C sub “t” greater than C sub “d". Or more plainly, thrust should exceed drag.