I'm documenting this so I remember how I did it...
I'm in the process of installing the inner skins/walls and there are several places with switch or outlet boxes set into the wall where I needed to cut out a 2"x4" or 4"x4" rectangle in the middle of a large panel. This is the method I used.
Outlet Box Inside Wall
The first step is the absolutely most important step. You need to accurately locate where the hole goes in the panel. Simple in concept but difficult in practice. Forget the "measure twice cut once" saying, you need to measure 4 times because you'll only get the chance to cut once and if you screw it up you will waste an expensive panel. Maybe even worse, you'll need to order replacement metal and wait for it to arrive.
Once I've measured and gotten the same answer multiple times I scribe the cutout on the backside of the panel. This means you need to make sure you do a mirror image layout. The reason for scribing the back is that the cutting tool (Knipex Sheet Metal Nibbler) shears up and leave a burr on the "up" side. Even though I remove the burr with a file, you want this burr on the side that doesn't show.
2"x4" Outlet Opening Scribed on Back of Panel
Using a step drill bit create a hole on the inside of the scribed area. The hole needs to be about 1/2" in diameter for the Knipex Nibbler to fit in. The edges of the drilled hole should just touch the outside edges of the scribed opening.
Note: you could use sheet metal snips for this step but they may deform the metal. The Knipex Nibbler cuts on both sides of the blade so there is little if any deformation. It's very similar to the Genesis electric double cut metal shears I use to cut the panels to size.
Hole Drilled Inside the Scribed Area
Repeat drilling a hole for each corner. You can actually drill holes just in the opposite corners but I found that drilling all four corners was most convenient and took little additional time.
Next use the Knipex Nibbler to cut along the scribed line from corner to corner. The Nibbler has a little sighting notch on the front that allows you to follow the scribed line fairly easily.
Cut Along the Scribed Line with Knipex Nibbler
Continue cutting along the scribed lines until the center section is cut out.
Scribed Area Removed
The Knipex Nibbler leaves a pretty significant metal burr that needs to be remove. Additionally there may be some sharp corners where the straight cuts intersect the drilled holes. Both should be cleaned up with a metal file. It only takes a minute and will prevent cuts and possibly sliced wire insulation in the future.
Remove any Metal Burr with a Metal File
After cleaning up the hole do a test fit of the panel in the trailer. With any luck the hole will align with the outlet.
Test Fit
Truth in Advertising:
When I decided to document this method I'd already done it several times and I thought I had it down. But here's how the test fit actually went:
The "Real" Test Fit
Yes I cut the hole 1" too high! Argh! I'd used a small framing square to measure and read the offset from the inside of the square vs the outside. After laughing at myself for a few minutes I decided my best option was to move the outlet up an inch so it fit the hole I cut. LOL. It took about 10 minutes but worked really well and there are no other outlets visible in that area so it won't be obviously out of alignment. No one will know and I'll probably forget the mistake I made in a day or two.
Installation of the interior skin is coming along and so far I'm happy with my progress.
Interior Skin Installation Progress
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