I want to cut a 12" diameter circle into the side of my van to install a window. My current plan is cut it by taking a sufficiently sized drill bit, drilling a hole near the inside edge of the circle, and using a jigsaw to cut the rest with a fine metal cutting blade.
I've also heard that you can accomplish this using electric shears, but I hear they are only useful for "gentle" curves. How gentle are we talking?
How would you tackle this problem?
Cutting curves in sheetmetal is a good job for a pneumatic nibbler. They can cut strait lines or curves with any radius and can be a little cleaner than a jigsaw because they support both sides of the material while punching out little bites.
If you don't have a decent air compressor, then they also have electric versions and even drill attachments that do the same thing. They can only cut sheet metal, so if you have to cut through a support or reenforcement, you will have to resort to a different tool for that. There is a learning curve to using these, so start with a smaller diameter hole to practice then move up to the actual hole you want.
If the only tool you have is a jigsaw, then make a radius guide. You'll need a fine-tooth metal cutting blade, and maybe several. A sabre-saw is not suitable for this approach.
Start by drilling a center hole in the van, and carefully measure inside and out to make sure you're clearing any framing members or reinforcements. Presumably you've removed any lining already and have clear access.
Drill a small entry hole on the circumference, just large enough for your jigsaw blade to start. Slightly inside of the final diameter.
Use a short length of scrap wood or metal, and drill a hole in one end. Secure the other end to your jigsaw so that it sticks out to one side but doesn't interfere with operation.
I would work from inside the van, so you're not dragging the jigsaw around the exterior paintwork.
Use ear protection because its going to be awful loud in the tin box.
Use eye protection because there will be sparks. Also have an assistant on the outside who can watch what you can't see, and kill the power if there are problems.
Bolt the radius-wood to the center-hole of your planned-hole, and work your way around taking your time on the cut.
If you had a Dremel-style cutoff wheel, then the same basic idea would work to hold your tool on the circle. A regular grinder is 90 degrees out so would not work the same.
I would prefer the nibbler personally.
A circle would require the van side be flat. It's not flat, it's curved. The curve gives it a great deal of strength. If it wasn't curved it would go "waggle-waggle" as you drove down the road. The shape is akin to a Pringles potato chip.
If your window depends on being a true circle in a flat medium, and not a Pringles chip, then the window will not fit.
The van metal is VERY thin - its strength comes from being stamped in a curve, not thickness. The #1 problem will be distorting the outside of the circle and creating yourself either ugliness or a need to do body work and paint. Almost any power tool will tend to do that, as will many hand tools.
With that in mind, I would tend to go old-school. First, as JACK advises, lay down masking tape and accurately mark the circle precisely on the tape. If you bodge it up, peel the tape, lay new tape and mark again. That's cheap/easy.
From there, I would use hand tools, such as a VERY fine tooth hacksaw, working carefully. Start it by drilling a hole well inboard of the edge. Either use the kind of hacksaw where part of the blade extends beyond the tool (and don't kink it).... or, cut a 4" diameter hole in the middle using coarser methods, cut the first inch with the bare blade (wear gloves, I show this in red) and then put the bow of the hacksaw through that hole. 12" is a large enough diameter that a hacksaw can make that curve. You'll have 45-ish lineal inches to cut, but that's not so far in such thin metal.
If I had to use a power tool, I'd use a Dremel with the little cutoff discs (you know the ones, they come in a 40-pack typically). The Dremel is too small to distort the sheet metal, and as always with those discs, they shatter when you are not careful, and patience wins the race. A hand hacksaw would be faster, but some people need to use power tools.
Make sure not to leave the masking tape on for more than needed, and cover it well if you must leave it. Over a few days, heat of the sun, and UV light from the sun, will damage the masking tape, and the damaged adhesive won't peel, and will leave very stubborn gunk on your paint job.
Aviation Tin Snips are also an option, however being manually powered they're a lot of effort.
If you're right-handed then you probably want the red-coloured ones, because they turn to the left as you cut. Left-handed users would want the green-handled ones in this instance.
Downsides are that you need a fairly large entrance hole, and like in @harper's answer you need to spiral into the final cut, which will make your cutout piece less-useful.
