I looked at the steel I'd been trying to cut yesterday and decided to try another route. I have plenty scrap metal to play with!
Heading out, with the intention of heading to Lowes (hiss, spit), I remembered there was a mobile home store on the way. Thus, I headed in and discovered vent covers. There were some nifty plastic covers with lifting plastic louvers. There was also a steel vent cover. I looked at that and realized I had probably found what I need in order to keep water out of my ventilation system.
The traffic heading to Lowes (hiss, spit) was awful. It was backed up as far as the eye could see. Thus, I headed in the opposite direction and looked in Humana Re-store. They had interesting things but nothing I currently need.
I had an interesting exchange of emails with my dad who responded to my comments about the excessive cost of hole saws. I'd seen hole saws of 4 inch diameter going for silly prices. He suggested something called a Tank Cutter. Mentioning it in the Lowes (hiss, spit) across from Humana Re-store drew blank looks from the tool in charge of the tool section. Looking it up on my phone, a few moments later, I found one advertised on eBay UK. I also found one at Home Depot at $25. Thus, I enquired at the Lowes (hiss, spit) customer service desk and the tool there, instead of checking in the computer catalog in front of her, rang the same tool that had been so useless before!
Leaving the circus of Lowes (hiss, spit) and their performing seals, I sat in my car and looked on the Harbor Freight website. Sure enough, they had adjustable hole saws and at $6.95. Needless to say, 40 miles of driving later and I was there. I bought two adjustable hole saws and a pack of 4 inch grinding disks for good measure. I figure if the hole saws disintegrate then I can have two goes and maybe a 3rd by combining the remains. The grinding disk is the same diameter as the holes I need as opposed to my usual grinding disk that's half an inch too wide. That's a last resort to get the steel cut right.
Heading home, I unloaded and commenced work. The new plan is to put plywood on the inside of the bus over a square aperture cut with the angle grinder. I figure there will be no water so wood should be fine. The outside skin will have an aluminum sheet with a hole cut in it.
The adjustable hole saw needed careful tensioning on the grub screws. Out of an abundance of caution, I set up an OSB shield between myself and the work as the cutters will fly off if the grub screws aren't tight enough. I had to pick them up several times. In fact, just about every screw worked loose over the time I spent working.
The first thing I did was to cut two pieces of plywood with 4 inch holes. After that, I went to work on the aluminum sheet. That was a very different story. It took forever to get half way through. Clearly the cutting blades aren't strong enough. In desperation I drilled holes along the length of the track and will get a fret saw to work to join it all up tomorrow.
By the end of tomorrow, I hope to have one vent completely installed, possibly with a fan attached. The other might take a while longer but I'm in no great rush. Yesterday will do just fine.
I leave you with two images. The first is of the stuff I bought today and the second is of my adjustable hole saw in use on my aluminum sheet.