You can draw the circle you want drill holes every 2" and then saw between the holes with a saw zaw or a jig saw with a carbide blade.

 

how an adjustable hole cutter? Or a Roto-Zip tool with a trammel attachment? Even a cheap set of hole saws from Harbor Frieght

 

HF's biggest hole saw is 5". I would need to take a rasp and open the holes up just a hair. For this task, I am not allergic to that idea… I don't mind trashing a HF hole saw.

 

The last time I bought a hole saw from HB was the LAST time I bought a hole saw from HB….Pretty much next to worthless.

 

Do not use a hole saw to cut a hole in Hardie board. All it will do is grind the teeth down to nothin. I'm speaking from experience, $25 hole saw now just a hole. use an angle grinder, it kicks up lots of dust, but it works great. Just have someone hold the shop vac next to the grinder and you'll catch most of the dust.

 

dbhost: sorry, I thought my HF hole saw set ha bigger than 5-1/4", my bad.
thewoodmaster: at $5.99 a set, it might have been worth it to get a few holes before having to toss the one size.
I do have an adjustable carbide tip circle cutter for cutting circles in ceramic tiles, and have used it in cement backer board, and also use the roto zip tool often with the carbide bit. I was offering the HF holesaw option as a cheap, one shot method.

 

If you are putting up the Hardi Plank and have a cement siding blade you can use your skilsaw to make a series of plunge cuts so you get something that looks like a cut up pie, than you use a hammer/pair of pliers to remove the sections. A grinder can do it to. A cement shear works also.

 

I cut holes in Hardi- Backer with a carbide bit on my roto zip with the vacum running next to it.

 

They make jigsaw blades specifically for cement board, which is what Hardi-board is. They also make hole-saws, but they are really expensive. I suggest the jigsaw.

 

So my thought process of using say my Jig saw with a RemGrit blade isn't off base? Those blades are kind of thick, so I was thinking about drilling a series of holes with my hand drill, then jig sawing hole to hole, cleaning up with a rasp. This will all be hidden behind a basically oversized Supurr vent anyway, I am just trying to get the port through the wall, and keeping creepy crawlies from coming back through…

I could probably manage with a cabide spiral cutting bit in my Dremel. No roto zip to work with. Had one, didn't use it for at least 10 years, gave it away…

 

dbhost, your right, using that blade, you will cut through it easily, (like a hot knife through warm butter.) If you draw the circle, place one good hole, insert blade and your done. The finish cut is pretty nice, I usually will hit it with some 120 grit sandpaper to make it pretty, but that would be all you need.

Just make sure to seal any area you have cut with the paint you are using. If you don't, it voids the warranty. Water will get in behind the paint and cause bubbling and peeling.

 

I was planning on hitting it with about 2 coats of the paint on the cut surface, and once the port is in place and mounted, sealing any gap between the port and the hole with that 50 year silicone caulk, and this is prior to putting the louver over top of that…

I am assuming I should drill pilot holes for the screws and possibly put a dab of silicone on them to seal the Hardie at the screw holes for the louvers as well. Is that right?

 

Yep, that sounds about right.