Help!! I have a tricky problem-- not blacksmithing, but tool-related. Putting a new roof on house, roofer left me with OSB (fancy particle board) sheathing under HD corrugated steel roof. Gotta cut a 14" OD hole in OSB for 2" setback from combustibles on rated 10" woodstove flue pipe, but then immediately above that a 10" hole in tin. Home Depot has: Ryobi, DeWalt, Rotozip spiral saws available. Any user evals on these tools much appreciated.

Spiral saws ok on wood products, don't think they will work on metal, sabre saw comes to mind. They kinda like a mini router, with all hand control to cut to mark

Jigsaw for the wood and tinsnips for the tin.
Bob

I tke it the tin is on the OSB. I think a sawzall with coarse metal blades may be the ticket. a sabersaw would be my next choice as irnsrgn said.

coarse teeth for light tin is a no, no, the tin can slip between the teeth and make a mess of very ragged torn and ripped edges. rule of thumb for teeth when sawing thin material or any material for that matter at least 2 teeth in the cut at all times.

Milwaukee Tools just came out with an "ICE EDGE TORCH" sawzall blade.

Ice Edge Torch

I've tried the rotozip and they work well. Last I remember cutting was some drywall around an outlet box. worked very well with the proper bit. obviously the osb is tougher and full of glue, not sure how well it would handle a high speed rotary tool. If you do decide to go that route make sure it's a decent variable speed (only the cheapest are not) and buy the miniature side mill bits they have available.

Now if I was personally doing it. there's an adjustable jig for power drills that allows you to cut some massive circles. I forget the name of it, but it is used to cut holes in the metal duct work for central a/c units. Simply put, you drill a hole in the center position, place the opposite end of the attachment in the hole. adjust for the radius you want to cut, and go. basically an infinitely adjustable hole saw.

Instead of having a blade of the exact diameter, you are pivoting off of the center point and using the rotary cutter to replace the saw. You could build one yourself by drilling a hole at one end of a bar and inserting a nail, and drilling a second hole the radius of your cut away. Wood would work but you would need a bushing on the drill bit side.

something similar to the idea, but specifically for rotary cutters Roto-Zip CRCT2 Circle Cutter Attachment
found the one I was talking about Malco - A Brand of Malco Products, Inc. | Products | Shear / Cutter Accessories | Hole Cutters
very handy indeed, though i'm not sure of the depth it would cut. should just make your own.

Thanks all, for the advice-- cutting the material is not the problem (I have an ancient recipro saw that'll do that in a flash), it's cutting the OSB from below without penetrating the tin lying smack dab atop it. There is lots of room to work. I mainly was/am curious as to what users think about how the various brands of spiral saws (which are just little hand-held routers), Ryobi, DeWalt, Rotozip, behave-- and hold up-- in actual use.

I've used Roto-zip, and own a DeWalt. I feel like my DeWalt has a bit beefier motor, but my brother's Roto-zip has oh-so-many more attachments available. There are none for the DeWalt (at least when I bought mine a few years ago). My recommendation to you is that unless the tool is going to be for a one time use AND you are strapped for cash stay away from the Ryobi. I had a few of their earlier tools and they were pretty good, but the ones I've bought over the last couple of years have all gone to xxxx. I'm in the process of replacing all my Ryobi cordless stuff with the new Makita lithium battery stuff.

How much room to the sides of the circular cole in the OSB do you have? The hole in the plywood doesn't have to be perfectly round. Mark an octagon, then set your circular saw to the depth of the OSB, and plunge cut in for each of the 8 sides and voila you're done with that part.
ML

Crisis resolved, if not exactly solved: CEO here at Entropy Research has spoken, says bag the spiral saw jazz and drill a xxxxed pilot hole above dead center on the flue, go up top, cut a bloody 14-inch hole around pilot hole with reciprocating saw (or that Sawzall in the ABQ Craig's list now for $50!!), insert pipe, slap on a goodly quantity of goop, slap on stainless flashing mit der stainless boot, and say to xxxx with it, life goes on.
Many thanks to all for thoughtful advice. I will file it away to use in next crisis.

Referring to the time honored comment "When you ask a question, you get 12 answers from 10 blacksmiths, I will add the following:

Drill a series of small holes around the circumference of the circle, then pull out the center plug. Smaller the drill bit, the smaller the holes it makes, and the smoother (a relative term) the edges of the hole. Then just use a burr, rasp, or what ever to smooth up the edges or correct any clearance problems.

This method can also be used to make an angled hole through material with thickness. Just adjust adjust the angle of the drill bit to match the desired cut angle.

Put a piece of masking tape around the drill bit to use as a depth gauge.

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One thing I will mention is to waterproof everything.

OSB (fancy particle board) is particle board using big chips instead of sawdust (my opinion). I would be afraid that the even after it acclimates to the local humidity, it will do all sorts of distortions and separations if it gets wet. You may want to build a metal circular ring to firmly attach, and caulk, to the tin roof as a channel for any water to bypass the OSB. You may want to also consider keeping any inside air from getting into the roof between the tin and OSB and condensing, therefore getting the OSB wet.