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First time cutting plywood using foam board as support, and it burns, melts, and produces enough smoke that I have to open the garage to ventilate it after every cut. There is no damage to plywood, but burning foam does not support the cut, thus leading to chip out. Here is the example of what it does:

foam burn example

The foam board is a major brand pink insulation from a big box shop. That seems to be the kind that I see being recommended for that purpose, but it appears to be a softer foam than the one it should be, the harder styrofoam-like substance (For instance, Kreg talks about rigid foam insulation). What chemical compounds should I get for this application, and what should I avoid?

I use 4600 rpm worm-drive Skilsaw with the track sled attached and thin kerf 40-tooth blade, so it shouldn't be an issue.

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    Adjust the cut to miss the foam. – Solar Mike Dec 12 '19 at 11:23
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    @SolarMike I think the OP wants the foam to directly support the bottom of the plywood to prevent chip-out. Unless they can adjust the depth of cut so that it is exactly the thickness of the plywood (with the unlikely assumption that the thickness doesn't vary), they have to cut into the foam at least a little bit. – Martin Bonner supports Monica Dec 12 '19 at 11:50
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    What exact blade are you using, and what is your depth of cut set to? – dwizum Dec 12 '19 at 14:13
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    Our panel saw used an 80 tooth finish blade and we did not have a backer under the cut but the pice was supported by the frame within 1/8 of an inch, the blade may be the problem as I think @dwizum is getting to. – Ed Beal Dec 12 '19 at 15:05
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    Which Freud blade though? To reduce heat and tearout, getting the right tooth shape and hook angle is important. Not all 40 tooth blades are the same. A high bevel ATB blade with a lot of hook will cut nearly perfect without any backing at all. You could switch to a plywood backer if the foam continues to melt. The blade is probably heating up too much in the plywood. Staying with a lower tooth count and a thin kerf will keep it cooler. I have cut a lot of thin plywood with foam as a backer but if you've got a material that's generating a lot of heat, it may not be a good option. – dwizum Dec 13 '19 at 15:34
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Never tried this, honestly don't see a point in doing it this way (expensive material, and yes, foam melts.)

Ignoring the option of using a blade that causes less problems, a supporting material that costs less and does not melt would be appropriate. OSB, or MDF for two, or hardboard, for another. Hardboard being the generic term for Masonite® which you might or might not know it better as. And I do recall mention of using it for exactly this purpose on difficult materials in some old woodworking text.

However, as a rule the combination of the correct (and sharp) blade and the correct depth of cut for that blade (so it's cutting into the surface, not stripping it off - this problem is usually associated with inadequate tooth projection from the cut material - the entire tooth down to the gullet should be sticking out below the sheet) are a more common solution that does not require a bunch of sacrificial backing material. A dull blade will tend to beat the wood more than it cuts it and aggravate this problem.

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  • I agree. Tearout occurs on top of the workpiece, so the support hypothesis fails. – isherwood Dec 12 '19 at 17:52
  • @isherwood, in my case, the tearout was on the bottom, which confused me, but led to resolution: since it is a worm-drive saw, I have put the blade the wrong way (rookie mistake). The support is not as much for tearout, but to give the sheet uniform sacrificial surface to work with. – theUg Dec 14 '19 at 21:17
  • @Ecnerwal, many reputable shops give that advise. I've even linked the video from Kreg Tools in my OP. Your advise about OSB/MDF/hardboard etc. is in the same vein, but the argument goes that foamboard may have higher upfront cost, yet it may last longer, and easier to handle, due to it being lighter. – theUg Dec 14 '19 at 21:22
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The culprit turned out to be my lack of experience. Previously, I’ve only used the regular, direct-drive circular saw, but had never used a worm-drive saw. When I had installed a new blade, I assumed that branded side of the blade should be visible just as it is on a regular saw. Since the blade on the worm-drive saw is on the left side, but rotation is the same, it should’ve been oriented the same way, so that branding faces inwardly.

I've discovered that by trying to figure out why I was having the tearout on the underside of the plywood which shouldn’t have been happening with a circular saw. Only then I have realised that my blade was reversed. It was an idiotic, but honest mistake, I would like to think.

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A method to prevent tearout that I've had success with was using 1.5 or 2 inch painters tape. Roughly figure out where you want to cut and place the tape over that area. Tape on both sides, pressing it down firmly. You use the wider tape to give you a margin of error. Mark your line and cut. Peel off the tape when finished. A blade with a finer cut (more teeth) should help too.

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