Blade sizes on circular saws

9 Jan 2020
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United Kingdom
I have the Dewalt DCS570N ( which runs a 184mm blade with 16mm bore and 65mm cutting depth.

I'm after a cement-fiber blade for a fairly small one-off project and I've never bought a replacement blade before. Do they have to be exactly the same diameter? Dewalt themselves sell specialist cement-fibre blades in 165mm and 190mm for instance
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They have to be the same size or slightly smaller - the "rule" is normally that blades should be at least 80% of the designed diameter, which on a 184mm saw would mean at least 147mm, so 165mm should be fine, but your depth of cut will go down to about 55mm. This is hardly an issue for cement board. More of an issue might be the blade kerf - if you can get a 1.5 to 1.8mm kerf PCD blade for a cordless tool then great, but if your kerf is wider than that watch out for your batteries getting hot, etc. By looking on eBay you might find USA market thin kerf saw blades - Ø184 x 16Ø bore is common in the USA whereas Ø190 x 30Ø bore is more common over here

The 190mm blade might work (I have an old saw which is designed for 156mm blades - but it will take 160mm blades as long as I remove the riving knife, a potential safety issue), but I wouldn't depend on it. So unless you can get an old 190mm blade to try it out I'd probably give it a miss

As to cutting cement board, it is an incredibly dusty thing to do. You need a serious extractor and a good quality mask to undertake this - the last time I did it was with a corded plunge/rail saw (better dust extraction than an open sided/conventional saw), a Festool CTM36 extractor with a pre-filter cyclone, and wearing a JSP Force 8 FFP3 mask (for which I had been face-fit tested). By break time on the first day all I could taste was cement. The job (12 and 15mm sheet) also seems to soak up power, so as I said, your batteries might get hot quite quickly if you are doing a lot and you will find that they don't last as long as they do on wood.
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Cheers @JobAndKnock yeah for cutting 20cm cuts on 8mm boards the cutting depth isn't a factor.
I'm planning to have a guillotine and just use the saw for angles or a few rips - I might get carbide jig-saw blades for fiddly bits.
This will all be done in a big outdoor space but I will get a decent mask - I am making a few cuts not cutting for long periods fortunately.
The carbide jig saw blades don't last that long, but they can do the job. I found the cheapo TopsTools ones sold on Amazon lasted almost as long as the Bosch blades on 12 and 15mm Versapanel cement fibre board. Bosch make a brilliant carbide grit cutter wheel for 115mm grinders which is very fast for rough cutting work (such as cutting access openings after fitting) - it cuts most of the things you put it near. e,g, aluminium, steel, cement fibre board, hardwood ply, OSB, etc and works happily on an 18 volt brushed cordless grinder, with the caveat that it eats battery charge
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I didn't think of using my little grinder. But, I figure I'll need a circular saw blade because even if I only have to rip one board that's going to be worth it!
I might even see if I can 'massage' the design slightly to avoid ripping any full-length planks since at this stage I still have the flexibility and I already chose the product (James Hardie)

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