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I need to cut out a shape (rectangular opening) inside a cement board. Was wondering if it can be done with a jigsaw and a special blade once I have pilot holes drilled in each corner.

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    Carbide blade should do it, perhaps slowly. A side-cutting router ("rotozip" et al) might also work; I know those will happily cut through tile. – keshlam Oct 23 '15 at 21:32
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    Just try to keep the dust out of your powertools. The dust from cement board and gypsum drywall is hard on powertool bearing and cause them to degrade more quickly. – JS. Oct 23 '15 at 22:15
  • I know, I already have a lot in my grinder from cutting brick and all over the expensive Makita battery – amphibient Oct 23 '15 at 22:45
  • If you have a grinder, get a diamond blade. Cut outside if possible and wear at least an n95 mask. – Aloysius Defenestrate Oct 24 '15 at 5:08
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There is no real experience needed when cutting an opening in cement board. I've seen openings for faucet handles created with nothing more than a well placed hammer strike (one hit each hole!). What ever tool you have available will suffice. I've drilled perforations with a masonry bit for a single shower valve. Oscillating tools with a carbide blade work. The primitive method is to cut the mesh with a utility knife and then force the board to break along the cut line. For a triangle cut not along an edge, mark the outline with a pencil, and drill a 3/8 inch hole at each of the 3 points. With an aggressive saw blade follow the lines to each 3/8 inch hole. If the edges are to jagged smooth with a rasp or a whet stone.

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I've cut out pieces in the middle of cement boards starting with a well placed whole saw drill bit for an opening, and following up that area with a sawzall shortblade for the actual shape. Grab a spray bottle with water to control the dust. It's a bit primitive but works well.. Good luck!

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Yes, it can be done... Personally, I would avoid high speed cutting tools, and simply use a drywall knife. It works very well. Leave one of the 4 edges uncut to simply score with a utility knife (as usual) to snap it. I've done this on several cement boards, and it works much better (imho) and without any dust.

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