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I'm looking for advice on how to efficiently cut a rough hole about 4" diameter and 2" deep in a concrete floor. My tools are: a new 4.5" angle grinder with a diamond-cup grinding wheel and a diamond blade; a corded hammer drill with 1" masonry bit; an 8 lb. sledge hammer; cold chisels; and miscellaneous others. I can buy additional tools if there's a good reason, but they wouldn't get much use beyond this one task.

Background: I'm shaving down some high spots on a concrete floor, as preparation to pouring self-leveling cement. I'm experimenting with using my self-leveling laser to continuously illuminate those high spots, so I know where to shave and when I can stop. Unfortunately my laser device emits its horizontal beam several inches above the bottom of the device, so even placing the device at the lowest spot of the floor causes the beam to run about 1.5" above the desired elevation.

As a workaround to the beam-elevation issue, I'm experimenting with creating a hole in the existing concrete that's wide and deep enough to hold the laser device and for its laser to run along the desired elevation. (I'll either let the self-leveling cement fill these holes, or maybe pre-fill them with something like Quikrete.)

I've successfully done this in one part of the basement, using just the angle grinder and diamond cup. (This was my first time using an angle grinder.) It took about one hour to make a suitable hole, but once I had the hole the technique of using the laser continuously worked great.

Then... I tried to make my second hole more quickly / efficiently. I first tried using my hammer drill and 1" masonry bit, but that was pretty slow-going, perhaps because the bit diameter was ambitious for that drill. So I switched to my first angle-grinder and diamond cup. I tried to be more aggressive by pressing down harder, and by coming in at an angle (as though I was cutting rather than grinding). Unfortunately I asked too much of the angle grinder, and about 10 minutes into this approach it emitted the smoke-puff-of-death from its vent.

I made sure my replacement angle grinder has a beefier motor (11 vs. 7.5 amp), and now I know better about how to avoid cooking an angle grinder. But this could potentially add time to the task of creating these holes in the floor, since I may need occasional breaks to let the motor cool.

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Just cut a series of parallel slices about a quarter inch apart. This should take barely a couple minutes with a diamond wheel. Since you're patching anyway, overcut somewhat so that you have full-depth cuts throughout the area needed for the laser device.

Now take a cold chisel and hammer and just snap them off. Start by driving the chisel downward to break the first plate off, then at an angle near the bottom of each plate.

If your grinder doesn't give you 2" of cut depth, do the work in two stages. I can't imagine the whole process taking more than five minutes.

  • I tried @isherwood's idea. I used a 4.5" 11-amp grinder with diamond blade, and a hand-held sledge + cold chisel. I found it took about 70 minutes to make the hole I needed. (The hole was a little oversized, so if I was more careful in planning I could perhaps have gotten it down to 50 minutes.) The shallow blade depth required 3 layers of cutting to get the depth I needed; in between layers I removed dust and de-dusted my grinder's vents. I'm guessing that a 7" grinder would have allowed just one layer of work, cutting my time to about 15 minutes(?). – Christian Convey Jul 3 at 19:19

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