I need to enlarge a few holes from 40mm to 50mm in a cinder block wall and in a concrete beam.
Chiseling will do much damage and need patching, can I rasp it with a wood rasp?
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You could use a heavy duty drill and a diamond encrusted bit, such as this one
They can be found on-line and in some big box stores for about $20.
When grinding or cutting very hard materials like concrete, you should use a lubricant to reduce heat. Usually a small constant drip of water at the cut point helps.
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This is the correct tool for rasping concrete but it won't fit in a 2" hole. I suppose you could ruin a file (and the better part of a day) to get it done; CMU's aren't that hard.
I'm adding my own answer since I tried to use the rasp.
It didn't work. The rasp can ease hard corners on cinder block and even a little bit on concrete, but it won't bite enough to enlarge a hole.
The method I use was a hammer drill with a chisel bit. I hold the chisel against where I want the edge to be and pressed the trigger very slowly. I did this so that it dig about 1 cm inside the concrete, then I move to the side and did that again making a contour. After that I used more force to break inside the contour. That way I could make a pretty good edge that requires minimum patching.