You can use one of two methods to make the hole wider in a porcelain sink. You can use a grinder to wear down the porcelain, or you can use a diamond grit hole saw to cut a new hole. The hole saw is definitely faster, but it's a little riskier because it generates more heat. No matter how you do the job, you have to do something to keep the porcelain cool so it doesn't crack.
Set Up a Water Station
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Porcelain is basically ceramic that has been fired at a high temperature to make it almost glasslike, and like glass, it's fairly fragile. When tile pros cut ceramic or porcelain tile, they usually use a wet saw, which continuously sprays water on the tiles as you push them through the blade. You need to use water in the same way when cutting or grinding a porcelain sink, or a hairline crack will quickly develop, and the sink will be ruined.
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An easy setup calls for a 1-liter plastic soda or water bottle that you can hang over the sink with a length of twine or wire. Drill two or three holes in the bottom of the bottle using an eighth-inch or narrower drill bit and then tape over the holes with duct tape and fill the bottle with water. Hang the bottle over the sink, put a bucket underneath the sink drain and position the bottle so that when you remove the tape, the water will flow around the area you're cutting, and you're ready to go.
Cut With a Hole Saw
You actually need two hole saws to accurately widen a hole in a sink. The first is a wood-cutting hole saw that is the same diameter as the existing hole, usually 1 1/4 inches, and the other is a diamond grit hole saw with the diameter you want the new hole to be, which can be 1 1/2 inches, for instance. Each hole saw has its own pilot bit, and you insert the bit and the saw together in your drill. Always wear goggles when working with a hole saw (or any other type of saw).
Use the wood-cutting hole saw to cut a disk from a piece of 3/4-inch scrap plywood or softwood lumber and then insert the disk into the sink opening and make sure it fits tightly. You may have to wrap duct tape around the perimeter to get it to stay firmly in place. Now, fit the pilot bit of the diamond grit saw into the hole created by the pilot bit of the wood-cutting saw, pull the tape off the bottle to start the water flow and begin cutting the porcelain.
The purpose of the wood insert is to keep the hole saw centered while you cut the porcelain, so if it starts to rotate with the saw, it isn't a problem as long as it stays in the hole. Run the drill at a low speed and avoid putting extra pressure on the saw while it's cutting. You may have to stop and refill the water bottle once or twice during this procedure.
Widen With a Grinder
A slower and less accurate way to widen the hole in a porcelain sink is to use a rotary tool with a grinding accessory. This method also generates a lot of heat, so you still need the water bottle to spray water on the porcelain while you're working, and don't forget the goggles. Run the rotary tool at about half speed and keep moving the tool around the existing hole at a uniform speed to be sure you widen it symmetrically.