How to increase the size of an existing metal hole in kitchen sink

Question

I want to install a sink soap dispenser but the existing hole in my sink is too small. From what I've found the universal diameter of the threaded piece that inserts into the hols is 1 inch. My hols is closer to 3/4 inch. So, how can I increase the size of an existing metal hole in kitchen sink? My research so far has led me to using a piece of clamped 2x4 as a guide for a drill bit but I cannot clamp the wood to the sink. Any ideas?

Answer 1

You should be able to use a knockout punch.

These are inserted through a pre existing smaller hole, and can cut sheet metal including stainless steel. They come in a range of sizes.

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Answer 2

I did exactly this recently using a standard drill, small bits, and a cutoff wheel in a rotary tool (Dremel). I marked the circle and drilled a series of small holes using high-speed steel bits of around 1/8" in size. I then cut between the holes with the cutoff wheel. It leaves a somewhat jagged hole, but it was easy and free and isn't visible once the dispenser is installed anyway.