Here I'm drilling a hole near the bottom of the kettle for the drain. The
key to drilling stainless steel is to use a slow speed and to not let the
metal get too hot. If you do, you will change the composition of the metal
and it will become brittle and lose its stainless properties (it will rust!).
I bought the hole saw at Ace
Hardware. Make sure you get one made for cutting metals.
It took me years to get comfortable with the idea of drilling a hole in my precious 10-gallon kettle, but when I finally did it and installed a ball valve, I regretted that I hadn't done it sooner. I had been lifting the kettle (after immersion chilling) and pouring the cooled wort into a carboy through a funnel. This is not very safe nor is it good for your back. I was so happy with the drain in my 10-gallon kettle that I didn't think twice about drilling holes in my brand new kettle and mash tun.
The hole left by the hole saw was rather sharp and had a ridge on the inside,
so I used a rat-tail file to smooth out the hole and remove the ridge.
I recommend ear protection during the drilling and especially during
the filing!