If you're using a router I would look at single O flute bits. Onsrud is the maker of the ones I have used. They are very tolerant since there is a lot of room on them for chip evacuation. I have cut 1/2 plate with a .100" stepdown and a 3/16" bit diameter. 1/4 would have been sturdier, but I needed to interpolate some .201" holes. You can spin them at 20,000 rpm without worry.
A little spritz of wd-40 is very helpful now and then, and you must not let them recut chips left in grooves.
About 30$ for a 3/16".