I would like to drill a regular (I think 3/8") knockout on a metal appliance housing. It is not tabbed like on a metal handy box. Which drill bit should I use?
Consider buying a step drill bit for drilling clean holes in thin materials. Step drills look like this:
The magic happens because they increase the size of the hole gradually, and each previous step holds the work steady as the next step cuts the material. You can find various demos on youtube, but basically if you've ever tried drilling a sheet material with a bit that is the end size you want, you tend to end up with either a triangular hole, a hole with ragged edges, or the drill bit cutting through the sheet quickly and turning it into something like a screw thread that the drill bit screws itself into, leaving a big mess
Note: in case it's not clear from some videos you might find that demo step drills, you can stop drilling when the hole gets to the target size which means one step drill bit can cover a range of needs - some electrical tape stuck on/wrapped round the bit on the "next size up" can help you remember where to stop, because it's pretty 'ard to read the numbers at hundreds of RPM :D
Step drills can also be used to soften the edges of a hole and remove sharps if you just gently push the next step against the work and cut a slight chamfer. Very handy things; you'll be able to find some videos comparing brand effectiveness I'm sure - a good bit may be expensive, but it's probably one of those things you want to lay out a little more on in return for a longer service life, neater cuts and not having to hand onto the drill until your arms are worn out
nigel222's absolutely right with an obervation that it's also possible to get bits that are perfectly conical and can hence drill any size of hole, albeit slightly less precise. In the past I've used these in tandem with a step drill; step first up to the size smaller than what you want, then you can open the hole up a little more, if the next step up on the step bit is too much.
I've also, on softer materials, reamed the hole slightly larger (when the step bit is in a pillar or fixed drill; it's a bit harder to be accurate if it's in a hand drill) by pulling the work around slightly - first cut a chamfer with the normal part of the step, and then back off a little and use the sides of the step drill to trim away the chamfer.
Don't forget too that step drills also come in mm sizes which might give you an acceptable "in between" size; imperial bits essentially go in steps of about 2.5mm and mm bits tend to go in 2mm
If you have an electrician buddy borrow his knockout punch for the size you need, probably for 1/2". If not you probably can rent one. They typically come in sets. You will need to drill a hole maybe 3/8" depending on the punch set. You then place the bolt through the die, then place the bolt through the hole you drilled, then thread the punch on the end of the bolt. Tighten with an appropriate wrench and it will give you the hole you need. Do not forget to remove the scrap piece of metal from the die. Here is the link: https://www.greenlee.com/us/en/punch--die-set1-2to-1-1-4-7235bb?w=2&c=161&b=86&r=30&p=1&l=4&v=c If you follow the link you will find there is a large varity of these available.
There are some connectors labelled 3/8" such as an Halex 05103B, the 3/8" in that describes the nominal size of the cable or flex being fitted to the connector. If you click the link the catalog page show in misc notes that it fits 1/2" knockout.
The nominal description of knockout size corresponds to raceway sizes the hole is cut for, the raceway sizes are measured by their approximate internal diameter, the hole is measured to the outside of fitting.
The holesaw or punch size for the exterior dimension of 1/2" raceways and the 3/8" fittings above is 7/8".
EDIT: If thick metal sometimes step bits will leave a tapered cut or if too thin when penetrating the ring you want it will pop through and the next ring will start peeling the next larger size, particularly if you are using a torquey drill. With experience you can overcome that, but for one hole a cheap hole saw should be just fine.
Easy / Fast: A corded electrical drill is used ensure power. I use a hole saw to drill a knockout in metal electrical fixtures. I start with a small hole so that the hole saw will stay on target. Ear and Eye protection must be used.
In my experience it worth every penny to purchase a good hole saw set, along with ear and eye protection.