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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?

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    Are you sure the KO needs to be 3/8" nominal and not 1/2" nominal (7/8" actual)? Also, I presume this appliance housing is sheet metal, no? Oct 21 at 2:12
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    Also, drilling sheet metal leaves sharper edges than on a junction box -- be sure to use a strain-relief that protects the cable and can be mounted firmly on thin metal. Oct 21 at 2:25
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    Consider using a nibbler tool for clean cuts in sheet metal. They make ones that are drill powered or even hand powered. Its certainly a lot slower than Gil's answer below, but you might get more use out of it.
    – JPhi1618
    Oct 21 at 3:01
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Consider buying a step drill bit for drilling clean holes in thin materials. Step drills look like this:

enter image description here

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.

enter image description here

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

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    You can also get conical cutters which are not stepped. The drawback is that the hole is slightly larger on one side than the other, and you don't have widths marked on the cutter. But apart from being cheaper (when I bought mine, decades ago) they may also cut more smoothly in some materials, and can do a hole of any diameter, not just one of the marked steps.
    – nigel222
    Oct 21 at 12:55
  • Another way to mark a step drill is to use a brightly coloured permanent marker (Sharpie) on a step. This can be cleaned off with common workshop solvents like denatured alcohol. That way you can use multiple colours and there's no chance of the tape snagging and coming off at an inconvenient moment
    – Chris H
    Oct 22 at 11:18
  • Top tips for step drills - use a press if possible (if not, really try to keep square to the workpiece), and be gentle. In steel, a drop of oil on the edge of the cut every few steps is helpful
    – Chris H
    Oct 22 at 11:20
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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.

enter image description here

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    If he doesn't have an 'electrician buddy', maybe he can rent the tool. Oct 21 at 4:50
  • A little bigger drill than 3/8 is preferred. Otherwise, it's difficult to remove the punched-out piece from the 3/8-24 threaded bolt in the punch. I use a 25/64" or 13/32" drill. Oct 23 at 15:22
  • How is this different from a hole-saw? Maybe this is better than hole-saw somehow?
    – gatorback
    Oct 23 at 19:49
  • @gatorback hole saws for steel do exist but are rather slow and messy. Punches for metal enclosures cu the metal like shears, and leave quite a clean edge
    – Chris H
    Oct 25 at 10:36
  • @Gil please consider posting a photo and a link for those of use who are not familiar with the knockout punch. Not sure if my Amazon link is representative
    – gatorback
    Oct 27 at 13:45
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7/8".

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.

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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.

enter image description here

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  • These work better on wood than metal
    – amphibient
    Oct 23 at 18:19
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    @amphibient True: wood is softer than metal, however, in my experience drilling out a ring is preferable to grinding out the entire disk with a step drill bit. I've drilled about a dozen knockouts using this technique: I am a "weekend warrior". Maybe the pros have a better technique
    – gatorback
    Oct 23 at 19:41

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