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I need to drill a threaded, countersunk hole in a piece of material. After drilling, should I use the tap, or the countersink bit first? Does it make a difference? Does it depend on the material?

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    @DMoore How can you tap a hole that doesn't exist? I'd think drilling must be the first step.
    – Tester101
    Jan 15, 2016 at 17:44
  • Step one: locate a Bridgeport.
    – Mazura
    Jan 16, 2016 at 1:24

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I would countersink and then tap for two reasons.

The first is the countersink bit could damage the first thread and make installing the bolt a problem.

The second is there is less metal to tap and therefore easier and less wear on the tap.

Happy Day!

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  • Use an awl and punch it to center mark your location (so the drill bit doesn't walk).

  • Drill a small pilot hole (to insure correct placement of the hole).

  • Enlarge the hole to the tap's specification.

  • Countersink (without the hole as a guide you run the risk of the countersink going off center).

  • Tap (run it through again if you had to readjust the countersink depth).

I never seem to get the depth of the countersink right on the first try, but you can just clean the threads back up with the tap if you have to hit it again with the countersink.

This procedure doesn't really change for different materials, mostly just the level of difficulty and the quirks involved working with them. I.e., aluminium likes to gall; hardened steel doesn't like to be drilled; wood is not normally going to tap, etc. Titanium? good luck with that.

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    Completely agree the keys are punching your awl right and the pilot is a must.
    – DMoore
    Jan 16, 2016 at 4:32
  • @DMoore - Agreed. I was fine with Tyler's answer save for that.
    – Mazura
    Jan 16, 2016 at 4:56
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You countersink first, then drill, then tap.

If you countersink after drilling the hole, the finish on the countersink dome will not be as good.

I would recommend drawing a circle indicating how wide the countersink diameter should be so when you make it you know when to stop.

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    Your answer would be much better with some explanation or reasoning to support it.
    – isherwood
    Jan 15, 2016 at 20:52
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    How do you keep the countersink from walking if there's no hole?
    – Mazura
    Jan 15, 2016 at 23:13
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    A countersink will not walk unless the material is hard, like metal and you are using a hand drill. In that case using a drill press is generally mandatory. If the work must be done with hand tools, then a prick punch should be used to set the point of the countersink. For a standup job on a hard material that is precision, it may be required to use a wall-drilling jig.
    – Tyler Durden
    Jan 15, 2016 at 23:32
  • If you're concerned about the finish, then drill pilot hole, countersink, drill final hole, tap. Otherwise, drill, countersink, tap as described above. The assumption here is that the material is metal, if it's being tapped.
    – Mark
    Jan 16, 2016 at 2:35

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