I need to make my bolts flush - they're M2 with nuts on the other side. The wood will have acrylic glued over the top so I need a flat surface.

I'm confused about how to select an appropriate countersink.

  • 3
    It's hard to tell where your confusion is... You just drill a hole slightly larger than the head of the bolt so the head will sit below the surface. – JPhi1618 Sep 6 '19 at 16:28
  • 1
    Unless you are positive that you are using the correct terminology, a picture is helpful. Are you asking about which countersink bit to use, or a countersink/counterbore method? – Mattman944 Sep 7 '19 at 0:45
  • which head style on your M2 bolts? standard hexagon, or some other? Clear acrylic, or opaque? – Jasen Sep 7 '19 at 11:22
  • @Jasen flat head bolts so they're flush. 3mm opaque black acrylic – Tom Sep 7 '19 at 12:00
  • @isherwood sorry, I mean nuts of course! – Tom Sep 7 '19 at 12:00

You want the diameter of the countersink bit to be at least slightly larger than the screw head. Larger is OK, for wood, larger bits don't cost much more. You then drill until the V-shape is slightly larger than the screw head. If I am drilling a lot of them on a drill press, I set the depth stop. If drilling by hand, I drill a little, then I test the depth by putting the screw head into the hole upside-down. If it is important for it to be exactly flush, work slowly.

Ideally the chamfer angle of the bit should be equal to the screw head angle. But for your application it isn't critical. My local home store only has 82 degree chamfer bits, this is close to the angle of most common flat-head screws.

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