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.
Screws – How to Countersink an M2 Bolt
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You took the wind out of my sails with that last paragraph. I was going to suggest using a nut/bolt.
As far as it causing problems when the tabletop is lowered, you can recess the bolt head. Yeah, you could make the cavity in which the bolt will stay on the underside of the tabletop, but maybe you'll drill too far or crack/scratch the tabletop if you chisel it out. It could end badly... which is why you should instead house the bolt head on the top-side of the cross member.
Brown = table top
Black = underlying cross member
The one on the left is what you were talking about. The one on the right is what you want to do.
Also, use a washer under the bolt head, etc.
Sounds like trouble to me. Superglue won't do you any good, as it's only as strong as what it's gluing to (and I suspect here it would be far less strong).
Some alternatives:
- Cut away a square of drywall and replace it with the same thickness plywood, glued to whatever is behind (e.g. that metal). Then apply a thin coat of plaster, paint, and then screw into it
- Find out just what that metal is, and consider screwing into it
- If the curtains will generally hide that bracket, consider gluing a thin piece of plywood to the surface of the drywall and screwing into that
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Best Answer
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.
https://www.homedepot.com/p/RYOBI-5-8-in-High-Speed-Steel-Countersink-A10CS82/205469295