Standard countersink drill angles

[LCHIEN]

I was in a discussion on counter sinks on another site, so I made this chart showing

• Standard US 82 degree flat head countersink
• Standard Metric 90 degree flat head countersink
• Standard 118 degree twist drill point (common retail drill)
• Standard 135 degree twist drill point (used by professional machinists)

which is why you need to make sure you have the correct countersink and angle so that the screw is spreading its load over the entire bearing surface and not just a thin line which will deform and become loose with stress and load.

US standard flat heads require a 82 degree countersink bit. Many ads for countersink bits don't specify the angle and are often 90 degree designed for M-series metric fasteners... these are often sold on eBay, TEMU and even Amazon. So verify the angle.

Also, as you can see by my diagram, its not a good idea to use an oversize drill bit to drill the countersink. While 90 degrees is close to 82, 188 or 135 is way more angle.

For softwoods like pine, cedar, the use of 90 degree with 82 degree screws will probably compress the wood to conform. For hardwoods and metal, you will end up with force concentration around a small ring rather than the conical surface intended to bear the load. leading to a visible gap, and potentially to the screw working loose.