Your 82 deg screws are making INITIAL contact with that 90 deg countersink only near the screw's shank. When you tighten them, depending on the alloy, the aluminum will probably distort to conform to the screw head and all will be well. But if it a hard aluminum alloy and the torque is not very large, then they will continue to make their primary contact near the screw's shank. But, if the torque is light, then this seems to argue that the forces are light and that this mismatched contact may be OK. BUT, of course, a properly machined hole that matches the screw is unarguably best.
As for the difference being negligible, In screw sizes larger than #0, I have seen gaps around the heads of 82 deg screws in 90 deg holes. My conclusion from this is that it is not negligible.
As for modifying the existing countersinks, if your screw heads are presently flush with the surface and you go over them with an 82 deg countersink, then the screw heads will be below that surface. This may or may not be acceptable in your situation.
As I said, a properly machined hole that matches the screw is unarguably best. So you, and all of us, should have both 82 and 90 deg countersinks.
One more thought on this comment by Booze Daily, "If they are countersunk to the dia of the screw head they won’t be flush after assembly." I strongly disagree. Screw heads are always rounded or otherwise dressed back from a sharp edge. If you rely on the diameter of the head for determining the diameter of the countersink, the screw heads will sit proud (above) the surface. You always need to go deeper than the screw head's diameter would suggest. Even the tables of screw sizes often list the actual diameter and not the theoretical diameter of a sharp edge. In practice, I simply bring a typical screw to the drill press or mill while I am countersinking and test the depth with that screw. Perhaps not the best procedure, but it does work. And if the angle of the screw is correct, then that procedure should work for other CS screws, from different batches or manufacturers.
I'm hoping that you guys can provide a little insight.
I have a new component that is essentially the design child of an existing component. It is attached with flat head screws (82 degree angle) but the countersink that they sit in is 90 degrees. I find that I have copied this mismatch in my new component, and I am wondering if there are any foreseeable problems likely to occur because of this. Both components being attached are anodized aluminum and the screws are alloy steel.
My thanks in advance!
