At the risk of posting something most people here already know: Our machinist (this was long ago) at work told me one day that if you drill a pilot hole first followed by a slightly undersize drill bit of the desired size to finish, ?the result be a hole just less than the drill size. He couldn’t explain why that happens. Part of the reason is I think that the drill cuts so little it’s acting like reamer and there is nothing forcing it off Center. I do this frequently.?
You probably also know that if a drill is sharpened with the point off Center it will cut a larger hole than nominal. I do that too when I need a little clearance.?
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On Mar 30, 2024, at 6:56 PM, tgerbic via groups.io <tgerbic@...> wrote:
?Almost everyone has the drilling accuracy problem. It is impossible to drill a 0.500 hole with a 1/2" drill bit in a drill press. All drill press chuck assemblies have a little play, or they would not be able to move up and down very easily, or not at all. Drill chucks are not guaranteed to grip perfectly true. Drill bits generally have some wobble or flex, and have a diameter tolerance range.?? Reamers in collets on a lathe or mill will get you a lot closer but it still may take a lot of effort to even get close to 0.500.
In the case of your face plate adventure, you could put hours of exacting machine practice and science into getting the chuck almost exactly aligned. Or you can accept the realities of the tools and equipment you have and make allowances to calibrate the assembly, the best you theoretically can, after assembly. The little bit of slop in the holes might be needed to slightly move the chuck to get the center of the jaws aligned with the centerline of the lathe spindle and tailstock center. Having no tolerance will mean you may never be able to get it right.
