"bill yohler" wrote:

but, other than 'drilling undersize and reaming', what's the best way
to drill a hole in mild steel and get the 'expected' size hole (aka:
have it yield the _same_ hole size as the bit size)?


What do you mean by "_same_"? There is no such thing as "exact" in
metalworking (or anything physical, for that matter); you can only work to
within a given tolerance. The smaller the tolerance, the more difficult it
is to achieve. The tolerance required will generally dictate the tools and
techniques needed to accomplish the task.

The thing to remember about standard drills is that they are *incapable* of
producing extremely round holes of precise diameter (i.e., they do not
operate to very close tolerances). In fact, drilled holes are often not even
particularly straight. Even a new drill of very high quality will produce
holes that tend to be trigon in cross-section and slightly oversized. (Note:
if you are using a cheap drill, you might as well just forget about even
getting close to your desired size.) The amount of oversize can be estimated
at +0.004" to +0.006" per inch of drill diameter, but this can vary
depending on a number of factors, including the geometry of the drill (how
well it was made), the material being drilled, the rigidity of your setup,
the concentricity of your drill with the spindle axis, etc.

The bottom line is, if you simply need to make a clearance hole for a bolt,
then drilling a hole with a standard drill bit is usually fine. But if you
need to produce a very round and precisely-sized hole (e.g., a running fit
for a shaft), then you are using the wrong tool (for the finishing
operation).

Regards,
Michael