On Fri, 09 Jan 2009 11:24:37 -0600, F. George McDuffee
wrote:

A tap depth of 1 to 2 diameters [in your case 1/4 to 1/2 inch]
will be adequate. Indeed, it has been frequently demonstrated in
most cases even a 1 diameter tap depth [assuming good threads and
a reasonable thread percentage] will result in fastener breakage
before the threads will strip when loaded to destruction.

Without knowing your actual design, it would appear best to drill
a counter bore large enough to pass the tap shank, if you will be
using the drill press as a tapping alignment jig. If you tap from
the other side, the counterbore can be just enough to clear the
tap, say 5/16 or 9/32. It can be helpful to put a heavy chamfer
[or counterbore] on the side where the thread exits/enters as
this will keep the surface smooth/level, as you will tend to
raise a bur where the trap enters/exits the material. If you do
this remember to take the chamfer depth into consideration when
calculating the length of tapped hole.
good advice

Gunner


"First Law of Leftist Debate
The more you present a leftist with factual evidence
that is counter to his preconceived world view and the
more difficult it becomes for him to refute it without
losing face the chance of him calling you a racist, bigot,
homophobe approaches infinity.

This is despite the thread you are in having not mentioned
race or sexual preference in any way that is relevant to
the subject." Grey Ghost