On Fri, 09 Jan 2009 15:54:38 GMT, "Norm Dresner"
wrote:
snip
I've got a bar of aluminum that I had to drill and tap several 1/4-20 holes
in. The bar is roughly 7"x1"x1.25" and drilling and tapping the holes
through the thickness (1") was a good refresher course, but the hole through
the height (1.25") has me stumped. Without thinking it through, I drilled
the hole and started tapping. But the tap's cutting length is only 1" and
the shaft behind it is too thick to pass though previously tapped metal.
snip
Actually very long tapped holes are not a good idea. The slight
variation in lead between the tap and the hole will frequently
cause the tap to bind/break. You are attempting to tap 5
diameters deep.

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.

Although soft, aluminum can be tricky to tap, so back off often
to break the chips, back out often and clean if the chips are
tending to pack in the flutes, use a tap wrench [a crescent
wrench is a sure way to break a tap] and use a good tap lube,
WD-40 if nothing else.

Good luck on the project, and let the group know how you make
out.


Unka' George [George McDuffee]
-------------------------------------------
He that will not apply new remedies,
must expect new evils:
for Time is the greatest innovator: and
if Time, of course, alter things to the worse,
and wisdom and counsel shall not alter them to the better,
what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman.
Essays, "Of Innovations" (1597-1625).