On Jan 1, 6:18*pm, wrote:

Any help please?

The only reason sharp taps break in correctly drilled holes is because
the tap is not square to the hole (or you bottom out the tap - silly
mistake).

I've got a large 1/2" drill (corded - tons of torque) with two gear
speeds, variable speed, etc (mine's a Metabo, but everyone makes
them). I've tapped down to M4 reliably (in high gear) under power
without the use of a square, and without breaking the tap with this
thing (soft cast iron, mild steel, 4140).

You have to gain a feel for getting the tap in square. It could take
several holes for you to get the feel, but.... You basically just
gently rock the drill as you start to drive the tap in, and you'll
feel it when the tap kinda *slides* in easily instead of getting a lot
of reverse torque from the drill - this means you're square. If you
don't get this feeling, you're not square and you need to reverse out
and try again - do NOT just ram it in! If you're not confident,
reverse and do it again. You have about four thread pitches from the
tip of the tap to *get it* or else you have gone too far to make any
further angular adjustment.

If the holes are quite deep and you're not too concerned about leaving
chips inside the hole, use a spiral-point tap (most ideal). Spiral-
flute, on the other hand, are good when you want the chips to come out
of the hole (blind hole, or you don't want to dig them out) BUT they
are weaker and typically more expensive than spiral-pointed taps (not
a good situation when you don't have a lot of experience).

Buy a new tap for this job, and don't use it on steel until you're
done (steel work will dull the tap much faster than aluminum). Always!
use lube when tapping aluminum (and pretty much any other metal).

Remember, you can chuck these taps in a reversing drill press, or
milling machine and do the work that way too. It inevitably takes long
to do this type of work on a machine as opposed to by hand, but you
don't have the issues with squareness.

Good luck. 5/16" taps are pretty strong. You shouldn't be breaking
them in aluminum.

Regards,

Robin