On Tue, 1 Jan 2008 18:58:41 -0800 (PST), wrote:

Hey Robin,

Thanks for taking the time to write that detailed post. I've got a
new sense of confidence now and I might try the hand drill tapping
machine with your advice.
mcmaster lists the following spiral point taps. I'm guessing 2 flutes
is more ideal than 3 (more room for chips to leave). I'm guessing
cheap $4.68 one will do fine unless someone tells me these coatings
are worth twice the price?

Tap Material Surface Coating/Treatment Thread Length Overall"
#Flutes Each
High-Speed Steel Uncoated (Bright Finish) 1-1/8" 2-23/32" 2
2523A412 $4.68
High-Speed Steel TiN (Titanium Nitride)-Coated 1-1/8" 2-23/32" 2
2762A48 $6.41
High-Speed Steel TiCN (Titanium Carbonitride) 1-1/8" 2-23/32" 3
2568A43 $9.75
High-Speed Steel Oxide-Over-Nitride-Treated 1-1/8" 2-23/32" 3
8300A19 $7.83
Cobalt Steel Uncoated (Bright Finish) 1-1/8" 2-23/32" 2
8779A31 $10.65

The only quibble I have with Robin's advice, based on my specific
experience working with these extrusions, is regarding spiral point
taps (gun taps) vs. spiral *flute* taps. I use gun taps for almost
everything, but tapping these extrusions is one exception. A gun tap
will work fine 98% of the time, but for some reason, 2% of the time
something goes wrong and the tap buggers the hole. Since I started
using the spiral flute taps I've had zero problems. I gave McMaster's
p/n for a 5/16-18 spiral flute tap in an earlier reply.

I'll second Robin's endorsement of the Metabo drill. I bought one on
his recommendation and love it. You should not need a special tapping
tool to cut 5/16 threads in aluminum as long as you have a drill with
good speed control and adequate torque.

--
Ned Simmons