I watched the video on building the Ox CNC mill and Mark gave a real good tutorial on how to tap threads into the ends of the extrusions. The tapping tutorial starts at about 47 minutes into the first video on the Ox build. I bought the tap mentioned in the video from Harbor Freight and it worked good until my battery operated drill's battery ran down. I didn't want to wait for the battery to charge so I got out my electric drill to finish the job. After tapping 2 holes with the electric drill, the tap snapped in the extrusion. Fortunately there was enough of the tap protruding from the hole to grab onto with a pair of lineman pliers and get it out. Reason for the snapped tap: The electric drill had to much torque and I had to little smarts to take it slow and easy. The technique and tap works good, but just like most things in this world, the tap doesn't respond well to abuse.
Thanks for the tip @rpleyva to be honest I no longer tap the holes and have been using the self taping screws. Its a lot quicker and they hold very well. The V-Slot holes are still a little tight (working on that) so you have to run these screws back and forth a couple times but they are much quicker to use. http://openbuildspartstore.com/self-tapping-screw/
