Check machinery's handbook for tap drill sizes for form taps. I highly recomend form taps or spiral point two flute tap for thru hole in aluminum. Alot of the other guys on here have great success with tapping around the speeds you said you tried, personally I tap 1/4-20 thru aluminum at 2500rpm and I haven't broke one yet. I've got around 110 holes on one form tap and it still looks as good as the day I bought it. I usually just have the flood coolant on but around every 20 holes or so I will coat the tap with a dry graphite film spray, I don't know that it does much after the first 1 or 2 holes but it can't hurt.
With form taps though just be sure you have the right size or a hair bigger hole drilled for the tap as a hair under will cause problems, I already coated one 5/16-18 tap with aluminum- not pretty. Also with form taps use a 90 deg spot drill and run it down far enough so that the tapped hole will be chamfered when all said and done, as the form tap displaces material, with no chf on top they will quite often roll material up above the top of the hole.
JP  |