tapping aluminum - tap clogging

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dansawyer

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I am trying to tap 1/2 inch deep 10-32 holes in T6061. The holes are drilled with a #21 drill and I am attempting to tap the first one. The tap is aligned well. The tapping was strange compared to steel from the beginning. The steel technique is to tap forward until build up is felt then to reverse and break up the build up. There was no sense of build up and release as the tapping proceeded. Tapping just got harder and harder. Reversing did not release any buildup.
When I removed the tap it was heavily clogged. There were no identifiable chips. The aluminum was pressed into both teeth and into the grooves.
Did I choose a too fine a thread? Should I switch to 10-24? (#21 is already large for 10-24)
Should I drill the holes out a few more thousands?
Is there a lubricant that will help>
 
Try a lube such as Tap magic or WD-40. You may have to back the tap out after a couple of turns and clear the threads. You could also try a spiral flute tap. They tend to have more space in the flutes. Usually used for CNC machines but can be hand tapped.
 
How about when the tap starts to feel harder to turn you back tap out and clear the chips from the tap and hole. Clear hole with compressed air and a rag to put around hole to catch debris... Re-start tap and count the number of turns of tap till you reach the same sense of snug as the first time and repeat above. Clear chips and go back in until you reach desired thread depth. You're wearing eye protection right?

Just how I would do it.
 
I have been using form taps in aluminum. They do not leave chips, and the threads are stronger. Choose your drill size specifically for form taps, and lube is good, as mentioned, WD-40, Tap magic, kerosene, also is good for aluminum lube.
 
Dry tapping aluminum is asking for trouble. It doesn't take much for aluminum the stick to steel cutting tools with an almost weld-like bond. WD 40 or some of the tapping fluids specifically designed for aluminum like AlumTap or Tapmatic for Aluminum work well at preventing galling.
 
According to GWizard, the #21 tap drill is giving you a 76% thread. You could open it up just a bit with a #20 drill and still get a 71% thread. Plenty of thread for aluminum and it would tap just a bit easier.
 
Use a sharp, quality tap. WD-40 for lube and to wash the chips out of the tap. #21 drill is fine.
 
Agree with all above- I remove the tap every so often and use an old toothbrush to clean the tap. Aluminum is somewhat "gummy"
Liquid wrench makes a good emergency tapping/cutting fluid for aluminum
mark
 
You didn't say what the threads look like in the hole. Are they there or wiped out?
 
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