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Counter Sink for Form Threads/Tapping

_MO_

Aluminum
Joined
Apr 28, 2009
Location
Seattle
Does anybody have a rule or ratio for counter sinking form threads? I know you need to sink a little more for form taps than cutting taps, but how much to keep the tap from pushing material above the top of the hole?

I'm thinking of using a 90deg tool because that would let me use the same tool I'll be using for chamfering/edge breaking.

If you need specifics I'm looking at 4-40 and 6-32 in 6061-T651. I'm guessing aluminium will push more material above the hole than something less ductile but please let me know different.

I've got a lot of holes to tap in these two sizes, so I'd like to get everything nailed down the first time if that's not horribly unrealistic :D.

MO
 
I don't know if there is a rule, because of the uncertainty over the tip diameter of the countersink or chamfer tool, and how much this truncates the virtual cone tip of the countersink.

On hole sizes that small, the adjustment amount to get the proper diameter countersink is going to be relatively delicate. Also, the actual diameter of the countersunk depression could vary quite widely, depending on what the designer had in mind.

So I'm not sure what rule you'd expect to discover, other than to do a test hole completely.
 








 
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