So for years now have been form tapping when I can, using various tapping heads on a VMC. Did lots of cut taps in the selfsame tapping heads until I discovered form taps, but mostly #6 and up sizes. The other day needed to use some 4-40 cut taps. No problem, done that in the past, but not a lot.
Drilled some test holes in 6061, in goes the first (High quality $$ tap), BAM! Crap What the...? So I get out a hand tap, first 6-7 turns not so bad, increasingly WAY tight after that, (need to go to 13 turns) No Wonder the tap broke!
Specifics: Greenfield GH2 EM-NI, black oxide, spiral flute bottoming tap. hole size .089, no taper measured. O.K. so the tap was designed for Harder Nickel/stainless, but I can see that making a big difference in tap life, not so much for tool pressure. Hmm... In the old days, we had like 4-5 taps. spiral, bottom, hand, coated, uncoated. Now there are 2 dozen choices, mostly for production gains.
So how do you guys do this. Do you create a hole size chart specific to material/tap size/tap style when doing tappng heads, rigid tapping with cut taps? Or did I just pick the wrong type tap? I haven't found, yet a chart showing using a larger hole for bottoming taps.
This doesn't seem to be as critical with the larger taps.
ca