Clay,
I've rigid tapped lots of 4-40's in aluminum, mostly bottom tapped, I think
part of the problem is the tap you are using, for aluminum, I use the bright
uncoated ones or TiN coated onse and seem to have few problems. I am a bit
surprised that you would have a problem with this, perhaps the tap is really
not setup for aluminum, but heck, if it's set up to cut through stainless, I
would think that 6061 would be no problem.
I have cheated from time to time and used 3/32 instead of .089, since if you
are engaging more than about 4-5 threads it will make no strength
difference. Usually what kills a tap when I am rigid tapping is if a chip
gets in the way and is "sucked in" because it was sitting on top of the hole
when the tap went in.
I assume you were using some sort of cutting fluid when you did the hand
tapping, and were guided.
I can look up some of the taps I normally use.
John
"clay"
citlink.net> wrote in message
news:wMFUj.1542$_01.1468@fe091.usenetserver.com...> 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
