Note: All of the below is in relation to aluminum. Mostly 6061, some
7075, MIC6, ATP-5, and rarely 5052. (All CNC)
On the light weight (1200ish pounds) bed mill I mostly use taps in a
tension compression holder at 500 rpm. I have one factory made one, and several shop made ones. I get about 1.5 threads over/under run at
worst. Usually better. I could turn faster, but over/under run is less predictable.
Occasionally I use (full form?) thread mills where a tangent entry into
the threads and one and a half rotations completes the hole to full
depth. They have one big advantage. Exact depth control for blind
holes. The big draw back is depth limit. Their reaches are typically
less than I would like to thread. They also tend to chatter. I'm sure
I just don't have the recipes down quite right yet. Within their reach
they do an acceptable job.
On the higher speed small bed mills I run single form thread mills. In
theory they can run multiple holes, but in reality I use a different one
for each of the common hole sizes I thread. They do an acceptable job
and they are available in long enough reach for any hole I need to
thread. I seem to recall I run them at 18000-24000 rpm, but I have
styles saved for each common hole and just select the geometry and plug
it in. They will thread a 1/2 deep 10-32 hole in about the same time as
the "larger" mill taps a similar hole, and if doing multiple holes its
faster.
For spiral point taps I've come to like Balax, but I have used others.
Whatever MSC's less expensive brand is (I forget) tends break off
unexpectedly with a low number of holes at hole number 9 in a row of 10
holes that tapped perfectly in holes 1-8.
For multi form I've been using Lake Shore Carbide and they seem to be
ok. I don't think I have tried any other brand. I might have a 100-ish
holes in a 1/4-20 so I have no idea as to life.
For the single form I use Rouge Systems Inc. Generally they get used
for 10-32 from a depth of 0.495" to occasionaly a depth of 0.625" and
1/4-20 from a depth of 0.625". The life is pretty good. Maybe good
enough as they are not expensive. I've got one that seems to be
approaching end of life as its threading slightly under size, but it has threaded maybe a couple thousand holes. At 3X magnification comparing
it to a new one I can't tell for sure if the points are rounded off or
not. My operator console does not record individual tool use time.
Now getting to the meat of the matter. I've been looking at trying a 3
tooth thread mill. I'm trying to wrap my head around whether or not it
would be any benefit in tool life or real world cycle time.
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