[sanddrag]

I am countersinking a #9 hole in 6061-T6 aluminum using a 6-flute Lakeshore Carbide "chatterless" countersink. Going until top of the hole is .411 in diameter. I know I have my feeds and speeds wrong, because I'm getting a ton of chatter, and the countersunk hole does not turn out well at all. I've tried 750 RPM at 2 in/min. What should I be running this for the perfect smooth countersink?

Or should I abandon it altogether and go for a single flute?

Thanks.

[fizzissist]

It'll load up with chips because there's little room in the flutes. You have to keep the feed rate up and peck it.

RPM is conservative in alum for a carbide, but you're ok.

We just got thru running a 6fl carbide C'sink in 1018, same rpm, about 3.5IPM feed. Worked just fine with 3 passes on a peck cycle. We were also going pretty deep with it too.

[6061-t6]

cnc? manual?

I rarely use anything more than a single flute for this type of operation.

[sanddrag]

HAAS Mini Mill CNC.

[6061-t6]

6000rpm, 10ipm sometimes too little rpm can be a bad thing...

[fizzissist]

6000rpm, 10ipm sometimes too little rpm can be a bad thing...

You've run a carbide 6fl at that rate?

Guys who ran it that fast before I got there broke it. Mind you, I agree with you in theory, but in practice we had to slow things down, and this was in steel.. but we had to slow it WAY down. Part of our issue was going from a .330 hole to a 5/8 top dia. That's a pretty big bite.

I think the real issue here is the total swept surface area x number of cutting edges, along with the lack of room for chip evacuation.

[txcncman]

Well, you found out that chatterless countersinks aren't. I usually try to use a spot drill with the appropriate angle and have my countersink finished before the hole is drilled.

[sanddrag]

Looking at some more numbers, I'm looking at trying 3400 RPM, 11 IPM, and peck it 3 times or so. How does that sound? Also, should I do something like a 0.2 second dwell at the bottom?

[TechGuy5002]

FWIW: HSS or Cobalt Countersinks for Alum may be a better option to avoid chatter and will likely have a better finish than Carbide. That said a constant even pressure will help reduce chatter. If the pressure is too light it causes excessive chatter, Too much feed causes it to build up with large chips fast, that also increase chatter.

[txcncman]

If you do a dwell, since you are using a 6 flute tool, calculate your dwell for the RPM to be slightly more than 1/6 of one revolution.

[rlockwood]

Well, you found out that chatterless countersinks aren't. I usually try to use a spot drill with the appropriate angle and have my countersink finished before the hole is drilled.

Agreed- I've also simply had better luck with single flute countersinks for this type of application. And likely not much need for carbide here either.