I'm trying to cut an appx 2" square pocket 1.7" deep (it ends up being a through hole, but I'm cutting it as a pocket and flipping it) in 6061 aluminum on a new Haas VF-1. I'm using balanced ER-32 collet chucks with appx 3" gage length. Very conservative DOC. Tried between 0.100 and 0.075. 50% stepover.

It has 1/4" corner radii. I've tried a couple things--

1) Drilling out the corners with a 1/4" drill to avoid leaving a lot of material for the 1/4" EM finishing pass. Using a 1/2" 4 flute Cobalt roughing endmill, appx 2.5" long to rough out the pocket. Tried every speed and feed combination I could think of, all the way from 3000 RPM to 8000 RPM or so. Started feeding at 20 IPM. Sounded absolutely horrible. Even dropping it down to 6-8 IPM gave me unbelievable chatter. the walls of the pocket were gouged so badly that the 1/4" finisher couldn't even save them.

2)Same thing as above. Swapped to a 2 flute HSS TiN High Helix endmill of the same length, in the hopes that it would clear chips/chatter less. Ended up running it at around 5000 rpm and 12 ipm. Still sounds horrible. Leaves terrible surface finish, but the finisher almost cleans it up.

3) Drilled the 4 corners. Drilled out as much of the 'meat' of the pocket as possible. Used a short 1/2" coated carbide endmill to get the first 0.850 cleaned out. The surface finish on this portion of the pocket is beautiful. I was running at around 100 ipm and 85000 rpm.

Next switched to the longer 2 flute HSS endmill to clean out the rest of the pocket. Terrible chatter still, untill I finally dropped the RPM's to about 2000 and the feed to 8 IPM. Not too much chatter, but this seems unbelievably slow. I have to imagine I can machine a pocket this shallow much quicker. Surface finish is ok, but still pretty terrible compared to a good carbide endmill.

Would switching to an extended length solid carbide endmill really improve the rigidity that much to eliminate all the chatter? What else can I try?

Thanks for your help.