Hello all, got a question for the experienced. I have been running my 1100 since May 2012 and its been exceeding my expectations since day one. Currently I am designing a new intake manifold for some drag/street cars and it requires quite a bit of machining of deep holes. The intake runners to be more specific. They are 3 1/2 inches deep and have some contours inside so drilling is not an option. Another part also requires me to do some (alot) of machinning at almost 5 inches deep. Both parts require I use a bit that is no bigger than 3/8 in diameter and a straight shank.

Now I ordered a Multi-Master carbide shank from Iscar that allows you to mount intechangable tips. It is 7.5 inches long and currently I have purchased end mill bits for it that are a 2 flute square cut specifically for aluminum. The parts are being made from 6061. And I have terrible chatter no matter what doc and ipm i try. I have tried 0.005 up to 0.040 doc and varied ipm from 5 up to 35. The finish is almost identical regardless of what settings I try. I tried spindle speed of 3800 up to max speed and also tried climb and conv milling.

Now I also have some cheap 6 inch long 1/4 dia. bits from shars that I have used here and there and tired a couple tested. Now the shars bits are ball nose not square. And I have one that is a 2 flute and one that is 4 flute. And the 4 flute will chatter a bit at lower ipm around 5-8. But once I get it up past 9 ipm it smooths out to almost no chatter and the finish is acceptable. I am considering ordering some either ball nose 4 lute bits for the Iscar tool holder or trying some 4 flute square end bits for the holder. But at about $80 each and you have to buy them in quantity of 2 I would rather hear from an expert what would be the better choice to order.

Sprutcam said the part will take 14+ hours to make with the 1/4 inch ball nose from shars, and after about 8+ hours the bit snapped. Which was to be expected from a $18 bit with over 4 inches of overhang. So we spend several hundred for the Iscar tool to get a better quality tool and speed up the production with a wider tool. But currently we are stuck with extreme chatter which I know If I keep running it like that the new holder will break very soon from the vibrations.

Any suggestions or ideas would be appreciated.

Also flipping the part is not an option.