Just because they make it doesn't mean it will work.

​We are carrying some of these new tool path strategies a little far. Last week I was asked to tap test a 3/8" 3 flute endmill with a 4" length of cut. They wanted to cut at full depth in aluminum. That is 10.66X the diameter! It was like measuring a wet noodle. There were no stable lobes at any speed at any axial engagement. Just because someone makes a tool doesn't mean it will work. About 40% of the carbide was removed from a cylinder to grind the flutes, so this endmill was the equivalent to a 0.225" rod as illustrated above. You can imagine how much that would flex. Even if we could find a speed to cut at, the wall would not be straight. So what to do? If the diameter of the tool could not be changed, you could go to a relieved shank with a shorter flute length and make more passes. If you select the right speed with minimum displacement (SLE) you would not get water-lining if that is a concern. If you still wanted to go in one pass, then increase the diameter (and stiffness) of the cutter. At 3/4" you would cut the L:D ratio in half. Adding more teeth would also increase the stiffness.