Hi, What happens in your screenshot is you are using too much on a depth of cut. You can see that your deflection and torque are very large. Though they seem to be below critical values, you just don't have enough chip clearance on your cutter to pull it off. Simply click either DOC or WOC label to have it suggest better number. There is a Performance slider on Speed and Feed Adjustments panel. For such long endmills it helps to have that one all the way to the left. Also make sure you specify correct work material and tool geometry data. I see you did not modify Flute lenght. Are you sure it is what it shows on the screen? Flute length is very important for deflection and proper DOC suggestion. Please let me know if this helps. Otherwise we would have to dig deeper. Regards.