I am really frustrated right now. In 9.1, I am trying to mill some holes in a model. I'm using the IGES file for a lower AR receiver. It's the one from www.cncgunsmithing.com. I know how to mill the rest of it, just can't seem to do the couple holes. I don't want to drill them, but rather do some sort of helical plunge cut. What would be the easiest operation for this?