Best milling tool for h55 copper tubes

Hey fellas, I have recently been moved onto a new T drill machine at my workplace. Its a CMS Italy model MDE machine. The endmills we have been using to pre drill holes for collaring with a flowdrill are not lasting very long before they start leaving lots of stringy chips along the diameter of my holes. I have a hunch that they are giving me cheap quality stuff purchased from Amazon. I’m drilling H55 temper copper tubes 1 3/8 ODx 0.055 wall thickness. The size of the holes being drilled is 6.6mm Any suggestions towards endmills that would last longer than 1500 holes before needing extra asjustment would be great. Thanks in advance