You’ll want to go with a long-length end mill designed for aluminum and choose polished flutes to help with chip evacuation. A 2- or 3-flute tool works best for this. For chatter, using a variable helix end mill will help. It disrupts the vibration of standing waves. Also, consider roughing the cavity with a larger tool, leaving a small stock for a finishing pass.

For aluminum 6061, keep the surface speed around 400-600 m/min, depending on your tool. Use a moderate feed per tooth—0.05 mm/tooth is a good starting point. Coolant is crucial. Flood cooling works well, but if chip evacuation is still an issue, consider air blasting or a high-pressure coolant system. A pecking or trochoidal milling strategy can also improve chip clearance while reducing heat buildup.