Milling: creating a smooth end finish in a slot

[Rob Meades]

Well, I'm not having much success here. I don't have my compressor yet so I thought I'd try a HSS/8% cobalt end-mill (4 flute, 7 mm flute length, 6 mm diameter shaft) which requires a lower speed. With the tool sticking out 20 mm to avoid my clamps, I've taken the HSM Advisor recommendations and then backed them off, so 5 mm/second feed rate and 0.5 mm depth of cut, tool running at 14000 RPM as recommended. As a reminder I'm cutting a slot in [343-460 N/mm2 tensile strength] brass plate.

Result: tool snapped off in the middle of the first pass. It looks as though the tips have worn quite quickly and then the tool has become clogged with brass:



Here are my tool details, material details and the HSM advisor screen to match. Where am I going wrong?



FYI, until it snapped, it appeared to be making quite a reasonable cut:



Rob