[The_Iron_Joe]

I'm using GWIZARD calculator to get my speeds and feeds. Haven't had any problems until last night. I tried to cut some 1018 hot rolled steel at 2mm deep with a half inch thick, four flute TiAIN coated carbide endmill, square/ flat cut. I was going at 3250RPM and 891.6 mm/minute.

I broke the first end mill, and chipped the hell out of the second. I'm using Fusion360 for all my cad/cam, and was using the 'adaptive roughing' strategy to remove material, which means the tool wasn't fully slotting the steel all of the time.

Any ideas what went wrong? Should I be using specific tool paths?

[Geof]

You may be going a bit fast. At 3250 rpm with a four flute cutter taking .001" per tooth you need about 18ipm, i.e. About half the speed you have (if my math is correct).