wrward

Not to derail anything but that business card for Paul Freiler's Historical Models brings back memories. I spent a lot of time in that shop(both locations) admiring the completed models on display and trying to decide which kit I could afford. Many of the customers seemed to be engineers from TRW or Hughes. Thanks Don!!

A little shot of WD40 on the cutting tool helps a lot when cutting aluminum. I suspect that chips welding or packing themselves into the cutting flutes is what caused the end mill to break. Any light oil will work, traditionally kerosene was the preferred cutting oil for aluminum.

I suspect that your endmill is running out in the holder. We used to measure the endmill with a micrometer and then check the runout of the tool in the holder and spindle so we could correct for the runout. If you don't check runout and correct for the runout you are liable to have undersize features. Example: 1/2-inch endmill that measures exactly .500-inch with .002-inch runout is effectively a .504-inch endmill. Edit: Thank you for a fascinating thread