Why: Think peanut butter. It takes a lot of pressure to shear any metal, including aluminum, and that pressure gets turned into friction and that in turn becomes heat. When aluminum becomes hot, it becomes VERY sticky, and will stick to the cutter’s flutes. When a cutter becomes clogged there is no place for the new chips to go, and then BAM, they break.
Fix: Go to ebay and buy coated cutters (TiN, AlTiN, etc.) Find the cheapest cutters you can find for aluminum, as long as they are coated. Most machinist think that the coating adds to the cutting edges and isn’t needed for soft materials like aluminum, but what most don’t understand that it ALSO adds a layer of friction protection on the cutting flutes (channels) that helps fight the material from sticking to it.
I am a retired Professor (and ex NASA) and I used to research and teach this information in one of my engineering classes. I also happen to be the guy who has the most experience cutting aluminum on the Shapeoko. I am also using the SAME 1/4 end mill since last October 2015 on my machine with hundreds and hundreds of cutting hours on that end mill, including cutting steel with it, but that’s another story. Of course I have 1/32, 1/16, 1/8 Flat and Ball end mills that I have also been using to cut aluminum, and I have NEVER broken an end mill cutting aluminum…NEVER!
Additional Lube: Go out and buy a can of WD40 and all a light layer to the material before you start cutting. This will also brighten the cutting surface, making it look shiny.
This is a great machine, and this is also a great community forum…we are here to help you avoid the mistakes…