I was recently asked to engrave into some aluminum sheet. I I have the most experience with wood but I’m never one to turn down the opportunity to learn something new. After three broken bits I think I have a few pieces of wisdom that I can pass on that might save you the same trouble.
Aluminum comes in many different formulations the most machinable and easiest on your tooling is 5051. The aluminum sheet that I was delivered to engrave was 6061 which is slightly tougher to cut but still machinable without lubrication.
Take extremely light passes. Try .003″ inch per pass if you’re using something like an 8th inch endmill. Always consult the manufacturer of your bits for recommended feeds and speeds. The larger the endmill you’re using, the deeper your passes can be. In general you want to start shallow to have success cutting aluminum on a machine like a ShopBot CNC router.
Use an appropriate bit. In general you can use any bit made for wood to machine aluminum but there are bits that are specially made for aluminum that will help with problems often associated with cutting it such as re-welding (where the chips that are flying off of your material melt and become re-attached to your cut. many people find great success cutting with o-flute style bits. These offer very quick chip removal as well as an extremely clean cut.
Pay attention to the direction of your bit. Is it an upcut bit or a downcut bit? I generally like to use an upcut bit to remove the chips efficiently. Although when cutting thin material (an 8th inch or below) I prefer a downcut bit to avoid lifting the material from the table. This can also be remedied by using a vacuum table or using generous amounts of double-sided tape to keep the aluminum stuck to your spoilboard.
Good luck and as with anything else the best instructor is a closely examined mistake.