There are a lot of factors that go into getting a good finish with aluminum. Here are a few tips that have helped me greatly:
1- Cutting Direction: With all non-ferrous metals you want to “climb cut”. That is to say, you want to feed in the same direction of the tool rotation. So when the flute contacts the material it is pulling it instead of pushing it. This is probably the single most important thing you can do to get a good finish but is tough to control using CAM programs that don’t do it automatically!
2- Material & Temper: Not all aluminum is the same. Even within the same grade! Some are good for forming and others better for machining. Grades that form better will machine like hot boogers. The grade you get at the hardware store usually sucks. You need to pay attention to the temper as well. For instance, 6061-“T6” will machine much much better than 6061-“T5”. The T6 temper makes the metal harder and form chips better so cutters are less apt to gum up. Starting with the right material with the right temper makes all the difference.
3- Speeds & Feeds: You want to make sure you are “cutting” and not “pushing” the metal. You want to make a good chip. So if you run your RPMs really high and feed really low you are not taking a cut and just making friction = heat = gummed cutters. If you were to run a spindle at anywhere near 10k RPM you would have to run really high feeds. To do that you need a really rigid machine, which these are not. So going lighter is not always the answer. You might want to try feeding faster and turning the tool slower. If you try to take a fart skin thin pass at high RPM you will just push the metal and swirl it up. The cutter needs to engage in the material. So take a cut.
4- Cutters: Anything considerably harder than the material will cut well. HSS works just fine. I do use uncoated carbide because you can get a little better finish and be a little more aggressive. However, most of this is based off of the rigidity of your machine. Given that most of the machines in this realm are made from aluminum themselves, you can only be so aggressive cutting the same or like materials. You don’t want to get a cutter with any coating that has Al in it. Like materials stick to each other. This also generates friction/heat. At best the coating will just wear off. It is more likely that it will gum up and break tool. Probably when you are almost done with your part. The TiN coating works good (gold in color), but is probably not needed unless you really have a serious CNC machine that is taking heavy cuts.
5- Experience: You will know when you have it right. When you are machining aluminum with the right parameters it sounds really good. It is a very satisfying sound. It also produces a surface finish second to none. I wish all parts could be made from aluminum…
Hope this helps! Good luck!