I am making some rack mounts out of (presumably 6063) 2x2x1/4 angle. My results are not good. I am using a new, sharp, two flute 3/8" end mill with WD-40 as cutting fluid. Spindle speed is about 2800; I started slower and went up to get better results. I am cutting both sides of the slot. I am experiencing tearout, vibration and a lousy finish.
Use a 5/16" end mill and cut it in two passes after you chain drill . I like NAPA penetrating lube in the spray can for cutting aluminum . Seems to lube a bit better than WD . Worst I ever had was some .125" slots in some 304SS . Pieces were for a dog harness so the dog could pull a small wagon . Chain drilling them helped a lot .
If it's unmarked extruded hardware store stuff, it'll be really gummy. Only way I've ever gotten clean threads with the stuff is to use some purpose-made cutting fluid for aluminum. With anything else, it built up on the cutting edge and tearout and a lousy finish was the result. Sounds like you're getting the same problem with buildup. The stuff I used was called Alumicut, was clear and kind of oily stuff, didn't smell much like any straight petroleum distillate, though. With this stuff I got threads that looked like mirrors, no tearout and no buildup. Sometimes it pays to go with the Right Stuff instead of what's on hand.
Generally I use kerosine as a lube for aluminium .Work well and prevents the stuff from sticking to the cutting tool. I have used it on my milling machine and with the wood router with carbide cutting edges .
Also on the router to keep the mess down I have used an ordinary wax candle ,just rub it along the line you are cutting on ,it will melt and lubricate the cutting edge.
Thanks, I would be interesting to try, but in my household the best result of opening a bottle with d-limonene would be a trip to the ER. d-limonene is a sensitizer to some people, with effects similar to someone with a peanut allergy having a peanut butter sandwich. You may want to limit your exposure. The company that made Alumicut, mentioned earlier, has apparently gone to the big refinery in the sky. Someone on the PM board mentioned olive oil as a substitute. I made identical cuts using olive oil and WD-40 as a lubricant. Olive oil is clearly superior. Smells better, too.
I bought a jar of citrus based hand cleaner a year or so back. Ended up with my arms looking like they'd been sprayed with boiling oil. Painful and not nice at all. I had thought I wasn't one of those people that got allergic to things :-(
The "Alternative Medicine Review" quoted doesn't have a lot of application in this instance. d-limonene is a sensitizer, like epoxy. The more you are exposed to it the greater chance you have of having a reaction. But, as my timber framing instructor used to say: "Do what you want, you will anyway".
If it starts well, and then gets worse as the cut progresses, then the work material is heating up. It doesn't take much warming at all for some aluminum alloys to get gummy. My usual fix is to reduce the depth of cut and increase the feed as much as I can. i have never liked WD-40, and use wither thread cutting oil for light work or water-based flood coolant for more concentrated cutting where the heat buildup is more of a problem.
The idea of keeping the feed rate up is to spread the heat around the material instead of letting one spot get hot.
2800 RPM for a 3/8" cutter might be too fast for this situation with poor cooling.
When slotting, I always use an undersize cutter, plow down the middle with relatively shallow step-downs in Z, no more than 1/2 the cutter diameter, but in this case probably .050" per pass. When I get it cut all the way through, I then move over and clean up the walls and widen the slot to the desired size. I almost always "climb mill" where the work is fed "with" the cutting edges, rather than against them. The problem with doing this is that if the machine has a lot of backlash, the table can be pulled into the cutter, breaking tools and damaging the work. On a tight machine, though, you get MUCH improved surface finish and less heating, as the cutter plunges into the uncut material rather than sliding along the just-cut surface until it presses through.