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  • aluminum

    I'm a beginner machinist in training and have a couple of questions.I want to know at what speed should you mill aluminum for a smooth finish ,also does this speed apply to all cutting operations? like using a boring head or a fly cutter?.Thanks for any help you can give.Neil.

  • #2
    Fast. It does depend on the alloy, a lot. Al can be machined dry with very good results if it is a high silicon alloy like 2024. 6061 is the most common alloy and it is very machinable. It is hard to go too fast in terms of SFM with a SHARP tool and high positive rake. Hone your tools with an Arkansas oil stone and never use them on steel. Keep a seperate set of tools just for Al.
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    • #3
      What Evan said.... I'd add that some of the softer alloys are quite "gummy" and tend to weld to the tool. For those, some kind of cutting fluid helps a lot. Kerosene works well, but if it gets hot enough to smoke try not to inhale too much of the fumes; it will give you a headache.

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      • #4
        By far the best cutting lube for Al is 100% ethyl alcohol. Convince your pharmacist that you aren't going to drink it.
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        • #5
          What's been said about sharp, rake and gummy.... I like the idea of keeping a separate set of cutters just for aluminum and I will start doing that.

          A harder aluminum will mill OK dry but the softer ones like 6061 (hardware store variety) do better with a coolant. The build-up on the cuting edge become work hardened and will gouge into your desired finish. I often use WD-40 and it produces good results. It's quite a bit more expensive than alcohol or kerosene but less flamable and does not give me headaches (which I am prone to).

          As for speeds and feeds, that can depend a lot on your mill. When I do aluminum on my Unimat (a very small machine) I have to use much slower feeds. The cutters I use with it are small (1/4" or less) so I try to compensate with as high a spindle speed as possible. Still slow going.

          On the full size mill the key is often how rigid the setup can be made. A 2 or 3 HP motor will allow aggressive cuts but the work must be absolutely secure or you invite disaster. You need to think in terms of not only the speed and feed but the cross section area of the chip you are cutting. A cut 0.03" deep with a 0.002" feed per flute is removing the same amount of metal as one that's only 0.01" deep but 0.006" feed per flute. Another thing about feeds is the diameter of the cutter. A 1" diameter, 2 flute mill will have a lot of room in the flutes for the chip and a fairly agressive feed will work OK. A 1/8", four flute mill will have very little room in the flutes and a very slow feed will be required to prevent breakage. Fly cutters have an almost infinite space for the chips (the size of your shop?). The way I work I find that a final cut of at least 0.001" or 0.002" gives a better finish than trying to cut less. But that may be just me and my equipment.

          When thinking about spindle speed you need to translate it into the linear speed of the cutting edge. So a 2" cutter would be run at a spindle speed that's 1/2 that for a 1" for the same linear cutting speed. There are tables but your experience with your equipment and cutters is the best guide. Start with moderate numbers and experiment on scrap or while making the rough cuts.
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!