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Newbie: Machining Cast Aluminum?

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  • Newbie: Machining Cast Aluminum?

    Ok, standard newbie disclaimiers apply...

    As an ongoing hobby, I currently build audio equipment (amplifiers, active crossovers, loudspeakers etc). Purchasing aluminum heatsinks, enclosures, knobs, carpet spikes, etc for the equipment is a major factor in the cost of my projects (60%+), and using 'generic' enclosures is getting old. I'm getting to the point that a basic mill/lathe setup may be cost effective.

    Like most hobbiests, I have a lot more time than money. While investigating the cost of aluminum stock, I stumbled across "backyard metalcasting". Would it be possible to cast my own stock to the sizes I need for heatsinks (say, 2"x6"x10"), then square up the cast on a mill? I can get AL scrap from a friend for next to nothing. It appears that most aluminum bar stock available is extruded, is there a difference when it comes to machining it versus cast Aluminum?

    All advice appreciated.
    Matt

  • #2
    Matt,

    Do you need lots of small/thin fins? If so, you may have trouble casting those at home.

    If you can use long lengths of parallel fins a small and inexpensive horizontal mill with ganged slitting saws or something like that might be worth looking at.

    On the other hand, watching for surplus heat sink extrusions to go on sale might be an easier way to go. If you could buy a big lot of stuff that was excessed by some other company you may not have to pay a lot for it.

    cheers,
    Michael

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    • #3
      matt;
      there is a world of difference between machining a casting which usally turns out as condition O very soft and gummy and a extruded shape that is tempered.
      granted this statement is very generic depending on alloy and temper...jim
      ...jim

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      • #4
        My thought was basically this: Build a small furnace, round up some 6061 scrap & melt it down. Have a steel mold of the size I want (say just over 2"x6"x10" to allow for cleanup), fill it up and let cool. Knock it out of the mold, and basically have a billet that size. Square it up on the mill to wind up with a 2x6x10 aluminum bar, suitable for milling into a heatsink.

        If you start with 6061 and melt it down, will the resulting bar (after cooling) be much softer than the original (I assume yes). Is there a way to re-treat it (at home) to restore some of the hardness (quenching & re-heating, etc.). Is the resulting cast worthless as far as machining?

        Thanks for the replies so far...
        Matt

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        • #5
          The few (very few) pieces I've cast machined nicely. (They were cast from other previously cast scrap.)

          But as indicated, my experience is real limited.

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          • #6
            Typically, extruded alloys are less suitable for casting as cast alloys. Ie if you want to cast something, remelt something that was already cast - although 6061 will melt and pour.

            The longer a casting sits after being poured (read: months to years) the harder it will become and so will be easier to machine.

            I have attempted to machine castings the day they were poured and it is virtually a waste of time, even a week and they are significantly better, after 6 months they are good machinibility, not as good as 6061 bar though.

            You can heat treat alloy yourself but it involves specific temperatures and time - This side of things I know nothing about so am not prepared to comment...

            Oh yeah, one more thing - not much of an issue for heatsinks, but aluminium will absorb iron from a steel cruicible changing its composition (and so alloy) you can not accurately say how an alloy will perform that has been cast from a steel crucible. You can prevent this somewhat by using a wash of refractory on the inside of the crucible.

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            • #7
              The bigger issue may be the ability of a cast heatsink to do the job. You may not get the heat transfer you'll need or expect.
              Not sure, but it's something to look into.
              Ed
              Ed Pacenka

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              • #8
                I guess I should clarify... I wasn't looking to directly cast a heatsink, but rather just cast a solid aluminum bar, clean it up, and mill the fins. I can calculate the heatsink specs and design accordingly. the hard part is getting a block of Aluminum that size for a reasonable price.
                For example, I'm woring on 2 2-channel 200 watt power amplifiers now. That means 4 heatsinks, each about 2.5"x7"x9". That's $200+ just in heatsinks, not to mention the rest of the 2 enclosures. I do 4 or 5 projects like this each year.

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                • #9
                  Hi Matt,

                  A steel mold for aluminium may not be a good idea, as aluminium will absorb iron from a steel mold.

                  A sand mold would be better. Try reading some metal casting books. You could make an open faced mold from green sand and pour a block to machine.

                  Yours, Jim

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                  • #10
                    Matt,


                    Have you priced Al. in the size you need ?

                    Perhaps you can make a heat sink from a thinner bar stock and rods press fit into holes ?

                    Just a thought

                    Lenord

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                    • #11
                      The short answer to your question and specific need is YES it will work just fine. We have been casting scrap aluminum mix's for years into blocks to machine into heat sinks. Don't ecpect them to be as pretty as machined bar stock but good enough for most jobs. Sand casting for you is likely the best solution unless you are going into a large production run. Then try useing brass molds. We can pump out over 100 cams an hour with a hinged one cavity brass mold we made years back. 6061 scrap works pretty well also for cheap casting jobs. Granted your end product wont hold up to MIL spec's but I don't think that is what you need either. If it is, then disregard...
                      Stay Safe

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                      • #12
                        Thanks everyone, for your replies. I think I'm going to give it a shot and see how it turns out.

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                        • #13
                          I've had very good experience machining cast aluminum. Just use a very sharp tool and experiment a bit to get the right rake and relief angles. Also, use the WD-40 for the finish cuts.

                          I've mainly used 6061 scrap for my castings. I also suggest avoiding the iron crucible as the aluminum will be embrittled by the iron.

                          I'm looking into heat treating the 6061 castings to strengthen them to the T6 state.

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