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Thread: I Didn't Like the Pulleys on My Drill Press

  1. #11
    Supporting Member Crusty's Avatar
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    There was some but it was widely separated small dots and to me didn't affect the integrity enough to be concerned about.

    I first intended to use Light Salt to degas the AL but several videos I've watched since have convinced me that it doesn't work so I just poured the metal trying to keep impurities to a minimum. Apparently you need a costly bubbler rig to achieve significant degassing. I did use a smallish tapered sprue along with the Campbell pouring basin design which eliminates gulps of air while filling and there weren't any voids that I saw during machining. The foam outgassing as it vaporized left a terrible surface though.

    That just reminded me that I forgot to take a picture of and mention that I made a fixture to turn wooden tapered sprue forms on my table saw.

    Another metal casting tip that I forgot to mention: Plan all your moves in advance so that you can setup for the pour and make it with no improvising or adapting during.

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    Last edited by Crusty; Nov 12, 2020 at 10:17 AM.
    If you can't make it precise make it adjustable.

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    bravo beau travail

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    Supporting Member Saltfever's Avatar
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    Quote Originally Posted by Crusty View Post
    . . . . Apparently you need a costly bubbler rig to achieve significant degassing. I did use a smallish tapered sprue along with the Campbell pouring basin design which eliminates gulps of air while filling and there weren't any voids that I saw during machining.
    Years ago I remember using some cheap tablets to degas. Can't remember the chemical but I think chlorine was part of the mix. Supposedly, it combined with the hydrogen gas in the aluminum to out-gas hydrochloride or sumptin equally as toxic. The "bubbler" was just a simple, small, steel cup with holes I drilled in it and welded to the end of a steel rod. Drop the tablet on top of the molten aluminum and then plunge the cup and tablet to the bottom of the crucible holding it there while it dissolved creating a lot of gas that would bubble up to the top carrying impurities with it.

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    Supporting Member Crusty's Avatar
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    Light salt is a mixture of sodium chloride and potassium chloride and the combination of the two breaks down under heat to release chlorine which entrains hydrogen as it evolves from the melt. From the videos I've watched testing the premise it doesn't work, however none of the testers melted the salt into a hard cake before plunging into the melt, a step reported to be essential.

    In any event, the presence of impurities and voids is more a function of feeding and venting system design than of formation of undesirable hydrogen in the melt and care in making these two factors perform optimally is rewarded by good castings. The inclusions are most often aluminum oxides which have formed during the pour and are not driven out rather than being due to hydrogen entrained in the pour.

    My dross skimmer is a flat plate which has also been domed to push a slug of light salt down into the melt but I didn't do that in my pours. Possibly I'll try it with a hard cake in the future but for now I'm concentrating on getting my fill velocity right along with correct filling and venting.
    If you can't make it precise make it adjustable.

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    Well done, Crusty!

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    Supporting Member Crusty's Avatar
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    Just poured a replacement pulley using the traditional greensand method with the previously mentioned features to improve the pour and I believe this is my best pour yet. No voids or inclusions that I've seen, no shrinkage defects and there's good metal underneath the dingle berries. The surfaces are much better than the lost foam attempts too.

    I Didn't Like the Pulleys on My Drill Press-new-pulley-blank.jpg
    If you can't make it precise make it adjustable.

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    Supporting Member metric_taper's Avatar
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    I remember back to metal shop class in 1973, we poured aluminum. I somehow recall the teacher using mothballs to degas. But I can't find any reference to that on the internet, only that it's naphthalene, C10H8. So a hydrocarbon. Found youtube video of guy using backing soda, that comments refute as BS. That indicates salts of Fluoride or Chloride are used. I did see a site where this can be purchased.

    Looks like you really don't need that.
    Your earlier post, you said you used a parting tool to cut the V groove faces. Did you have to do some grinding to give clearance for the blade in that angled slot? Trepanning operations are always a pain.

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    Supporting Member Crusty's Avatar
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    The part was turned while being revolved on it's bore so it was just plunge cuts from the side. I tried the mandrel with expanding collet turning between centers and I was able to rework the pulleys with straight bores easily to minimal runout.

    As I said, light salt - a mixture of sodium chloride and potassium chloride, which liberate chlorine and that in turn entrains hydrogen. Many internet metal casting fan boys are about to have a stroke over degassing when the problems are primarily due to aluminum oxides formed and entrained into the melt by poor metal delivery systems. It's likely that your teacher used a degassing pellet which looked like a mothball.
    If you can't make it precise make it adjustable.

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    Supporting Member Crusty's Avatar
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    I got the replacement pulley that I cast yesterday rough machined today. I made the largest pulley a little bigger than before to get my lowest rpms even lower and I left the unimportant surfaces as cast rather than taking meat off to pretty them up and weaken the part.

    I wish I had tried mandrels and expanding collets long ago because they're working great to turn these parts with features that need to remain concentric to the bore. I've known about them for a long time and just never got around to trying them and now that I have I see the potential for improved machining.

    I Didn't Like the Pulleys on My Drill Press-new-pulley-roughed.jpg



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    If you can't make it precise make it adjustable.

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