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Thread: Pulley Shrink Cavity

  1. #1

    Thumbs up Pulley Shrink Cavity

    Hi again, I'm having a heck of a time getting a 4 step solid (with out belt grooves) pulley, 2,3,4,5" for my drill press to come out with no shrink cavity.
    Tried 5 different times today to cast it, with risers and spruces centered, on the edge, going in on the side,,,
    Anyway, what is the correct way to do this, I read about how but still dont get it.

    Thanks,
    C

    EDIT: I think "Sink Marks" is the correct name for my problem, as far as what I'm reading right now.
    If You Cant Be Handsome Ya Gotta Be Handy

  2. #2
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    Have you considered making the back hollow or does it need to be solid? Or alternatively if the shrink position is predictable, you can modify your pattern so the shrink occurs on a sacrificial part of the casting.
    Any pictures?

    Pete

  3. #3
    Moderator DavidF's Avatar
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    Hollow out the inside of the pattern.... or one big riser in the center of the 5" pulley...
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  4. #4
    for 20bucks. buy one and be done with it. https://www.amazon.com/dp/B00004RAO1...a-500503160312
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  5. #5
    Senior Member Wolfcreek-Steve's Avatar
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    Quote Originally Posted by corky View Post
    EDIT: I think "Sink Marks" is the correct name for my problem, as far as what I'm reading right now.
    Corky, years ago I worked for a company that did plastic injection molding, it didn't take long as a production trouble shooter to realize that shrink is a law of physics. (impossible to get away from) You can design around it, move it, live with it, swear about it, but in the end, it is always there. That being said, what is it about your "sink marks" that you can't live with? You are machining after casting so it can't be affecting the roundness of the part. Balance? Aesthetics? Chasing perfection?
    If it is something you really can't live with, casting with shrink bobs is one method of "designing around" the problem, optimizing wall thickness helps also. Pouring at the optimum temps is very important.
    BTW I understand why you are casting rather than spending $20, I'm goofy like that also.
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  6. #6
    Thanks Guys.
    Have nothing to take pics with right now.
    I know I could go buy one, I have bought things before.
    Its the learning and the brain work that satisfies me. Just want to know the correct way if I run in to this problem again.
    The reason it bothers me is a little Perfectionism, If its not perfect I need to know why and how it happened and try to remedy it but in the end its all good.
    Wolfcreek-Steve has it right.
    So, I'll machine the cavities out and next time increase the sprue size.
    Thanks Again Guys,
    C
    If You Cant Be Handsome Ya Gotta Be Handy

  7. #7
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    Hi Corky! Glad to see you're casting!

    The reason pulleys you buy have the hollow center with ribs is partly to save money but mostly to avoid shrinkage. Making it longer where the shrinkage is will allow you to machine the shrinkage out, but hollow with ribs is still better, but more pattern work.

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