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Thread: Achievable milling and Sink EDM profile tolerances

  1. #1

    Achievable milling and Sink EDM profile tolerances

    I am designing a component where there is an inner form constructed from a radius that is offset from the centreline. As such, the profile is not a true hemisphere, but two half-hemispheres with a flat in the middle.

    I believe this means that using a lathe is out, and so the options will be to either spark erode it using a graphite probe, or to mill it using a fine ball nose to finish machine it.

    As such I am trying to understand realistically achievable and best case achievable tolerances for a 3D surface profile such that I can produce a drawing for it.

    Having considered the worst case application, I need to be aiming for a 3D surface profile tolerance of 0.060mm maximum. Gut feeling says this is tight but can be done, but then again I'm not experience in the programming or machining of it.

    Can anyone with experience share their thoughts? Am I on track, or way out?

    Comments and criticisms gratefully received.



  2. #2
    Technical Fellow
    Join Date
    Feb 2011
    Hi Preo,

    Welcome to the group, some details will help.

    How big are we talking here? Is it 4" or 6-feet?

    What is the worst surface finish you can accept?

    Where is the "flat" between the hemispheres? Since you say it can't be done on a lathe, is it perpendicular to the pocket center-line?
    If so, that brings in another round of complications.

    Can you post a pic of what you are trying to achieve.

    Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.

  3. #3
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Material and thinnest section would also be helpfull..

  4. #4
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Rochester Pennsylvania
    Tighter the tolerance the more $$$...
    A good tool and die shop will discuss your "sketch" and let you know what can be done without extreme effort. If your partial circles are smaller diameter than the distance between the two finished partial circles then you just offset the bore and bore twice. One bore to the left and a second kicked to the right? Gut feeling is that this isn't the case and that's why you're asking. Sooo... Without knowing what you're making etc. is wire EDM might be the way to go. Rough it in or if it's a draw die or something then buy a preformed nib then wire cut with a light finish cut and then polish to size. Holding .002" is like falling out of bed for a tool and die shop on small tools but like Dave said... If it's a huge part then you might be talking serious cabbage to hold .002".

    Good luck...!

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