Thinking about datum shift

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  • Thinking about datum shift

    I have a part with a cylinder along Z and a cylinder along X. The cylinder along Z is called out Position .002 (M) to the other cylinder (M) as primary datum.

    I understand that the tolerance zone will grow depending on the features departure from MMC. This is drawn up as a diametric tolerance zone, but the cylinders can only be off in one direction (Y). How can I visualize the tolerance zone?

    Also, the primary datum is at MMB. This would imply datum shift. I cannot simply calculate the deviation from primary datum at MMC and add that to my .002 position tolerance, correct?

    Untitled.jpg
    Last edited by JacobCheverie; 05-21-2019, 09:36 AM.

  • #2
    No. Your feature will shift only in Y until the datum's MMB is reached. At that point, if the deviation of the feature about Y is not at 0 at that point, then it will count against your TP tolerance.

    Comment


    • JacobCheverie
      JacobCheverie commented
      Editing a comment
      Okay, so the MMB is the actual mating envelope of the datum feature simulator at MMC? If so, then I have a mile to move that feature to reach conformity. -A- has .010" total tolerance. Am I thinking of this correctly? I am trying to go through ASME Y14.5 and get a thorough understanding.

    • louisd
      louisd commented
      Editing a comment
      I'm confused. Print doesn't mention -A-. If you are inferring -D-, .010" of shift isn't much, for a cylindrical axis that long.

    • JacobCheverie
      JacobCheverie commented
      Editing a comment
      louisd I apologize, I meant -D-.

  • #3
    Bad Callout, Primary has 2 Jobs, Level and Origin, Max Material, doesn't apply, What happens If you lay a part on a surface plate and Datum A is a Plane with Max Material? Do You get to add Shims? kinda screw with it's main objective, leveling. If it's a cylinder. whether it's 8 inches round or 2 inches round, it will level to center line. That's like telling the machinist, "Put that part on a three chuck jaw, tighten it up, Then loosen the jaws about 3/4 of a turn and start machining, because you have Max Material "
    (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
    They say "Nobody's Perfect." I must be Nobody.

    Comment


    • louisd
      louisd commented
      Editing a comment
      I disagree with this. This is a GOOD callout, as it supports an economical product and design. 0.010" of MMB shift on that axis isn't much when you start trying to fit the entire length of that datum into a datum simulator. Picture the datum simulator a really long slot, like an umbrella holder.
      Drop that rod into the umbrella holder slot. With the C Datum always perpendicular to the slot, you can rattle that rod around freely, If you can make a pin at LMC go through C Datum hole, (within 0.002" of datum simulator centroid), your part passes.
      Last edited by louisd; 05-21-2019, 11:41 AM.

    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      Oh I see louisd these must be IKEA products. As yes it is a "good callout" Let me re-phrase that. Poor engineering or a s_h_i_t product. That kinda of callout would lie under the thinking of, I can dig a hole in my backyard for a tree, I can start from the top surface or I can crawl under ground and start from the bottom and work my way out. So you say "good part" I still say "no"
      Last edited by KIRBSTER269; 05-22-2019, 09:27 AM.

    • louisd
      louisd commented
      Editing a comment
      I promise you kirbs, there's a happy medium between "Poor engineering or a s_h_i_t product", and in order to be profitable, manufacturers and engineers often find it.

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