Midplane Between Two Radii

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  • Midplane Between Two Radii

    If this was your primary datum, how would you replicate it in PC-DMIS? My only thought would be to do an iterative alignment on one side only...but that would throw everything out of location based on the width of the part. Could I do a best-fit alignment incorporating each side (I've never done one before)?

    asdf.PNG

  • #2
    Do you have scanning capabilities, or just a touchprobe? I you have scanning, I would construct a scan on one side, covering as much of the surface as possible. then, I'd copy/paste with pattern-> flip Y that same scan to the opposing side.
    -From there, you could do several things. you could simply try to construct a midplane from the two scans, (my guess is this won't help much), but you'd have to define the hits of each scan to do it correctly.
    construct -> plane -> midplane -> SCN1 & SCN2. then within the constructed feature you'd add .hit[1..SCN1.numhits], to end of SCN1, and the same for SCN2.

    If that fails I'd construct a handful of theoretical planes sectioning part along X. I'd then construct intersect lines for each plane, and construct mid-lines for each set of intersects. and finally stitch the line data into a constructed plane.

    For the implied origin that this inverted torus can imply, I'd construct a single cast point of the two scans.

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    • #3
      I would call the designer to explain him what is a cmm measurement, and how bad is his definition !

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      • Lukas
        Lukas commented
        Editing a comment
        Hahaha it is that it's an op sheet drawn by our internal engineer. The customer drawing uses a different datum that isn't machined yet. Tomorrow is my last day so it won't be my problem anymore, but I thought I'd ask if this is possible to program.

      • JEFMAN
        JEFMAN commented
        Editing a comment
        Engineer or not, the drawing must have the same meaning for the designer, the machinist and the controller.
        If that does not mean anything, you have to explain it to the designer...
        It's rarely fun, but it's a way of improvement!

      • Lukas
        Lukas commented
        Editing a comment
        In theory it could be done though, right? For instance, if those radii were much fuller, then I suppose I could create a mid-line between the two cylinders. The engineers here are too stubborn--it's not worth the effort. I am just going to select a different datum that is as true to the customer drawing as possible.

    • #4
      You're right, it's possible... You can obtain this :
      midplane.JPG
      It's a datum, if the measurement is bad (less than my draw, but just a little wrong), what will be the result ?
      An engineer who can't admit his errors is a bad one !

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      • #5
        In all due respect, this is beating a dead horse here, if your last day having anything to do with inspecting this product is tomorrow. BUT I presume the A datum is not explicitly defined as a midpoint between a width of curved features on the print. My guess is there's some information left out of the picture here. Plus, we don't know any other view of this part.
        A datum might be a torus, like I presumed. You can get 5dof controlled with a torus.

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        • #6
          XactMeasure can use freeform surfaces (scans, sets) as datums - not sure what it locks, though. Can't be primary, I suspect.

          Edit: A quick test seems to indicate a set only locks XYZ, no directions.
          Last edited by AndersI; 08-29-2019, 03:57 AM.
          AndersI
          SW support - Hexagon Metrology Nordic AB

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          • #7
            Originally posted by AndersI View Post
            XactMeasure can use freeform surfaces (scans, sets) as datums - not sure what it locks, though. Can't be primary, I suspect.

            Edit: A quick test seems to indicate a set only locks XYZ, no directions.
            Well, you gave me an idea. I created a mid-plane between the scans. See my pics below. However, it looks crooked like JEFMAN's drawing. Thoughts on this? Anyway, I thought I'd just post this for learning purposes. It still doesn't seem like it's possible to use this datum. I'm using the ID bore instead.

            Capture.PNGCapture2.PNG
            Attached Files

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            • #8
              What about scanning both sides and do a bestfit alignment on the scan(s)? That should take care of the direction(s). If you actually *need* a midplane as a datum feature, you can construct it afterwards, from the current alignment.
              AndersI
              SW support - Hexagon Metrology Nordic AB

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              • Lukas
                Lukas commented
                Editing a comment
                I don't need an actual mid-plane. Yes, that is what I was hoping would be possible. I've never done a best fit alignment (only iterative). Iterative doesn't allow you to select a primary datum with drastically different point vectors. Does best fit?

              • AndersI
                AndersI commented
                Editing a comment
                Yes. You actually *need* drastically different point vectors if the bestfit is 3D - you need to lock all three directions...

              • AndersI
                AndersI commented
                Editing a comment
                Note: When you do it with the alignment, you need to use legacy TP, or create fake datum features from the alignment for XactMeasure.

            • #9
              How's this look for a first attempt at best fit? I used the two scans, a midplane between the left and right sides, and a point on top of the part. My deviation was at .0008 so I set my deviation threshold to .0009 lol...

              https://i.postimg.cc/t4FgVPnr/Capture.png

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