Snap Points / Gap Only

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  • Snap Points / Gap Only

    Well, I would post this in the TIPS & DESCRIPTIONS section, but there isn't one (yet?).

    SNAP POINTS:

    1) Not a great idea to use for ITERATIVE alignment points
    2) Great idea for all other VECTOR points in the program

    Snap Points simulate a perfect machine measuring a vector point, the perfect machine being able to STAY exactly on the approach vector, not deviating by as much as a micron. Snap points are a short cut for something YOU could do yourself with about 6 more steps.

    a) Vector point nominal IJK used to create a 3-D line going through the nominals of the vector point
    b) Plane perpendicular to the 3-D line is constructed THROUGH the actual measured point
    c) 3-D line and PERP. Plane are intersected giving the reported XYZ location of the point. This puts the point EXACTLY on the nominal vector line eminating from the nominal XYZ location of the point.

    By using SNAP point, the formula SQRT(Xdev^2+Ydev^2+Zdev^2) will match exactly the "T" axis as reported by Pcmid

    By NOT using SNAP, you must use Xdev*I+Ydev*J+Zdev*K to get the matching "T" axis value.

    Both formulas are correct for the RADIUS of a SPHERE, which is really what the "T" value will represent, the distance from the center (XYZ NOMS) to a point on the surface (XYZ ACTUALS). HOWEVER, if NOT using SNAP, the ACTUALS will not match due to the 'drift' of the CMM, thus requiring you to use the VECTORS (IJK) of the point in the calculations to determine the "T" value.




    GAP ONLY

    Gap only acts like a SNAP POINT for edge points. When doing edge points, ESPECIALLY on sheet metal, you SHOULD be taking a surface sample hit so you can control the depth of the hit on the edge. There is shear and breakage on the edge of sheet metal, you SHOULD be touching on the shear portion.

    To enable you to do this, time after time after time, WITHOUT changing the program, use a surface sample hit and the proper depth (small value if the shear is on the 'top', a value = (stock thickness - a small amount) if the shear is on the 'bottom'. Once you determine the proper depth to hit the shear everytime, you need to ensure that the CMM knows where the surface of the part is, that is what the surface sample is for.

    To report the edge point, you CAN do XYZ & T and T will give you the (T)otal amount the edge point is off nominal by, along the edge vector. HOWEVER, if using surface sample hits, the XYZ values will NOT jive with the "T" value. I almost always (99.995% of the time) will report the "S" axis at the same time, this gives you the SURFACE deviation (like the "T" of a vector point) as well as the (T)otal edge deviation and eliminates the need to also take a vector point (2 for 1, eh?).

    IF you want to report JUST the trim edge itself, you can use GAP ONLY (in the FEATURE LOCATION dimension window. What this will do is allow you to use the surface sample hit so you can control the depth of the edge touch, but it will then 'project' the actual point BACK INTO THE NOMINAL surface and then report the XYZT of the edge point AS IF THE SURFACE CHECKED ABSOLUTELY PERFECT, thus giving you only the GAP, as if you were doing a feeler check to a hard gage.


    It sure would be NICE if there were a section set up for this kind of thing. If the TITLE described EXACTLY what the post was dealing with it would make it MUCH easier for newbies to find QUICK easy help (since the search don't work so well). This could be the first such post in a TIPS and DESCRIPTIONS sections
    sigpic
    Originally posted by AndersI
    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

  • #2
    Kudos to Matt, Thanks for helping, pal!

    TK
    sigpicHave a homebrew

    Comment


    • #3
      MUCH REP TO MATT!!

      I have always had questions as to when to use SANP and when not to.

      Great info!
      Xcel & MicroVal Pfx & Global 37mr4 thru 2012mr1sp3
      Contura Calypso 5.4

      Lord, keep Your arm around my shoulder and Your hand over my mouth. Amen.

      Comment


      • #4
        Note:
        Blue arrow reports the t value
        Green arrow is the surface
        DR Watson shut me down again !!!! :mad: Smoke break:eek:

        Comment


        • #5
          Originally posted by Jeffrey Lovely
          Note:
          Blue arrow reports the t value
          Green arrow is the surface
          Good point, helps you to 'see' which is which when creating the feature. Lets you know if you have picked it right if you have top, bottom, and edge surfaces in the cad file. If it 'grabs' the edge surface, then you will see that it is trying to use the edge as the surface and the surface as the edge. This is just ONE of the reasons why I always divide surfaced parts into different layers, top in layer '1', bottom in layer '2', and the edges in layer '3'. Then, I can just show the 'top' if that is the side I am programming from. No mis-grabs by Pcdmis getting the wrong surface, then having some yutz wonder why it is off 'almost exactly stock thickness' (if they notice that at all!).
          sigpic
          Originally posted by AndersI
          I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

          Comment


          • #6
            Snap Points

            Matt,

            Are you sure about Snap Points being OK for all other vector points. As I posted in the T-Val thread as far as I know the Snap Points are not raw data values that can be used for any type construction or spatial analysis. They are a special calculated value for sheetmetal applications. I had an issue of inaccurate data being reported and it was specifically do to using snap points. I haven't tried to use them in any of the latest releases, but I don't believe they can just be substituted for standard points. That's why they don't work for iterative alignments, non working plane contruction, etc...

            let me know what you think.

            Thanks, Mike
            Job Function:
            Quality Engineer/Programmer
            Machine Type:
            Global
            Software Version:
            V2010 MR1
            CMM Experience:
            25+ yrs.

            Comment


            • #7
              Originally posted by mchough1
              Matt,

              Are you sure about Snap Points being OK for all other vector points. As I posted in the T-Val thread as far as I know the Snap Points are not raw data values that can be used for any type construction or spatial analysis. They are a special calculated value for sheetmetal applications. I had an issue of inaccurate data being reported and it was specifically do to using snap points. I haven't tried to use them in any of the latest releases, but I don't believe they can just be substituted for standard points. That's why they don't work for iterative alignments, non working plane contruction, etc...

              let me know what you think.

              Thanks, Mike

              Well, I do not use them for iterative alignments (and I rarely DO iterative alignments) simply because they ARE 'altered' points and when you are doing an iterative alignment you are trying to find the part, not check it. AND, as I am in sheet metal, I have never had an issue with them. I do not do very much construction of features from points, and the ones I do construct ARE in 'perfect' reference to a workplane (DCC alignment features, plane and lines that SHOULD be a perfect match for the manual alignment features). BUT, for construction of features, you may be right. I have never had an issue with it, but like I said, I construct VERY FEW features from points and those are almost always perfectly to a workplane for a DCC alignment of a fixture. I do not use constructed sets nor do I make AND dimension scans. For 99.999% of the inspection work I do, it is EDGE, VECTOR, and HOLE (hole, round slot, square slot) and for all those vector points, SNAP works perfectly.
              sigpic
              Originally posted by AndersI
              I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

              Comment

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