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Iterative alignment and using XactMeasure

BWIZZLE
Alright here is my problem/question.
I have a part where Datum -A- (Z axis) is not prismatic, however datum -B- (X axis) and -C- (Y axis) are.

I do not have much experience with Iterative alignments other than that I have been to the Level 2 Hexagon class (Just don't use them or have the need too).

My thoughts were, this is a good time, to try an Iterative alignment.
I believe I have that part of the program correct:
I created 6 auto vector points on top (Datum -A-),
then 5 auto vector points on left side (Datum -B-)
then finally 5 auto vector points on the front of the part (Datum -C-).

When I create the Iterative alignment I pick top 6 points as level (Datum -A-),
5 points on left rotate (Datum -B-),
then 5 points on front as origin (Datum -C-).

After completing this Iterative alignment my trihedron doesn't move from before the alignment.
So I constructed planes out of the five points for datum -B- and -C- respectively.
I then origin my "X" to Datum -B- and my "Y" to datum -C-.
How do I origin my "Z", and how do I assign my datum -A- for exact measure? Where did I go wrong? Please any help would be greatly appreciated.
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  • Alright here is my problem/question.
    I have a part where Datum -A- (Z axis) is not prismatic, however datum -B- (X axis) and -C- (Y axis) are.

    I do not have much experience with Iterative alignments other than that I have been to the Level 2 Hexagon class (Just don't use them or have the need too).

    My thoughts were, this is a good time, to try an Iterative alignment.
    I believe I have that part of the program correct:
    I created 6 auto vector points on top (Datum -A-),
    then 5 auto vector points on left side (Datum -B-)
    then finally 5 auto vector points on the front of the part (Datum -C-).

    When I create the Iterative alignment I pick top 6 points as level (Datum -A-),
    5 points on left rotate (Datum -B-),
    then 5 points on front as origin (Datum -C-).

    After completing this Iterative alignment my trihedron doesn't move from before the alignment.
    So I constructed planes out of the five points for datum -B- and -C- respectively.
    I then origin my "X" to Datum -B- and my "Y" to datum -C-.
    How do I origin my "Z", and how do I assign my datum -A- for exact measure? Where did I go wrong? Please any help would be greatly appreciated.

    Sorry, but I just had to edit your original post for clarity.

    There are three issues here:
    1) You missed in Level 2 that Iterative doesn't require you to set origins at all, it takes the nominals and actuals of each point and fits a coordinate system to minimize the error between the two. This is why you must have nominals from a CAD file or a print. If you do this with jogbox-created points, the nominals and actuals are exactly equal (in Startup coordinate system of course) and no useful alignment is done. The trihedron doesn't move because no fitting is being performed.

    2) There is a disconnect between Iterative's basic methodology of near-random surface points as 'reference datums' and XactMeasure's GD&T-specific usage of discrete geometric features as 'real datums'. You must have discrete planes, cylinders, circles, etc features to assign datum letters to.
    So, is A a plane? Then measure it as a plane, and assign it datum letter A.
    XactMeasure will create it's own coordinate system fit to these datum features to report what you need of it.

    3) You wrote, "Datum -A- (Z axis) is not prismatic" - what do you mean by that? Is it a tilted plane whose vector is not pointing in +Z?
    That's not a problem. In the regular alignment dialog there is an Offset field in the Rotate section where you can enter in a decimal degrees offset to rotate your coordinate system around an axis of your choice. No iterative is needed.
    As you observed, tilted A plane then becomes a poor choice to set your Z origin on - a better one would be an imaginary Constructed Line where A plane intersects the B side plane.
Reply
  • Alright here is my problem/question.
    I have a part where Datum -A- (Z axis) is not prismatic, however datum -B- (X axis) and -C- (Y axis) are.

    I do not have much experience with Iterative alignments other than that I have been to the Level 2 Hexagon class (Just don't use them or have the need too).

    My thoughts were, this is a good time, to try an Iterative alignment.
    I believe I have that part of the program correct:
    I created 6 auto vector points on top (Datum -A-),
    then 5 auto vector points on left side (Datum -B-)
    then finally 5 auto vector points on the front of the part (Datum -C-).

    When I create the Iterative alignment I pick top 6 points as level (Datum -A-),
    5 points on left rotate (Datum -B-),
    then 5 points on front as origin (Datum -C-).

    After completing this Iterative alignment my trihedron doesn't move from before the alignment.
    So I constructed planes out of the five points for datum -B- and -C- respectively.
    I then origin my "X" to Datum -B- and my "Y" to datum -C-.
    How do I origin my "Z", and how do I assign my datum -A- for exact measure? Where did I go wrong? Please any help would be greatly appreciated.

    Sorry, but I just had to edit your original post for clarity.

    There are three issues here:
    1) You missed in Level 2 that Iterative doesn't require you to set origins at all, it takes the nominals and actuals of each point and fits a coordinate system to minimize the error between the two. This is why you must have nominals from a CAD file or a print. If you do this with jogbox-created points, the nominals and actuals are exactly equal (in Startup coordinate system of course) and no useful alignment is done. The trihedron doesn't move because no fitting is being performed.

    2) There is a disconnect between Iterative's basic methodology of near-random surface points as 'reference datums' and XactMeasure's GD&T-specific usage of discrete geometric features as 'real datums'. You must have discrete planes, cylinders, circles, etc features to assign datum letters to.
    So, is A a plane? Then measure it as a plane, and assign it datum letter A.
    XactMeasure will create it's own coordinate system fit to these datum features to report what you need of it.

    3) You wrote, "Datum -A- (Z axis) is not prismatic" - what do you mean by that? Is it a tilted plane whose vector is not pointing in +Z?
    That's not a problem. In the regular alignment dialog there is an Offset field in the Rotate section where you can enter in a decimal degrees offset to rotate your coordinate system around an axis of your choice. No iterative is needed.
    As you observed, tilted A plane then becomes a poor choice to set your Z origin on - a better one would be an imaginary Constructed Line where A plane intersects the B side plane.
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