hexagon logo
Register Sign in
  • Replies 22 replies
  • Subscribers 221 subscribers
  • Views 210 views
  • Users 0 members are here
Options
Related

Does Translating The Trihedron Ruin "True Zero"?

Bearpierro
Hello All!

I'm having a debate with a fellow programmer today on the effects of moving the trihedron away from the CAD's zero point.

This is in reference to a True Position measurement of a hole located at X0 Y14.5.


So there are two different methods to get our alignments and then dimension the true position:

Method 1
"Locate" The Part, by tapping a point on X, Translating the trihedron to that point, tapping a point on Y, Translating the trihedron to that point, etc for Z.

This obviously moves the trihedron around a bunch, and because of that physical features need to be used on the part to "return" the trihedron back to X0, Y0, Z0 on the drawing for the true position measurement to be taken.

Method 2
Leave the trihedron at X0, Y0, Z0 of the CAD model and NEVER translate it. Instead, tap points around the part and align without ever moving the trihedron.


Now, once dimensioned, both of these methods give me the same Nominal Measurement Values, X0, Y14.5.

Both of these methods use the same features to create their Iterative and Best Fit alignments.

Is there any difference in the end?

Please let me know if that wasn't clear enough, and thanks in advance for any help!
  • Your initial alignment locks the part down from moving, so anything you do after is moving around the part, don’t be fooled by being able to move the CAD model virtually, your program represents the mating part or parts. So you can move your trihedran around, and the part never moves. Even when using Xactmeasure or Geo Tolerances, if your alignment isn’t the same as your Datum structure, if you put your cursor on that callout, you’ll see a temporary trihedron on the screen. Technically your moving the mating part around when changing the coordinates of the trihehdron
  • Your initial alignment locks the part down from moving, so anything you do after is moving around the part, don’t be fooled by being able to move the CAD model virtually, your program represents the mating part or parts. So you can move your trihedran around, and the part never moves. Even when using Xactmeasure or Geo Tolerances, if your alignment isn’t the same as your Datum structure, if you put your cursor on that callout, you’ll see a temporary trihedron on the screen. Technically your moving the mating part around when changing the coordinates of the trihehdron

    I may need you to dumb this down for me. If I'm reading this correctly it means that movement is happening regardless when we make alignments throughout the program, so it shouldn't make a difference which method is used?

    For some added clarity, both methods are using a READ POINT alignment to establish the initial zero on the part.
  • If you translate/offset your Y axis by the 14.5", you have NOT changed the part's 0 point. You have just shifted your perspective, as now the original 0 point would appear to be 0,-14.5,0.
    There is nothing wrong with translating your alignment/triheadron to see the values so they match to the print.
    Unless you physically move the part, the 0 point will be the 0 point.
  • If you translate/offset your Y axis by the 14.5", you have NOT changed the part's 0 point. You have just shifted your perspective, as now the original 0 point would appear to be 0,-14.5,0.
    There is nothing wrong with translating your alignment/triheadron to see the values so they match to the print.
    Unless you physically move the part, the 0 point will be the 0 point.

    So a follow-up then, I think the concern is that I can never get the Trihedron location back to the PERFECT 0,0,0 location given by the CAD model itself. Even though my nominals in both methods are correct and the same. Is that true?

    Or is it true that if the nominal distances from location to location match in both methods then I must have that correct position?
  • Technically, as written, the Y14.5 standard is written so that you align to the datum structure in your Feature Control Frame (lets say you have POS | Ø .014 MMC | A | B | C |).
    You constrain all degrees of freedom that A will constrain (lets say it is a plane, that is two rotations and a translation).
    If you are measuring two dimensional features (not points for itterative but circles or the like) you need to align that plane now prior to measuring the circles.
    Then you measure B, lets say it is a plane also, so it constrains the final of three rotations and another translation.
    This leaves one translation for C, it can be whatever you want. Doesn't matter.

    You now have zero zero zero. Name it ABC so you remember what it is and can recall it whenever you want.

    Now, you translate your origin (from your example) zero on X and 14.5 on Y.

    This moves your trihedron.

    You then inspect the actual position of your feature compared to this "TRUE" position you translated to.

    This doesn't help a machinist move the feature to fix it, but this is what you are to do, technically.

    Then you recall ABC and continue measuring.

    You do NOT re-measure the features of A B C and redo the alignment.

    Basic dimensions are just that, basic moves from zero zero zero to get to the feature. That is why they can be added and subtracted.

    Part says this hole has a .010 position to A B C, but the basic is from some random hole in a bolt pattern? YOU add/subtract as necessary to measure the hole compared to true position FROM A B C. The distance from that other hole means nothing. Only the distances and angles from A B C.

    It concerns me that you say you can't get back to zero zero zero.

    I measure 1500 dimensions on a manifold, zero A B C and recall it, bouncing all over the place for compound angle holes, and I get back to zero every single time.

    I'm more worried that your part is so far from perfect that it doesn't match the model enough to reconcile on the first alignment anyway (which might explain you doing an iterative to begin with), of you are re-measuring the datums but not using the same features/points as the first time. maybe?

    If you don't have angle holes, you don't have to move the alignment to check the hole.
    If you use ExactMeasure or GeoTol you don't have to move the alignment to check the hole.
    In both cases, though, the software IS moving the alignment to check the hole in the background and you don't see it.

    Make a ExactMeasure or GeoTol position and then click into it. You'll see a yellow trihedron pop on the screen.
    That is the alignment the software is using to measure the hole's position against true position.

    If you can't move it yourself and get back to where you were, you need to look at your alignments and see where you went astray.

    I said I do it on large, complex manifolds.
    I didn't say I got it perfectly right the first time, every time.
    It is very easy to hit the enter key before you clicked on the button to apply something in an alignment, or do an angle in the wrong order with a translation.
    If you are having a repetition problem, it's almost always in the alignment.
  • in reply to Bearpierro
    You can recall your original alignment at any time. This will return you to your 0 point.
  • Technically, as written, the Y14.5 standard is written so that you align to the datum structure in your Feature Control Frame (lets say you have POS | Ø .014 MMC | A | B | C |).
    You constrain all degrees of freedom that A will constrain (lets say it is a plane, that is two rotations and a translation).
    If you are measuring two dimensional features (not points for itterative but circles or the like) you need to align that plane now prior to measuring the circles.
    Then you measure B, lets say it is a plane also, so it constrains the final of three rotations and another translation.
    This leaves one translation for C, it can be whatever you want. Doesn't matter.

    You now have zero zero zero. Name it ABC so you remember what it is and can recall it whenever you want.

    Now, you translate your origin (from your example) zero on X and 14.5 on Y.

    This moves your trihedron.

    You then inspect the actual position of your feature compared to this "TRUE" position you translated to.

    This doesn't help a machinist move the feature to fix it, but this is what you are to do, technically.

    Then you recall ABC and continue measuring.

    You do NOT re-measure the features of A B C and redo the alignment.

    Basic dimensions are just that, basic moves from zero zero zero to get to the feature. That is why they can be added and subtracted.

    Part says this hole has a .010 position to A B C, but the basic is from some random hole in a bolt pattern? YOU add/subtract as necessary to measure the hole compared to true position FROM A B C. The distance from that other hole means nothing. Only the distances and angles from A B C.

    It concerns me that you say you can't get back to zero zero zero.

    I measure 1500 dimensions on a manifold, zero A B C and recall it, bouncing all over the place for compound angle holes, and I get back to zero every single time.

    I'm more worried that your part is so far from perfect that it doesn't match the model enough to reconcile on the first alignment anyway (which might explain you doing an iterative to begin with), of you are re-measuring the datums but not using the same features/points as the first time. maybe?

    If you don't have angle holes, you don't have to move the alignment to check the hole.
    If you use ExactMeasure or GeoTol you don't have to move the alignment to check the hole.
    In both cases, though, the software IS moving the alignment to check the hole in the background and you don't see it.

    Make a ExactMeasure or GeoTol position and then click into it. You'll see a yellow trihedron pop on the screen.
    That is the alignment the software is using to measure the hole's position against true position.

    If you can't move it yourself and get back to where you were, you need to look at your alignments and see where you went astray.

    I said I do it on large, complex manifolds.
    I didn't say I got it perfectly right the first time, every time.
    It is very easy to hit the enter key before you clicked on the button to apply something in an alignment, or do an angle in the wrong order with a translation.
    If you are having a repetition problem, it's almost always in the alignment.

    I think there's a miscommunication here. I believe what I'm doing is correct, using features to create alignments should give me the proper XYZ coordinates to the drawing. What my coworker is telling me is that the XYZ origin (the original trihedron position set in the CAD) should be set when the CAD is created and not be moved. He claims that by moving it I am never going to have it accurately placed to the drawing. Is that right?

    I appreciate you going to this length by the way. It's a lot of info to take in.
  • If I shift the CAD model in a direction, after my alignment, then take points on the shifted CAD model I will potentially have erroneous data based on the direction I moved the model. However, once a CAD model is located and translated, there really isn't much reason to translate it again.
    IE shift a cylinder that is running along the Y axis with a Z translation will give you incorrect vectors using auto points, shift the cylinder along the Y axis and your vectors won't likely be incorrect but your linear distance could cause you to probe before/after the area of the cylinder you were attempting to measure.

    Is this what you are referring to?​


  • I think there's a miscommunication here. I believe what I'm doing is correct, using features to create alignments should give me the proper XYZ coordinates to the drawing. What my coworker is telling me is that the XYZ origin (the original trihedron position set in the CAD) should be set when the CAD is created and not be moved. He claims that by moving it I am never going to have it accurately placed to the drawing. Is that right?

    I appreciate you going to this length by the way. It's a lot of info to take in.


    that is NOT correct. the CAD zero can and will often be meaningless to the datum structure. or how you want to mount the part on the CMM and set up the part. I rarely (read NEVER) leave the CAD trihedron in it's original origin. Most of the design engineers I encounter have NO regard for how the part is to be manufactured or inspected. If you have not run into this, it is probably only because you have not been doing this long enough. Do not concern yourself with moving it, if you need to, do it. Don't let the less informed convince you otherwise. Feel free to point your "co-worker" to this forum and have him ask about his concerns of moving the zero point of a CAD model
  • If I shift the CAD model in a direction, after my alignment, then take points on the shifted CAD model I will potentially have erroneous data based on the direction I moved the model. However, once a CAD model is located and translated, there really isn't much reason to translate it again.
    IE shift a cylinder that is running along the Y axis with a Z translation will give you incorrect vectors using auto points, shift the cylinder along the Y axis and your vectors won't likely be incorrect but your linear distance could cause you to probe before/after the area of the cylinder you were attempting to measure.

    Is this what you are referring to?​

    Not quite. So my method for starting a program would be this:

    Manually move my probe to 0,0,0, which is usually the center of a hole on the part. (Close enough by eye)
    Create Readpoint at that position and align to it.
    What I was taught to do during 101 training was to then "Locate" the part by taking a point on top and setting it as my Z Origin, take a point on the "back" as my Y Origin, and on the "left" as my X Origin.
    Then I do my iterative alignment, because these parts are somewhat complex, and best fit, etc.

    My coworker is telling me that I shouldn't be moving the trihedron at all in these alignments, because the origin should be correct at the beginning when it comes from the CAD.
© 2024 Hexagon AB and/or its subsidiaries. Privacy Policy | Cloud Services Agreement