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Composite true positions in Legacy

Lake Monster
I'm being required to use Legacy GDT reporting for a program.
I haven't used legacy before and need help on how to measure/report these composite true positions using it.

Datum A is a surface
Datum B is a bore
Datum C is a bore
I have my alignment set up to ABC.

In TP#1 how do I ignore C to get the AB alignment?
and how do I get just B for the composite?

In TP#2 How do I get the composite with no datums?

Can Legacy do MMC bonus or MMB datum shifts?


TP #1:

{"data-align":"none","data-size":"medium","data-tempid":"temp_22355_1648740249085_596"} ​
TP #2:

​ {"data-align":"none","data-size":"medium","data-tempid":"temp_22356_1648740215078_727"} ​ ​​
​​
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  • Looks like I'm a little late to the party.

    I don't want to upset anyone but I must point out that it is impossible to create a composite position or profile command using legacy dimensions and have it be fully compliant with ASME Y14.5. This is because of the rules governing how the lower segments work. Besides the fact that you will not be able to correctly apply datum shift where material modifiers are applied to datums, there are other problems. Only the upper segment functions "as normal" in that its datum reference frame (DRF) is allowed to constrain rotation AND translation and, if necessary can be fully constrained (locking all 6 degrees of freedom). The lower segments only allow the pattern to rotate and translate when there are no datums referenced. When datums are referenced they are only allowed to constrain rotation - so the pattern can only translate relative to the DRF. Under ASME, datums must also be fixed in orientation and location relative to each other which is not possible to achieve using traditional alignment commands - which is why you get different results using legacy than you do with the geometric tolerance command.

    This post shows the ASME rules governing composite feature control frames: https://www.pcdmisforum.com/forum/pc-dmis-enterprise-metrology-software/pc-dmis-for-cmms/504603-coaxial-holes?p=504628#post504628

    The geometric tolerance command is designed to handle composite feature control frames of up to five segments. It performs the complex datum fitting and calculates the material boundaries whilst fixing datums in orientation and location to each other as per the ASME rules. It simulates how hard gauging would work.

    Thanks Neil. I have been trying to be careful to point out the limitations of trying to report a composition position using legacy mode, but I worry that I haven't been clear enough on that. I'm glad you brought more emphasis to that.

    Let me be extra sure to say that reporting composite positions using legacy mode in the manner I described will only approximate the measurements to the ASME standard. I think it can be useful to know for troubleshooting and reporting unusual callouts that are not intended to comply with the ASME standards, but generally I think it is best just to stick to reporting using the GeoTol command.

    Also, Neil has a very good point about the rules for the lower segment of a composite position callout. The more I think about it the more I realize the example I was working with wasn't a great general example for trying to approximate composite position tolerances using legacy mode. The upper segment was not fully constrained and lower segment was fully unconstrained. That was what was specified in the original question, but applying that to a bolt hole pattern could lead someone to make the wrong assumptions about which DOFs are constrained by the lower segment. Really, the only time I have encountered a callout like that was on co-axial features. Normally, the lower segment calls out at least primary datum. In such a case one may think that would mean only three degrees of freedom are constrained, but since it is part of a composite callout, there are actually more DOFs being constrained under the hood.
Reply
  • Looks like I'm a little late to the party.

    I don't want to upset anyone but I must point out that it is impossible to create a composite position or profile command using legacy dimensions and have it be fully compliant with ASME Y14.5. This is because of the rules governing how the lower segments work. Besides the fact that you will not be able to correctly apply datum shift where material modifiers are applied to datums, there are other problems. Only the upper segment functions "as normal" in that its datum reference frame (DRF) is allowed to constrain rotation AND translation and, if necessary can be fully constrained (locking all 6 degrees of freedom). The lower segments only allow the pattern to rotate and translate when there are no datums referenced. When datums are referenced they are only allowed to constrain rotation - so the pattern can only translate relative to the DRF. Under ASME, datums must also be fixed in orientation and location relative to each other which is not possible to achieve using traditional alignment commands - which is why you get different results using legacy than you do with the geometric tolerance command.

    This post shows the ASME rules governing composite feature control frames: https://www.pcdmisforum.com/forum/pc-dmis-enterprise-metrology-software/pc-dmis-for-cmms/504603-coaxial-holes?p=504628#post504628

    The geometric tolerance command is designed to handle composite feature control frames of up to five segments. It performs the complex datum fitting and calculates the material boundaries whilst fixing datums in orientation and location to each other as per the ASME rules. It simulates how hard gauging would work.

    Thanks Neil. I have been trying to be careful to point out the limitations of trying to report a composition position using legacy mode, but I worry that I haven't been clear enough on that. I'm glad you brought more emphasis to that.

    Let me be extra sure to say that reporting composite positions using legacy mode in the manner I described will only approximate the measurements to the ASME standard. I think it can be useful to know for troubleshooting and reporting unusual callouts that are not intended to comply with the ASME standards, but generally I think it is best just to stick to reporting using the GeoTol command.

    Also, Neil has a very good point about the rules for the lower segment of a composite position callout. The more I think about it the more I realize the example I was working with wasn't a great general example for trying to approximate composite position tolerances using legacy mode. The upper segment was not fully constrained and lower segment was fully unconstrained. That was what was specified in the original question, but applying that to a bolt hole pattern could lead someone to make the wrong assumptions about which DOFs are constrained by the lower segment. Really, the only time I have encountered a callout like that was on co-axial features. Normally, the lower segment calls out at least primary datum. In such a case one may think that would mean only three degrees of freedom are constrained, but since it is part of a composite callout, there are actually more DOFs being constrained under the hood.
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