I have been tasked with explaining some items concerning a program I inherited concerning Xactmeasure Profile. I have a two part question. Attached is a pic of the profile dimension results and the D datum results. First, the call out is |Profile|0.010|A|D(M)|B|. A is a plane, D is an intersect of a cylinder and plane (constructed circle for size) and B is a line. The attached results show the D Datum size at 0.4683 and the nominal at 0.472. I am trying to figure out the Bonus of 0.0113. Is it the bonus of the feature plus it's position tolerance, see second attachment? I have looked over 2.8, 4.11.5, 4.16.2, 4.16.4 & 4.17 and now my head hurts. This one I am confused as the standard shows the other way around, Datum is profile and feature is position.Second, if datum A is zplus and B is in the X direction, why whould I have a Y shift? Please let me know if you need more info. My previous job only used legacy reporting and this one has a lot of parts utilizing this boundary condition.
I'm interested in finding out how pcdmis is calculating true position if a datum is called out to have MMB.
FIFY
Depends on if you use Legacy or Xactmeasure. I do not know enough math to explain what's going on behind the curtain exactly, even if I knew what it was, which I do not.
I do know that both attempt to replicate a hard gage. However Hexagon admitted that Legacy did not always handle the MMB for all datums correctly. I do not know the details of which specific instances did not work right.
If you are using Xactmeasure it is supposed to be doing it all correctly. I have not heard any credible assertions otherwise.
MMB for a Datum is really all about duplicating a functional assembly IMNSHFO. 99% of the time I ignore MMB. If we can make a part that is good without using the material boundary shift, it is a good part. When our manufacturing department wants to be able to use that MMB, I tell them we need to build a hard functional gage per the FCF and ASME Y14.43-2011. IMNSHFO this is the only way to know for certain if the parts pass of fail the FCF requirement unless you are a math genius who can work with the types of equations needed to calculate datum shift.
Many people are more easily satisfied and have no qualms about using their CMM and Pc-Dmis to report things like MMB, Runout, and even Concentricity (with a straight face!). In most cases the CMM is "good enough" and unless it is a critical item I don't necessarily think it is "Bad" to do so, but I do think people need to recognize the limits of what the CMM and software can really do, then decide if that is adequate for their application or not.
If you are still really interested in learning more about these maths you should have a look at ASME Y14.5.1M-1994.
I'm interested in finding out how pcdmis is calculating true position if a datum is called out to have MMB.
FIFY
Depends on if you use Legacy or Xactmeasure. I do not know enough math to explain what's going on behind the curtain exactly, even if I knew what it was, which I do not.
I do know that both attempt to replicate a hard gage. However Hexagon admitted that Legacy did not always handle the MMB for all datums correctly. I do not know the details of which specific instances did not work right.
If you are using Xactmeasure it is supposed to be doing it all correctly. I have not heard any credible assertions otherwise.
MMB for a Datum is really all about duplicating a functional assembly IMNSHFO. 99% of the time I ignore MMB. If we can make a part that is good without using the material boundary shift, it is a good part. When our manufacturing department wants to be able to use that MMB, I tell them we need to build a hard functional gage per the FCF and ASME Y14.43-2011. IMNSHFO this is the only way to know for certain if the parts pass of fail the FCF requirement unless you are a math genius who can work with the types of equations needed to calculate datum shift.
Many people are more easily satisfied and have no qualms about using their CMM and Pc-Dmis to report things like MMB, Runout, and even Concentricity (with a straight face!). In most cases the CMM is "good enough" and unless it is a critical item I don't necessarily think it is "Bad" to do so, but I do think people need to recognize the limits of what the CMM and software can really do, then decide if that is adequate for their application or not.
If you are still really interested in learning more about these maths you should have a look at ASME Y14.5.1M-1994.
