Used exct method to check composite true position.

Hi, all cmm members
I am really confused the composite true position report by using exct method. I have attached my handy drawing( bad draw skills tbh) and cmm reports. Please check it.

1. Why do the report show start pt and end pt (see red circles) for each cylinders? Why not cmm use the same points or best points to create the results?

2. As you can see my attachment, cyl7 and cyl12 has .020 true position to datum -A- & -B-. Then we have two dimensions to datum -A- which are 29.5981 and 29.6003. I understand it. But how come the FCFLOC2 is not repeatable the same checked dimensions and give me another two different dimensions to datum -A-? It does not make sense to me.

3. Should I always use legacy method to check true positions? CMM will give me better results? By the way, I am using PC-DMIS 2022.2 model.

Any experts cmm programmers have any ideas, please let me know. I really appreciate it.

Thanks

Matthew D. Hoedeman over 1 year ago

I think it uses the WORST end by default since the entire thing must pass, so, the worst end must pass, so that is what is shown.
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cmm@slaerospace.com over 1 year ago in reply to Matthew D. Hoedeman

Yep, thanks for your quick replying. But why the location checking use the start points for cylinders and .005 concentricity use end points to check? It is supposed to use WORST end to check everything, isn't it?
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Matthew D. Hoedeman over 1 year ago

I would guess it is because it is TWO different evaluations, location is worst at the start, but concentricity is worst at the ends. Think // to \\, if one has more 'lean' to the left and one has more lean to the right, the concitricity could easily be worst at the better end than that of the location.
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cmm@slaerospace.com over 1 year ago in reply to Matthew D. Hoedeman

I think you are right, Matthew. Thanks for your explanations. I think I understand it how pcdmis calculate the composite true position.

For the first location checking(.020"), PC-DMIS project the WORST PTs to datum -A- and -B- and calculate the true position.
For the concentricity checking(.005"), PC-DMIS only finds the worst points between two cylinders and calculate the true positions.

Let me know if I think wrong here.
Thanks again your help.
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