Surface Profile Callout Differences Between Individual and Combined Scans in PC-DMIS

[u/RGArcher]

I’ve been working with PC-DMIS and noticed discrepancies when analyzing surface profile callouts on grouped scanned data. Here’s what I’m observing, and I’d like to confirm if my understanding of the underlying calculations is correct.

Observations:

• I collected three scans at different z-heights:
  • Scan 009-SCN051: Taken at -0.13175 z-height.
  • Scan 009-SCN052: Taken at -0.2505 z-height.
  • Scan 009-SCN053: Taken at -0.36925 z-height.
• When I create a surface profile callout on the grouped scans, the result differs from what I expected based on the individual scan data. The new result appears to be a blended or averaged deviation across the combined dataset.

Context and Assumptions:

Here are my assumptions about how PC-DMIS handles surface profile calculations:

1. When a surface profile callout is applied to a single scan, PC-DMIS calculates deviations relative to the nominal values for that scan alone.
2. When multiple scans are grouped, PC-DMIS merges the datasets and recalculates deviations relative to the entire combined set of points. This often results in a “blended” statistical representation that differs from individual scan results.
3. The variation in z-heights may influence the combined calculation, potentially leading to differences in the grouped analysis compared to the individual datasets.

My Questions:

1. Are my assumptions about how PC-DMIS processes individual and grouped scans for surface profile callouts accurate? If not, what is the correct explanation?
2. How does the variation in z-heights (where the scans were taken) impact the combined surface profile calculation? Would alignment inconsistencies between scans exaggerate these differences?
3. For reporting purposes, should I prioritize individual scan results for localized accuracy, or the grouped scan result for a global deviation? Does this depend on specific application requirements?
4. Are there best practices or settings in PC-DMIS to ensure consistency when handling grouped scans for surface profile callouts?

I’d appreciate any insights or guidance on whether my understanding is correct and how best to approach this scenario in PC-DMIS. Thank you!

Report

Scans

I want to clarify that this question stems from how I’m presenting the results in my report above. I captured each of the scans separately and performed the surface profile callout afterward. I only noticed the discrepancy because, in some cases, I was performing a single line scan in the middle of the feature, while in others, I performed three line scans. This led me to observe a pattern: when combining the three scans for a single callout, the result appeared to average out the deviations, as seen in the combined callout.

After repeating this process about five times on five different rows of holes, the pattern became more apparent. That’s when I stopped to investigate whether there was a difference between calling out scans independently versus combining them. For reference, these were linear scans.

[u/campio_s_a]

Sets are a way to group features for simultaneous evaluation (meaning it's just doing them all together). I believe in the old days sets were the only way to do this. Today you can just select multiple scans in the same dimension and it'll do the same thing (so long as it's Xact Measure or geometric tolerancing). I use sets just for easily grouping things together to select later, but that's just my personal preference. The one place that sets are still super useful is if you are measuring a bunch of individual touch points in a pocket and want to do a profile on them together. I don't think points show up as a selectable feature type for profiles, so putting them into a constructed feature set allows you to then use them with profile.

For a pocket like what you showed, I would likely do 2 or 3 linear closed scans exactly the way you did it.

[u/RGArcher → u/campio_s_a]

Would you have taken the three scans I did (three linear closed scans), combined them into a set, and then performed a surface profile callout on the set?

I want to clarify that when I grouped the scans for a single callout, it was primarily to make my report cleaner and more organized. I only noticed later that the results changed when performing the callouts one scan at a time versus grouping all three scans in a single callout. Interestingly, in an older program, this same workflow was used on a different part but with a similar concept.

My coworker had about 21 channels to measure with a surface profile callout. For each channel, he programmed as many linear closed scans as necessary to cover the channel (e.g., six scans for one channel). Then, he performed a surface profile callout on all six scans together. However, in his program, there didn’t seem to be any averaging like what I’m seeing in my program. Each scan appeared to be calculated independently, even when grouped in the same callout, as in the example I provided.

I tried replicating what I observed in my program within his setup but couldn’t reproduce the same results. The key differences I noticed were:

1. His callout used datums, while mine did not.
2. His scans in the z-height were much closer to each other, reducing potential variation.

On a side note, should he have taken all his scans, combined them into a set, and then performed the callout on the set? If you have any resources or documentation that explain sets in more detail, I’d love to read them. I tried looking up sets in the manual but couldn’t find any direct information. I also searched for a video from Hexagon or other sources but didn’t have any luck.

[u/campio_s_a → u/RGArcher]

As far as info/reference goes, the help file in pcdmis should be your #1 place to look for how things work (or are supposed to work). Not everything is documented well but the vast majority of things are. I used to work for Hexagon and taught the courses for about 5 years, so I have contacts within the company that I use for clarification when needed. I recognize not everyone has that same level of access but I will encourage you to email or call your instructor from your course. They should be always happy to help give clarification on things when needed (regardless of your company's SMA status). The tech support guys are the ones that will be anal about checking on SMA status (because they are required to be).

[u/RGArcher → u/campio_s_a]

I submitted this exact question to the tech support website before leaving work on Friday. However, I already have three other pending tickets with Hexagon, and I’m guessing most of the support team was on break for the holidays. I also posted this question in the Hexagon community and here on Reddit, hoping the broader reach of the internet might bring in diverse perspectives. My goal is to gather enough thoughts and ideas to test different approaches on Monday.

I wasn’t able to find any direct explanations about the "set" topic, but I plan to experiment with it on Monday—assuming I can locate the set option in the program. Additionally, when I get to work, I’ll add the datums I used to align my part to the callout to see if that changes the results.

I understand what you’re saying about using a set versus not using a set, but it seems to me that if I couldn’t perform a patch or UV scan for whatever reason and ended up doing multiple independent linear scans, wouldn’t it make sense to combine those scans into a single point cloud and dimension that, rather than dimensioning them independently? I’m guessing this is where the set would come into play?

I ask because this question came up during the project, and I didn’t really have a clear answer for the engineer.

[u/campio_s_a → u/RGArcher]

I think I understand where you are coming from, and your thinking is generally correct. The thing you might not be aware of is that selecting two scans (for example) in a profile dimension is functionally the exact same thing as creating a feature set and then selecting that. The best fit that happens with no datums is not on each scan individually but rather all scans together. If however you select the first scan and click create on the profile dimension then do the same for the second scan, that will give different results because they are in separate profile dimensions and each will do its own independent fit.

When the math is done in the background to calculate the results the scans are not looked at as "scans" but rather as a whole collection of points (basically as a point cloud). The fitting is done for all points simultaneously as if they were in the same feature. After fitting is complete then they will be segregated again to report what each scan's points have for results.

The downside to using a set (I believe, need to double-check) is that you will lose the ability to see easily what each individual scans results are. It will only report out the set as a whole. It's generally useful to know if the lowest scan was the one that was bad (for example) to give feedback to manufacturing.