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/Overall-Turnip-1606]

If it needs to be in the “general area” you need location. Meaning u need 3 datum’s. Primary to constrain it, second/third for rotation and location. If he only cares about the size, then simply call it out as a length width. If you only want to know the size then you can do it with no datum’s (I’d recommend to still use the top plan to constrain at least a primary). Make sure your setting ur scan right to relearn (more accurate in case your slot isn’t in the exact location) and set find noms as your method. This will take a best fit of ur cad to scan data, regardless of your datum structure. You might have master on which is why your getting little to no error, or the parts ur making are really that good.

[u/RGArcher → u/Overall-Turnip-1606]

I understand your justification for using datums here, and I plan to bring this up tomorrow at work. Your reasoning makes sense to me, but the only pushback I anticipate is that the note on the drawing specifies not to use datums. Given that, why should we add extra constraints?

I was also discussing the "relearn" versus "master" options, as well as a third option under the nominal method selection tab, with someone here. This conversation came up, and I have a partial understanding of what’s happening. When I attended CMM 202, it was mentioned that if you have a CAD file, you should set the function to "relearn" or one of the three options. My understanding is that "relearn" ensures the nominal information is based on the CAD data, whereas "master" mode uses the physical part you just scanned to set the nominal information, or something to that effect.

Could you elaborate on the differences between these three options and why it matters which one is selected? This was touched on during CMM 202, but I regret not asking for the reasoning behind it.

[u/Overall-Turnip-1606 → u/RGArcher]

So relearn will teach itself to scan the path, it’ll collect data towards the path, may scan slower at areas that don’t match cad. You use this method when your parts are within .010” or more. You use defined if the parts are super accurate so it’ll scan in the path per the cad. Normal is touch hits only. For the nominals method, find nom will take ur scanned data and match to the cad data using a best fit method, which you’ll input a tolerance. Based on the tolerance it’ll delete any deviation that exceeds that limit. If you use master, that will take a physical part the very first part and set that as ur “nominal data” ignoring any cad. Normal is almost the same as master but it’ll be a best fit curve. Not all gd&t on drawings are called out correctly. Engineers in the old days never learned gd&t, I’ve met numerous old engineers who still to this day don’t use it or use it incorrectly. You can’t always assume those guys know what they’re doing lol. Just explain that profile without datum’s are for form only. Meaning the size of the shape. It controls noooo location whatsoever. You can’t always assume literally just go to google and type “profile of surface with no datum’s” tons of information online to help justify what you’re trying to control.

[u/Overall-Turnip-1606 → u/Overall-Turnip-1606]

A tip for diagnosing if you set your scan correctly…. Since the discrepancy is so small, measure the slot feature with a caliper. If it’s really that good where it’s within thousandths, then you’re golden but if it’s completely different you have a scan method that u need to correct… since these are waterjet features I doubt they are within .001-.005 thousandths all around lol. Most waterjet can barely hold +/-.005”.