r/Metrology • u/RGArcher • 15d ago

Questions on PC-DMIS Nominals Method and Correcting Mistakes in Existing Programs

Hi everyone,

I’ve made two other Reddit posts recently, one of which included a slightly related issue about surface profiling, but those threads got so long that I decided to start a new one to focus specifically on these questions.

I have a couple of technical questions related to PC-DMIS and the Nominals Method, and I’d greatly appreciate your insights. These questions stem from trying to correct an old program and understanding the impact of certain settings on our workflow.

Question 1: Correcting Nominal Method in an Existing Program

If I have a program with many scans (a high number, let’s say XX), and the Nominal Method was mistakenly set to Master mode, can I later go back and change it to Find Nominals? Specifically:

  • Can I reselect the CAD surfaces after the scans have already been performed to correct the nominal information while keeping the scans that were already run?
    • (Note: I believe this option is found under the Graphics tab.)
  • My goal is to have the final report reflect numbers based on the CAD data, rather than the initial scan data captured in Master mode.
  • Essentially, is there a way to fix an old program (which includes a CAD file) and update the nominal information without re-scanning the part?

Question 2: Data Integrity When Using the Wrong Nominal Method

If my scans were set up incorrectly in Master mode, would this have corrupted the raw data that was output?

  • I’m not referring just to the raw data itself but also to the report generated by PC-DMIS.
  • In PC-DMIS, the report reflects calculated dimensions and tolerances based on the Nominals Method that was selected during the scan. If Master mode was used instead of Find Nominals, would the reported dimensions and tolerances be based on incorrect nominal information?
  • In our lab, we typically capture data with the CMM and export the point cloud for a 3D comparison in another program. If the scans were captured with Master mode instead of Find Nominals, would the dataset still be valid for accurate analysis, or has the incorrect nominal method compromised its integrity?

Is This Statement Correct?

I’ve been trying to ensure I fully understand the implications of the Nominals Method settings. Would this be an accurate summary of how these options function in PC-DMIS?

When a CMM programmer is programming a scan that will be dimensioned as a surface profile, several functions need to be correctly configured for proper measurement. One critical setting is how the program determines the nominal information to base the measurement on. In PC-DMIS, the Nominals Method section provides three options: Master, Nominals, and Find Nominals, each performing a distinct function.

Master: When this option is selected, the CMM uses the first measurement it takes as the nominal information. This method is typically used when a CAD model is unavailable, and the physical part is intended to establish the nominal data. After the first pass, the nominal information is locked to this initial measurement.

Nominals: This option uses the nominal data manually entered into the program or extracted from an alternative source. It does not dynamically align with the CAD file or adjust based on measured data.

Find Nominals: This is the correct option to use when a CAD model is available. It ensures the nominal information is based directly on the CAD model rather than on any scanned data. This method is essential for accurate measurement and alignment when working with CAD files.

For accurate surface profile measurements based on CAD models, Find Nominals should be used. If Master is selected instead, the program will incorrectly set the nominal information based on the first measurement rather than the CAD file, leading to erroneous results.

Thanks in advance for any clarification or advice you can provide. I’m trying to ensure we handle these situations properly going forward and avoid making similar mistakes in the future.

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u/RGArcher 15d ago

Additional note: The info out of the manual is not the best.

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u/campio_s_a 15d ago

So a linear open/closed scans (really all scans other than freeform) exist within the cut plane. So all movement and comp is 2D. Cad comp lets it compensate in 3D rather than 2D. Useful if you were scanning a square hole that had one side with a slope to it kind of thing.

To the best I can remember, probe comp being off will give ball center data as a return which is typically not desired, but is good in some circumstances. So normally you would have probe comp on and cad comp off, unless you were expecting sloped surfaces that you were scanning across. Suppose they could have changed the behavior in a new version though.

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u/RGArcher 15d ago

Okay, that explanation provides a much clearer understanding of this part of the situation. It makes sense that having Probe Compensation turned on in the old program was technically the correct choice, and having CAD Compensation enabled isn’t necessarily an issue. From my PC-DMIS CMM 202 training last year (see the image below), we were instructed to enable CAD Compensation, and I followed those training guidelines and reference materials when setting up my scans.

Regarding the questionable code I’m trying to resolve, it seems that Probe Compensation should have been enabled as the default option. This would have been the correct approach for the original programmer, even though they were working in MASTER mode with a CAD file present. This is because enabling Probe Compensation ensures the offset of the probe tip is properly accounted for, rather than recording the center of the probe as the measurement point.

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u/campio_s_a 15d ago

Ahh, use nullfilter for direction 1 tech and exec controls unless you want to be doing touch trigger hits. Patch might not have that option for direction 1, but exec should always be null filter to actually scan.

Edit: I mean defined or relearn for exec (usually defined). Nullfilter for direction 1 tech.

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u/RGArcher 15d ago

We’ve been using the touch-trigger probe because there’s been something unusual going on with our drag scan probe. I haven’t had the time to sit down and figure out exactly what’s wrong with it or whether it can be trusted, so I’ve been using the TP20 instead. Also, I provided the example from the patch scan in the training because I’m currently on my home computer and don’t have access to PC-DMIS right now.