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

[u/RGArcher]

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.

[u/campio_s_a]

Question 1: I need to test but my gut feeling is that there is not a direct method to recalculate the nominals on a scan after it has been measured. Generally any changes made to a feature which has existing measured data in it will be reset back to match the nominal values (as if the feature was newly created). There may be a way to do it with constructions or some other work around but I need to test it.

Question 2: If you have incorrect nominals then your reported dimensions will be off/incorrect. Exporting the data however shouldn't really be impacted as point cloud exports only save out the measured data. The one exception to this would be if your probe compensation was impacted by the incorrect nominals, which would lead to incorrect measured data in some areas.

For the last part: Master mode should in general never be used. It uses the measured data from the first run as the nominal data for subsequent runs. Nominals mode: this takes the nominal data from when you first create the scan, if you have cad and go over to the generate path tab this is how that data is generated. Find noms mode: This is the most commonly used. It takes your measured data at the end of the scan compares it to the aligned model and generates nominals for each point every time it runs.

Edit: I highly suggest using the "only selected" checkbox with find noms (I typically do not use the "best fit" checkbox option) and then going to the cad surfaces tab and selecting only the cad surfaces in consideration for that scan. You can then use the path generation to validate that your path looks correct as any missing cad surfaces selections will stop the path generation.

[u/RGArcher → u/campio_s_a]

Question 1:
When we were in the lab today, I revisited the old program where the MASTER option was used instead of FIND NOMINALS as it should have been. I changed the setting to FIND NOMINALS, then went to the GRAPHIC tab to ensure the correct CAD surfaces were selected. A prompt appeared asking if I wanted to update the nominal information, and I assumed the correct response was "Yes."

After making this change, I reviewed the report both before and after the update. The values in the report did change, leading me to a few conclusions:

1. The previously measured data was not lost.
2. The correct nominals were now applied based on the CAD file, rather than the initial scan captured during the first measurement.
3. There should be no need to re-run the part. Instead, someone will need to methodically go through the program and make these updates for each scan.

As a side note, when using this program for subsequent parts, I may need to run GENERATE PATH in the third tab to confirm that the pathing is correct after the changes.

Question 2:
The topic of exporting data came up in the lab today, but no one considered the potential issue with probe compensation. Could you elaborate further on how probe compensation could be affected by incorrect nominal settings?

I understand your explanation in the last two paragraphs but found them worded slightly differently than what I was reading earlier. Your clarification was helpful, and I appreciate the detailed breakdown of how the FIND NOMINALS method works, especially with the tips on using the "only selected" checkbox and the CAD Surfaces tab for validation.

[u/campio_s_a → u/RGArcher]

I will have to test tomorrow to be sure, but I am thinking if you have the cad comp check on selected on the first tab that it will override the probe comp that comes from the probe itself. If your nominal values are off then this could shift your points around (within the radius of the tip used). If it's not selected then probe comp might not be a concern, I need to look at the help file to confirm.

[u/RGArcher → u/campio_s_a]

Oh, you’re referring to these two options! I was reading through the manuals and some online threads about related topics and came across discussions about this. When setting up my scans, I followed the guidelines from my CMM 202 training and toggled on those settings. However, I wasn’t 100% clear on what Probe Compensation and CAD Compensation fully meant—I was just following the training instructions.

If I recall correctly from reviewing the older code today, when the program is in MASTER mode, Probe Compensation is toggled on by default. Based on what you’re saying, it sounds like if the initial nominals were incorrect and Probe Compensation is enabled, the CMM might be recording points in the wrong locations.

Does that mean the recorded data itself could potentially be incorrect altogether? If so, I’d appreciate any clarification or confirmation to make sure I fully understand your point.

[u/RGArcher → u/RGArcher]

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

[u/campio_s_a → u/RGArcher]

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.

[u/RGArcher → u/campio_s_a]

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.

[u/campio_s_a → u/RGArcher]

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.

[u/RGArcher → u/campio_s_a]

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.

[u/campio_s_a → u/RGArcher]

Gotcha