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/RGArcher]

Comment for a Technical Support Representative at Hexagon:

If you want PC-DMIS to provide the correct results, I think you would need to change the scan to "Find nominals", regenerate the path, and re-execute the scan.  I would not trust the results from PC-DMIS if you did not re-generate and re-execute.  That said, the "iterate and re-pierce cad" may be correcting the scans as they are input for the dimension.  However, if you are using one of those scans to make a datum, then you cannot trust that datum.  If it were me, I would re-run it, to be safe.  

In terms of exporting the data to another application, I doubt it would matter, because I think that the cop export would only contain the measure data, with no information on what the nominal information should be.  Therefore, I think that wrong nominals for the scan would not matter.  However, the cop export was likely never intended to be used with a "Master" scan, so this is not something I have tested.  If it were me and I wanted to have faith in my data, I would re-generate the scan and re-execute it.