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mbonka2-Explorer
2-Explorer
November 19, 2014
Solved

Your model accuracy?

  • November 19, 2014
  • 6 replies
  • 18149 views

Hello,

l have a long time problem with model accuracy. l understand it´s different from company to company.

Company no. 1 is specialist about fine optic lens with high accuracy.

Company no. 2 is specialist about rought machining with lower model accuracy.

Our company produce a air-ducts. It means 5000 mm long duct, DIA 1500 mm with 2 mm wall thickeness ---> it´s a big difference between lenght and thickeness.

Rules that l have to keep:

- all models have same model accuracy

My information about accuracy:

- absolute is better than relative

- set absolute = 0,002 looks fine?

Questions:

- do you have any expierences with model accuracy?

- could you share your "standard" model accuracy?

Thanks for information

Milan Bonka

    Best answer by TomU

    Absolute accuracy is important when:

    • copying data from one model to another (copy geom, publish geom, etc.)
    • model size is large relative to feature size
    • exporting data for manufacturing (cnc milling, turning, etc.)

    The allowed values for both relative accuracy and absolute accuracy are based off the model's bounding box size. If the accuracy dialog won't let you enter the value you want, temporarily change the model size, set the accuracy, then change it back.

    The config.pro option "default_abs_accuracy" will always be accepted, even if the bounding box calculation wouldn't normally allow it. We have this set to "0.0001"

    The config.pro option "accuracy_lower_bound" will allow smaller numbers to be entered for the accuracy values without needing to change the model size, but it shifts the upper limit down as well. We have this set to ".000001"

    As a general rule you want the effective accuracy to be at least half of the smallest feature you want to resolve. So if you want to be able to see/measure something .001" you need the accuracy at .0005" or smaller. Keep in mind that relative accuracy is not the same as effective accuracy. When relative accuracy is used, the effective accuracy is a calculation between the relative accuracy value and the bounding box size.

    Absolute accuracy can increase file size, memory usage, and regeneration time. PTC still recommends relative accuracy, but in the tool & die world it seems pretty common to standardize on a single absolute accuracy setting for all models.

    6 replies

    T
    TomD.inPDX17-Peridot
    17-Peridot
    November 19, 2014

    My experience with model accuracy is that it is a huge problem all around. I understand how it can make calculations better, but it should have remained hidden from the user and self adjust as the user needed it.

    I enable absolute accuracy and allow accuracy down to .00005" When a model give me trouble, I try this setting. even so, once I change to absolute, and it takes on my default of .00005, when I change it to something less stringent like .0001, I get a message that I can only use .0012 to .00087 or some weird thing like that. But I can switch back to .00005!

    Next thing I learn from PTC is that sheetmetal has absolute accuracy set by default. But not if you convert to sheetmetal! All in all, it is simply a very poorly implemented performance feature.

    For the most part I can work with default relative accuracy. But I have to remember that some failures may be accuracy issues. If I needed only one value for my designs for say analysis or other functions, I would go mad!

      M
      msteis1-Visitor
      1-Visitor
      November 19, 2014

      My experience with accuracy is a miserable nightmare! I tend to re-use a lot of parts and assemblies, and also have to work with inches and millimeters in the same assembly. I usually start out with the default relative accuracy, then cut it down when I have problems.

      Other than having feature failures by simply changing units, my worst complaint is that I have accuracy issues that don't cause features to fail, they change dimensions, re-locate features, or just don't work, but don't fail. So if I don't notice them, I make scrap. It's a total nuisance, time waster, and scrap generator.

        S
        23-Emerald III
        StephenW23-Emerald III
        23-Emerald III
        November 19, 2014

        I generally consider accuracy a time related function. Sure, I can solve just about all the accuracy problems by going to absolute accuracy at the lowest possible value, but that approach adds time to regeneration. I work with large assemblies, time is always my enemy, I can't afford to make a rule that demands the most time, no matter the outcome.

        So I think your "Rules that l have to keep: " is a problem. Accuracy is just another tool in the toolbox. Usually, you have reasonable parts that work relative accuracy levels and then you have parts that are long and skinny that don't work with relative accuracy. Why would you want to use extremely precise accuracy for parts that don't require it, that just adds time to your regeneration. Time is cost added and if it's just because you have an arbitrary rule, it's wasted money. Consider coming up with rules and reasons for adjusting accuracy instead of trying to have one rule that fits all situations.

        I do agree with Antonius that accuracy can sometimes be infuriating and it doesn't always seem consistent in the values it allows. There are also some considerations when merging parts that requires accuracy to be the same across all the parts being merge.

          M
          mbonka2-ExplorerAuthor
          2-Explorer
          November 19, 2014

          Thanks for respons. Nightmare is the best word...

          My rule about same accuracy is a generall rule from the reason of merging surfaces. It´s not absolutly strict rule only recommended. Sometimes l have to change accuracy from reasons that Antonius wrote.

          l know two really skilled company in Czech republic that use absolute accuracy 0,002 and second company use 0,01. But the difference is 50x ---> so l´m confused what to choose. Original PTC is relative 0,0001 and it´s not good for me (many failed featers etc...).

          T
          23-Emerald IV
          TomU23-Emerald IVAnswer
          23-Emerald IV
          November 19, 2014

          Absolute accuracy is important when:

          • copying data from one model to another (copy geom, publish geom, etc.)
          • model size is large relative to feature size
          • exporting data for manufacturing (cnc milling, turning, etc.)

          The allowed values for both relative accuracy and absolute accuracy are based off the model's bounding box size. If the accuracy dialog won't let you enter the value you want, temporarily change the model size, set the accuracy, then change it back.

          The config.pro option "default_abs_accuracy" will always be accepted, even if the bounding box calculation wouldn't normally allow it. We have this set to "0.0001"

          The config.pro option "accuracy_lower_bound" will allow smaller numbers to be entered for the accuracy values without needing to change the model size, but it shifts the upper limit down as well. We have this set to ".000001"

          As a general rule you want the effective accuracy to be at least half of the smallest feature you want to resolve. So if you want to be able to see/measure something .001" you need the accuracy at .0005" or smaller. Keep in mind that relative accuracy is not the same as effective accuracy. When relative accuracy is used, the effective accuracy is a calculation between the relative accuracy value and the bounding box size.

          Absolute accuracy can increase file size, memory usage, and regeneration time. PTC still recommends relative accuracy, but in the tool & die world it seems pretty common to standardize on a single absolute accuracy setting for all models.

            T
            TomD.inPDX17-Peridot
            17-Peridot
            November 19, 2014

            Tom, that is the best explanation I have seen to date on accuracy.

            How did you come by this information?

              T
              23-Emerald IV
              TomU23-Emerald IV
              23-Emerald IV
              November 19, 2014

              I will say that I'm glad Creo doesn't let you change accuracy on a feature by feature basis. Catia allows that and it causes all kinds of downstream issues when exporting the data to other systems. It drove me absolutely nuts. Feature won't make, no problem, just decrease the accuracy of that feature until it does. Looks great on screen, just don't try to import the geometry into any other system!

                tbraxton
                22-Sapphire II
                tbraxton22-Sapphire II
                22-Sapphire II
                November 19, 2014

                One important consideration is to understand the absolute accuracy has units of length and relative accuracy is a dimensionless parameter. So when using relative accuracy it does not translate into the physical world to resolve a dimension which would have units of "L". In addition the relative accuracy is dependent on the model size and therefore is not fixed. Relative accuracy is exploited to speed regeneration of geometry.

                Two important points from my experience the first of which has been disclosed here previously.

                If you model parts that are manufactured you should be using absolute accuracy. The value depends on what you are making. I have designed mems parts that are manufactured using micrometers as the units of length so the accuracy must be set finer than for many other types of parts. I have a mems start model set appropriately.

                If you are sharing geometry via copy geom, merge etc. within Pro/E match all of the model accuracies (absolute). Not doing this can really screw up mold design and other mfg design/fab operations in the Pro/E design environment.

                M
                mbonka2-ExplorerAuthor
                2-Explorer
                November 20, 2014

                l think my question was answered more than successfully.

                l´m proud of learning from Creo masters like you and l wish to be in one office with so skilled colleagues like you.

                All of you have a nice day...

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