NX Calculation of mass, center of gravity appears to be wrong

A solid body with a large number of complex surfaces give different result running Measure function for solid body on different occurrences of pattern feature copies. This suggests that pattern feature does not produce exact copies of the original body.

Also, if you calculate on a thin (sheet metal) body with complex form, like the helix shaped spiral of an ice auger, measure result of weight, center of gravity, etc. does not seem to be correct. How to get a Measure result in these situation of good quality?

All calculations in a CAD system where complex surfaces are involved are approximations. NX offers the ability to adjust accuracy of a calculation on a body or surface. Working with the correct calculation accuracy is vital.

There are 3 main factors to consider when setting calculation accuracy:
- Complexity of geometry (mathematical complexity)
- Number of faces/objects to be calculated (performance)
- Position/orientation (calculation is performed based on Absolute CSYS)
The higher accuracy you calculate with, the less will position factors affect the result, i.e. you get less difference between copies of the same body in different position/rotation.

Depending on the shape of the body being calculated, sensitivity to accuracy setting is different.
- A thin section body (shell) demands a higher accuracy calculation to give a good result.
- A body which have relatively thin sections and non-symmetrical shape demands a higher accuracy calculation to maintain a good result.

In the other end there is performance, calculating with an unnecessary high accuracy will give you a performance hit, without giving you any better-quality result.

To exemplify this a model with over 120 faces, many with complex shape were used. This body have been used to create 2 pattern feature instances, so there are now 3 bodies, assumed to have the exact same properties. In the example calculation below "mass" is used for this comparison.

I ran a calculation with Measure command, selecting solid bodies. Also moved the bodies to different positions between the measurements. Here are the results for the mass (kg). To start with the NX standard accuracy (0.99) were used.
--------------------
Position      1                 2               3

Body 1   76.1043    76.1321    76.1385
Body 2   76.1329    76.1272    76.1329
Body 3   76.1270    76.1337    76.1284
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Then I tested to increase the accuracy of the calculation, found under File --> Preferences --> Measurement, Calculation and Accuracy, Body group or in the Settings section of the Measure command.

There are 3 levels of accuracy, Standard (0.99), Fine (0.999) and Ultra Fine (0.9999). Running a test with the 3 bodies in their first position through all 3 accuracy levels renders in the result
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Body                 1                 2               3

Standard    76.1043    76.1329    76.1270
Fine             75.9913    75.9933    75.9921
Ultra Fine   75.9934    75.9930    75.9931
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At Ultra Fine accuracy level there is only a small difference between the 3 bodies at 4th decimal level, i.e. the mass of the body can be stated to be 75.993kg

To summarize, there is no actual difference in mass on the bodies for the different pattern occurrences in this example but using default calculation accuracy (0.99) you will see different results depending on the bodies position/rotation.

You can increase the accuracy to avoid this effect, but depending on the degree of increase and complexity of body, you will also see a performance decrease. Most likely performing a similar test on another body will perhaps indicate a different accuracy level as optimal, but the overall picture could be assumed to be the same.
If you decide on an accuracy level, (which is different from default), that will suite your products and needs in general, you can set that accuracy in NX customer defaults.
You find this under Analysis --> Measurement --> Accuracy tab, Accuracy group

The " Accuracy" parameter in NX customer defaults is session specific, i.e. if you make a change to it in customer defaults, it will have influence on both existing and new part files.