Region property, Principal Axis Wrong

[jinglisVMSZQ]

Hello. I am comparing the second moment of inertia for different cross sections to determine what style we want to use for a frame. I have been getting these with the region properties of a sketch cross section. 

I ran into a snag when I compared angle iron at different angles. the second set was rotated 90 degrees, so you would expect Ix and Iy to be reversed, but they are coming up the same. See attached pictures.

[andrewdroth]

"About the Principal Axis"

'each of three mutually perpendicular axes in a body about which the moment of inertia is at a maximum.'

That hasn't changed for that shape. Those numbers are relative to the axis it gives the shape, not the origin axis.

[swalton]

Did the sketch coordinate system change?  Remember that the region properties are calculated with respect to the base sketch coordinate system, not the part or assembly coordinate system.

[jinglisVMSZQ]

Thanks for that, I should have remembered that from school. In that case, region properties are also calculated relative to the sketch coordinate system, how would I go about making the  sketch origin at the centroid so that I can calculate  Ix and Iy for a less uniform shape? 

The problem is, I work in food processing equipment, for sanitary reasons, our framing cannot have flat parts. I want to analyse a C channel rotated on 45 degrees, so i cannot just assume it will be the same since it is not uniform.

I am able to get the origin close, but Is there a way to do this automatically? or do i just need to play with dimensions?

[andrewdroth]

I've always used AutoCAD instead of IV for region properties, it just seems to work better.

Start by defining your profile then using the REGION command to make it have area. If it's hollow you can use SUBTRACT to cut one region from another.

Then use MASSPROP to determine your centroid relative to the origin. Shift the region by that amount. Do another MASSPROP command and now you have moment of inertia numbers that match the neutral axis of the member. 

[andrewdroth]

Like this.

[johnsonshiue]

Hi! This is another bug, reported as INVGEN-31298. It has been fixed on 2020.2 update.

Many thanks!

[tecnico17JXDDT]

Well this is still happening as of 2022.3.1...

45 deg:

0 deg:

How can I get region properties to show region properties from the coordinate system, and not max values?

Thanks!

[johnsonshiue]

Hi! The original bug here is about MOI disrespecting the sketch coordinate system. It has nothing to do with the radius of gyration (ROG). Inventor calculates ROG based on principal axes. If you want other places, you need to transform it yourself.

Many thanks!

[chris_davis]

Steve,

I'm having a similar problem where it seems that the coordinate  systems changes from on sketch to another. From the snippets you can see that the views are the same but the x and y axis are not they same?  Why would it do that?

Thanks

The Sketches are in the same part. 

 

 

[SBix26]

I don't know the details of how Inventor decides where and how to place a sketch coordinate system, and probably couldn't understand them if they were shown to me.  But sometimes Inventor's decision on placement is less than helpful.  That's when you may decide to alter it manually.  Exit the sketch, then right click on its name in the browser and choose Edit Coordinate System.  You will see this prompt  in the lower left corner of the screen.  Following the prompts you can relocate and re-orient the coordinate system to your liking.

In my experience, Inventor usually gets it right.  The situation you show, where there is quite a bit of non-orthogonal geometry, is one where it may require manual intervention.

---

Sam B

Inventor Pro 2025 | Windows 11 Home 23H2

[chris_davis]

Sam Thank you for the response.  I will adjust the coordinate of one Sketch to match the other sketch. That will hopefully solve my issue.

Thanks.

Chris Davis