Autodesk Inventor

[#1]

Hi!

My company recently installed Inventor 2013. In previous versions we used a part with negative density similar to seawater to calculate submerged weight and CoG.

In IV 2013, it seems that the ability to calculate negative mass is not longer available.

This is how it worked in pre 2013 IV, when I could specify a negative density.

What I have is a spool made of steel. This spool is modelled as an iAssembly with four configurations:

-In air, empty

-In air, filled with glycol

-Submerged, empty

-Submerged, filled with Glycol

For this example, I now need three parts:

-Steel Pipe

-Glycol derived from the inside volume the pipe

-Seawater buoyancy derived from the outside volume of the pipe.

Each part has its own density, and the assembly weight and CoG correspond to the assembly configuration (with or without glycol) and environment (in air or submerged).

The seawater buoyancy represents the water displaced by the total spool assembly, regardless of the contents. To achieve correct weight in the submerged state, the buoyancy material has to have a negative density.

I do not want to make any of the mass parameters static, and I also fear that the CoG might not be updated correctly when using that method.

As I no longer can define a negative density, my question is if anyone can think of an optional method to achieve this without overriding the mass value.

[#2]

When checking this from Inventor 2010 t0 2013, mass could be overridden in iProperties manually using negative value. But for the density, it could not be overridden in material within style manager manually or iProperties using negative value. So we are not clear how customer overrides the density in previous 2013 releases. Could you give us more details?

[#3]

Hi, River Cai.

Please see attached screenshot from IV2011 SP1 Material editor.

The density is defined as -1.026 g/cm³.

When a part with this material is assembled with other parts in IV2011, the total mass and CoG behaves in a way that is mathematically correct.

[#4]

When looking into the image, the value will turn red when setting negative value for density, and Save button is also disabled. That means this can't be done in R2011, right? Currently Inventor 2013 keeps the similar behavior with it when setting the negative value for density. In R2013, it will not turn red but reset the value close to 0 when using negative value.

[#5]

Yes, I agree.

I did not try to make a new material, so I didn't notice that the save button was greyed out. I only assumed it displayed negative values with red colour.

I believe the buoyancy materials were created in IV2009, and they have been in our library since then. I do not think the text was coloured red at that time.

Anyway, the materials have worked as we wanted until IV2013 (we have not used 2012, so I do not know how the behaviour would be there).

We now have several assemblies and drawings that are "corrupted" due to this change in density behaviour.

Maybe you could try the attached part in IV2011 and see if the models appears with negative weight on your side?

It seems that what we have assumed was an undocumented feature, in fact was a bug...

Thanks

Thomas Hauglien

[#6]

*It would be nice if we could have it as a feature again, though...

[#7]

Hi Thomas,

Allow the material to have negative density may give you convenience for your calculation, but this is not so reasonable from physical aspect. And currently even you could not have negative density you still can override the mass value to be negative, which might be a workaround for you in your calculation. The workaround can be like below:

1. Add a property in the document which you want its mass to be negative to indicate this:

Document.PropertySets.Item("{D5CDD505-2E9C-101B-9397-08002B2CF9AE}").Add(True, "NegativeMass")

2. When you need to do your calculation, you can check if a part has the property you set, if yes then set its mass value to be negative.

Dim oPartDoc As PartDocument
    Set oPartDoc = ThisApplication.ActiveDocument
    
    On Error Resume Next
    Dim oProp As Property
    Set oProp = oPartDoc.PropertySets.Item("{D5CDD505-2E9C-101B-9397-08002B2CF9AE}").Item("NegativeMass")
    If Err = 0 Then
        ' If the NegativeMass returns True
        If oProp.Value Then
    
            Dim oMassProperties As MassProperties
            Set oMassProperties = oPartDoc.ComponentDefinition.MassProperties
            
            Dim dMass As Double
            dMass = oMassProperties.Mass
            
            oMassProperties.Mass = -dMass
            Debug.Print oMassProperties.Mass
        End If
        
    End If

3. Then do your calculation with the overridden negative value.

4. After calculation, reset the mass value to be positive as normal.

oMassProperties.MassOverridden = False

Hope this works for you.

[#8]

I agree with Thomas, this new behaviour is not very good. We need due to some reasons materials with density 0 and this is also not possible in version 2013. These small differencess (mostly undocumented) between Inventor versions are very annoyning and for example in this case we do not see any benefit. Only troubles .

[#9]

We should still be able to set material density to 0. Can you double check it there? Any questions please let me know.

[#10]

Thanks, Rocky Zhang.

Although I am quite a novice in scripting, I finally managed to make an OK script with your input.

It seems that this is the way to do it for now, but I would really like the ability to define a negative density again.

Does anyone know if this is a necessary limitation within the software or just something the programmers forgot to include?

Can someone please verify the following summary of the gradual degradation of negative density usability?

- 2009: Materials with negative density can be created and used

- 2010-2012: Materials with negative density can be used if already existing.

- 2013: Density <0 is ignored

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