Assembly CG

[bigBboris]

When I try to assign mass properties to assemblies thru Mass Properties dialog box, center of gravity moves out of the model.
Any idea how to fix this situation?
I am using SW2008, SP 5.0

Boris.

 

[CorBlimeyLimey]

Are you able to assign the material or mass properties to the individual parts instead of the assy?

Do you know where the CoG should be?

What shape is the assy?

 

[ctopher]

Is your origin located within your part/assy?

Chris
SolidWorks 08, CATIA V5
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[TheTick]

center of gravity moves out of the model

Perhaps this reflects reality? i.e. Where would the centre of gravity be for a U-shaped part?

 

[bigBboris]

It is electronic assembly with hundreds of components, bug and small. For major components material is specified. Total mass calculated does not match actual weight, like 30% off. When Assigned Mass Property box is unchecked, CG is in reasonable location.

 

[dgowans]

It is electronic assembly with hundreds of components, bug and small.

Sounds like the stuff we make...

 

[TheTick]

Does your assembly have any stray components that are not visible?

Do any of your components have stray bodies?

 

[SincoTC]

As you say, "When Assigned Mass Property box is unchecked, CG is in reasonable location", this indicates that when the box is checked, so that you can specify the Mass of the complex assembly, you will also have to specify the position of the CG. However, the axis offsets have to be measured from the Assembly's Origin point (Usually the same as the Origin of the Fixed, first Part of the Assembly), which is not necessarily where you're measuring the balance point from on the real thing!


Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2009x64 SP3.0 Intel Core i7 2.94Ghz, 12Gb Ram, NVIDIA Quadro FX3700 Driver: 6.14.11.8246
SW2009x64 SP3.0 Intel Core 2 Duo 3.17Ghz, 8Gb Ram NVIDIA Quadro FX3700 Driver: 6.14.11.8246

 

[bigBboris]

All assemblies are simple shaped boxes, some components were modeled in SW, some were brought as IGES. I don't believe I have stray bodies in my models.
Wow! It looks like I have unic proble. No one have heard about something like this!

 

[SincoTC]

I don't really think that you've got a unique problem, just a combination of a couple of common ones related to the way the parts and assembly are built in relation to their origins and planes. Also, I've used imported models where the Origin is way outside the model, so I think you'll need to check a few things to sort it out.

I expect your "electronic component boxes" will be mated to a some form of Tray of PCB as a Base part and this will be the Part with which the assembly was started. This part should be Fixed and the Part name will be prefixed (f). If it isn't fixed then take a look down your Feature Tree, collapse it and check all the Part names for this prefix, if you find one then it's probably best to Right Click on the name and select Float (All Configurations).

Make sure that the View menu is set to show Planes, then Go Back to this "base part" of the Assembly in the feature tree and show its three main planes. Now do the same for the Assembly's three planes, my guess is that they're nowhere near each other and this is the cause of your problem. If they are way off, and this makes sense regarding what you're seeing, then as you can't move the Assembly Origin, you will have to move the "base part" to the assembly origin by making coincident mates between the base part's three main planes and those of the Assembly ( if this part WAS already fixed it will need to be set to "Float" before you can add these mates). If all the other parts are sensibly mated to the base part, then they will all move together. Now if necessary you can tweak the Assigned Mass properties CG co-ordinates to get it to match your measured values.

Hope this helps.


Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2009x64 SP3.0 Intel Core i7 2.94Ghz, 12Gb Ram, NVIDIA Quadro FX3700 Driver: 6.14.11.8246
SW2009x64 SP3.0 Intel Core 2 Duo 3.17Ghz, 8Gb Ram NVIDIA Quadro FX3700 Driver: 6.14.11.8246