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Drafting Mass, Inertia, CG automatically

Drafting Mass, Inertia, CG automatically

Drafting Mass, Inertia, CG automatically

(OP)
Does anyone know of a way to automatically calculate the Mass, Inertia, and a Center of Gravity and add a call-out onto an assembly print that automatically updates each time the part is changed?

On a side note...

Is there a true way to draft the center of gravity?  I've only seen it on one other print and I doubt the method I saw is a part of any ANSI spec (or equivalent).

Thanks

Keegan  

RE: Drafting Mass, Inertia, CG automatically

I assume that you're talking 'Moments of Inertia', correct?

Well Mass and Center of Gravity can be done, with a little work, but Moments of Inertia will be a bit trickier.

But first, what version of NX are you running?

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

Keegan mentions NX6 in another thread.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
I use NX6.  

Thanks!   

RE: Drafting Mass, Inertia, CG automatically

(OP)
NX6

And yes, Moments of Inertia.

Thanks,

Keegan

RE: Drafting Mass, Inertia, CG automatically

With the center of gravity do you have to give each part its own material from the density point of view?  Going to watch this thread Wondering how this is done in NX6.  Good thread

RE: Drafting Mass, Inertia, CG automatically

Yes.  I'm sure that John will be better able to explain than I would.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Drafting Mass, Inertia, CG automatically

I'd imagine that you could create a macro to do what you need. I don't know how you'd show the moments of inertia on a drawing exactly, but some of the other stuff is readily set up to work with expressions and the centre of gravity can be saved as a co-ordinate system. In later versions setting up an associative measurement under Analysis>Measure Bodies will create a point at the COG and maintain expressions for volume, area, mass, weight and radius of gyration. Any of these can be displayed on drawings using relational annotations.

The good news is that these expressions are automatically updated every time the part is changed.


The bad news is that these expressions are automatically updated every time the part is changed.

Because it takes some time. However the measurement is just a modelling feature that can be suppressed until you need it just as long as your mindful of how you're managing the process you'll be fine. I operate that way all of the time.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

RE: Drafting Mass, Inertia, CG automatically

OK, here's an example of a Drawing where I've got a note reporting the Weight of the Master Part as well as the Center of Mass (shown as a point in the drawing).  Note that any edit the Master Part model will be reflected in an updated drawing (if the update doesn't occur immediately, perform a Tools -> Update -> Update for External Change from the Modeling Module).  Now if you are simply editing the model, there is not much else that you have to do, but if you add any additional features, you will need to go into the Part Navigator and reorder the Measure Body feature to once again be the last feature in Timestamp Order.  Also, if you edit the Material assignment this will also update the Mass values.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
Awesome!  It can be done!  Could you break down how you made that model?  I get the P33 expression but I am not sure how you knew it was P33.  I also do not understand how you made the center of gravity point.

I'm thrilled it can be done though!!!

Thanks,
 
Keegan

Oh and also I found a COG symbol:
 
http://www.virginiadot.org/business/resources/Cells-Symbols.pdf
Page 2, second line down  

RE: Drafting Mass, Inertia, CG automatically

The p33 expression was whatever was assigned to the Mass when you created the 'Measure Body' feature, which can be done either inside the Expression dialog using the 'Measure Bodies' option at the bottom of the dialog, next to the f(x) option, or you can use the Analysis -> Measure Bodies... with the Associate option toggled ON and you will get the same Measurement expressions.

As for the 'smart' COG point, that's created automatically when you create the 'Measure Body' feature (in fact, the 'Point' IS the feature), but to make sure it gets included in your drawing, you will have to manually add the Point object to the 'Model' Reference Set.

What I've done is added the required Expressions and Attributes to my default Part Templates so that if I ever need an Attribute, for whatever reason, which tracks the mass/weight of a model, most of the hard work has already been done.  All I have to do is create the Measure Body feature and edit one Expression linking it to whatever p-number was assigned to the Mass/Weight value of the model.  Note that depending on whether you're working with Metric or Imperial models, you will need to do things slightly different if you wish to get the normally used values for what we generally consider as being the 'weight' of an object.  In Metric parts, you use the value for 'Mass', whereas with Imperial parts (AKA English units), you use the value for 'Weight'.

To help you out, I've attached copies of both my Metric and Imperial default parts with the predefined items already in place.  All that I do is build my model and when I it's complete I create the Measure Body feature, go into the Expression system and edit the expression 'Mass' ('Weight' in an Imperial part) setting it equal to whatever p-number was assigned to the value for 'Mass' ('Weight' for an Imperial part) measurement expression.

As for the Drawing, once you've created your drawing, create a note where you set the value equal to an 'Object Attribute' (using the 'Relationships' option) and then select the Component in your Master Model Drawing (while in drafting, you will need to use the Assembly Navigator to actually select the Component) and select the 'Weight' attribute.

Attached are copies of my two Part Templates (they are in NX 5 format).  You may also wish to look a the other default attributes which I include as well as the default display setup and so on.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

John,

I check out you model and I do much the same thing except that I take the measurement at the drawing level. I suppose its much of a muchness either way, but I wondered what anyone thought of the relative merits of doing it either way.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

RE: Drafting Mass, Inertia, CG automatically

(OP)
   So far what you have shown is gold!  So to pry any further I feel like I am taking advantage...

I've gotten the COG drafted and automatically updating and I've also added the weight as a text field in the title block.  Now I am down to adding the Inertia.

I would need to use either the calculations from the "Moments of Inertia (WCS or Centroidal)"  These are the results I am getting and the output that I want to appear on the print.  

Moments of Inertia (WCS)
Ix, Iy, Iz            =  346.14661714892,  677.16928209864,  345.37266325727

Moments of Inertia (Centroidal)
Ixc, Iyc, Izc         =  345.80884898495,  677.16927178071,  345.03490541115


INERTIA 677.16927178071/(32.174*12) = 1.75392468 lb-in-sec-2


And Hudson, as far as drafting the inertia, I just want it to appear as a text field in the title block.  


Thank you to all of you so far and this website!  

Keegan  

RE: Drafting Mass, Inertia, CG automatically

(OP)
Hudson,
    For me (or the company I work for), I think it makes more since to put it in the drawing file as well.  I say this because once all that info is in the drawing I shouldn't have to go back to the model to get those calculations.  

It also makes it easier to modify the model without having to wait on the Mass properties to update.  Once the modifications to the model have been made and finalized I can then update the drawing and mass props.   

I'm a bit scatter brained at the moment so hopefully that makes since!  

Thanks again to all of you!

Keegan   

RE: Drafting Mass, Inertia, CG automatically

As John says he does it using a template and that makes some sense but I work the way that I do for the very reason that you stated.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

RE: Drafting Mass, Inertia, CG automatically

John,
Can you comment on how to apply the Associative "Measure Bodies" approach for assigning a MASS parameter to parts, to Machinery Library parts?

For example, if I have an assembly containing a bunch of Machinery Library screws, how do I obtain their masses?  What if one wants to use a custom material for the Machinery Library parts (and thus a different weight than default steel)?

In general, how does one handle this issue with Family parts?

Thanks.

RE: Drafting Mass, Inertia, CG automatically

You would have to assign a Measurement feature to each of the Part Family Template files as well as the expressions to drive the attributes, which would then be inherited by the each family member as they were created.  If these were done using consistent names then they could in turn be linked to the upper level assembly and utilized however you wish to do so.

That being said, if your goal was simply the overall weight of an assembly, including all of the component parts (would include the standard or purchased parts, such as fasteners from the Machinery Library), you could create the Measurement feature at the assembly level (including a Master Model Drawing) and simply select ALL of the components.  Now this will result in a single value for the entire assembly.  Now if changes where made to the master parts such as changing the size or the material assignment, this would automatically be reflected in the Measurement expressions at the assembly level as long as the original components were still part of the assembly (even if they've been modified).  Now if new parts were added or existing components removed/replaced, then the Measurement feature would have to be recreated as we do not yet have a automatic mechanism which would 'recollect' (reselect) the components to capture any new/replaced/removed components whenever there were any changes made to an assembly.

Anyway, you can get a fairly good arrangement in place with the current tools, which can handle the normal updates, with just a little extra manual intervention needed for a few of the more exceptional situations.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
That's a good question Potrero!  And thanks John, I've never thought of doing things that way!   

RE: Drafting Mass, Inertia, CG automatically

(OP)
Am I doing something wrong?  

I have a fairly large assembly and if I Measure the bodies and select "Associative" it gives me the results I want (except inertia) in about a minute after selecting the bodies.

(yes there is always a big but)

But... when I click OK, it sits there for about 15 - 20 minutes. I don't get it because before hitting OK, the results are displayed and, I'd think, should already be stored and calculated, so why the huge delay just to create a few expressions?

Any suggestions ju all?

Thanks!!

Keegan  

 

RE: Drafting Mass, Inertia, CG automatically

@ingallspw, I've definitely noticed what you're describing but unfortunately can't give an answer as to why it behaves in that manner.  Someone else?

RE: Drafting Mass, Inertia, CG automatically

Have you changed you default...

File -> Options -> Save Options...

...so that the 'Generate Weight Data' is toggled ON for all your parts?  If this is not toggled ON when you last saved your part files, none of the Weight Data will have been saved and therefore it will need to be recalculated whenever it's requested.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
That's good to know but on load it only takes about a minute extra (and I am happy to know there is a way around even that little bit of time).

But... that's not what causes the hang up.


It's upon APPLY or OK.  Here are the steps to get me to that hang:



Analysis > Measure Bodies

I select the bodies I want to measure.  (Associative is already checked)

After a minute, the information window pops up with all the calculations.

I select OK on the Measure Bodies dialog box.

Now I wait 20 minutes.

After 20 minutes, it is in my Part Navigator.  As long as if I never "edit parameters", that is the only time I see the delay.
  

Keegan

RE: Drafting Mass, Inertia, CG automatically

When you say a 'fairly large assembly', how large is large?  Are we talking hundreds or thousands of Components?

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

The system really ought to look at dealing with weight data for assemblies somehow more efficiently. The saved weight data works only for components with solids in them, assemblies are assigned no mass and that to me seems to be a shortcoming for any of us who display mass on parts lists for example.  

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

RE: Drafting Mass, Inertia, CG automatically

(OP)
John,

In the model that is hanging at the moment, there are 67 parts.  

34 of the parts are simple extrusions (such as solid rivets and a sheet metal plate in the center).

The other 33 solid parts are actually two part files, arrayed, with lots of complex surfaces.

________________________________________________________


As for Hudson's suggestion, I know for us end users its always easy to say it should work better but in this case I'd agree.  What I'd suggest is that the mass properties feature allow you to determine what is going to be calculated.  

eg: In this instance I do not need, Surface area, Volume, Radius of Gyration, Products of Inertia, and Radii of Gyration.

So to speed things up, give us the option of calculating these results.  If we don't need them, don't take the time to process them.

And if I am understanding Hudson, he just needs the weight.  If I get the mass properties of the individual component, NX calcualtes these results Speedy Gonzales like.  Now take it over to an assembly and understandably it takes a while.  But if all he wants is the weight (nothing else) then it's just a simple equation for the solver to create.  

I know, dang pesky end users and their suggestions!  

For anyone reading this, that is new to NX , its a great CAD system and this website is full of reasons why!  So don't take this as all complaints... just trying to learn as I go and hopefully it will get even better!

Thanks,

Keegan

RE: Drafting Mass, Inertia, CG automatically

Quote (ingallspw):

to speed things up, give us the option of calculating these results

This initially sounds like a good idea, but due to the nature of the calculations - won't give much of a speedup. The time consuming part is the volume calculation: NX divides your model into small cubes and adds up the volumes; it splits the model into smaller and smaller cubes until your specified tolerance is met. The more 'organic' your part is the longer this process takes; if you model large boxes then it is lightning fast. Weight is then calculated as volume * density. The other parameters are simple 'one time' (no iterations required) calculations that depend on weight and/or volume.

I'm not sure how the surface area is calculated, if it is an iteration (similar to the volume) then you might see an improvement if there is an option to turn this off. But NX may use the same cubes to estimate surface area that it used for the volume calculation, so surface area might 'come along for the ride' with the volume calculation. The rest of the calculations you basically get for free once the volume is calculated.

RE: Drafting Mass, Inertia, CG automatically

If this is only happening with one assembly, I would suspect that there is a problem with the file (or one of more of the component parts) itself rather than with the code which is adding the Mass Property (Measurement) Expressions to the assembly model.

I talked to our developer responsible for this code and he states that you should not be surprised that it actually takes a bit longer to create the Measurement Expressions, after you hit the OK button, than it took to perform the initial mass property calculation when you first started to select objects.  The reason for that is because, despite what might be logical, we first RERUN the calculations before saving the results (I know, this seems like a waste of time because we already displayed the results temporarily and technically they should still be good 2 seconds later when you hit OK, but that is just the way the code works, however this is a very small part of the time it takes to complete the task so it's not really a problem).

The extra time that it DOES take has mostly to do with the fact that we need to create what we call an 'object record' in the Assembly file which the Measurement Expressions are actually assigned to since for them to be associative and automatically update if changes are made, they have to be part of a smart object or feature (even if you can't see or 'touch' this feature, it still needs to be created).

Note that we don't create this feature when the initial calculation is done for a couple of reasons.  A) you might change your mind and add or remove selected objects and since we don't wait for you to do anything before displaying an initial result, we wanted to keep this working as fast as possible.  B), you may decided to NOT save the results permanently (you've toggled OFF the 'Associate' option) so why waste any time, after displaying the temporary results, creating a feature record which we are just going to have to throw away anyway.  Therefore, we wait until you hit OK and then check to see if the Associate option was toggled ON and if it was, we then recompute the information, create the Object Record, assign the Measurement Expressions to it. add them to the list of Expressions in the Assembly file, thus finishing the task.  Now because we are actually creating something permanent, we have to do some additional checking to make sure that we have all of the data needed to complete the task (which might require some additional time if for example the components had only been partially loaded or there were some Interpart links that needed to be checked) so it does take some additional time, above and beyond the actual mass property calculations, which as you already seen, is very fast indeed.

Now what is the typical time you should expect?

I did some testing with a couple of average sized assemblies.  I had a couple in the 150 component range and for both of them, with the worst case scenario, it took about 2 seconds (these tests were done on my 3 year old Dell M65 laptop) for the initial computation of the mass properties and about 15 seconds, after I hit OK, to actually process and save the results as Measurement Expressions in the Assembly.  I then tested this using an assembly with about 370 components, where it took perhaps 3 seconds to compute the data and 27 seconds to actually save it and complete the operation.

So, if your assembly only had 67 components and it's taking you 20 minutes to complete the task, I suspect that we have problems somewhere else.

As to what you can do, try doing a Part Cleanup on all of the components and re-save them and see if that helps.  If it does not, then you may need to send this assembly file to GTAC so that our developers can look at it as this should NOT be taking this long no matter how complex your parts may be, something else is slowing this process down, but we would have to see the parts to determine what the real cause is as I suspect that what you're seeing is only the symptom of something that may not be related to Mass Property Calculation at all.

Anyway, I hope this helps.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
Wow, Thanks Jon!  I appreciate all the work and effort you put into this!  

(For all of you reading this I challenge you to get this kind of customer interaction from PTC/Pro-E even calling them directly!)  

I'll have to call GTAC.  I just tried creating a new assembly using very similar parts, on a different PC, and I still have the same results.  The components that I am running the process on are very "organic" as Cowski called it so maybe that has something to do with it.  

And to reply directly to Cowski, I still would like to see an option to Not calculate volume if it is not necessary to calculate the rest of the values but your explanation was awesome and explains a lot!  Thanks!!  

And I'll take one last stab...  any ideas on how to get the Inertia to calculate automatically as a value?  Or do I have to wait for NX7 to do that?! :)

THANKS TO ALL!

RE: Drafting Mass, Inertia, CG automatically

Inertia would have to be derived (computed) using the values provided by the system when you perform the Measurement.

And there are no changes in what is computed nor in how the Measurement function works in NX 7.0 and NX 7.5.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
http://www.siemens.com/plm
http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.
 

RE: Drafting Mass, Inertia, CG automatically

(OP)
As an FYI to everyone...

I mentioned it taking 20 minutes to calculate the Mass properties.  Potrero also reported this issue.  Recently I loaded up NX7.  It also has the same issue.  


However, this is the response I just received from GTAC:

"It has been determined that the performance problem is due to eleven (11) calls of the Parasolid function MASSPR due to the associativities of the Mass Properties feature".

The fix will be available in NX605 which is ready for the customer around March 2010.

RE: Drafting Mass, Inertia, CG automatically

One supposes that it may be in NX-7.01 whenever that comes out as well.

Best Regards

Hudson

www.jamb.com.au

Nil Desperandum illegitimi non carborundum

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