Material Density question

[gwbruce]

Ok I have been following Too Tall Toby and his speed modeling tournament on YouTube. I really enjoy trying my hand at doing the same models in Alibre that everyone in the tournament are doing in other CAD systems. It is really helping me with my modeling in Alibre as sometimes I have to figure out the workflow that differs from the much more expensive CAD systems. I am a daily SW user at work. However I seem to have run across something that is making me scratch my head. I have modeled this part from last weeks competition.



The supplied material is Plain Carbon Steel with a density of 7800 kg/m^3. Which to me converts to 0.281793 lb/in^3. I set up a custom material using the correct method I believe. However, no matter what I do when I select this material and check the properties, I get the wrong mass. When I look at the properties screen, I see the material is plain carbon steel as selected, but the density is not correct.
it shows a density of .281541. No matter what I change the density to it always shows this. Am I doing something wrong or is this a bug? Am I missing something? If I was using Alibre in the competition this could cause me to lose.

It was really great to see Alibre represented in the very first round and Ty almost pulled it off. Hoping that maybe next year other Alibre users will try their hand at speed modeling. Something like this might be a little discouraging.

Thanks for the helping me understand this.

 

[DavidJ]

I've verified that there is a difference in the displayed density between material library and the physical properties tool. Have not checked so far if this affects the result. Have logged for Development to take a look.

 

[jroy]

Well, I just drawn that contest sample to check and I came up with 17.44181 Lbm. The correct answer on the in the contest is 17.461 Lbs. I had to create a new material library for the plain carbon steel since its not part of Alibre's library. I do wonder what the discrepancy is from, is there a problem with the floating point math in Alibre?

Hope to find out soon....

 

[jroy]

After some quick revision I'm now at 17.44367 Lbm...Still not the same answer as on the contest...

Jocelyn

 

[gwbruce]

After some quick revision I'm now at 17.44367 Lbm...Still not the same answer as on the contest...

Jocelyn

When you do the check to calculate the mass look at the dialog box and see what it shows for the density of the material. It might not be what you have put in for your material density. That is what I am seeing.

 

[DavidJ]

I've done some tests with simple geometry. A 100mm cube with material density set to 7800kg/m3 gives exactly 7.8kg as mass for the part (as expected).

Switch mass units for the part to lbm and the answer doesn't quite match what a units converter gives in lbm for 7.8kg. My guess is that Alibre has used an approximation internally for the unit conversion. The density value in lbm/in3 also differs between the 2 locations as shown in post #1 - that could suggest differing conversion factors, or just differing levels of precision in different parts of the software. May be linked to the material library and the physical properties tool being developed at different times.

Until this is fully understood and fixed, for critical cases use kg as the mass units for physical properties tool (these are native in the material library), and convert to lbm if needed outside of Alibre.

 

[jroy]

When you do the check to calculate the mass look at the dialog box and see what it shows for the density of the material. It might not be what you have put in for your material density. That is what I am seeing.

The part density is as entered in the new library created. Or perhaps it is not modeled properly, but I am pretty certain that I did. Anyway, I'm not going to spend an hour on took me 12 minutes to model...LOL.

 

[DavidJ]

For next v27 build precision of the kg to lbm conversion factor is increased.

 

[jroy]

Hi, had a hard time to sleep over this...
So after a last revision the mass come out to be:



Which is still below the 0.01 lb tolerance of the contest.

GWBRUCE, perhaps we need to compare our models to see what the differences are, to get to the bottom of this.

Have a nice day,

 

[gwbruce]

GWBRUCE, perhaps we need to compare our models to see what the differences are, to get to the bottom of this.

I am at work right now and don't have access to my files or Alibre. When I get home tonight I'll post up my model and we can maybe see what our differences are.

 

[jroy]

For next v27 build precision of the kg to lbm conversion factor is increased.

Sorry David, you might want to hold off such a request to Alibre. I just did another sample contest model and did time got the right answer. Obviously something wrong with the previous model.





Have a nice day,

 

[gwbruce]

Jocelyn try doing one of the models using the IPS settings and using the plain carbon steel. If you convert the 7800 kg/m^3 to lb/in^3 you may not get the correct answer for your mass. This is where I was finding a difference. If you stick with grams for you final output mass it appears to work.

 

[jroy]

Jocelyn try doing one of the models using the IPS settings and using the plain carbon steel. If you convert the 7800 kg/m^3 to lb/in^3 you may not get the correct answer for your mass. This is where I was finding a difference. If you stick with grams for you final output mass it appears to work.

I see what you mean, the displayed density in the physical property window is different from what is selected. But I'm still getting the same resulting mass as shown below.
I created another material to protect this library. I will now remove it.



Don't know what to think, must an error on what is displayed in Alibre.

Have a nice day,

 

[gwbruce]

I see what you mean, the displayed density in the physical property window is different from what is selected. But I'm still getting the same resulting mass as shown below.
I created another material to protect this library. I will now remove it.

View attachment 39322

Don't know what to think, must an error on what is displayed in Alibre.

Have a nice day,

Exactly. I created a custom material with the correct density in lb/in^3 and what you are seeing in your dialog box for density is what I see not what I have in the selected material. Also I get the same result of the 17.444 lbm. Just don't quote understand it.

 

[Max]

Without reading this too in depth, we looked into it on the possibility something may be wonky and found the conversion factor from metric to imperial to have what some people might consider too few significant digits. In v27 the significant digits has been increased by 2-3x. The build containing that fix has not been posted in the Beta and will not be, as we are shooting for gold build in a day or two for a release on the 14th. So we are basically out of beta but have not shut it down since some people are enjoying it.

 

[HaroldL]

The build containing that fix has not been posted in the Beta and will not be, as we are shooting for gold build in a day or two for a release on the 14th. So we are basically out of beta but have not shut it down since some people are enjoying it.

Well, if Beta is done I will be removing it from my computer and try to get my Space Navigator radial menus working again. I hope that when v27 is released that I can get it installed alone without it disabling the radial menus.