Adding weight (not mass) as a property

[scotty71]

Because I work in much larger items than most of the folks that would use this - I work in excavation safety (shoring) and systems are composed of large steel tubing, plates, beams, etc.  All of it is A36, but I really have no use for mass - I am concerned with weight of individual components.  For example, I might use  HSS 5x5 x 3/8 square tube: it weights roughly 22.5lbs / Lf.  

Also - my product is largely static and I'm not concerned with mass in the sense that some designers would. Such as things that move and mass is  genuine consideration.  

It also helps me with cost consdieration since I buy steel by the pound - not the "mass".   

Thanks...

Scott

[TrippyLighting]

Really ?

http://www.colorado.edu/physics/2000/periodic_table/mass.html

[Anonymous]

hey scotty71, I have to admit (embarrassingly) that I always have to hit the books on mass and weight when dealing with english units because even after four years of mechanical engineering school it still doesn't sit right with me. (Luckily I usually have it right but it still makes me uneasy!)

suffice to say that for all intents and purposes your pounds mass will equal your pounds weight

to reconcile:

weight is defined as the force on a mass due to acceleration from gravity

32.2 pounds mass is defined as 1 slug

f = m*a

pounds force = slugs * feet / second^2

a = 32.2 feet / second^2, due to gravity on earth

x pounds weight = 1 slug * 32.2 feet / second ^2

on earth 1 slug weighs 32.2 pounds weight on earth

therefore 32.2 pounds mass = 32.2 pounds weight on earth

1 pound mass = 1 pound weight on earth

[scotty71]

Thank you kindly for the little math. That brings back some early physics lessons.  I did not go to school for engineering - I have degrees in finance and economics - but math is still math.  However, I have spent my entire life in the family construction business and now manufacturing. While I may not have formal training in engineering, I have worked with highly skilled engineers and also the experience that I KNOW what things can and cannot do (section modulus, stress risers, etc).  I work out the design, then get my PEs to put in the math. 

See attached screenshot. This lays out the problem I'm experiencing. Illustrated is a 5" x 5" square tube, 3/8" wall.  Roughly, it weighs 22.5 lbs / foot.  The drawing is exactly 12" long, so it should be 22.5# or 360 oz.   For what I do, the weight is important. Particularly because I need to work out the materials cost and it nice to have the weight (or mass) laid out for me. 

Okay - enough of my yabbering:  I believe I applied the correct physical material, A36 steel (which is what we use) and as you can see - it gives me a much different number. In fact, just applying ANY steel physical material should give me a number closer to what I should see.

  I have tried other physical materials from the library - but I cannot get it a number thats even close.  So - it may be that my problem lays there. As near as I can tell - I've set up my preferencs correctly,  but the more I look at it, the stranger it gets.  Maybe I need to build a material from scratch... or....

Maybe I need to make that a new thread....   

[Anonymous]

Well, for 5" x 5" x 3/8" you should get a cross sectional area of 6.94 in^2, then with your 12" length you arrive at a volume of 83.3 in^3

At your target weight of 22.5 pounds per foot we're expecting a material density of 0.27 pounds mass per in^3

A36 density appears to be 0.28 pounds mass per in^3 as per Matweb datasheet for A36 plate.

It sounds like Fusion is doing some awful math for you with the density and your expectations are totally valid.

But you already knew that! 🙂

As a side note; I wish we could DEFINE the mass (not density) on our parts. Why? Because sometimes you draw a basic form to act as a much more complex part, and the mass will never be correct! 

[Anonymous]

Hi Scotty71, this is a very good post. Lots of people do not know the differences between MASS and WEIGHT. Your post generated responses that clarified these relationships! In a point you were right. MASS is calculated and WEIGHT could be more phyiscal than calculation. But in the case of Fusion360, the MASS in "Property" is calculated. But I think they can improve that by listing the Changes in MASS due to certain correction factors from the Real Materials on Earth. For example, due to the environment on Earth, the actual WEIGHT of a big block of material could be heavier or lighter than the calculated MASS. So I would say for your purposes, the MASS in "Property" could be done according to your own adjustment. Why not setup a "Control Experiment" where you use an actual Block of Material that you are handling, measure the exact size. Use the same materials in Fusion360 at the exact size. Get the Calculated MASS from "Property". Than you know the deviation and you can use the same logic to come up with a WEIGHT of the Block of materials that you used to handle.  I would say if your accuracy is within "+" or "-" 5% or any acceptable deviation that you might agreed on. That will be quite safe.

In a Way I do agree with you. WEIGHT might need to be added to property but then the Fusion 360 team might have to set up some numbers that you can plug in yourself from your own "Control" Experiments. Than the program can use your "Deviation Percentages" to come up with a "Weight" of your Block within your acceptable "Accuracy of say within 5%". The team might have to consider this options on their update roadmap.

I think this is a very good post. I used that to estimate the wight of my PMMA products. I would say a calculated MASS of 278.23gm in actual is about 310gm. I used that to estimate the real Weight of my product from that MASS. I need to use that to calculate Costs before production. This is important as I know how much I wanted to sell the product but I also know now much I wanted to spend on the Materials.  

Hope this will help you out at this moment!

[Anonymous]

Oh Scott, I have to clarify "Accuracy of within 5%". That is based on the corrected Weight from your "Control Experiment" from the MASS in "Property".

In other words, once a deviation percentage is found, somewhere down the line you will know how accurate these corrections were. I would say if these calculated numbers are within a certain percentages of say 5% it would be quite acceptable. My 5% accuracy is due to my own acceptable cost variation of plus or minus 5%. I hope I had clarified this "Accuracy" matter.

[LMD001]

Hello Kingson,

Assigning "water" as physical material to a cube of 100 mm X 100 mm X 100 mm gives a mass/weight of 0,9983 kg which is the exact density of a litre water at 20°C, so my guess is that when you asign a material with the correct density, the calculated mass/weight will be really close.

Not sure why Scott is getting these results, I did the pipe in Metric and got a, converted, result of 22,7 pounds/foot which is close to his estimate of 22,5 pounds/foot and the difference is probably because I used std steel as material.

As far as I can see the math Fusion 360 does (in Metric) is correct. I read a post some weeks ago that Fusion 360 uses "centimeters" as internal unit, so probably also "kilogram" as weight unit, if so maybe there is a Metric / Imperial converting error.

Best regards,

Ludo

[Anonymous]

Hi Ludo, I believe you are also correct. Materials like Polyethylene  (PE) comes in Low Density, High Density and a few other variations. Their respective  Weights at a given volume are different. That is why I suggested to get a set of Controlled correction figures and use those to make correction on the calculated Mass from "Property". Materials also have variations in the real world. So a corrected MASS to come to an actual weight for me is quite important. But I would accept a 5% deviation.

[kb9ydn · Accepted solution]

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@scotty71 wrote:

See attached screenshot. This lays out the problem I'm experiencing. Illustrated is a 5" x 5" square tube, 3/8" wall.  Roughly, it weighs 22.5 lbs / foot.  The drawing is exactly 12" long, so it should be 22.5# or 360 oz.   For what I do, the weight is important. Particularly because I need to work out the materials cost and it nice to have the weight (or mass) laid out for me. 

Okay - enough of my yabbering:  I believe I applied the correct physical material, A36 steel (which is what we use) and as you can see - it gives me a much different number. In fact, just applying ANY steel physical material should give me a number closer to what I should see.

I'm not sure what's going on but the stated density for A36 in your picture (3.468E-5 oz/in^3) is completely wrong.  3.468E-5 = 0.00003468.  The mass calculation is correct for the stated density, so my guess is that there is an error in the material library or something.

C|

[scotty71]

This is what I figured.  Late last night I modified the densisity of the A36 setting and got more acceptable results.  My setting is measuring in cubic feet (not a problem, still easy enough) so after I changed that, things made more sense.  

Now if I can just get it to stop reading in ounces -  I have the Prefs set to read "lbsmass", but for some reason, Fusion360 is feeling I should read it another way.

Thank you to all of you for your help on this. 

[Anonymous]

Hi Scotty thanks for posting. It picked our minds to find out more about what we had been doing everyday without even noticing it. I think you just have to use similar materials in "Property" and use some correction figures to compensate the differences. As I mentioned before, using Acrylic (PMMA) in Property the difference between Mass and actual product weight is quite big.

[LMD001]

Hi Kingson,

A PMMA 1 litre cube gives a weight of 1,19 kg, this reflects the density that Fusion 360 uses for PMMA (1,19 kg/litre) the calculation Fusion 360 does is correct. As you know the density variations of PMMA do not account for the about 12% difference you are getting in real world products. Left unnoticed your profit is also off by a rather big percentage.

If the 12% difference is consistent, you could edit the density parameter of the PMMA (Physical Material) to reflect this difference and avoid the need for compensation.

Of course you know best if such an approach would work for you.

Indeed nice catch Scotty!

Kindest regards,

Ludo

[Anonymous]

Thanks!

[Anonymous]

Hi Ludo, I am back to the computer so we can talk better. Your analysis is correct. We are all talking about a finished product which should be exactly the same as the model in Fusion 360, theretically speaking of course.

The 12% increase in weight of PMMA is mainly due to the nature of Plastic Injection Molding Method. Assuming that the mold is built  without any modification needed from the original design. And the Mold is milled out at the exact dimension, Nothing added, no pressure releasing holes needed, no wall thickening nor thinning needed. The final product that came out of the machine will still have a different weight than the calculated MASS. Injection always added something into the products. A little weight will always be added. It is because of the nature of Plastic Injection, no matter now perfect the machine is, the products that came out of the machines will all have slight deviation in weights.

That was what I was talking about. A product designed in Fusion360, has a perfect calculated Mass.  But the production cycle can only make perfect products at top quality at a certain accepted tolerance in weight and quality.

I always added 10 to 15% on top of the Calculated Mass.  Strangely speaking, we try to limit our wastage to a maximum of 10% in the whole production cycle, within the same order.

I had found that these correction figures that I used can be within an accuracy of 5% after production. That is an acceptable figure. I do not advise changing the density of PMMA in Fusion because that would not be too nice, wouldn't it?  That is like changing the physical or chemical properties of the material.  I myself had been using a correction percentage to compensate the differences and had been working quite well for me. 

Thanks for bringing all these up.. It had been a pleasure discussing this topic at this forum. Isn't this forum great? Fusion 360 is indeed a great community driven program!