using weight in expressions

[druzil69]

I am new to this site but haven't found anything on what i am trying to do yet. Basically I have a final unit assembly i need to put a base under. The base is created from I-Beam. I want to set up an expression that either changes the dimensions of the i-beam according to the unit weight or suppresses/un-suppresses different size i-beams. Either is fine. However i can't seem to get the i-beams to change either way after the weight changes. Below are examples of the different expressions i created but dont seem to work. Any ideas what i am doing wrong?

Beam_Heighta if(A_Unit_Weight<=1000)(12.25)else(Beam_Heightb)
Beam_Heightb if(A_Unit_Weight>=1001&&A_Unit_Weight<=5000)(18.59)else(Beam_Heightc)
Beam_heightc if(A_Unit_Weight>=5001&&A_Unit_Weight<=10000)(24.3)else(Beam_Heightd)
Beam_Heightd if(A_Unit_Weight<=10001)(36)else(Beam_Heightb)

12" I-Beam if(A_Unit_Weight<1000)(1)else(0)
18" I-Beam if(A_Unit_Weight>1001&&A_Unit_Weight<5000)(1)else(0)

 

[cowski]

Is this exactly what you have so far? If so, I'm surprised it even let you enter the equation for Beam_Heightd because there is a cyclic reference in there. Perhaps the Beam_{h|H}eightc typo let it through.

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[druzil69]

thats exactly what i had in there. I saw that reference and meant to change it before posting this and forgot. I have since changed my expression.

 

[cowski]

Also, you will have problems if 1000 < A_Unit_Weight < 1001 or 10000 < A_Unit_Weight < 10001.
These formulas would catch all values up to 10000:

Beam_Heighta if(A_Unit_Weight<=1000)(12.25)else(Beam_Heightb)
Beam_Heightb if(A_Unit_Weight>1000&&A_Unit_Weight<=5000)(18.59)else(Beam_Heightc)
Beam_Heightc if(A_Unit_Weight>5000&&A_Unit_Weight<=10000)(24.3)else(Beam_Heightd)

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[cowski]

You want one part in your assembly to drive geometry in another part in the assembly, to do so make sure interpart modeling is turned on (customer defaults -> Assemblies -> General -> Interpart Modeling) assuming the site standard allows it. If you are working across levels of the assembly, you may need to force an update (Tools -> Update -> Interpart Update -> Update All).

What version of NX?

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[druzil69]

NX version 6

 

[druzil69]

ok i will have to try that. Is there anyway i can do it all inside the same part file?? the final unit assembly is like an "automation". I have about 10 parts in it all done with sketches. I change 6 inputs and all the sketches update accordingly through expressions. Now I need to make the I-beam base update with it as well but it will be based on the size & weight of the unit assembly. I hope that makes sense.

 

[cowski]

I have about 10 parts in it all done with sketches.

Just to be clear: do you have 10 separate part files added as components to this assembly, or do you have 1 file with 10 different sketches in it representing 10 different parts?

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[druzil69]

10 different sketches, representing 10 different parts. i would like to either keep the base in the same file if possible.

 

[cowski]

I tried to recreate your expressions in your first post, but NX would not accept the formula for Beam_Heightd, it gave a cyclic reference error.
Make sure the delayed after update option is turned off (Tools -> Update).

Are you using units on any of these expressions? Perhaps if the units don't work out it won't apply the values (ie trying to drive a length in a sketch with an expression that has units for pressure).

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[druzil69]

not sure if this will help but i tried to upload the file i was messing with. I was trying to get the weight of the cylinder to drive the I-beam size somehow.

 

[cowski]

The expressions do not evaluate correctly because NX is trying to compare a constant with a weight value (technically a mass value, but we'll ignore that for now).

Create an expression such as weight_condition_1 and give it a value of 1000 lbm (you will have to use the dropdowns to assign the units on the expression). Now change your beam height expressions to be something like:
Beam_Heighta if(A_Unit_Weight<=weight_condition_1)(12.25)else(Beam_Heightb)
You'll see that it now updates correctly when the weight (mass) of the cylinder changes.

Optionally, you might also want to assign the Beam_height expressions unit values of inch to better capture your design intent.

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[druzil69]

Well look at that. Its exactly what i was trying to do. Thank you so much!!!! Just a question i guess, why do i need that intermediate step "weight condition". why can't i just link the beam height directly to the unit weight value??

 

[cowski]

The expression A_Unit_Weight is defined in terms of units, the number 1000 in your equation has no units associated with it. To create a logical comparison, both items you are comparing must have equivalent units. Is 500 lbs > 8? Well, that depends on the units associated with the value of 8: 8 grams? 8 tons? 8 amps? A true, false, or you can't do that! answer depends on the units.

I created an intermediate expression that had the same units as A_Unit_Weight. This is a quick & easy way to create a consistent comparison. Perhaps there is a way to add units to a value directly within your equation. I have not tried that.

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