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Chord line distance used for cut applied when unfolded

Question asked by David Wiberg on Jan 7, 2019
Latest reply on Jan 8, 2019 by David Wiberg

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Hello.

As part of a course I'm trying to model a real life object and have encountered a problem which I'm not sure if it is expected or not. Let me describe/motivate the steps I have performed and what I find strange:

Created a sheet metal part from a sketch which consists of a 175 deg arc, i.e. not a complete half circle. Note that the sketch defines the outer diameter.
Unfolded the part to add a cutout which I wanted to dimension when the part is in an unfolded state.
Folded the part and realized that the measurement isn't applied as I expected as the distance I have set is used as shear chord line instead of arc length.

Is this expected behavior?

File is attached for reference.

Best regards
David

Jeff Parker Jan 8, 2019 1:00 PM

Correct Answer

David,

This is how most all parametric CAD software works in regards to sheet metal. One thing that will clarify this is that the fold/unfold features calculate the bend allowance for each feature. So if you unbend, it calculates the material to remove to show an accurate flat pattern, and the fold feature calculates the material to add when it is stretched. It is not just a simple visualization of the part with no calculations. So this is really not a limitation of the software, just a misunderstanding of how these two features work.

You are correct that you will need to use a formula to calculate the correct length in the flat to determine a precise value for the arc length.

Please see this page of the help for the formula and explanation when using a K-Factor for the bend allowance.

2019 SOLIDWORKS Help - K-Factor

Regards,

Jeff Parker

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Jeff Parker Jan 7, 2019 6:23 PM

David,

Yes, this is expected behavior since you are dimensioning it in the flattened state. So the value is typically the chord length and you will have to calculate the correct value to get a specific arc length based on thickness, k-factor/BA/BD, radius, and angle of the segment.

Regards,

Jeff Parker

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David Wiberg @ Jeff Parker on Jan 8, 2019 12:38 PM

Thanks for the answer Jeff.

My follow-up question is why you expect this from the program? Why shouldn't the program do the calculation to follow the arc? It seems counter-intuitive to me. My expectation for the folding/unfolding operations is that I should get "views" of the part in different states, much the same as if I manually folded/unfolded a piece of sheet metal. In this case, this doesn't seem to apply. Is there a good motivation for this limitation?

If I were to do the calculation manually, I most likely would need to move some of the sheet metal parameters you mention (e.g. K-Factor) into variables and create a formula which I could use for the dimension in order to produce a robust model of the part. Otherwise I guess I would need to manually update the dimension if I change e.g. K-Factor in the future.

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Jeff Parker @ David Wiberg on Jan 8, 2019 1:00 PM

David,

You are correct that you will need to use a formula to calculate the correct length in the flat to determine a precise value for the arc length.

Please see this page of the help for the formula and explanation when using a K-Factor for the bend allowance.

2019 SOLIDWORKS Help - K-Factor

Regards,

Jeff Parker

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David Wiberg @ Jeff Parker on Jan 9, 2019 10:21 AM

Thanks for the follow-up Jeff,

I noticed that I've written shear line in the original title and question, while I meant chord line (I blame it on neither CAD nor English being my first language...). For my first example, where I had calculated a cut to be located 90 degrees from the start, this was true. It seems however this was only "luck" as if I change the thickness the position in the folded view changes. This was the part which seemed most strange.

Upon doing some more testing now I see that the result is not related to chord line but rather sheet metal and bending properties. Thanks for pointing me in the right direction.

Best regards

David

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4 Replies

Jeff Parker Jan 7, 2019 6:23 PM
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Correct Answer
David,

Yes, this is expected behavior since you are dimensioning it in the flattened state. So the value is typically the chord length and you will have to calculate the correct value to get a specific arc length based on thickness, k-factor/BA/BD, radius, and angle of the segment.

Regards,
Jeff Parker
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- David Wiberg @ Jeff Parker on Jan 8, 2019 12:38 PM
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  Correct Answer
  Thanks for the answer Jeff.
  
  My follow-up question is why you expect this from the program? Why shouldn't the program do the calculation to follow the arc? It seems counter-intuitive to me. My expectation for the folding/unfolding operations is that I should get "views" of the part in different states, much the same as if I manually folded/unfolded a piece of sheet metal. In this case, this doesn't seem to apply. Is there a good motivation for this limitation?
  
  If I were to do the calculation manually, I most likely would need to move some of the sheet metal parameters you mention (e.g. K-Factor) into variables and create a formula which I could use for the dimension in order to produce a robust model of the part. Otherwise I guess I would need to manually update the dimension if I change e.g. K-Factor in the future.
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  - Jeff Parker @ David Wiberg on Jan 8, 2019 1:00 PM
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    Correct Answer
    David,
    
    This is how most all parametric CAD software works in regards to sheet metal. One thing that will clarify this is that the fold/unfold features calculate the bend allowance for each feature. So if you unbend, it calculates the material to remove to show an accurate flat pattern, and the fold feature calculates the material to add when it is stretched. It is not just a simple visualization of the part with no calculations. So this is really not a limitation of the software, just a misunderstanding of how these two features work.
    
    You are correct that you will need to use a formula to calculate the correct length in the flat to determine a precise value for the arc length.
    
    Please see this page of the help for the formula and explanation when using a K-Factor for the bend allowance.
    
    2019 SOLIDWORKS Help - K-Factor
    
    Regards,
    Jeff Parker
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    - David Wiberg @ Jeff Parker on Jan 9, 2019 10:21 AM
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      Correct Answer
      Thanks for the follow-up Jeff,
      
      I noticed that I've written shear line in the original title and question, while I meant chord line (I blame it on neither CAD nor English being my first language...). For my first example, where I had calculated a cut to be located 90 degrees from the start, this was true. It seems however this was only "luck" as if I change the thickness the position in the folded view changes. This was the part which seemed most strange.
      
      Upon doing some more testing now I see that the result is not related to chord line but rather sheet metal and bending properties. Thanks for pointing me in the right direction.
      
      Best regards
      David
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Chord line distance used for cut applied when unfolded

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