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Modeling a variable pitch and vaiable inner diameter of helix structure with an intermittent tangential straights.

Question asked by Sreenath Perumal Ganeshbabu on Jun 7, 2018
Latest reply on Jun 8, 2018 by Sreenath Perumal Ganeshbabu

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Good Morning Solidworks Users,

I have a question on modeling a part to support my quality control team at my company. I have to model a spring which has a variable pitch and a variable inner diameter of helix structure for every revolution. Added to this, i need to insert a straight which is equal to 1/8 * coil diameter and it must be tangential to both the adjacent helical arcs so that i can model a complete tangential helix structure. The straight must be inserted in the structure for every 180° revolution of the helix and the coil has 3 revolutions in total. Can this be done in Solidworks 2013?

I tried doing this and i'm not able to make the straights tangetial to the second adjacent helical curve. Help needed.

All valuable suggestions and inputs are welcome. I would be glad to have your support on this. Thanks in advance and have a great days ahead.

Regards

Sreenath Perumal Ganeshbabu

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Dwight Livingston Jun 8, 2018 8:30 AM
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Sreenath

The "variable pitch and a variable inner diameter" part of your question makes me think your question needs more definition. Anyway, I expect you want to create all the straight line segments first and then connect them with splines. The "style" splines would work best for this, but since you have SW2013, you will have to make do with regular splines. I don't know more to say without learning more about those variables.

Dwight
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- Sreenath Perumal Ganeshbabu @ Dwight Livingston on Jun 8, 2018 11:36 AM
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  Good morning Dwight,
  
  Thank you so much for your response. But here is the definition of the problem. So the spring must have 3 revolutions and i want to include a straight of .0234 inch for every 0.5 revolution of the spring. first revolution of the spring has an inner diameter of .75 in, second revolution has an inner diameter of 1 in and third revolution must have an inner diameter of 1.125 in. So basically you can imagine this model as a conical spring. Constraints which we have are don't use splines for modeling and just use curves and straights. These are the constraints of our inspection machine. Or else the software doesn't accept the model.
  
  Right now am not able to make the curve after every straight to be tangential.
  
  Suggestions are welcome.
  
  Regards
  Sreenath
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  - Jeremy Feist @ Sreenath Perumal Ganeshbabu on Jun 8, 2018 11:44 AM
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    A helix CANNOT be accurately drawn as an arc - it would appear your inspection tool(s) are not up to the task at hand. or, perhaps, there is some detail of the problem not coming through.
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  - Dwight Livingston @ Sreenath Perumal Ganeshbabu on Jun 8, 2018 1:15 PM
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    Sreenath
    
    What is the pitch for each? And is the .0234-inch straight as measured along the straight or as projected on a plane perpendicular to the spring axis?
    
    Dwight
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    - Sreenath Perumal Ganeshbabu @ Dwight Livingston on Jun 8, 2018 3:53 PM
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      Dwight,
      .0234 in is measured along the straight.
      
      Regards
      Sreenath
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  - Dwight Livingston @ Sreenath Perumal Ganeshbabu on Jun 8, 2018 1:55 PM
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    reenath
    
    I can see using arcs. The image below shows one approach. In a 3D sketch, create a series of straight lines in the spriral you want, then at every intersection add arcs tangent to the pair of lines. Pretty tedious, but it will get you there. I expect there are ways to simply this, but I'm not seeing it right away.
    
    Dwight
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  - Kevin Chandler @ Sreenath Perumal Ganeshbabu on Jun 11, 2018 8:19 AM
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    Sreenath Perumal Ganeshbabu wrote:
    
    Good morning Dwight,
    
    Thank you so much for your response. But here is the definition of the problem. So the spring must have 3 revolutions and i want to include a straight of .0234 inch for every 0.5 revolution of the spring. first revolution of the spring has an inner diameter of .75 in, second revolution has an inner diameter of 1 in and third revolution must have an inner diameter of 1.125 in. So basically you can imagine this model as a conical spring. Constraints which we have are don't use splines for modeling and just use curves and straights. These are the constraints of our inspection machine. Or else the software doesn't accept the model.
    
    Right now am not able to make the curve after every straight to be tangential.
    
    Suggestions are welcome.
    
    Regards
    Sreenath
    If I'm understanding correctly, this isn't geometrically possible, at least not readily so:
    A helix isn't an arc. Arcs are 2D trig functions. A helix is a 3D trig function (it's three functions, actually)
    With your disjointed structure, the number of equations is 3 for each revolution, plus 6 more for the straight segments
    You want a straight section every 180° which are supposed to be tangential at both ends
    Looking down the helix axis, the projected helix is circular.
    At 180° of the current radius, a straight segment has to be inserted at the same radius
    Tangent to a circle is square to the radius, so the straight travels outward ("off on a tangent" is the cliche)
    This straight segment is a projected chord.
    How can a tangential line somehow swing back and become tangential again to the same projected radius?
    At 360°, there's an increase in radius that can be "bridged" by a straight section, but its length must computed by the 3D tangential constraints, if they're even mathematically possible.
    To make the straight sections even probable at 180°, I think their projected axis has to be square to the projected radius (tangent at the midpoint)
    Then the segment distance from the helix axis must be computed and there must be tangential segments on each end of the straight segment
    Plus you must compute the axial angle of the the straight.
    #4 applies to the straight segments at at 360° where the radius is increased, except the math and geometry are more involved due to the radius change
    Exception to #4 is that the straight projected axis can't be square to the radius
    
    Cheers,
    
    Kevin
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Timothy Taby Jun 8, 2018 11:56 AM
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Does SW 2013 have the variable pith helix/spiral command (i'm using 2017 and it does)? If it does then you should be able to make your helix using it. To put straight section in you will have to do a section of helix, then sketch the straight section, then do another section of helix. I would sweep your profile one section at a time instead of trying to make a single helix out of the sections and then sweeping it.
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- Sreenath Perumal Ganeshbabu @ Timothy Taby on Jun 8, 2018 3:53 PM
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  Timothy,
  
  Thanks for your response. But the problem which i face is section 1 + a straight + section 2 is what you are asking me to do. I did the same and i'm not able to make section 2 tangential to the straight. Suggestions are welcome.
  
  Regards
  Sreenath
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Dan Pihlaja Jun 8, 2018 1:41 PM
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Here are a couple of areas to read up on:
How to define a custom thread path
variable diameter helix in sweep

Basically, the method is to use surfaces and their intersection with a helix and other geometry to define your path.

Maybe this will help you?

If you could share a couple of screen shots or even a drawing of what you are trying to accomplish, it would be a lot easier to help.
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- Sreenath Perumal Ganeshbabu @ Dan Pihlaja on Jun 9, 2018 9:27 AM
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  This is a screenshot of what i'm trying right now. I'm creating section 1 using helix command for .5 revolution and sketching a straight and then i'm using the reference points generated along the center line of the actual helix structure modeled using helix command and using those selective points and its coordinates i'm created individual segments to cover the rest of the spring section so that all the arcs and straights are tangent to one another.
  
  I am actually recreating the actual part that we manufactured in our plant and then creating STEP files for those. So structural dimensions are important for us.
  
  This is a bit time consuming way of doing and if you guys have any simplified approaches then shoot them up.
  
  Regards
  Sreenath
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Modeling a variable pitch and vaiable inner diameter of helix structure with an intermittent tangential straights.

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