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

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.

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

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

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:

1. A helix isn't an arc. Arcs are 2D trig functions. A helix is a 3D trig function (it's three functions, actually)
   1. With your disjointed structure, the number of equations is 3 for each revolution, plus 6 more for the straight segments
2. You want a straight section every 180° which are supposed to be tangential at both ends
   1. Looking down the helix axis, the projected helix is circular.
   2. At 180° of the current radius, a straight segment has to be inserted at the same radius
   3. Tangent to a circle is square to the radius, so the straight travels outward ("off on a tangent" is the cliche)
      1. This straight segment is a projected chord.
   4. How can a tangential line somehow swing back and become tangential again to the same projected radius?
3. 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.
4. 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)
   1. Then the segment distance from the helix axis must be computed and there must be tangential segments on each end of the straight segment
   2. Plus you must compute the axial angle of the the straight.
5. #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
   1. Exception to #4 is that the straight projected axis can't be square to the radius

Cheers,

Kevin