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How do I join a helix to a line using an arc, with the line being perpendicular to the helix axis?

Question asked by Tijs Robinson on Apr 17, 2018
Latest reply on Apr 19, 2018 by Tijs Robinson

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Hi all,

Was hoping someone could help me with this problem I've been battling with for a couple of days. I have a 3D sketch with a helix (created by converting entities). The axis of the helix is vertical (the Y axis), the line is sitting on the top plane along the Z axis. I have an arc connecting the two but if I make the arc tangent to both the helix and the line then the sketch is over-constrained. If I remove the relation between the helical spline and the spline from which it was created, then I can have the arc tangent to both the line and the helix, but the helix is under-constrained. Any ideas on how to tackle this? I'm pretty sure that an arc that is tangent to both is logical and ought to be possible. I've attached a picture of the sketch and the part itself.

Thanks,

Francis

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coil.SLDPRT.zip34.5 KB
coil.png56.4 KB

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M. B. Apr 18, 2018 9:07 AM
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To do as you described, an arc tangent to both line and helix, the line cannot be on the same plane as arc and the helix endpoint. Create a 3DSketch, insert a line parallel to a plane perpendicular to the helix axis, insert a 3 point arc connecting the end of line to end of helix. Then make tangent relations between the arc, helix, and line. To get the arc endpoint coincident with the helix endpoint, you may need to use Convert Entity on the helix. The correct solution will have a line offset down a little from the helix endpoint and the arc will be at an angle matching the helix angle.

See attached.
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- coil.SLDPRT.zip52.1 KB
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- Francis Fowler @ M. B. on Apr 19, 2018 6:13 AM
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  Thanks for that M.B. I didn't mention it earlier, but I also wanted a line and arc at the top of the helix in a similar arrangement. Your solution does indeed work, and I think I actually drew this configuration on Monday. I had dismissed this design as not allowing me to define the overall height between two lines, but I'm not sure this is correct. I should be able to sketch the helix, arcs and lines, find how much of the "height" the arcs take up and subtract this from the overall height desired, this will give me the height I need to make the helix. I'm not sure if there is a nice way to do this in SolidWorks (i.e. tell S.W. the overall height and have it find the helix height), or if I will just have to do the calcs myself and plug numbers in. Perhaps I should have a look at that "Equations" thing in S.W.
  
  Initially I was hoping to have lines at set heights, arcs of set radius and have SolidWorks find the arc orientation and helix intersection points necessary, but I think the software is just not designed to do this.
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  - M. B. @ Francis Fowler on Apr 19, 2018 8:40 AM
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    This part has the lines the same distance apart as the length of the helix.
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    - Tijs Robinson @ M. B. on Apr 19, 2018 4:21 PM
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      Hmm, that's interesting. This is what I was trying to achieve the other day, but SolidWorks would simply not let me. I think you have to add relations in just the right order sometimes, otherwise S.W. gets confused about things that actually make perfect sense.
      
      Anyway, thanks for the help,
      
      Francis
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Erik Bilello Apr 18, 2018 1:22 AM
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I'm not thinking too hard about this, so I may be missing something at this point in the day.
But, to achieve what I think you are wanting (a smooth transition between the line and the helix) I think you will need to insert a spline. An arc is a segment of a circle, all the points of which will be in the same plane.
If you look closely at the attached part, I have inserted an arc, which is only tangent to the line, and a spline which is tangent to both the helix and the line. I also inserted a line segment tangent to the end of the helix. I added a reference plane defined by the line and the end of the helix (any arc tangent to the line and connecting it to the end of the helix will reside in that plane). You can see that the arc is in that plane and that the helix sweeps up out ot the plane. So, the arc cannot be tangent to the helix and the line. The tangent line segment dives under the plane and highlights how geometry coincident to the plane can't be tangent to the helix.
If you want the transition between the helix and the line to be a true arc, you will need to vary the helix (like a spring with a flat end) so that the end is tangent to the plane which the arc will be in. That way the tangency between the arc and the helix only has to deal with planar geometry.

Erik,
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- Andy Grant @ Erik Bilello on Apr 19, 2018 1:26 PM
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  I like your suugestion, Eric.
  
  However, I would make one suggestion, if I may. I'd use the "style spline" as it's easier to define.
  
  Andy.
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Roland Schwarz Apr 18, 2018 8:24 AM
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I wasted so many hours on this problem. Then I had an epiphany as to why it wasn’t possible.

For an arc to fillet two lines, the two lies must be coplanar. If you have a line that itches a helix and goes off tangent to the helix‘s defining cylinder, then a tangent line for any further point on the helix will not be coplanar.
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How do I join a helix to a line using an arc, with the line being perpendicular to the helix axis?

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