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How to define a spiral around a hollow cylinder, without "overhangs"

Hi all, I'm trying to model a rounded "thread" around a hollow cylinder, to create a collet for a flexible spiral vacuum hose (). I've gotten to the point of sweeping a circular shape around the cylinder and then hollowing it out (sweeping around the hollow cylinder was causing the thread to "poke" out of the inside surface as well). However I can't find an "elegant" way to avoid the thread appearing above and below the top and bottom of the cylinder (see image). I've thought about adding a couple of extruded boxes set to "remove" to cut out the spillage, but is there a better or more elegant way?


  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 5,060
    Extrude/remove is the best option here. Are Loft 1 & 2 hidden? Can't see what they would be for?
    Senior Director, Technical Services, EMEAI
  • eric_pestyeric_pesty Member Posts: 1,308 PRO
    You could try a "Split part" (by mate connector) as well using the end of the cylinder.
  • MichaelPascoeMichaelPascoe Member Posts: 1,554 PRO
    edited November 2022

    For threads, I usually extend the helix beyond the cylinder. Then split it like @eric_pesty said. You can probably smooth out the transitions with fillet after it is split then joined to the cylinder.

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  • Gabriele_CannizzaroGabriele_Cannizzaro Member Posts: 9
    Yes they're hidden, they are there to create and hollow a "funnel" shape between two hollo cylinders. So you'd recommend (simplifying brutally) a couple of oversized cubes above and below the cylinder set to remove?
  • Gabriele_CannizzaroGabriele_Cannizzaro Member Posts: 9
    I've tried splitting the helix extrusion using the cylinder face then deleting, and not only it works great but to me this is a more elegant way than using solids to remove. However, the fillet only seems to work on one of the ends (see first image), the other one fails with a "failed to fillet selections" error, but the config of the two fillets is exactly the same. Is this because of some "direction" issue?

  • Gabriele_CannizzaroGabriele_Cannizzaro Member Posts: 9
    To be clear, the effect I'm trying to achieve is the thread so to speak "easing in" from the surface, ie progressively appearing out of the cylinder on each end (or at least one) in a smooth transition (eg over a couple of cm), so that when the cylinder is screwed into the spiral hose it goes in smoothly.
  • S1monS1mon Member Posts: 2,104 PRO
    I might try boolean adding the two solids together before adding that fillet. The way the path wraps back on itself is not helping. You could also try a smaller radius and sneak up on what works.
  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 5,060
    Senior Director, Technical Services, EMEAI
  • Gabriele_CannizzaroGabriele_Cannizzaro Member Posts: 9
    I managed to achieve a result a bit closer to my goal by extending the spiral further than the cylinder and playing with the position of the initial circle that creates the thread extrusion:

    I moved to an inner thread design because it suited the specific application better, but I also need an external thread one so I'll keep working on that.

    @NeilCooke thanks for the link, it looks quite close to what I'm trying to achieve, I'll fork it and play with it, I don't quite understand how it works yet.
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