What method to progressively sweep a helix inwards in its one rotation?

Hi V5,
how to sweep a helix inward during its next 360 deg rotation ?

What method does this ?

If one has a helix, how is the last rotation taken progressively inwards during the 360deg rotation of it.

Such that any triangle thread on it vanishes into the inner solid its sitting around.


Project it on to an appropriately designed cone?



Now thats a neat trick, do some maths and establish cone angle. It needs to depart the helix tangentially and that would do so as well.

Now someone needs to add that into a bolt thread making video !


Spiral command Rhinoceros Help - Spiral | Rhino 3-D modeling (mcneel.com)

As @davidcockey said, it’s called a Spiral. See the Help for details on how to use it.

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Thanks guys, so spiral would also do it. enter same pitch etc as helix and create the last turn.

Which in theory is the easier to construct, the project to cone or the spiral matching the pitch and so on of the helix ?



Spiral IMO…

Probably what Mitch said - especially after you’ve done half a dozen or so.

Why not try both and let us know what you think?

Project does not work.
Spiral Steve.3dm (2.0 MB)

Blue is helix
Green is spiral
Purple is helix projected onto cone
Magenta is helix pulled onto cone

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Creating a rectangular surface with a diagonal line, then flowing the diagonal line onto the cone also works. Result is essentially the same as Spiral.
Spiral Flow Steve.3dm (2.0 MB)

Of course you are right. I spoke too soon. I was imagining “projecting” parallel to the cone’s base (perpendicular to its axis). But as you show that’s not how the project command (or, for that matter the pull command) works


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in its one rotation?

In its own rotation you mean?

Hi Steve,

If you are talking about the final turn of a thread on a bolt, I believe you will find that the helix radius stays constant to the end. The reason that a casual glance leads you to think the final turn spirals in is that the end of the bolt has a chamfer that progressively cuts away more of the top of the V as you get closer to the end.

So to get the effect, make a solid bolt, then make a cutter by boolean differencing a cone from a cylinder and then boolean difference the cutter from the bolt.



Hi Jeremy,
aha !
I have seen videos of folk sweeping the curve in

an apparent Rhino tutor ?

and took that as to be the need, and theur method was criticised but no mention of the fact that it wasnt needed at all. However what you show makes such as they do unnecessary.
My brain says if metal moves inwards the helix would remain out in space, however your image shows that as not the case.
I must do what you show to see it for myself.

we need a definitive guide to bolt thread making !

I then look at the other end of the thread and see they have milled around the thread and it simple gets milled or ground/turned, whatever one would call it, out of existence without going inwards.

My item has the thread vanishing just before the slope starts, or perhaps its a tad up the slope, so a bit of experimentation moving the slope up and down is needed.


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Try and find a large bolt such as an M20, then use your vernier (assuming the edges are sufficiently feathered) to measure the diameter at the base of the thread at several points along the length, including near the very end and you should see it remains pretty constant.

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You should also see a video on how a thread is created in a thread turning operation on any CNC machine. To me, moving the helix in, while aesthetically “correct”, breaks the reality of how your underlying geometry definition is produced. Even on a proper higbee thread, the tool that turns or thread rolls the helix doesn’t just “bite down” at the end. Infact, they often taper outwards (at the screw/bolt head end), the opposite of what the modelling strategy may be by crushing the helix.


That is a fictitious feature of thread making.

Haven’t you ever successfully shortened a fastener and properly deburred it to function correctly?

‘Spiraling the helix inwards’ has nothing to do with it, and is completely unnecessary.

1.) no.


Hb which theory is correct?

Music to my ears :notes: Finally someone who knows :sweat_smile: