Why can't Sweep2 stay in its constraints?

In this particular scenario, I like the simplicity of the sweep2 surface, but it cannot stay inside its constraints.
Why is that ?

Sweep.3dm (59.7 KB)


you can build a couple of more section curves.
Otherwise the geometry itself, the degree of the arc, will create a bulge.

See attached file with some section curves. Sweep_re.3dm (86.0 KB)

Best regards,

Hi Thomas - I guess the answer is there is no constraint there as far as Sweep2 is concerned. The simple way out is to loft the line and the arc (straight sections) and MatchSrf for Tangency/Position (other end).

Sweep_Loft.3dm (65.4 KB)


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Pascal, yes that worked. Thanks.


It doesn’t quite work in every scenario though.
In this new example the loft is fine until you try to match it to the bottom black guide curve.
(The goal is again not to cross under the x-axis on the top view at any point and still have a simple surface)sweep.3dm (23.5 KB)

Hi Thomas - use the ‘Preserve isocurve direction’ option in MatchSrf - does that get what you need?


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I suppose it works as long you matchSrf to the guide rail curves only. Once you try to do a third match to the guide surface it changes the geometry again … but we could argue if the guide rail curves were already curvature continuous to the guide surface there should be no need to do the third matchSrf.

Hi Thomas - this is the result from

  1. Loft, straight sections
  2. MatchSrf, once, for tangency/position.

Sweep_Loft2.3dm (36.6 KB)


I am afraid I can’t replicate that result.
You loft the guide curves first ? or between the surface and the little arc ?
Are you using the refine-match option ?
Are you using the multipleMatch option within the command ?

Hi Thomas - if the match in your image is from the loft (arc to edge) to the planar surface edge, you’ll want the settings reversed - match for Tanngency and ‘Preserve other end’ at Position.


ok, thanks a million for that video !
So, you are not touching the top and bottom guide curves at all. I’ve been trying to match those all this time.