Can't get rid of this crease

I’ve got these curves and whenever I try sweep 2 rail or network srf I always get creases. I’ve tried exploding it and match surfacing it but that usually makes things worse.

The crease happens where the two outer curves meet at a point. So I tried smoothing that out with a blend surface but then I can’t even make a sweep or network surface :disappointed_relieved:

Chan someone explain why this crease happens?

SWEEP 2 RAIL.3dm (29.0 KB)

Thank you in advance

Hello - the inside part of the path has a kink in it - the two segments of that arc shape are not tangent.

Command: _Gcon
First curve - select near end
Second curve - select near end
Curve end difference = 0.000 millimeters
Radius of curvature difference = 0.544 millimeters
Curvature direction difference in degrees = 9.880
Tangent difference in degrees = 2.843
Curves are G0.

SWEEP 2 RAIL_PG.3dm (87.8 KB)


Hi Pascal

Than you for your response. I can’t believe these two curves had a kink. It was originally just an arc and all I did was move the control points up so I’m not sure how it became two curves… :open_mouth:

__EDIT: Okay I played around with the arc some more and it seems like any arc will have a kink at the middle if I try to move the control points around like I did. I managed to fix it by just deleting that point from the start but then it changes the geometry. I could use crv2view, but I watched a tutorial where it said that was bad because it makes a ton of control points. If I just change the degree of the arc to 3 or 4 that doesn’t work, but it seems like if I rebuilt the arc with more points and a higher degree that works.

I got rid of the crease up there but just redrawing the arc, but I still have two creases at the bottom. Any idea how to get rid of them? I made the attached image surface with network surface.

Your curves are degree two, have tangent breaks and the patch layout does not lead to proper four-sided NURBS surfaces.

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Hello - With the curves you have I’d say it will be tough to make clean. I’d rearrange the large cross section curve and make a new arc at the fat end that is properly tangent to the larger outside rail.

SWEEP 2 RAIL_PG.3dm (202.2 KB)


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Hi Pascal

Thank you for solving the crease. May I ask how you built your surfaces? I’m trying to recreate your work but cannot do it.

At the fat end I’m assuming you did a sweep1 with two profiles in red and the rail in cyan?

How did you create the black line in the middle? I tried tweeningtwocurves but that didn’t work. I also tried just sweeping the joined red lines up the two curves (blue and white) and splitting it by isocurve at the midpoint, which got me close but not perfect.

I tried matchsrf (first starting with position+tangency as going upwards from that just made things weird) but it didn’t fix the crease.

Even when I delete your surfaces and remake them with sweeps, mine don’t come up smooth. See attached picture and file

SWEEP 2 RAIL_PG.3dm (622.4 KB)

Let’s see… I made the initial sweep then split it in two (the middle curve is an extracted isocurve), shrank the resulting surfaces (ShrinkTrimmedSrfToEdge in this case) then MatchSrf from the split/shrunk surface to the round sweep1 surfaces’ edge for tangency. The other half of the split surface I left alone since there is a kink dictated by the curves - that should fade out as it approaches the middle. I think I also added a few knots to the split surface as well, in the long direction to help keep the tangency along that edge in case I needed to re-match back along that long central edge… I think.


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Hej jshethj,

you could do it this way, using simple degree 5 curves. But with your curves, you can only have tangency (G1 continuity) and also not maintain that along the entire common boundary, see screenshot/file.

SWEEP 2 RAIL_PG.3dm (441.1 KB)

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Thank you!

I now got it to work!