Why does offset go squiggly on curves treated to fitCrv and also rebuild?

this is result of a straight line, radiused corner shape projected onto a complex cowling skin,

I then treated it to fitCrv, trying to minimise any shape disturbance. I need to make something that sits on the inside of the skin which is 3mm thick. offset this 3mm and squiggles.

treat it to Rebuild after exploding it, not possible before explode, two different values, squiggles each time…


Offset squiggly.3dm (68.0 KB)


I don’t understand what you’re trying to do… Do you?

It seems you are just pulling things out of your cookbook without really knowing what they do, and then wondering why it doesn’t work.

Offset is a mostly 2D command, when you use it on 3D curves it will give you unpredictable results. Using FitCrv and Rebuild without knowing what they do and without checking the result will also be unpredictable.


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If you have projected this onto a surface, it seems offsetsCrvOnSrf would be what you are after in this case. I wouldn’t mess with fitCrv or rebuild if your intent is to keep to projected curve on the surface you projected it onto.

I didnt realise offset was for 2D. I will update my little black book. I must read up on the do’s and donts of offset, I am aware of its ease of ruination on surfaces, so prefer to use it on curves, sort out the mess then surface them.

If I project the simple line and arc shape onto my surface, then need to create a solid from that with its one surface 3mm offset inwards and its other surface 13mm inwards, how should I do that ? My instinct is to take the result of project and offset it inwards 3mm, then a further 10mm , surface them afterwards, but one mustnt use offset.

I had projected a simple line and arc shape that you see here to the surface, then as it developed measles, I tidied it up with FitCrv, as FitCrv doesnt alter the curves shape as Rebuild can, I am always after keeping the exact shape within tolerance if poss.
Then I surfaced it following all dos and donts learnt from here.

then I offset it and it went well.

Needing to offset it a further 3mm I thought I would use the curve again.

I threw rebuild at this just to see if it solved the squiggles, I also looked at Curve graph, so its a lost cause, because its 2D…

As I discover what shouldnt be used, I remember such, …forming quite a list in fact.


If you projected a flat, planar curve (looks like from the Front view) onto the surface, the best way to get it offset on the surface in Front view projection would be to offset the planar curve in Front view your 3mm and then project the result back to the surface. That would pretty much guarantee success (I haven’t seen the surface though).

Arcs and lines offset in their native planes as arcs and lines, so no complexity is added there. However it does look like you did something to the original (magenta) curves, they’ve been rebuilt with quite a few points. If you project them to the Front plane and then offset them, the result is still not awful, though.