Hi gizmo,

> Could someone explain to me what's going wrong?

To get a smooth sweep juncture, it is not enough to just have 2 points touching one another. To give you a bit of an exaggerated example, here are 2 curves that touch each other at a common point just like points you have labeled in red above:



Even though these curves touch at a common point, if you sweep them you will get this result:



You can see more easily in this case why the pieces do not mate up even though the 2 curves connect at a common point. It is because the curves are not totally smooth with one another.

You are running into this exact same issue with your curves - your pieces do not share a common tangent similar to the above, although your one happens to be much closer to being smooth so it is harder to see it initially.

A curve's tangent direction is defined by the end point and the next point to the inside.

If we look at these 2 points:



And then these 2 points:



Creating 2 extended lines that go through each of these points (here I used construction lines to do this):



Initially that looks pretty much like they are aligned, but if you zoom in:



You can see there is actually a deviation there - those curves do not have a common tangent so they are causing the same kind of effect as in the 2 sharp lines that I showed above.

I've attached a version of this curve pad_trued_tangents.3dm, where I lined up all the points to be exactly along a straight line (I drew in an extended line and snapped them all to it), with this version you should now be able to make a sweep where all the pieces join up exactly.

- Michael