Hi Bill - that's related to the small scale of your objects. Basically your curves there are too close to the default 0.001 fitting tolerance. When a sweep is made, the sweep is refined with more sections added as needed until the generated surface is within 0.001 units of the ideal result.

But in your case there the line that you are sweeping is 0.02 units in size, and that's not too far off from 0.001 units so a 0.001 unit gap on that sweep is a pretty noticeable amount - that's where that bulge and broken seams are coming from. If you draw in a little line for the broken seam, you'll see that the line is something like 0.0004 units in size, so it's within the 0.001 unit tolerance.

You should get a better result if you increase the scale of the object by 10 times so that they're not so close relatively to 0.001 units.

That's what I did in the attached version - if you repeat your sweeps with the attached file you should find it won't cause that kind of bulge.

I have been gradually changing various operations to use an automatic tolerance scaled to the size of the objects involved. Sweep does not do that relative tolerancing yet but I should be able to tune it up at some point in v3 to do that and then it would work better at its current size. But until that point you will want to avoid doing sweeps at that smaller scale.

- Michael