Thanks, yes google is very useful here-
“I understand what you said about the smooth transition. In fact, this is usually a problem that requires a processing process to solve, such as polishing. Actually, in the model, the presence of intersecting lines does not mean that this surface is not a smooth transition. For example, if you pass “Tangent command”, although it must be smooth because of tangent, but it will also have intersecting lines. I’m sorry I can’t communicate in your language, but Google Translate may also be able to convey my basic message. If you do not understand, please tell me, I am willing to show you through the picture.”
Thank you, I would be grateful for any picture you are willing to share, I have tried everything everyone suggested here, including drawing splines, but that doesn’t work, because there are a potentially infinite number of splines required to intersect the existing ones, at what would have to be all points along their length, in order to create a fully smoothened ( is that a word?!) surface. At least I can see by now that there isn’t some absolutely basic feature that I have missed.