Having a little trouble with a baluster model. It’s easy enough to put a square end on a turned baluster, but I would like there be a transition between the two. The picture shows a 70 degree incut which makes a convex surface at the top of the square portion leading into the turned portion. I can post more pics if you need.

Any thoughts on how to fill in the missing surface? Thanks!

Make the turned part with the full diameter, then intersect a square prism having the correct across-flats dimension with the spindle (you can either construct the prism in place or off to the side and then move it into place). Then select the faces of the prism and right-click > Intersect Faces with Model. Finally, remove excess geometry.

You can also accomplish this with the Solid Tools Intersect command, creating a figure that is the intersection of a turned spindle and a square prism (both of which must be solids).

You can also try using the Bubble Skin Extension.
Make the outline for 1/8 of the area, and then copy-scale for opposite side, and then rotate-copy around.
But @Gully_Foyle’s solution is the more efficient and simpler way for this case.

How can we go on without a gif.
I was just thinking if you are drawing the profile anyway, just use the negative to cut the post with the solid tools subtraction.

The sandbox technique begs a question: how did you obtain the arched lines at the sides of the square part? Maybe using the follow-me and intersect method? They aren’t circular arcs (hyperbolas!) so I don’t think you can draw them with native tools.

I’m not sure if you are asking me or not but here’s an attempt at clarification.
In my earlier image, both curves were done using the follow me tool. I did one more using an arc similar to the parabolas. The results are nearly identical.

i think the followme version is the one giving the closer to accurate results… sandbox-> from contours is just basically filling in a hole with no concern of roundness… with follow_me, you can at least feed it the correct profile.

Yes, indeed. We’re dealing with the surface of a cone, nothing very exotic. It is not a compound curve and therefore not really convex. Convex is like the surface of a sphere, curved in two directions (a compound curve).

I asked rhetorically (and didactically) for the benefit of anyone reading this topic

The technically correct line where the turned cone meets the square post is a hyperbola. When the curve is very shallow, as Shep drew (a very broad cone), a circular arc can approximate it fairly well. But if the cone is more steeply tapered, at what point is the circular arc no longer “close enough”? You really can’t tell unless you generate the hyperbola. There is no way to do so using built in tools, but the follow-me plus intersect method does it as quickly and accurately as SketchUp is capable of.

Of course, as Shep illustrates, once you have the technically correct curve you can use sandbox to fill in between it and the center circle if you prefer triangulated surfaces vs quadrilaterals.

I was more concerned about the corners - where Sandbox Tools didn’t generate edges that went all to the edge - so you ended up with triangles oriented “against the flow” of the rest of the triangles.

It might cause some shading deviance - there Follow Me version should ensure a smoother result. Might be nore noticeable if rendered with a shiny material.