I'm not seeing the issue. Can you list the steps taken from the start?
I did this:

1. add curve, move from origin, add screw modifier.
2. render
3. Convert to mesh from curve,
4. render

I don't see any difference.

In T51451#432952, @Joel Godin (FloridaJo) wrote:

I'm not seeing the issue. Can you list the steps taken from the start?
I did this:

1. add curve, move from origin, add screw modifier.
2. render
3. Convert to mesh from curve,
4. render

I don't see any difference.

using the .blend file attached:

• select render in new window from render tab
• hit render as it is and saving in slot 1 (the object is a curve+screw mod+subsurf+mirror)
• change to slot 2
• convert the object to mesh
• hit render again and see the difference

Couple of things I noticed. (may or may not mean anything)

1. If I create a similar object from curve from scratch, it does not do as your file does.
2. On your file if I apply 'location' it changes the object shape.
3. If I go to the curve object data panel and change from 3D to 2D and back to 3D the viewport render matches the final render.

I think there may be a clue in 3) but I'll defer to someone more knowledgeable on this.

In T51451#432978, @Joel Godin (FloridaJo) wrote:

Couple of things I noticed. (may or may not mean anything)

1. If I create a similar object from curve from scratch, it does not do as your file does.

• the same for me

2. On your file if I apply 'location' it changes the object shape.

• the same for me also here on my file, it seems it change the origin to center of world space and I don't know why

3. If I go to the curve object data panel and change from 3D to 2D and back to 3D the viewport render matches the final render.

I think there may be a clue in 3) but I'll defer to someone more knowledgeable on this.

• do you mean on my file or on a new? Because on my file it completely change the curve shape and I can't go back to original if I do as you wrote, I've to undo the operation.

I think there's something wrong with the management of curve origin position, and maybe with the curve+modifiers,
but are only my suppositions

On 3) I mean I hit the 2D tab and it changes to the rendered shape (the smaller circle) and when I go back to the 3D tab, it does not change back, it stays the smaller (more correct) shape.

There is no bug, here.

You just did not set same number of steps for screw modifier for Viewport and for Render.
There are only 6 steps for Render against 72 for Viewport.

So, subdivision surface modifier will affect the resulting geometry differently, producing a more important shrinking on an hexagonal shape than on a circular basis made of 72 sides.
You just have to precise same number of render steps to obtain same result.