I added a boss to one side of this part.
I want to mirror that boss on the opposite side.
The Partial preview shows what is expected.
But the actual mirror is wrong.
I am assuming it is due to the way the feature was created.
Up To Surface is tough with mirrors because it tries to actually mirror how the feature was built, meaning it tries to terminate the feature on your same surface. That's why (not using geometry pattern) it sorta goes in the wrong direction. It starts from the right point, but it still terminates on the same face because it's trying to really mirror the "recipe" for the mirror, but can't actually pick the opposite face.
Partial preview shows what you want, but only because it's doing a quick-and-dirty visual representation "guess" of throwing temporary surfaces up there that geometrically mirror the faces belonging to the feature.
Typically in this situation, you'd use "geometry pattern". This would work if your feature didnt' terminate in an existing face, which ends up creating a "modified" face. If (just for this purpose) you change your extrude to offset from surface (either by going past the existing surface or stopping short of it), you'll be able to use the geometry pattern. Un-checking geometry pattern still won't work though.
So yeah, unfortunately you'll have to figure out a way to modify your modeling method here. I assume you have no difficulty with getting the thing modeled, you're just trying to understand why this feature doesn't work...
I am using 2019 so I had to attack it a little different since you are using 2020
Correct on both counts. I've already got it sorted a different way, I was just curious if I was doing something wrong.
Your explanation makes perfect sense. A circular pattern behaves the same way, both with the preview and the final part.
At first I thought it may be the extra material from the offset from the sketch plane.
So I change it to sketch plane, which is on the Right plane, and changed it to up to vertex:
You could also do a midplane with the value linked to the overall width and eliminate the mirror.
Thanks for the attempts at modelling. I've got that part covered, and it seems that some of you are missing the gap in the middle. Maha's picture shows it correctly.
I think OP expects reason not the final result.
You don't need to change anything. In your cases (symmetrical part) when creating a mirror, use "Body" as an object to be mirrored.
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