Part of offset goes the opposite direction

I created the arch cutout in the plywood sheet by tracing around another object. I then wanted to enlarge the arch with the offset tool. The screenshot sequence shows what happened. Why?

I was able to avoid the problem by removing the right half of the arch and mirroring the left half. Then, the Offset tool worked as expected.

If you try the attached file with SU 2018 Pro, you should encounter the problem.


plywood arch.skp (50.2 KB) **

Not for me it doesn’t. However, I do notice that whilst you have a true arc on the right hand side, it has been exploded on the left leaving just a series of segments. That may have something to do with it. It does make picking the right bits to offset hard.

Yes indeed old coot I see exactly what you do. Sketch up does have a few issues with offset, in your specific setup exploding the curve on the weird side fixes it.
You shouldn’t need to explode it but that’s the way it is just now.
I can give you other examples if you want them.
Just know it’s not your fault.

Offset turns out to have a forest of special cases that make it very difficult to get right! The developers keep refining it, but it seems that some glitches make their way through each time.

Just know it’s not your fault.

…except that I somehow managed to make the construct non-symmetrical (one arc, the other segments). I gotta remember to watch out for that.

That can happen if you use a Mirror plugin - in older versions of SU at least. I think that later versions of TIG Mirror and/or SU keep a mirrored arc as an arc, not just line segments.

Your profile doesn’t say what version of SU you use - can you update it, for future use, please?

Profile updated. Thanks, all, for your guidance.

For anyone interested, the problem did go wrong with 2018. It seems that 2019 had a relevant fix in it.