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) **