MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.

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Running a knife in the groove won't free the sphere because the blade is at the wrong angle. You need to use a small flat bladed screwdriver and pry the outer edge of the sphere inward towards the center. Then you should hear it crack loose. I've uploaded a cross section photo so you can see what's going on better.