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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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Ha ha ! That's ok :-)
Some filament spool center hole are not full, like perfect cylinder, they have ridges on sides and the center part is actually bigger, like the SaintSmart spool. So the cylinder is for those kind of spool because the 3 bearings will not be perfectly aline with the edges of the spool.
Not sure if I'm clear!!!!!! I'm french, you'll have to excuse me for the explanation if it's not clear.