Hello,

What distance and shape parameters are you trying to use for each part? I'm sure Michael has an explanation for it, however you can use those fillet parts to make a watertight solid. Make a Copy of your original solid, Hide the one copy of the solid, run the Fillet operation on the copy of the original solid. You should be left with fillet parts. Separate the fillet set and clean it up so it is symmetrical. Now Join the fillet set into a joined surface. Now you can select the inner edges of the fillet set and Join them, these curves will act as your trimming curves for the unfilleted solid. Unhide your original unfilleted solid and Separate it. Use the trimming curves you created to then Trim the surfaces, leaving the inner trimmed surface is removing the outer. Now unhide your fillet joined surface, and Join it to the inner trimmed surfaces. You should be left with a watertight Solid. Hope this helps.