Because that ball is spherical, as long as it fits into the bushing it will rotate fine. No rounded out part needed. Mine right now is a brass bushing that I ground out with a dremel stone that happened to be the exact correct diameter. The ball fits into it like a glove, and it swivels just fine.

 

i don't have very accurate measuring tools or i would pull mine out and measure them. It was just a though since some people have been asking about them and the vendor hasn't come through.

 

so the base of the id has a sperical bottom in the bushing? if so, since it would be made out of delrin, using a ball nosed end mille and finishing the bottom of the hole shouldnt be a problem on this guys.

i doubt this is a molded part since it would be more profitable to make this out of delrin roundstock on a CNC compared to a pour type fabrication.