[Cura fills hollow cylinder where shaft is supposed to go]

Oh awesome. That did the trick perfectly! Thanks so much!!

[Cura fills hollow cylinder where shaft is supposed to go]

I could use some help slicing. The circled area is supposed to be a hollow cylinder with one square side so that it can slide on to a keyed shaft. However, Cura is filling it in as can be seen by the arrow. If I turn support off it doesn't fill it, however, I need support on because the model gets much wider at the top. Any help would be much appreciated. I have attached the stl file as well. 

ender_3_extruder_knob__groot.stl