Sits on z-axis rod and guides filament so it doesn't flop around near the print area.After printing, i had to enlarge the hole that the goes into a little bit using a drill bit. This might have been caused by a z-axis problem I encountered at the start of the print where the axis would not raise enough after each layer. The print started to squish. I put some lubrication on the z-axis rod and manually adjusted the z-axis, and it printed straight for the top half. Didn't reprint the object. Make a comment if the hole needs to be expanded. In the model, it should be 9mm in diameter. In reality, the diameter should be about 8.3mm, so 9mm allows for some shrinkage.