Reduce free play at the head rod ends
It gives a smoother movement and less noise when the directions changes forth and back.
I originally designed this to reduced a bit the useless friction that the standard plywood losanges were applying to the bearing and/or the X/Y rods.
This may not be significant. Also, a hole in the object would allow the bearing to be oiled also. To do so, you can also use the batman derived object on thingiverse(!), or as below:
Update: switched to these better end caps
|Better, with a Nylon hub and free play adjustment (jwag55's excellent upgrade)|
It is also a very good illustration why open licences such as Creative Commons are good, since this is a design by jwags55 inspired by mine (above). So I benefit from a free smart upgrade because I did not ask for money not prevented anyone from improving it.