Polyholes

When 3D printing with fff, the retraction and the way it's extruded makes holes smaller than wanted. One way to couteract it is to create bigger holes, but it's not the only way: you want play with the geometry. Strait lines are printed precisely and so by reducing the number of "sides" of the circles AND by putting them at the "exterior" (don't cut corners) of the hole, you can have functional holes.

In the picture below, drills fit just right into the polyhole but they can't enter their round hole, it's too narrow as expected.

Idea and mathematics from http://hydraraptor.blogspot.com/2011/02/polyholes.html