This part connects directly to the metal gears of two servos. You will need to heat up the printed part.It is derived from tmbg's 3DOF Mini Quadruped femur.stl. It can be used as femur for walking robots.Print the part with at least 4 perimeters. Change the "stab" variable to about 3x your printers nozzle size to adjust the wall size. The "cold" printed part should not fit on the servo.To heat up the surface of the printed part I'm using an alcohol torch from Buffalo. You can squeeze that alcohol filled plastic bottle to have a tiny flame tip. Heat up the inner surface where the connecting gear will be. Then gently press down the heated part on your metal geared servo. Then wait till it cools down. Finally fix the part with a screw. Disclaimer: It seems to work with nylon geared servos, too. But I have not tested this method entirely! The nylon gear may not fit to the regular horn after this procedure. Maybe it helps to cool down the nylon gear before applying the heated part to it. If the nylon melts, your servo may get damaged. Take fire safety precautions, because plastic is flammable. And let an adult watch over that torch tool.