Assembly Assistance for the Printrbot Simple - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 58

Figure 3.20 More bits that make up the extruder assembly.
You begin wrapping up your assembly by inserting the small driver gear over the
axle of the extruder motor. Notice it has small grooves on it (teeth) that will grab
onto the filament and help feed it into the hot end.
Insert the hot end, secure it with a couple of M3 bolts, add the last few wooden
pieces used to direct the filament into the hot end, and you're done! When you've
finished your Simple, it should look like the one in Figure 3.21 .