Assembly Assistance for the Printrbot Simple - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 57

Figure 3.19 Starting the extruder assembly.
In additional steps you assemble smaller bits (some using wood glue) that end up
looking like the item you see in Figure 3.20 . This little collection of parts puts ten-
sion on the plastic filament as it is being fed into the hot end, so go slowly, making
sure all the pieces and parts are oriented properly, and attach it carefully!