Mounting Electronics - Printing in Plastic: Build Your Own 3D Printer - page 291

Now carefully push the motherboard onto the Arduino Mega. If you didn't use a fourth screw in the
lower right-corner of the Mega, place a spacer underneath it to provide support as you insert the
motherboard; you can remove the spacer when done. Figure 16-6 shows the motherboard inserted over
the Arduino Mega.
Figure 16-6. Motherboard added to the Mega