Testing and Printing - Printing in Plastic: Build Your Own 3D Printer - page 384

You will notice that the X and Y show minimum and the Z shows maximum. This is because the X
and Y axes will home at the start of their axes (near the front left corner of the table) and the Z will home
at the top and not the surface of the table top. As an axis homes (moves toward its limit switch), keep the
mouse over the Stop button—just in case you see that part of your machine may be getting close to the
limit of its movement. Each mechanism (Table, Extruder, ZY Plate) will move quickly towards its
respective endstop and then stop immediately when the moving mechanism triggers a limit switch.
When you home the Z-axis, you will also need to determine the Z-axis height (the distance from its
maximum raised position when the endstop is pressed to the flat surface of the tabletop). You will need
to run a Gcode set of instructions to determine this height. On ReplicatorG's menu bar, click File 
Scripts  calibration  Thing-O-Matic calibration.gcode, as shown in Figure 20-15.
Figure 20-15. Run a special bit of code to determine the Z-axis height.
When the script opens, press the Build button shown in Figure 20-16 on the toolbar.
Figure 20-16. The Build button will prepare the script for calculating the Z-axis height.