By convention, the center of the build platform is assumed to be the point (0,0,0) in XYZ space. The X, Y, and Z home offsets tell the printer the location of the X, Y, and Z endstops in relation to the build platform’s center.1 For instance, the typical Z home offset value is 0.0 mm. This means that when the printer raises the platform until is activates the Z endstop — that is, when it “homes” the Z axis — it raises the build plate up to the extruder and then lowers it by the home offset value.
Similarly, the X and Y home offsets tell the printer how far an extruder that has hit the endstop is from the center of the build platform, giving your printer a general “sense” of where it is so that it is able to accurately find the center of the build platform. This relationship is depicted in Figure 4.1. This is important as the printer does not have sensors that tell it where the extruder or build platform are located. When it begins a print, the printer “homes” the axes — it moves the extruder along the X and Y axes until it hits the endstops and then it raises the platform along the Z axis until reaching that endstop. Since the resulting position is well-defined, your printer tracks the movement of the plate and extruder so that it always “knows” where they are. As this information is lost whenever you power cycle the printer, homing is a necessity at the start of every print.
The home offsets may be set from the printer’s front panel as explained in Section 3.7.10. They may also be set from ReplicatorG, MakerWare, and MakerBot Desktop using the “Machine” and “MakerBots” menus and accessing the onboard parameters.
As most slicers center a print, if your prints do not appear well-centered then your home offsets are likely incorrect because they effectively define (0,0,0) for your printer. To change the X offset, it is important to note that the X endstop is to the right of the center. So, if prints are centered too far to the left, you should reduce the X home offset value. Likewise, if the print is centered too far to the right, then increase the value. If the prints are centered too far forward then decrease the value of the Y home offset; if the center needs to be moved forward, then increase the value. Consult Table 4.2 for a list of the default home offsets for many printers. Note that Cupcakes and Thing-o-Matics do not have default values for their home offsets. Owners of those two printers set their home offsets by running a calibration script such as that supplied with ReplicatorG.
Note If you have a dual extruder setup and printing with the left extruder seems off, then the problem is likely in the toolhead offsets, which are described in Section 4.2.
More care is needed in changing the Z home offset; if made too large, the nozzle and build plate can sustain damage. Since the Z home offset describes the position of the nozzle in relation to the build plate after homing, increasing the offset tells the printer that the plate is farther from the nozzle than it might actually be. For example, if you change the home offset value to 1.00 mm and the print begins at 0.15 mm (which is typical for models sliced at 0.30 mm layer height) then the printer will move the build plate up 0.85 mm. Which, if the build plate was actually closer to the nozzle, can drive the plate into the nozzle. For this reason, understanding, forethought, and planning are much more important in changing the Z offset value than the X and Y values. A deeper discussion of the Z home offset may be found in Section 9.1.
Note that you should be cautious and thoughtful in changing the Z home offset. Nevertheless, here are two situations in which you may wish to change the Z home offset: if you have adhesion problems with the first layer of your print, then you may set the Z home offset to a value such as 0.05 mm, making the first layer of the print start 0.05 mm closer to the build plate than it would have otherwise; or, if the extruder is clicking a lot on the first layer since the nozzle is too close to the platform and the filament cannot exit it, then you might decrease the Z home offset to a value such as -0.03 mm.