The Z home offset plays a critical role in the success of your print. Namely, it influences a print’s first layer whose proper adhesion to the build plate is critical. When the first layer is too far from the build plate, the print will not properly adhere and may detach mid-print, destroying the build. And, if the first layer is too close to the build plate, the extruder may jam.
How does the Z home offset affect this? If you are not using software-based auto-leveling, then it does so by defining the initial position of the build plate.1 Typically, the Z home offset has the value 0 and thus states that after homing, the build plate is at position Z=0. More generally, after homing the build plate is at the position
where Hz is the value of the Z home offset.
When you slice a model, your slicer generates instructions which place the first layer at an initial height, Zi. This height will not be zero. Rather it is some height larger than zero; often it is one-half the layer height, but that varies between slicers and is often configured in your slicing “profile”.
To complete our mental picture of the first layer height, we must finally take into account the small gap we created between the tip of the nozzle and the top of the build plate when we leveled (“trammed”) the build plate. Often we use a sheet of paper, whose thickness is about 0.1 mm. This small gap must also be taken into account. Let us use the symbol g to represent the size of this gap. See Figure 9.1 for a diagram of these values.
From Figure 9.1 we see that after homing, the build plate is at position Z = Hz while the nozzle is actually at Z = Hz - g. Note that Z values decrease as you move upwards in that diagram. Increasing print heights are had by lowering the build platform and thus moving downward. The distance between the build plate and nozzle in Figure 9.1 is the difference between their Z values,
which simplifies, as we expect, to the value g, the gap.
With these values defined, we can now determine the critical vertical distance, D, between the extruder nozzle and build plate for the first layer of a print. It is
That is, for the first layer the build plate is moved from its homed position Hz to the initial printing height, Zi. The vertical distance between those two positions is Zi - Hz. Add to that the leveling gap, g, and you have equation 9.1. Alternatively, the value D may be derived by using the difference between the initial printing position Zi and the extruder position (which remains fixed),
which again yields Equation 9.1.
For example, when Hz = 0 mm, g = 0.1 mm, and Zi = 0.15 mm, the initial distance between the extruder and build plate is 0.25 mm.
Now for the fun. Suppose you are having a problem with a print not adhering well and you know it needs to be started ever so slightly closer to the build plate. Your choices are to change the slicing profile and reslice (Zi), relevel the build plate with a smaller gap (g), or to adjust the Z home offset (Hz). Adjusting the Z home offset is quick and easy and can be done from the printer’s front display (Section 3.7.10). To decrease the initial distance, you wish to decrease D. To do that, increase Hz. For example, change it from 0 mm to 0.03 mm. That causes D to change from 0.25 mm to 0.22 mm and thus the initial distance was decreased by 0.03 mm.
Similarly, if you want to increase the initial distance, you can decrease Hz. Yes, it is okay to use a negative value for Hz.
In changing Hz, be careful to not change it too much. Be careful to not accidentally enter a large value such as 0.5 mm. You do not want to generate a value of D that is so small that you damage your build plate, extruder, or both.
Finally, note that all slicers assume there is a slight leveling gap, g. ReplicatorG and MakerWare assume a gap of approximately 0.1 mm. Other slicers may assume a different leveling gap. It is a good idea to learn what gap your slicer assumes. If you consistently have issues, it may be that either you need to create a different leveling gap, or you should change your slicing profile to use the gap you have created.