The Three Axes and Layer-by-Layer Printing

X, Y and Z

Every printer works in three directions at right angles to each other:

Axis	Direction	What it does in a print
X	Left and right	Draws the layer, together with Y.
Y	Front and back	Draws the layer, together with X.
Z	Up and down	Steps up one layer height after each layer is finished.

Formlabs describes an FDM printer as moving the nozzle in the X and Y axes, and in Z between layers. Which part physically moves differs by machine. On Prusa's MK4S, for example, the X axis carries the print head left and right on two smooth rods driven by a motor and a belt, and the Z axis moves it up and down on threaded and smooth rods. The printer types are compared in Cartesian, CoreXY and Delta Printers; the result on the part is the same.

How the printer is told where to go

The slicer turns the model into G-code, a text file of moves. In Marlin firmware, a G1 line is a straight move to a target position, written with X, Y and Z coordinates in millimeters by default. FDM printers add a fourth axis, E, which Marlin's documentation describes as the position of the filament fed into the extruder. So a line such as G1 X50 Y20 E2.5 means: move to X 50, Y 20 while pushing filament forward. An F value sets the speed. You rarely need to read G-code, but it explains why a printer is really just a machine that follows coordinates. Firmware is covered in Marlin, Klipper and Proprietary Firmware.

Layer by layer

1. The slicer cuts the model into horizontal slices, each one layer height tall. See Choosing a Layer Height.
2. For each slice it plans paths in X and Y: outer walls, inner walls, then infill.
3. The printer draws the first slice on the bed.
4. Z moves by one layer height, and the printer draws the next slice on top of the last.
5. This repeats until the part is done.

The key consequence: every layer has to rest on the one below it. Prusa puts it plainly: printers cannot print in mid-air. That is why overhangs and bridges need care; see Supports and Overhangs. How finely a printer can place each layer and line is a separate question, covered in Resolution, Accuracy and Precision.

Why FDM parts are weaker across the layers

Inside a layer, each line is a continuous strand of plastic. Between layers, the new line is laid on top of the cooler line below and has to fuse with it. Formlabs states that in FDM, the layers are not bonded together as strongly as the extruded lines themselves, and describes FDM parts as anisotropic: their strength depends on direction. Prusa's design guide makes the same point with a comparison to wood, stronger in one direction than another, and says structural parts should be printed in the orientation that best serves their strength.

In practice that means:

• A part pulled or bent so that the layers peel apart fails first, along a layer line.
• A part loaded along its layers, so the force runs through the continuous lines, is stronger.
• A tall, thin peg printed standing up snaps more easily at a layer than the same peg printed lying down.

The fix is usually orientation: decide which way the load runs, then turn the part so the layers lie along it. See Print Orientation. How much weaker the layer direction is depends on material, temperature, cooling and the part itself; it is a Measure it value for your setup, not a fixed ratio.

Resin is different

Resin layers bond chemically to each other as they cure. Formlabs describes SLA parts as highly isotropic, with no difference between X, Y and Z at the molecular level. Orientation still matters on resin, but for surface quality, peel forces and supports rather than layer strength. See Resin Orientation and Supports.