When you examine the firmware or documentation for some printers, you will see that some use endstops at both ends of the axis, referred to as the minimum endstop and the maximum endstop. Other printers (most RepRap variants) use only minimum endstops and a setting in the firmware for maximum travel. Care should be taken when adjusting endstops or changing the firmware. If the endstop switch does not work or the software defines an invalid end position (too large), the printer may cause the axis to crash into its mechanical components; or worse, cause the motor to continue to move the axis beyond physical limits.
This can cause serious damage. It is best to get your axis movement completely configured before attempting to home or print for the first time. We consider the cube (or cuboid) defined by the axes movement to be the build volume. That is, a printer can create any object that fits inside the build volume. Most printer CAM applications take the maximum build volume into account when slicing objects. For example, MakerWare will not print objects until you scale them to fit inside the build volume. So how does the printer move the axes? This is an area that can vary among printers. Some use smooth rods with bearings to support the mechanical parts of the axis, whereas others use wheels running in tracks. Bearings can be made from plastic, oil-infused bronze, or special ball bearings (called linear bearings). Movement of the axis is accomplished with an electric motor and some form of belt. This is the most common mechanism and can be found in printers ranging from the basic Printrbot Simple to the higher-end professional consumer (dubbed prosumer) MakerBot Replicator series printers. Typically, two of the axes are belt driven (X and Y) and the Z axis is driven (moved) via one or more threaded rods or lead screws—a special type of precision threaded rod. For example, both the RepRap iteration 3 and the MakerBot Replicator 2 use threaded rods to move the Z axis. In the case of the RepRap, two commonly threaded rods are used, and in the case of the MakerBot Replicator 2, a single lead screw is used. Each axis on the Prusa Iteration 3 rides on two smooth rods with bearings (typically linear bearings). It uses a belt-driven X and Y axes and a threaded rod mechanism for the Z axis. Notice in the drawing that the X axis is the part that moves the extruder (located in the center of the photo) left and right. The Y axis moves the build plate (the part with the binder clips attached) forward and backward. Both axes use a belt driven by an electric motor. If you look between the two rods, you will see the belt and idler pulleys. The Z axis uses two electric motors and two threaded rods to move the Z axis up and down. You can see the motors mounted at the bottom of the frame. The small vertical rods are the threaded rods . Whichever mechanism is used to move the axis, the geometry of that mechanism must be known and entered into the firmware. For example, the size of the drive pulley (the one mounted on the electric motor) and the number of teeth per millimeter of the belt are critical to determining how far the firmware must turn the motor to move the axis a fraction of a millimeter. Similarly, the movement of the threaded rods or lead screw must be programmed in the firmware. Lastly, the most vital part of the mechanism is the electric motor, called a stepper motor. A stepper motor is a special type of electric motor. Unlike a typical electric motor that spins a shaft, the stepper is designed to turn in either direction a partial rotation (or step) at a time. Think of them as having electronic gears where each time the motor is told to turn, it steps to the next tooth in the gear.12 Most stepper motors used in 3D printers can “step” 1.8 degrees at a time. Another aspect of stepper motors that makes them vital to 3D printers (and CNC machines) is the ability to hold or fix the rotation. This means that it is possible to have a stepper motor turn for so many steps, and then stop and keep the shaft from turning. Most stepper motors have a rating called holding torque that measures how much torque they can withstand and not turn. Up to five stepper motors are used on a typical 3D printer. One each is used to move the X and Y axes, another is used to drive the extruder (E axis), and one or two are used to move the Z axis.
