3D Printer Linear Motion Systems

Needless to say, a 3D printer’s linear motion system is one of the most important of the many mechanisms that make up a 3D printer. The linear motion system on a 3D printer is used to move the hot end and/or built platform along the x-axis, y-axis, and z-axis.

A 3D printer’s linear motion systems consist of two main parts:  the motor that turns electrical energy into rotational motion, and some kind of mechanism to turn the rotational motion of the motor into linear motion.

About Stepper Motors

Every single 3D printer on the market today uses stepper motors to drive their linear motion systems. Therefore, it is extremely important to understand how stepper motors work if you are going to build your own 3D printer. The beginning part of the YouTube video below by Thomas Sanladerer does an excellent job of explaining how stepper motors work.

So basically, a stepper motor is a specialized type of motor that allows a controller to tell the motor exactly how much to spin. In other words, the 3D printer electronics can control the position of the stepper motor very precisely. This is important for 3D printing because the ability to set the exact position of the stepper motors translates to setting the exact position of the extruder.

Linear Motion Systems

There are basically two different linear motions systems used in the vast majority of 3D printers on the market today, although there are a few experimental systems that will be discussed later on. Most 3D printers use combinations of lead screws and timing belts for linear motion.

Lead Screws

Lead screw systems are a simple means of translating the rotational motion of the stepper motors to linear motion of the 3D printer build platform and/or extruder. A lead screw linear motion system consists of some kind of threaded rod, which is rotated by the stepper motor, and a mating nut that moves up and down the threaded rod as it rotates.

You can see the lead screw in the middle with a nut that provides motion.

Almost all 3D printers use at least one lead screw in their linear motion system to control motion along the z-axis. Typically, a lead screw will be used to lift the build platform or the extruder up one layer at a time during the build process.

There are several different types of lead screws, each with their own advantages and disadvantages. The three most common types of lead screws are ordinary threaded rods, trapezoidal (ACME) lead screws, and ball screws. Each of these lead screw times has its own set of advantages, but there are some advantages that all three types of lead screws share. First, lead screws are capable of delivering a large amount of force. The mechanical advantage provided by screws is the entire reason screws exist at all. Second, screws are said to be self-locking. This means that if the 3D printer loses power, the screw will stay right were it is, without moving. This is part of the reason lead screws are typically used for vertical movement of the build platform. It does not matter if the z-axis motor loses power; the build platform will stay at its current position.

All lead screws also share some disadvantages. First, they all require periodic lubrication to reduce ware and increase efficiency. Second, all lead screws wear more quickly than other linear motion systems. Third, because of the metal-on-metal contact between the lead screw and nut, lead screws are noisy. Last, lead screws are prone to backlash. This is the reason lead screws are not usually used for the x-axis or y-axis. These axes switch directions very frequently and because of the backlash most lead screws suffer from, every time the screw changes direction, the positional accuracy decreases. This can get to be a big problem for a long print.

As for the unique benefits and drawbacks of each type of lead screw, ordinary threaded rods are the lowest performance, but also the cheapest, type of lead screw. Technically, threaded rods are not lead screws at all because they are not designed for use in linear motion systems. Their intended purpose is to attach things. Threaded rods have the benefit of being very inexpensive, and readily available. However, again because threaded rods are not designed for linear motion, they are not as accurate as the other lead screw types, they have a relatively large amount of backlash, they wear quickly, they require lubrication, and they are very inefficient. This last point about efficiency is actually very important. Threaded rods only have an efficiency around 20% to 30%. This means a bunch of power is wasted on fighting friction, it means the threads will were quickly, and it means moving the lead screw is taxing on the motor.

ACME lead screws are a kind of middle ground between threaded rods and ball screws. Unlike ordinary threaded rods, ACME lead screws are designed for linear motion. ACME lead screws are much more accurate than threaded rods, and they have smaller backlash. The trapezoidal thread profile also makes ACME lead screws more resistant to wear. However, ACME lead screws, like threaded rods, are still very inefficient and they require lubrication. Last, ACME lead screws are a lot more expensive than threaded rods, and the ACME nuts are more expensive than regular nuts as well.

Last we have ball screws. Ball screws have the best performance among the three lead screw types, but they are also by far the most expensive. Ball screws use a specialized type of nut that is kind of like a ball bearing. Ball screws are again much more accurate than threaded rods, and have a smaller backlash. Also though, ball screws are much more efficient than either threaded rods or ACME lead screws, about 70% efficient. For this reason they consume less power, are less demanding of the motors, and wear less. Ball screws, however, require regular lubrication to function properly and are, again, very expensive.

Timing Belts

Belt drives are used on the x-axis and y-axis on the vast majority of 3D printers. A belt drive consists of a timing belt with teeth, a toothed pulley which is attached to the motor, and a carriage attached to the belt. When the motor turns, it turns the pulley. The teeth on the pulley interface with the teeth on the timing belt so that when the motor rotates the pulley, the timing belt is pulled in the direction it needs to go. A carriage is typically attached to the belt such that it moves back and forth with the belt.

Belt drives have several advantages over lead screws for use on the x- and y-axes:

• Belt drives are typically less expensive than specialized types of lead screws like ACME screws or ball screws.
• Belt drives are better suited for long travel lengths since the timing belt can easily be made as long as necessary to achieve the desired travel distance for the axis.
• Lower maintenance than lead screws; no lubrication required.
• Capable of much higher speeds than lead screws.
• Low backlash

This last point is of particular importance. Assuming that the belt drive system has been set up and tuned correctly, a subject we will discuss in a moment, belt drive systems have very low backlash. This makes them good for use on the x-axis and y-axes, which change direction frequently. The low backlash means the linear motion will not lose positional accuracy over time. The second to last point is also important. Belt drives can move much faster than lead screw systems, meaning prints get done in less time.

Here you can see the belt drive used on the Ultimaker 2.

Belt drives for linear motion of course have some downsides. Most importantly, a belt drive system achieves its low backlash and high accuracy only when the belt is in tension. In other words, if there is slack in the timing belt, it will ruin the accuracy of the entire system. For this reason, 3D printers using belt drive systems must incorporate some mechanism for keeping the belts in the proper tension at all times. The belts must be tight enough to avoid backlash and any kind of oscillation. On the other hand, if the belts are too tight, the motors will not be strong enough to move them and the system could miss movement commands. Therefore, one of the drawbacks to using a belt drive system is that it requires more work to tune correctly.

Compounding this issue is the fact the timing belts have a tendency to stretch a bit over time. Therefore, a 3D printer operator will need to periodically re-tension the timing belts to keep the system working well.