Many of the machines we use in a makerspace require controlled linear motion and positioning. Examples include 2D and 3D printers, laser and vinyl cutters, CNC and manual milling machines and lathes, etc. In these machines either the tool or the work piece must be moved in a controlled manner along one or more linear axes. There are two main aspects of linear motion: the mechanism that causes the change in position, and the mechanism guides the movement along a straight line. Here’s a little background on some of the choices available and factors to consider when choosing a linear motion mechanism for your project.
There are a number of mechanisms that can be used to translate the rotational motion offered by common electric motors to the linear motion needed by our machines. Stepper motors (and some servo motors) are relatively easy to work with, and they can be controlled for position and not just for speed or torque. Below are the most common types of mechanisms used to translate this rotational motion into linear motion. In addition to these there are also some purely linear mechanisms like hydraulic and pneumatic cylinders and linear electric motors that are less commonly used in machines like those listed above.
Rack and Pinion: Rack and pinion is a good default choice for linear motion, especially for larger machines with axes over a few feet long. It handles high speeds well, does not lose precision or top speed with length, and can handle considerable forces. The main downside of rack and pinion is backlash, which can be reduced with added friction and expense. Racks and pinions are available in metal, nylon, and other materials. For any given length and capacity, the cost of a rack and pinion is usually more than a belt drive but less than a ball screw, both of which are considered below.
Belt Drive: Normally we think of belts being using to drive one rotating pulley with another, but they can also be used to create linear motion. The smooth v-belts and serpentine belts we think of that drive the fans, AC compressors, and power-steering pumps in our cars always have some slippage, and are not good choices for precise motion or positioning. However, timing belts engage their pulleys with teeth that prevent slippage, similar to bicycle chains and sprockets, enabling precise control.
Linear belt drives are commonly used for to move the print heads in inkjet printers, and they are particularly good choices for other low-force, light-weight machines that require fast motion, like 3D printers, laser cutters, mat and vinyl cutters, small CNC engravers, and plotters. Although there’s no hard limit, as axis length increases belt drive probably becomes less optimal while rack and pinion becomes more optimal. This is because as the axes get longer it takes more belt tension to maintain precision, and because larger means heavier, so the machine itself gains inertia even without high cutting forces.
Ball Screws: These offer the highest precision, and are an especially attractive choice for heavier, higher-force machines like CNC routers and milling machines with axes not more than around 4 feet long. The major advantages of ball screws are their precision and zero backlash. They also handle high forces with little friction, and can often be directly driven without the need to gear them down. The downsides of ball screws are their expense and their speed limit.
Ball screws have a “critical speed” which is an upper rpm limit before the screws start to vibrate excessively. The critical speed is related to resonant frequencies, so it is lower for longer ball screws and higher for thicker ones. Some machines have massive ball screws not for strength but because they need higher resonant frequencies to handle higher RPMs.
One consideration when selecting a ball screw is the “lead,” which is the linear distance traveled with one rotation. Higher-lead ball screws reduce the RPMs for a given linear speed, which is good for avoiding the screw’s critical speed and for using stepper motors which provide higher torque at lower RPMs. Lower-lead ball screws are more precise and are effectively geared down for lower-torque motors.
Leadscrews: It’s worth noting that ACME leadscrews with bronze or nylon nuts (or even regular hardware-store threaded rods and nuts) may be used in place of ball screws on slow-movement machines. ACME screws continue to be the main choice for manual milling machines because they can’t be back-driven (they are self-braking). Other than this, the main advantage of leadscrews over ball screws is their lower cost. Their downsides are speed, friction, and backlash. Although backlash can be minimized in various ways that all add friction, it makes them less attractive than ball screws for high-precision computer control. Because of the inherent friction there’s probably also a lower maximum practical lead at any given screw diameter when compared to ball screws, and a lower RPM limit before heat becomes an issue. For some machines traditional leadscrews may be a good choice, but cheap imported ball screws or high-quality used ones may not be much more expensive.
While the linear actuators above control motion and positioning along one linear dimension, we usually need guides for each linear actuator to restrict motion in the other two linear dimensions and the three rotational dimensions. There are a bunch of home-brew designs for linear guideways, some using skateboard bearings rolling along angle-iron. Many people have had success using relatively inexpensive round-rail linear bearings available on eBay from China, and they are probably better than most home-brew rails but are more expensive. A step up from these is to use linear motion guides like those from Thomson, NSK, THK, and Hiwin, and they can sometimes be found used on eBay at similar prices. They come in a variety of sizes, including miniature ones for very small machines. Some are designed to only be loaded from only one direction while others can be loaded from any direction. For CNC machines look for linear guides with two carriages per rail for more rigidity. You may find a deal on a long one with four carriages and cut it in half with an abrasive cut-off wheel (the rails are hardened steel).
Depending on the design of the machine, it may be attractive to use a prebuilt combo unit that contains a ball screw and linear guides in one assembly. This is what I did for my CNC router. NSK calls them Monocarriers, and THK calls them LM Guide Actuators. I used a THK KR33 for my Z-axis, and NSK MCM08 units for X and Y. They were less expensive used on eBay than buying similar quality ball screws and linear guides separately, and they were easier to build with. These combination units are available with one or two carriages, and like the linear motion guides above, having two carriages greatly increases rigidity.