Choosing a Drive Train: Leadscrews and Ball-screws
Linear motion is made when electrical power is put through a coupled device into a drive mechanism, called a drive train. Choosing the right drive train is about balancing cost with performance, not just buying the newest available technology. There are several types of drive trains you can choose from, each with their own pros and cons. Today we will be looking at two types of drive trains, leadscrews and ball-screws.
Leadscrews are the most common linear drive for at-home CNC routers. Cheaper CNC routers use a hardware store threaded rod that is referred to as a leadscrew, but they are not true leadscrews. Real leadscrews are in an imperial measurement, called Acme, or a metric measurement called Trapezoidal. The two types are very similar aside from their differing measurements. The real difference is the angle between the threads, with Acme at 29 degrees and trapezoidal at 30 degrees. A leadscrew is basically a high-quality threaded rod. They come in many different sizes and pitches so there are a few things you need to know to understand them. Make sure to know the major and minor diameters of the leadscrew, whether you have a right handed or left handed thread, what the pitch and lead of the leadscrew is and how many starts the leadscrew has. Leadscrews can be very difficult to understand at first, especially when you are just getting started. But once you have your head wrapped around the concept it will help you to understand ball screws and how a CNC router is able to move. Leadscrews provide strength more than anything else, which is important. In a CNC router you need strength because your machine will encounter significant resistance when going through wood or metal at the speeds the machine operates at. In terms of speed leadscrews are not the best. While other options, such as a rack and pinion or a belt drive, are faster they usually do not have as much strength as a sturdy leadscrew. Leadscrews are best used on a small or medium-sized CNC router since they produce good speed, strength and power transmission when cutting wood and light metals. However, keep in mind that leadscrews wear down quickly and that their backlash will increase over time.
A ball-screw is a linear drive using a hardened steel rod that is either precision ground or cut with circular leads or grooves. A matching ball-nut is matched with the same lead/groove as the screw and includes tubes that recirculate. As the ball-nut or ball-screw spins the ball-bearings run down the matching leads and screw. They then go back to the top through the recirculating tubes and repeat the process. The ball-bearing is sealed to keep debris out and it is also easy to lubricate. Ball-screws are fairly easy to take apart and clean as well. As the leads wear down larger ball-bearings can be used to cut down on backlash. Alternatively, two ball-nuts can be put opposite of each other and spring loaded to reduce the backlash. Many ball-screws are cut with two sets of leads, that way you can switch to the second set if the first set wears down the ball-nut. Many CNC routers use a single, center aligned ball-screw to drive the moving gantry. The bridge framework is shaped like a stretched “O”, connected to profile rails on either side of your machine which wrap below the table. The ball-nut and ball-screw connect to the middle of the framework under the table. The whole design is simple, using a single drive, with few if any tuning issues and having the screw below the table makes debris buildup almost non-existent. They do need lubrication from time to time and tend to be more expensive than some other options but this is generally outweighed by their having high density, efficiency and accuracy along with low wear. Ball-screws are a very problem-free piece of equipment that you can use.