An overview of the two most common drive systems in EDM machines today and what differences they bring to the EDM process.
Ballscrews have been with us for many years and have proved to be accurate, reliable, easy to maintain and easy to fit many different applications without major changes in design. A ballscrew gives some torque advantages that we need in motion when moving loads of various weight and speed. Where is this used in EDM? Heavy load. This could be large parts, or a large work tank filled with water. As we see machines change to accommodate larger work, maintaining control of the motion becomes an issue. With the linear motor, we have a different concern: with limited torque, how do we move the mass with accuracy?
If we look at the physical differences in the two systems, we see that they both have the same basic components. The linear motor has a coil, a magnet, a motion control and a drive system with feedback for location. The ballscrew has a coil, a magnet, a motion control and a drive system which may or may not have a feedback for location. So which is better? To answer that question, we need to ask, “Better for what?”
For accuracy, both work well with a slight edge to the linear motor for large radii (6″ and over) when .0001″ is needed.
For longevity, again, both work well with a slight edge to the linear after 10 years of use. If you have a ballscrew that is core-cooled and scale feedback, then you can expect it to last for decades of use.
For speed, hands down this goes to the liner system, but in EDM where is speed needed? Two decades ago, we used this speed to aid flush in the sinker EDM, and on the wire, we saw reduced wire breakage due to the responsiveness of the abnormal discharge spark. Now, in both cases, we have power supplies that respond to flushing, and spark conditions where we no longer need the speed to counter the situation. In my section on sinker EDM, I will explain in more detail how flushing has changed the need for speed.
For torque, linear motors need massive amounts of electricity to generate torque. This then requires cooling and large linear motors and transformers. With ballscrews, as it has been for decades, torque is developed at the screw by pitch and diameter, and requires little power consumption comparatively. But again, where does EDM need torque? In the case of large tanks and large parts, the ballscrew is a go-to design that works.
For maintenance, if we look back at both systems, we see major improvements and benefits that have made maintenance less of a concern and keep peak performance consistent. Some ballscrews have core cooling and auto-lube that keep wear to undetectable for the life of the machine, and linear motors have so few moving parts that there is nothing to wear at all.
For cost, linear motors have the advantage here because the magnet plate and the coil plate are flat and do not require bearings or shafts. Ballscrews do cost more, but over the decades, we have learned to manufacture a precision performance ball screw that can give both speed and torque.
For cost of operation, in EDM, the cost of electricity that is used by both systems is so low that I cannot find any data to support an advantage of either system.
For production, we must be concerned about the movement being on the same position of the ballscrew day in and day out. However, with careful maintenance management, it is less of issue. With linear, because nothing is touching, we have no concerns about wear. Whether to choose liner or ballscrew should not be a large consideration when deciding which type of drive system you buy. You may want to focus on other concerns in your purchase or machining process.