A servo milling machine is a machine tool that uses a rotating cylindrical cutter to move along multiple axes and create gaps, holes, and other details in the workpiece to turn it into a finished mechanical part. Most machines operate in three to five axes, providing much greater precision and detail. In this article, we want to focus on a key component of any milling machine: the engine. Of course, servo milling machines are equipped with a servo motor that drives the machine spindle. Let's see what are the advantages of a servo milling machine and how it compares to models with a stepper motor.
High-quality parts start with the milling engine. Servo motors are the best choice for axis motion control in machines built for high speed and precision operation. Factors such as excellent torque, high-resolution motion and feedback, and stability help to avoid results such as poor surface finish, accelerated tool wear, and out-of-specification shaping. So what are the advantages of a servo milling machine?
Rotary and angle encoders for servo milling machine drives provide detailed and precise control feedback with a thousand times higher resolution than is possible with stepper motors. Absolute and incremental encoders not only allow for the precise control motion and speed required in today's multi-axis machines and complex part geometries but also provide operators with greater visibility into the operation. These encoders are easy to install and wire, have a short overall length, and can even incorporate safety features such as those operated by limit switches.
Servo milling machine motors are available in a wide variety of configurations, sizes, and capabilities. This makes it possible to find the ideal rigidity match to the size and load of any machine tool. This is important because the inertia ratio between the motor and load is critical to maintaining position accuracy during retraction and to counteract vibration in heavy milling applications; Servo motors give machine tool builders the ability to fit the largest possible motor into the lightest table possible.
When it comes to accelerating a table or spindle, the relationship between torque and the inertia of the motor itself is critical. Due to their unique capabilities, servo motors offer more flexibility in terms of torque and the motor's inertia, which allows designers and engineers to adjust the acceleration capability as needed.
Servo motors that are optimized for milling machines reduce the interference that can themselves introduce into machining operations. Motor torque invariably changes as the shaft rotates, but a suitable servo motor can reduce the effect this has on the part, namely contour errors and shaded finishes.
There are three types of servo motors, brushed DC (two-wire) and BLDC (brushless DC) and AC sinusoidal, both have three stator wires and identical appearance, the difference is BLDC has two windings powered at any time, hence brushless DC, a sinusoidal alternating current has all three windings fed by three phases separated by 120 degrees. All servo motors should also have one other wire connected to the frame as ground. The other wires in these motors would be an encoder and for BLDC and AC SS they would also be conductors to the drive for commutation.
Servo motors on milling machines are continuously running motors that have built-in encoders for distance measurement. Control systems are more complex and require more wiring, but the final machine usually has a higher capacity than a stepper motor driven machine. Also known as axial or torque motors, servo motors use fewer electromagnetic poles - typically less than a dozen - and rely on closed-loop communication for accuracy and speed control, meaning there is a bidirectional signal between it and the control. In other words, when turned on, the control receives the exact position signal. There is no need to reset to zero. Rotary or right angle encoders are the main feedback devices used to facilitate this.
Stepper motors in contrast to servo milling machine motors operate in an open loop, meaning that their position is dictated solely by the voltage they receive; there are no sensors to control location or relative motion. Their relatively simple installation makes them a reliable choice when combined with the right application. The machine does not have a measuring system to verify that it has actually moved the distance, it simply assumes that it did because the motors are oversized so that it is rare for an engine to skip steps in a properly designed machine.
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