CNC plasma cutting systems are a wonder of modern manufacturing. They have drastically simplified and improved how items are produced by providing precise and consistent product output. By feeding a design into the CNC’s control computer, products can be automatically milled, welded, ground, and cut to precise measurements.
These machines depend on powerful and reliable components to ensure consistent and efficient function. Amongst these components, few are as vital as the servo or stepper motors that make up the main moving parts.
Deciding on a machine run by servos or steppers can be daunting without prior research. Here we will explore these motor types and where they perform best.
CNC Machine Motors
The motors in a CNC machine serve one major purpose: convert the electrical signals sent by the controlling computer into movement. The design for the product is programmed into the computer based on the required parameters of the material. These designs are then translated into 3D space so the machine can enact movements corresponding to the design.
The output of this design means engraving or milling a specific shape out of a piece of metal or wood or welding and securing two or more pieces together. Regardless of the output, the tool will need to make a combination of precise movements to complete the job. The motors in the CNC are responsible for moving the tool properly.
What are Servo Motors?
Servo motors are actuators that create movement along an angular or linear position. They can move precisely at high rotations, usually at RPMs in the thousands. They operate as part of a closed-loop system, meaning they will move according to set instructions until the desired outcome is met. Once it reaches this outcome, the servo will return to its original point.
This type of motor is most used for larger and faster machines. They can work at higher speeds and greater power levels than a stepper motor without losing any torque. This combination of control and power makes them much more precise, which is ideal for industrial applications. Projects requiring large and exact components are best made using CNCs that utilize servo motors.
However, servo motor CNC machines will initially cost more than a stepper motor. They also require a great deal more maintenance to ensure consistent working quality.
What are Stepper Motors?
Stepper motors are brushless DC electric motors with divisions in each of their full rotations. These motors can be divided into as many divisions, called steps, as the designer wishes. Each power input sent to a stepper motor tells it to turn a specific number of these steps, with the ability to stop at any instant. For example, if a stepper motor with four steps is programmed to rotate through six steps in sequence, it will turn one and a half times.
These motors run on an open-loop system, meaning they run only as much as they are programmed to run. They can operate without an encoder or resolve, which are necessary for closed-loop systems. This makes systems incorporating stepper motors cheaper and less complex than servo motor systems.
Due to the stepped nature of their rotations, stepper motors cannot run at faster than 1,200 RPM. While this is slower than a servo, they do not need as much maintenance during their use. Plus, stepper motors are cheaper to run than servos.
Deciding Factors
The job you need to perform will determine the type of motor (and CNC machine) that best fits your usage. Since certain jobs will fare better with one motor over the other, it is important to choose the correct one. The choice will also affect other aspects, such as the equipment you need and the overhead the job will incur.
Consider the factors below to decide which is most relevant for the job.
The Size of the CNC Machine
Knowing which motor will fit in the available machine is the first step to knowing which is best. Typically, stepper motors run tools with a linear motion that covers no more than a 4′ x 4′ area. This means belts, ball screws, and lead screws.
Some machines can run on stepper motors with cutting areas of 4′ x 8′ or 5′ x 10′, which will be appropriately cheaper. However, anything larger than this, or with longer gantries prone to flexing, will need servos. If you’re going to machine heavy materials, you will also need servos.
Stepper motors are ideal for smaller machines, but servos will be better with bigger or heavier ones.
The Method of Motor Control
As mentioned above, there are two main methods for controlling the motion of the motor: open-loop and closed-loop. Some jobs rely on precise cutting within limited spaces, such as when cutting or carving lengths of wood or metal. In these cases, an open-loop system will rely on everything running smoothly to produce accurate work. If there is any stuttering or jamming of the motors or gantries, the piece will not be machined correctly.
When working with a closed-loop system, servos are the best choice. If they are likely to be disrupted as they perform their functions, a servo-run motor will continue its cutting routine despite the disruption, preventing the material from being wasted or ruined.
The Speed and Torque of the Machine
By nature, stepper motors are limited in their operational speed. Thus, stepper motors are the only option for any machine running at 1,200 RPM or lower. Steppers can also hold a torque of between 30 oz-in and 1,500 oz-in.
Servos will run significantly faster, usually starting at 2,000 RPM, and as such, will hold a much higher torque. This makes them better for higher-speed machines and any machine with a heavy gantry. The weight of the gantry must be considered when using the machine. If it or the load will be unstable or weighty, then stick to servos.
Servo and stepper motors are vital to CNC machining, no matter the job. Ensuring that the correct type of motor is being used according to the job is just as important. Fortunately, it is an easy task to determine what is needed.
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