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January 27, 2021

Servo vs. Stepper motors in CNC work

By scribe


servo motor and stepper motor

When it comes to purchasing a CNC machine, selecting the right equipment with the right features and capabilities can make all the difference in your overall success. Variations in size, speed, precision, and price all factor in to the machine’s compatibility and fit with your unique application.

Among these key factors is the machine’s drive motor – specifically in choosing between a servo motor and a stepper motor. Both can work well in the right situation, but knowing which one to go with can be a complex decision.

Here is a breakdown of the key differences between servo motors and stepper motors to help determine which will work best for your specific application.

Stepper motors

Stepper motors generally provide the most value on small-footprint, small-work-envelope machines not in need of high speeds or torque. Most stepper motors run open-loop, which can help eliminate the added cost and complexity of adding an encoder or a resolver to the mix.

Stepper motors are capable of generating high torque at zero speed, but torque tends to decrease as speed increases. And speed limitations are commonplace with stepper motors, which generally operate most effectively at 1,200 RPM or lower.

From a high-level view, stepper motors can serve as a compact and economical solution that is dependable for small-machine applications.

Servo motor + planetary drive combo

For larger-footprint machines that typically utilize rack and pinion drive systems, ShopSabre combines clear-path brushless servo motors with planetary drives. In doing so, the motors and drives combine to create a very compact design solution while bringing closed-loop feedback technology into the mix. This combination tends to work best for CNC plasma work and for less-demanding CNC routing.

Servo motors

Once a CNC machine reaches a certain size and speed, it begins to require more power to work effectively. This is where servo motors come into play.

Servos are best for applications that require improved positioning precision and resolution. They run faster than stepper motors (usually at several thousand RPMs) and also provide more consistent torque across the entire speed range of the machine.

In many of our CNC machines, we use Mitsubishi servo motors, which are also used by many of the finest machine tool builders in the world.

Closed-loop systems

Mitsubishi AC servo motors utilize a closed-loop system, which means the servo drives always know what is happening during operation and can react instantly and accordingly as needed.

You can think of the closed-loop system concept as being similar to a GPS navigation system. If you’re driving and you miss a turn, the GPS will automatically update your route to ensure you still reach your destination successfully.

Without the rerouting capability, you would continue to drive in the wrong direction until you eventually became lost. This is fairly in line with how a stepper motor works. Stepper motors don’t have encoders, so there’s no way for them to relay positional data back to the drives. This is known as “losing a step,” and once a stepper motor becomes lost, the next move can be completely random, which can have catastrophic results for your project.

In most closed-loop drive systems, encoders are constantly sending data back to the drives and enabling them to recognize any issues that arise, ensuring corrections can be made instantaneously without stopping the machine.

Say a piece of wood were to fall off the table and become jammed between the gantry and the table, causing a mechanical problem. Closed-loop feedback motors can sense this abnormality immediately and can monitor the other drive motors. As soon as the abnormality occurs, everything stops to avoid further error.

Absolute encoder

The main reason this is able to happen is because of the absolute encoder – a device located on the end of the servo motor that sends data directly to the servo drive. The encoder allows the servo drive to know exactly how the servo motor is positioned.

In our closed-loop systems, the encoder inside the motor is made of glass. This is important because glass doesn’t respond very dramatically to surrounding temperature changes. Some other brands use plastic encoders to save money, but these are not as accurate. Plastic can change size as the motor heats up, which can negatively affect positional accuracy.

ShopSabre servo drives also check positions about 10 times more per revolution than the typical competitor. Our drive systems are much more accurate and can respond to problems much more quickly than competitors’ machines.

Buy your second machine the first time with ShopSabre

ShopSabre offers an industry-leading collection of CNC routers and CNC plasmas driven by either stepper motors or closed-loop servo motors with encoder technology for higher speeds, higher accuracy, and less maintenance.

Our team of experts is here to work with you to determine the type of motor and machine that will best suit your needs. We aim to provide hobbyists and shop owners alike with unmatched precision, efficiency, and versatility. Whatever your goals may be, we are dedicated to helping you produce outstanding results at a fraction of the labor and cost.

Our Industrial Series (IS) and PRO Series CNC routers provide industrial capability for a fraction of the price, while our SideKick CNC plasmas deliver professional quality at an entry-level cost.

Here are a few of the reasons ShopSabre is able to develop and produce such revolutionary CNC technology:

  • Experienced in-house engineering talent
  • The ability to totally fabricate and test designs onsite
  • The ability to place the designs into production in-house

These are just a few of the advantages of being an American machine tool manufacturer. We offer commercial financing on our CNC equipment to help meet the needs of your budget.

Shop our unmatched collection of CNC routers and CNC plasmas in Minneapolis today to experience CNC the way it was meant to be.

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