How to choose your stepper motors

Choosing a Stepper Motor

The motor is the heart of your CNC router. The size and type of the motor you choose will determine the speed, precision and accuracy that your machine is capable of. First you need to understand the terminology used with motors. RPM (revolutions per minute) tells you the rotational speed a motor can provide measured in how many revolutions the motor makes in one minute. Torque refers to the rotational strength of your motor and is measured in force times unit length (ft-lb., oz.-in., etc.) When you are selecting your motor be sure to check the units that are used to measure torque. A torque-speed curve is a chart that shows the relation between RPM and torque and is useful for judging how the motor will perform. Sellers often quote a specific torque, the figure they give being the maximum torque for that motor. Using the torque curve you can estimate what the torque would be at any given RPM. This will allow you to choose the ratio that will allow for the best performance from your motor when you are designing your own machine. Your motor will also have a power rating, giving you the voltage rating for the motor and the current per chase. A higher power rating means a higher work output along with more electricity use. Be sure you know your power rating so that you have a sufficient power supply for your machine.

Stepper motors have their pros and cons, with the alternate option being a servo motor. On the upside stepper motors are cheaper than their servo motor equivalents. They are also more versatile due to ease of use and they are more reliable since they do not require an encoder (which could fail) like a servo motor does. Stepper motors are easy to set up, generally just needing to be plugged in to work, and they provide better torque at low speeds. They also last longer since the bearing is the only part that sees any real wear. Repeatability is kept high since a stepper motor requires little if any tuning and they are very unlikely to suffer damage from a mechanical overload. Finding a stepper motor is also easier; they are more readily available to the public than servo motors. They also work well in direct drive mode, a feature that servo motors lack. The upsides to a stepper motor are plentiful but there are a few negatives as well. Steppers motors are somewhat limited in the available sizes you can find them in. While a stepper motor is better at low speeds a servo motor does have better high speed functionality. Servo motors also have a superior power to weight ratio and a better torque to inertia ratio, giving servo motors a better overall efficiency. They also tend to produce less heat and noise than a stepper motor. Both types of motors have ups and downs but for a hobbyist a stepper motor is often seen as the better choice.

The driver in your stepper motor will have different modes of operation. The first mode is the wave drive which energizes one phase of the motor at a time. This mode is inefficient and produces less torque than other modes so it is rarely used and may not even be available on some modern drivers. The full step method energizes both phases constantly, allowing the motor to get up to full rated torque at all positions. For example, if a motor has 200 steps and one pulse equals one step 200 pulses will give you 360 degrees in motor shaft rotation. The half-step mode energizes one coil, then two coils and then one coil again. Alternating between phases gives the motor higher resolution. Torque will vary depending on step position in this mode but high-end drivers make up for this by increasing the current when only one coil is in use. There is a lot to know about stepper motors, just as there is with any part of a CNC router, but this information should give you a better outlook about what to expect and what to prepare for.