Choosing an appropriate stepper motor for your CNC project is important. If you choose a motor that is too small, you may experience very slow maximum speeds and torques, and even missed pulses. On the other hand, if you choose a motor which is too big, it may require a powerful and expensive power supply and driver.
T is torque
F is force (usually measured in oz.)
r is radius (usually measured in inches)
All stepper motors have a torque rating, and this should be the first rating you look at. This is also known as holding torque. If the motor is powered on but not rotating, the holding torque is the amount of torque necessary to turn the shaft against the motor’s will. Stepper motors also have a maximum current rating, meaning that the holding torque number is usually given assuming the maximum rated current is flowing into the motor.
Of course, as a stepper motor increases its speed, the amount of torque it can apply diminishes quickly. This will be discussed in another article, but for now, just know that bipolar stepper motors have the best torque-speed characteristics.
Visualizing how much torque you need
Torque is a difficult measurement to estimate. Imagine you have a stepper motor shaft, and a one inch stick is attached to the center of the shaft pointing outwards. Now imagine that the motor is bolted onto a table, and you can put your finger on the table to stop the motor, since your finger comes between the one inch stick and table. A 16 oz. in. stepper motor would apply a force of one pound on your finger, since 16 oz equal one pound and the radius is one inch. One pound of force isn’t very much, and would probably not be useful in any reasonably sized machine. But now imagine a 160 oz. in. motor. That would apply ten pounds of force onto your finger in this scenario. Now we’re starting to talk about real forces. Yes, this imagination method isn’t very exact, but it can help you get some idea of how powerful these motors are before you buy them. When in doubt, it’s best to go with a more powerful motor. To give you another idea of how to view these, the stepper motors I use for my CNC machine are 280 oz. in. These motors are as big as my palm. Important! Some suppliers give torque ratings in other units like lbs-in, N-m, N-cm, etc. So make sure to convert these torques to units which you are familiar with. Also, the order of units doesn’t matter for torque. lbs-in is the same as in-lbs.
Maximum current rating
Stepper motors also have a maximum current rating you should absolutely pay attention to before buying. This rating means that if you apply more than the maximum amount of current to the motor for a long period of time, you risk damaging the motor. Yes, you can run motors below the maximum rated current, but if you do, you’ll basically be leaving performance on the table since you won’t be using your motor to its maximum potential. So if you have a 2 amp stepper motor driver, you shouldn’t pick a stepper motor that has a maximum rated current of 4 amps. That would just be a waste of money.
You will also want to pay close attention to degree rating. For most stepper motors, a rating of 1.8 degrees is given, meaning that for every full step, there is a 1.8 degree difference. This means that if you half step the motor, it will take 400 pulses to rotate the motor shaft exactly once. Beware that cheaper and smaller stepper motors have far fewer steps. Again, a 1.8 degree rating is very common.
Stepper motors usually specify a NEMA number. This rating doesn’t necessarily indicate the strength of the motor, but rather, the general size and shape of a motor. For example, stepper motors usually have four holes at the front of the motor for mounting purposes. Each NEMA number has different characteristics that dictate dimensions of the motor such as these mounting holes. There are some things that NEMA does not indicate, such as the length of the motor. So you could have two NEMA 23 motors, but one of them might be .5 inches long while the other could be 3 inches long. Of course, the 3 inch long motor would be vastly heavier and more powerful.
Where to buy stepper motors
There are lots of places to buy stepper motors. If you’re just creating one CNC machine, you may want to look on eBay. There are some very cheap motors on eBay that people sell because they don’t need them any more. If you want to buy fresh, new stepper motors, I don’t blame you. There are lots of companies that sell them. For very small stepper motors, the kinds you would find in a floppy disk drive for example, you may want to try suppliers like Digi-Key or Jameco. For larger stepper motors like 260 Oz-in and above, I personally use Probotix, since the prices are fairly good. Other companies like National Instruments also sell stepper motors, but at largely inflated prices. Choosing to buy stepper motors from the wrong supplier can cost quite a lot of money, so be sure to allocate at least an hour to investigate which company gives you the best bang for your buck for your particular needs.