The choice of a stepper motor it is not always an easy matter. Usually the question is: why should I spend 80-150 US$ for a good nema23 stepper motor when I can find the same motor in a surplus market for 5 (or even less) US$?
There are many points you have to consider:
Point 1)
Please consider:
- - a recent motor has been built using different technologies and materials. One of the main problems of a stepper motor is the magnetic hysteresys created by the stator of the motor itself. A recent production motor is always built with materials characterized by a low hysteresys. I can really prove that two motors with the same nominal tension and a same phase/current powered by the same driver and tension have a dramatically different maximum rotation speed (obtaining with our tests a triple maximum speed with a 2006 motor and an 1985 old similar stepper);
- - a new production motor is (90% of the cases) an hybrid motor, this means that we can use it with a unipolar or bipolar controller;
- - a recent motor has an holding torque really higher than an old one.
A stepper last for a life so the investment could be taken in a serious consideration.
Point 2)
If a motor is used to drive a screw, a 200 steps motor can be considered the right choice but if the motor is connected to the axis by a timing belt the power needed by the motor must be, ofcourse, higher, involving questions and other solutions regarding the size of the motor itself (and power supply, and power controller ...). So, if it is not absolutely necessary, use screws to realize the movements.
For small cnc machines (pcb prototype maker, light plywoods) the use of a timing belt can be also considered but, in my opinion, to obtain the necessary accuracy the stepper motors should have a higher number steps/turn. On the market 400/800 steps motor can be easily finded at relatively affordable prices for an hobbyst driving them with a microstepping controller to obtain an higher precision.
Point 3)
The nominal tension of a motor.
Usually, to obtain good performances by a stepper motor it must be driven at 10-20 times its nominal tension. Therefore a motor with a very low nominal tension will be more desiderable because a lower tension will be necessary to obtain the best performance. Many online catalogs show lists of their producs very clearly (like JapanServo's) giving to the buyer a clear situation of theyr production. If possible the choice should be done for the lower nominal tension motors (for the current we can manage with ourcontrollers). Other companies like Minebea prefers to declare only the nominal current/phase.
In this picture (regarding three different Minebea hibryd motors) it is possibile to notice immediately the differences of speed and torque obtained providing the same tension (24V).
It is a clear that not always to a motor with an higher phase/current corresponds a faster or more powerful motor. At low frequencies, in this case, the smaller stepper motor presents the best torque performances.
These qualities must be always remembered when we decide to build a cnc machine just to obtain the best performances by the item we have paid for.
