The US-based National Electrical Manufacturers Association creates standards that are developed by the manufacturers through a voluntary consensus to provide descriptions of how things should be. NEMA ICS 16-2001 (Motion/Position Control Motors, Controls, and Feedback Devices) describes stepper motors and is the NEMA standard usually referred to when talking about the NEMA standard in the context of RepRap, stepper motors and 3D printers.
In section 220.127.116.11 of that standard, NEMA chose to label stepper motors (e.g. "NEMA 17") with the size of their faceplate in tenth of inches. So a "NEMA 17" has a 1.7 inch by 1.7 inch faceplate. By standardizing the stepper motors (the faceplates, flanges and screw holes), you know ahead of time that one NEMA 17 motor will fit into the mounts of another NEMA 17 without having to redesign anything. This makes swapping components easier.
The first working RepRap used NEMA 23 Stepper motors for positioning and a different kind of motor for the extruder. As of 2013, most RepRap designs call for NEMA 17 Stepper motors or NEMA 14 Stepper motors for both positioning and extruding.
Full NEMA name
The full NEMA name of a stepper motor is written as:
...where the letters have the following meaning:
|DD||Diameter / Faceplate size||inches·10|
|MM||Mount type|| C: Flange with slots;|
D: Face with tapped holes;
CD: Face flange with holes
|I||Insulation class|| Maximum operating temperature:|
A: 221 °F;
B: 266 °F;
F: 311 °F;
H: 356 °F
Class B is the most common type for 60 cycle motors in the US. Internationally
Class F is the most common type for 50 cycle motors. Generally speaking, going 10 °F above the maximum temperature will reduce the motor life by half.
|VVV||Phase voltage rating||voltage·10|
|SSS||Steps||steps per revolution|
|W||Winding code|| How many internal wires the external wires are connected to:|
A: 2 wires;
B: 3 wires;
C: 4 wires;
D: 5 wires;
E: 6 wires;
F: 8 wires
- NEMA 34: 3.4" diameter stepper motor.
- NEMA 34D: 3.4" diameter stepper motor, with a flange.
- NEMA 34CD: 3.4" diameter stepper motor, with a flange and face tapped holes.
- NEMA 34D016 3.4" diameter stepper motor, that is 1.6" long, with a flange.
- NEMA 34D016-016B053200A 3.4" diameter stepper motor, with a flange, that is 1.6" long, has a phase current of 1.6 A, class B insulation, 5.3 V phase, 200 steps per revolution, and type A winding connection.
Some (all?) NEMA 11 motors (is this specified in the standard?) have blind mounting holes about 4.5 mm deep, tapped to accept M2.5x0.45 metric bolts. The 4 mounting holes are in a square 23 mm (0.905 inch) center-to-center.
Some (all?) NEMA 14 motors (is this specified in the standard?) have blind mounting holes about ???? mm deep, tapped to accept M4 metric bolts. The 4 mounting holes are in a square 26 mm (1.024 inch) center-to-center.
Some (all?) NEMA 17 motors (is this specified in the standard?) have blind mounting holes 4.5 mm deep, tapped to accept M3x.50 metric bolts. The 4 mounting holes are in a square 31.0 mm (1.220 inch, about 7/32) center-to-center. The holes in the part that needs to be bolted to the NEMA 17 motor typically need to be reamed out with a 1/8" drill bit to allow the M3 bolt to pass through. The drive shaft is typically 5 mm dia.
Some (all?) NEMA 23 motors (is this specified in the standard?) have through mounting holes, a smooth 5 mm (0.2 inch) diameter. The 4 mounting holes are in a square 47.1 mm (1.856 inch, about 1+7/8 inch) center-to-center. The drive shaft is typically 1/4 inch (6.35 mm) dia.