For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
When building a CNC router or upgrading a 3D printer, the first question is usually: "Is NEMA 17 enough, or do I need NEMA 23?"
Most beginners look at the Holding Torque and stop there. This is a mistake. A NEMA 23 motor isn't just "stronger"—it is physically different in ways that affect your speed, your driver choice, and your machine's ability to avoid missed steps.
If you choose a NEMA 17 for a heavy gantry, it is far more likely to overheat or lose steps under cutting load. If you choose NEMA 23 for a fast 3D printer, it might actually run slower than the smaller motor. This guide explains the engineering limits of each frame size.
Table of Contents
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1. Physical Difference (The Frame Size)
"NEMA" is just a standard for the mounting faceplate, defined by the National Electrical Manufacturers Association.
- NEMA 17: Faceplate is 1.7 x 1.7 inches (42 mm). Shaft diameter is typically 5 mm.
- NEMA 23: Faceplate is 2.3 x 2.3 inches (57 mm). Shaft diameter is typically 6.35 mm (1/4") or 8 mm.
The Limit: The biggest limitation of NEMA 17 is the 5 mm shaft. If you try to push high torque (over 0.5 Nm) through a belt drive, the radial load can flex the small shaft or wear out the tiny internal bearings quickly.
Figure 1: NEMA 23 (Right) has a much larger rotor and shaft, allowing for 3x-5x more torque than NEMA 17 (Left).
2. Torque & Speed (The Inductance Trap)
Here is the counter-intuitive physics: Larger motors are often slower. In real machines, the higher inductance and rotor inertia usually limit usable speed unless higher voltage drivers (48V+) are used.
NEMA 17 (High Speed, Low Torque)
Because the rotor is small and light, it has low inertia. NEMA 17 motors can accelerate incredibly fast, making them perfect for 3D print heads that change direction constantly.
NEMA 23 (High Torque, Lower Top Speed)
NEMA 23 motors have massive Inductance. As speed increases, the motor fights against itself (Back EMF), and torque drops off sharply. To run a NEMA 23 at high speed, you need higher voltage (24V is rarely enough; 48V is recommended).
Engineering Note: Torque curves matter more than holding torque. A NEMA 17 may retain more usable torque at 800 RPM than a poorly driven NEMA 23.
Rule of Thumb: If your machine needs to move light loads very fast (like a delta printer), use NEMA 17. If you are pushing a heavy router spindle, you need the raw torque of NEMA 23, even if it runs slower.
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3. Driver Compatibility
You cannot just swap motors without checking your drivers.
- For NEMA 17: Standard stick drivers (A4988, TMC2209) found on 3D printer shields are perfect. They handle the 1.0A - 1.5A current easily.
- For NEMA 23: These motors often draw 3.0A to 4.0A. A standard 3D printer driver will typically overheat or enter thermal shutdown. You need external industrial drivers like the TB6600 or DM542.
4. Selection Summary
| Feature | NEMA 17 | NEMA 23 |
|---|---|---|
| Face Size | 42 mm x 42 mm | 57 mm x 57 mm |
| Shaft Diameter | 5 mm | 6.35 mm (1/4") or 8 mm |
| Holding Torque | 0.2 - 0.5 Nm | 1.0 - 3.0 Nm |
| Required Driver | A4988 / TMC2209 (Small) | TB6600 / DM542 (External) |
| Best Application | 3D Printers, Laser Cutters | CNC Routers, Mills, Plasma |
Quick Selection Rule
- 3D Printers: Almost always use NEMA 17. NEMA 23 is too heavy for the gantry.
- CNC Routers (Wood/Aluminum): Use NEMA 23. NEMA 17 will miss steps during cutting.
- Voltage: If you upgrade to NEMA 23, plan for 36V–48V for good high-speed performance.
Recommended Components
Sizing the motor is the easy part. Managing the client is the hard part.
You calculated the torque perfectly. But did you calculate the "Scope Creep"? The Sheet Mechanic teaches you how to handle the messy reality of engineering projects—where clients want double the speed for half the budget.
The math makes the machine move.
The Sheet Mechanic makes the project move.
About the Author:
This article is written by a mechanical design engineer specializing in automation components, motion control sizing, and CNC machine design.
This article is written by a mechanical design engineer specializing in automation components, motion control sizing, and CNC machine design.
As an Amazon Associate, I earn from qualifying purchases.
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