Why Are Most CNC Machines Use Stepper Motors?

• 1
  Stepper fault table: symptom, cause, action
• 2
  Why CNC machines use stepper motors on the shop floor
• 3
  Torque, speed, and where steppers run out of margin
• 4
  Mid-band resonance and the chatter it causes
• 5
  What open-loop accuracy actually delivers
• 6
  When a servo is the cheaper answer
• 7
  Step by step: isolate a lost-step fault

A stepper motor moves in fixed angular increments. Give it 200 full steps per revolution and each pulse from the driver turns the shaft 1.8°. The controller counts pulses, so the position it commanded is the position it believes the axis reached. There is no encoder between the command and the result.

That is the whole reason why CNC machines use stepper motors for entry and mid-range 3-axis and 4-axis work. The drive electronics are simple, the motor is cheap, and the wiring is four wires plus a power pair. A servo axis needs an encoder, a tuned loop, and a drive that can fault on following error.

The trade is honest. A stepper delivers its highest torque at zero speed and loses torque as speed climbs. On a benchtop mill drilling 6 mm holes in 6061 aluminum at 1,500 rpm spindle speed, that low-speed torque is exactly what the Z axis needs. On a high-speed contouring pass at 10 m/min, it is not.

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  Open loopNo feedback device, so a missed step is invisible to the controller.
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  Low-speed torquePeak torque near standstill suits drilling, tapping, and heavy Z moves.
• 3
  Simple wiringFour phase wires and a DC bus, no encoder cable to route.

Every stepper has a pull-out torque curve. Torque is flat and high up to a few hundred pulses per second, then falls as inductance limits current rise in the coils. The knee usually sits between 300 and 800 rpm on a NEMA 23 frame running 48 V. Past that knee, a stepper can still turn, but it has little reserve.

The practical rule is to size the axis so the worst-case cut uses no more than about half the available torque at that speed. If you must climb a steep ramp, add a belt reduction instead of pushing more current through the driver. Current adds heat; a 2:1 reduction doubles torque at the screw and costs one pulley.

Watch the supply voltage too. A 24 V bus clips the torque curve early. Many benchtop machines jump to 48 V or 60 V and recover usable torque at 600 to 1,000 rpm without changing the motor.

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  Knee pointTorque drops sharply past roughly 300 to 800 rpm on NEMA 23.
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  Current vs heatRaising amps adds heat; a reduction adds torque without heat.
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  Bus voltage48 V to 60 V keeps more of the curve usable than 24 V.

Steppers have a mechanical resonance, usually between 100 and 300 full steps per second. At that speed the rotor oscillates, torque dips, and the axis may skip a step or make a rough howl. It shows up as a bad surface finish on one feed range and clean cuts either side of it.

Three fixes work. Switch the driver to 1/8 or 1/16 microstepping so the excitation frequency moves away from the mechanical resonance. Fit a damper on the rear shaft. Or change the acceleration ramp so the axis passes through the band quickly instead of crawling in it.

Do not confuse this with a mechanical problem. If the noise is present at every speed, check coupling runout and screw end-float first. Resonance is speed-specific. If it only appears in one narrow rpm window, it is electrical.

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  Microstepping1/8 or 1/16 moves the drive frequency out of the band.
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  DamperA rear-shaft damper absorbs rotor oscillation on larger frames.
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  Ramp speedPass through the resonance band quickly, do not dwell in it.

A stepper is not inherently inaccurate. A 1.8° motor with a 5 mm pitch ball screw and 1/8 microstepping gives a theoretical resolution near 0.003 mm per microstep. Real accuracy is worse because microsteps are not perfectly linear and because lost steps are unrecoverable.

The achievable number on a well-built 3-axis machine is around ±0.01 mm to ±0.02 mm on position, provided the cut stays inside the torque budget. That covers most aluminum prototypes, plastic fixtures, and sheet metal brackets. It does not cover tight-bore bearing seats or mold inserts that need ±0.005 mm.

Lost steps are the failure mode that matters. One stall event during a deep pocket can shift the whole part by 0.3 mm, and the controller will keep cutting as if nothing happened. That is the cost of open loop, and it is why any job with a tight tolerance needs an in-process check.

• 1
  Resolution vs accuracyMicrostep resolution is finer than the real positional accuracy.
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  Typical windowAbout ±0.01 mm to ±0.02 mm when torque margin is kept.
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  Silent failureA single stall shifts the part with no alarm raised.

A servo costs more up front. It becomes the cheaper answer when scrap from a lost step costs more than the drive. High-mix production with expensive titanium or Inconel blanks, unattended lights-out runs, and any axis that must hold position under a changing load are the classic cases.

Servos also win on speed. Closed-loop control keeps torque usable well past the stepper knee, so a machine can rapid at 30 m/min and contour at 10 m/min without stalling. If your cycle time is limited by how fast the axis can accelerate, the loop is the limit, not the motor.

The middle option is a closed-loop stepper. It keeps the simple drive but adds an encoder that faults on following error. It costs less than a full servo set and removes the silent lost-step failure. For a machine that mostly runs aluminum and needs one shift of unattended time, that is often the right call.

• 1
  Stay with stepperAluminum and plastic work, attended shifts, loose tolerances.
• 2
  Go closed-loop stepperNeeds fault detection without servo drive cost.
• 3
  Go full servoHigh rapids, hard materials, unattended or lights-out runs.