If your CNC router, mill or lathe loses steps under load, stalls on rapids, or runs rough at low speed, a servo upgrade is usually the right answer. Stepper motors are simple and cheap, but AC servo systems close the loop: the encoder feeds position back to the drive, so the motor corrects itself in real time. The result is higher torque at speed, smoother motion, and no lost steps.

But "servo" covers a wide range. Here is a five-step process to pick the right motor and driver for your machine — no engineering degree required.

Step 1 — Match the torque

Every servo motor lists two torque values:

  • Rated torque (continuous) — what the motor can hold indefinitely. For a hobby-to-light-industrial CNC, 1–3 Nm covers most router and mill axes.
  • Peak torque (MAX) — short bursts for acceleration and cutting. A motor rated 1.5 Nm with a 3 Nm peak handles aggressive rapids without stalling.

A quick estimate for a ball-screw axis: required torque ≈ (moving mass × friction coefficient × lead) ÷ (2π × efficiency). For a 50 kg gantry with 0.1 friction, 10 mm lead and 0.9 efficiency, that is roughly 0.9 Nm — so a 1.27–1.5 Nm rated motor with headroom is a sensible starting point.

Step 2 — Check the speed

Most AC servos are rated 3000 RPM. Your axis speed = motor speed ÷ reduction. If your lead screw is 10 mm, 3000 RPM gives 30 m/min rapid — more than enough for most machines. If you need more torque per revolution (heavy axis, rotary table), add a planetary reducer (2:1–10:1) instead of buying a bigger motor.

Step 3 — Pick the encoder

  • Incremental encoders need a homing routine every power-up.
  • Absolute encoders (e.g. 17-bit, 131072 counts/rev) remember their position after power loss — no re-homing, no crash on restart. Worth the small premium on machines you run daily.

Step 4 — Match the driver

Your drive must support both the motor and your control:

Motor spec Common match
Voltage 220V AC single/3-phase (check your control cabinet)
Command Pulse/dir (step-servo replacement), RS485 Modbus, or EtherCAT
Encoder feedback Incremental or absolute — drive must match encoder type
Cable Motor power + encoder cable, usually 3 m or custom

A complete kit (motor + drive + cables, pre-matched) removes the biggest source of setup errors — mismatched encoder wiring.

Step 5 — Check the mechanical fit

Match the flange size to your mount: 60 mm (400W), 80 mm (750W), 110 mm (1.5–2.3 kW), 130 mm (2.3–3.8 kW). Verify shaft diameter and keyway against your coupling, and leave room for the rear encoder cap and brake option if you plan a vertical axis.

Compatibility checklist

  • Rated torque ≥ your calculated axis torque × 1.2 safety factor
  • Peak torque covers acceleration phase
  • Rated speed ≥ your target rapid speed ÷ reduction
  • Encoder type matches your drive and control
  • Command type supported by your controller (pulse/RS485/EtherCAT)
  • Flange and shaft fit your mount and coupling
  • Supply voltage matches your cabinet (220V)
  • Cable length and connector type fit your machine

FAQ

Servo or stepper for my first CNC?

For light engraving, a closed-loop stepper is enough. For production cutting, high rapids, or heavy axes, servo pays off in speed and reliability.

Can I reuse my stepper driver?

No — servo drives are motor-specific. Use the matched drive from the kit.

Absolute encoder or incremental?

If you dislike homing routines and worry about power-loss position, choose absolute (17-bit). Otherwise incremental is fine.

220V or 110V?

Most AC servos run 220V. If your shop only has 110V, look for 110V-capable drives or add a transformer.

What does "matched kit" include?

Motor, matching drive, motor power cable and encoder cable — pre-matched by the manufacturer, so you skip the wiring guesswork.

Shop servo motors and matched kits in the Servo Motor collection — or see a complete 400W/750W servo kit with C30S driver as a reference spec.