Rotary Encoder Selection for CNC Machine Tools: Resolution, Accuracy, and Interface Requirements

CNC machine tools (lathes, machining centers, grinding machines) demand position feedback that simultaneously achieves sub-micron resolution, accuracy within ±2–5 µm over the full axis travel, and reliable operation in environments with cutting fluid, metal swarf, and vibration. 

The encoder technology selection must balance these requirements against the available installation space, the required interface protocol, and the service life expected in a production environment.

CNC Servo Axis Feedback: Requirements

A CNC servo axis (X, Y, Z, or rotary axis) uses the encoder for closed-loop position control. 

The controller executes a programmed position command (G-code) and uses the encoder feedback to correct any deviation from the commanded position.

Resolution requirement:

For a typical precision machining center with a required positioning accuracy of ±2.5 µm:

• Encoder resolution must be significantly better than the positioning accuracy target.
• Rule of thumb: resolution ≤ accuracy / 10.
• Minimum resolution: ≤ 0.25 µm.

For a rotary encoder on a ball screw drive (20 mm pitch per revolution):

• 0.25 µm linear resolution = 0.25 / 20,000 revolution = 0.0000125 revolution.
• Encoder resolution = 0.0000125 × 360° = 0.0000045° = 16 arc-seconds per count.
• In bits: 360° / 0.0000045° = 80,000,000 counts → > 26 bits.

For a rotary encoder measuring angular position directly (rotary table or 4th axis):

• Required angular accuracy: ±2 arc-seconds for precision rotary tables.
• Required angular resolution: ±0.2 arc-seconds → encoder must output at this resolution.

Accuracy requirement:

The encoder’s inherent accuracy must contribute less than the total system accuracy budget. With a ±5 µm total axis positioning accuracy target, and allocating ⅓ to the encoder: encoder accuracy ≤ ±1.7 µm.

For a 10 mm diameter ball screw, 1.7 µm linear error corresponds to 0.061° angular error = 220 arc-seconds. A magnetic encoder (±0.1° = ±360 arc-seconds) is insufficient. An interferential optical encoder (±2 arc-seconds equivalent) is 110× more accurate than needed.

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The appropriate technology for most CNC servo axes:

• Glass scale optical encoder on the rotary encoder: ±2 arc-seconds (±0.00056°) inherent accuracy.
• OR metal tape linear encoder directly on the axis (bypasses the ball screw accuracy).

CNC Spindle Feedback: Speed and Position

CNC spindle drives have different requirements from servo axes:

Speed feedback (velocity control):

• Spindle speed: 0–30,000 rpm (high-speed spindles).
• Speed accuracy: ±0.1% of set speed.
• Resolution: incremental encoder, typically 1,000–5,000 PPR (pulses per revolution).
• At 30,000 rpm: 5,000 PPR × 4 edges × 30,000/60 = 10,000,000 counts/second = 10 MHz count rate → requires high-speed incremental output.

Angular position (for synchronized operations — tapping, thread cutting):

• Single revolution synchronization: Z index pulse to align tool with thread start.
• Absolute position within one revolution: single-turn absolute encoder or resolver.

For high-speed spindles, magnetic encoders are commonly used: speed only, no precision position requirement. 

For synchronized spindle applications, high-resolution incremental encoders with clean Z pulse are required.

Linear Scale Encoders for CNC Accuracy

The most accurate CNC positioning uses linear glass scales directly on the machine axis rather than rotary encoders on the drive motor or ball screw. This eliminates all drive system errors:

• Ball screw pitch variation.
• Drive belt stretch (belt drives).
• Backlash and reversal error.
• Thermal expansion of the ball screw.

A glass scale encoder directly measures the position of the machine table relative to the machine base, the actual parameter of interest.

Typical linear glass scale specifications:

Coolant Resistance and IP Rating

Machining environments — particularly turning, milling, and grinding — use cutting fluids for lubrication and chip removal. The encoder must resist:

• Flood coolant: Direct liquid flow across the encoder body during machining
• Coolant mist: Airborne fine mist that penetrates small gaps
• Metal swarf: Fine metallic particles that can abrade seals and contaminate bearings

Encoder IP requirements for CNC:

For optical encoders in CNC applications: the encoder scale must be separately sealed from the coolant environment. External linear scales use a sealed strip housing with a spring-loaded slot seal that allows the readhead carriage to traverse while excluding coolant.

Interface Protocol Selection for CNC

CNC controllers from major manufacturers use specific encoder protocols:

For retrofit applications using non-OEM controllers, SSI or BiSS-C encoders with standard RS-422 incremental output provide the broadest compatibility.

For new designs with open control architecture: BiSS-C provides the best combination of update speed, error detection, and bidirectional parameterization.

Before you leave, you might find it interesting to dive deeper into: 

• Design Guidelines for Encoder Interface Cables: Specifications and Common Errors, 
• discover the details of Sensor Alignment for Small-Diameter Rotary Encoder Scales: Tolerance and Mounting Guidance, 
• or check out our analysis of Servo Motor Velocity Control: Speed Feedback Methods and Velocity Estimation Techniques.