encoder slit tolerance

Optical encoder disc tolerance directly determines the accuracy of position and speed feedback. Even tiny dimensional deviations of slits, outer diameter or flatness will cause signal jitter, waveform distortion and reduce encoder resolution. Manufacturers must strictly control multiple tolerance items during fabrication.

1. Slit Width Tolerance

Slit width is the core optical feature. Consistent slit width guarantees uniform light transmission and stable square-wave signals.

• For high-resolution metal encoder discs made by photochemical etching, typical slit width tolerance: ±0.003 ~ ±0.01 mm.
• Laser-cut encoder discs usually have wider tolerance due to heat-affected zone, generally ±0.015 mm or higher.
• Rule of thumb: the finer the slit, the stricter the tolerance requirement for high-precision encoders.

2. Slit Position / Pitch Tolerance

Pitch tolerance means the deviation of the angular spacing between adjacent slits. Pitch error will lead to periodic signal error and angle measurement deviation. For industrial high-precision encoders, pitch tolerance is commonly controlled within ±0.002° to ±0.01°.

3. Outer Diameter & Inner Hole Tolerance

The center mounting hole and outer circle decide coaxiality after assembly.

• Inner hole tolerance: usually H7 / H8 for precision fitting with rotating shaft.
• Outer diameter tolerance: ±0.01~±0.03mm, depends on disc overall size. Large runout caused by poor OD and ID tolerance will generate periodic signal fluctuation at high rotation speed.

4. Flatness Tolerance

Flatness is critical for thin metal encoder foil. Warpage will change the gap between disc and optical sensor. For ultra-thin etched metal discs (0.03–0.1mm), typical flatness tolerance ranges from 0.005mm to 0.02mm over the effective optical area.

5. Thickness Tolerance

Thickness variation affects mechanical stiffness and dynamic stability. Etched metal encoder disc thickness tolerance: ±0.002 ~ ±0.005mm for thin foils.

6. Key Factors Affecting Tolerance

1. Manufacturing Process: Photochemical etching delivers tighter and more repeatable slit tolerance compared with laser cutting for micro slits. Laser cutting brings thermal deformation and edge burning risk.
2. Material: Kovar and nickel maintain better stability under temperature variation, helping preserve tolerance. Stainless steel is more cost-effective for general tolerance levels.
3. Disc Diameter: Larger encoder discs are harder to keep flatness and pitch tolerance.
4. Post-processing: Improper cleaning or handling may introduce bending and deformation, breaking flatness tolerance.

7. Tolerance Inspection Methods

• Optical comparator / video measuring machine: test slit width, pitch, outer and inner dimension.
• Laser flatness gauge: check flatness and warpage.
• Rotary runout test: verify assembled coaxial performance.

Summary

The main tolerance items of optical encoder disc cover slit width, pitch, hole dimension, flatness and thickness. Photochemical etching is the preferred process when ultra-tight slit and pitch tolerance is required. Confirm your tolerance budget with the manufacturer at the design phase, especially for high-resolution miniature encoder projects.