Overview

A custom molybdenum code wheel is a photochemically etched encoder disc that carries high-resolution slits and tracks for optical rotary and linear encoders. Molybdenum (Mo ≥ 99.95%) is chosen for its high stiffness, low coefficient of thermal expansion and dimensional stability at elevated temperature, so fine graduations do not drift as the disc warms or spins. Etching dissolves the metal through a photoresist mask rather than scribing or punching it, so slits leave the line with steep, burr-free walls and no machining stress in the thin web.

For an encoder designer, a custom molybdenum code wheel addresses three recurring limits: graduations down to a few tens of microns with uniform pitch, a thin disc that still resists wobble at high rotational speed, and thermal drift that would otherwise amplify angle error. Geometry is defined by artwork, so a new line count or track pattern is a new film rather than a new die. Supplied from 0.05 mm to 0.5 mm stock, wheels are typically stress-relief annealed and blackened to stabilize flatness and raise the read-head signal-to-noise ratio.

Parameter Window

• Available Materials: molybdenum (Mo ≥ 99.95%), Mo-Cu composite, stainless 304, Invar on request
• Material Thickness Range: 0.05–0.5 mm
• Tolerance: ±0.005 mm on graduation pitch, ±0.02 mm on outline profile
• Minimum Feature: 0.05 mm slit width and 0.06 mm line width at 0.10 mm stock
• Surface Options: stress-relief anneal, lapping/polish, matte blackening, anti-oxidation coating
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Artwork & DFM Review — Convert the track pattern to vector artwork and confirm slit-to-thickness ratio, pitch and concentricity.
2. Material & Temper Selection — Match molybdenum purity, rolled condition and flatness to line count and rotational speed.
3. Cleaning & Lamination — Degrease and micro-etch, then laminate dry-film resist with no bubbles over fine slits.
4. UV Exposure — Expose both faces through registered film so tracks align to the center bore on a shared datum.
5. Development & Inspection — Develop the resist and inspect slits for pinholes or scumming before etching.
6. Controlled Etching — Spray-etch in molybdenum-matched chemistry; time, temperature and conveyor speed set sidewall steepness.
7. Stripping, Anneal & Blackening — Strip resist, vacuum anneal to stabilize flatness, then matte-blacken to the target reflectance.
8. Pitch & Concentricity Verification — Measure pitch, slit width and bore concentricity, then protect and tray-pack.

Core Advantages

1. Steep slit walls: Burr-free etched slits give clean read-head waveforms and low interpolation error.
2. No hard tooling: A new line count or track pattern is a new phototool, keeping short runs and revisions economical.
3. Dimensional stability: High-stiffness, low-expansion molybdenum holds flatness and graduations through speed and temperature.
4. High line counts: 0.05 mm slits with 0.06 mm lines hold at 0.10 mm stock for high-resolution wheels.
5. Optical surface match: Matte blackening gives low, uniform reflectance that lifts read-head signal-to-noise ratio.
6. Batch repeatability: A custom molybdenum code wheel from one approved film etches identically from first article to volume lots, simplifying qualification.

Typical Applications

• Incremental optical encoders: Main code wheels where even graduations set duty cycle and interpolation accuracy.
• Absolute encoders: Multi-track absolute-position discs where pitch and concentricity set positioning accuracy.
• Servo and robot joints: Thin, low-inertia wheels for motor feedback and joint angle measurement.
• Precision rotary tables: Reference gratings in measurement tables and optical index heads with low thermal drift.
• Grating and displacement scales: Fine-slit substrates for linear and circular scales at high line counts.
• High-stability angle sensing: Temperature-stable wheels for inertial measurement and precision tracking mechanisms.

Design Notes & Process Considerations

1. Slit versus thickness: Keep the smallest slit at 0.05 mm or 1.2 × material thickness, whichever is larger, to avoid merging.
2. Pitch and concentricity: State pitch tolerance and bore concentricity, since eccentricity becomes a once-per-revolution angle error.
3. Line-to-space ratio: Specify light-to-dark ratio, usually 1:1 to match the read-head photocell, and control duty-cycle spread.
4. Flatness callout: Specify flatness per 25 mm and overall bow; thin molybdenum distorts during anneal if not fixtured.
5. Surface and reflectance: Note matte-blacken or polish and target reflectance to prevent stray reflections at the read head.
6. Panelization and marking: Group wheels on a shared registration grid with orientation marks to speed assembly and read-head alignment.