Manufacturing
How Chrome-on-Glass Encoder Discs Are Made: From Substrate to AOI Inspection
A chrome-on-glass encoder disc is made by photolithography: the glass substrate is lapped and polished, chrome-coated, coated with photoresist, exposed through a precision mask, developed and etched; the outer diameter and bore are then laser machined and every disc is inspected by machine vision (AOI).
The glass encoder disc is the classic transmissive code wheel. Its accuracy comes almost entirely from how it is made. Here is the process we use at ENC7, step by step, and why each step matters to the encoder designer.
The process at a glance
| Step | What happens | Why it matters |
|---|---|---|
| 1. Lapping & polishing | Substrate flatness and surface quality are controlled | A flat, clean surface is the base for uniform coating and sharp lithography |
| 2. Chrome (Cr) coating | A thin chrome layer is deposited; it will form the opaque areas | Opaque-area optical density sets signal contrast |
| 3. Photoresist coating | A light-sensitive resist is applied over the chrome | Uniform resist gives uniform line widths |
| 4. Exposure | The graduation pattern is exposed through a precision mask | Transfers line count, track layout and index position |
| 5. Development | Exposed (or unexposed) resist is removed | Reveals the pattern to be etched |
| 6. Etching | Chrome is etched away where the disc must be clear | Produces the final transparent/opaque graduations |
| 7. Cleaning | Resist and residues are removed | Contamination on the track zone causes signal defects |
| 8. Laser cutting | Outer diameter cut and bore machined by laser | Small heat-affected zone, low edge chipping, controlled bore position |
| 9. Chamfering | Edges finished | Reduces chipping risk in handling and assembly |
| 10. Inspection | 100% AOI, vision measurement, optical density testing | Every disc is checked before shipment |
| 11. Packing & shipping | Discs packed for transport | Prevents scratches and impact damage |
All of these steps run in our own factory, with Class 10–10,000 cleanroom production.
Why photolithography
Compared with mechanical scribing, photolithography gives:
- cleaner line edges — the waveform is closer to ideal, which reduces harmonics and interpolation error on sin/cos discs (sin/cos signal quality);
- more uniform pitch — lower graduation error around the revolution;
- higher light/dark contrast — larger signal amplitude and better signal-to-noise ratio.
It also handles complex patterns such as absolute multi-track codes, which need accurate alignment between tracks.
Why the bore is cut after the pattern
Eccentricity between the pattern center and the bore center produces a once-per-revolution angular error of about e ÷ r. At a 20 mm reading radius, 5 µm of eccentricity is roughly 52″. That is why the bore should be machined referenced to the pattern, not drilled first and patterned later. For laser bore machining, machine vision first locates the pattern center, then the bore is cut. Read more in encoder disc eccentricity.
What we inspect
- AOI machine vision on every disc — graduation integrity, pitch, and surface defects (scratches, spots, chips).
- Vision measuring machine — critical dimensions such as outer diameter, bore and pattern-to-bore concentricity.
- Optical density testing — transparent and opaque areas.
If you are setting up incoming inspection on your side, see our incoming inspection checklist.
Specifying a glass disc
To quote a glass encoder disc, we need the substrate preference, outer diameter, bore and thickness with tolerances, line count or bit count, reading radius, concentricity requirement, readhead wavelength and opaque-area optical density. The full list is in our 20-parameter RFQ checklist.