Photo-Etched Encoder Discs for Robot Joints: Concentricity Before Slot Count

A photo etched encoder disc can carry thousands of clean transitions and still deliver poor angular accuracy if the pattern is not concentric with the shaft datum. On a robot joint, radial runout converts the entire track into a repeating position error. The first drawing question is therefore not how many slots fit around the circumference. It is which feature locates the disc to the axis that the bearing actually establishes.

Photo etched encoder disc accuracy begins at the datum

If the optical track is dimensioned from an arbitrary outer diameter while the part is assembled through a bore or three mounting holes, the supplier has no unambiguous functional reference. The outer profile may only be a handling edge. The bore or bolt-circle pattern normally needs to control the graduation track, and the inspection setup should reproduce that constraint.

Total indicated runout at the scale track combines pattern placement, bore position, bore clearance, hub fit, fastener shift and disc flatness. Tightening only the etched-pattern tolerance does not remove clearance between the disc and hub. A practical drawing separates the contributors: radial position of the track to the primary datum, position of mounting holes, allowable bore clearance, flatness in the read-head zone and the assembly method used to center the disc.

Renishaw's encoder-accuracy guidance identifies scale eccentricity as a once-per-revolution error, while HEIDENHAIN describes optical scanning of fine graduation structures. Those principles still apply when the scale is metal rather than glass. Pattern quality and installation geometry have to be treated as one system.

Define how concentricity will be inspected

Do not specify concentricity without a measurement method. A vision system can fit a circle through selected slot centers relative to the datum bore. A functional gauge can reproduce the hub fit. A coordinate machine may be useful for mounting features but inefficient for thousands of narrow apertures. Agree on which slots are sampled, how the fitted circle is calculated and whether the disc is free, vacuum-held or clamped during inspection.

A long chain of coordinate dimensions also hides the functional relationship. Basic dimensions from a common datum system, combined with position or runout requirements appropriate to the assembly, are usually clearer. The goal is not to decorate the drawing with tight numbers. It is to make the measured result predict the reading head's experience.

Slot width, web width and thickness move together

Photochemical machining opens an aperture from both faces. The sidewall is not perfectly vertical, and lateral etching creates undercut. As the metal becomes thicker, a narrow slot becomes harder to hold without a noticeable difference between the mask opening and the finished entrance and exit. Slot width, web width and material thickness must therefore be reviewed together.

Thin stainless steel and nickel alloys are common candidates because they combine stiffness, corrosion resistance and clean photo-patterning. The final choice depends on disc diameter, unsupported span, hub design, temperature and optical finish. A thinner foil permits finer geometry but can dish during handling or coating. A thicker disc handles better but pushes narrow slots closer to the process boundary.

State where the critical slot width is measured. For a transmissive reader, the entrance dimension, exit dimension and projected optical opening may not be equivalent. If two-sided artwork is used, front-to-back alignment contributes to edge position. Representative measurements should cover several angular locations rather than only one average.

Flatness and finish can undo accurate artwork

Local flatness over the optical track keeps the air gap stable. Global free-state flatness may be less useful when the center is clamped and the outer ring is free. Define the supported condition used for measurement. A washer or hub that introduces a slight cone can move the track axially even when the loose part passed inspection.

A dark finish may reduce stray reflection, but coating thickness and edge build can change the optical threshold on fine slots. Chemical etching avoids the raised shear burr of stamping, yet an etched edge still has a profile and can retain residue if cleaning is poor. Validate contrast, cleanliness and air gap with the actual emitter and detector.

When another process is the better choice

Photo etching is attractive for flat, thin encoder patterns because artwork revisions do not require hard tooling and the full pattern is created at once. It is not automatically right for every encoder. A glass scale made by photolithography can deliver finer graduations for metrology-grade systems. Laser cutting may suit quick prototypes with coarser slots, although heat input and taper need review. Stamping becomes economical at high volume when the pattern tolerates progressive tooling. Machining is stronger when the disc needs a thick hub, steps or other three-dimensional features.

The comparison should include centering hardware, coating, inspection, design revisions and annual volume—not only the piece price of the flat disc.

Key Takeaways

• Control the optical track from the functional shaft datum, not a cosmetic outer edge.

• Budget pattern placement, hub clearance, fastener shift and flatness as one eccentricity stack.

• Review slot width and web width against material thickness before freezing the drawing.

• Specify inspection conditions and optical finish with the read-head geometry in mind.

• Compare photo etching with glass lithography, laser cutting and stamping at assembly level.

FAQ

What should be the primary datum for a metal encoder disc?

Use the feature that locates the disc to the rotating axis—often the bore or mounting pattern—and control the optical track from it. An outer diameter should be a datum only if it performs that locating function.

Does photo etching produce a perfectly square slot wall?

No. Chemical attack creates an etched sidewall and some undercut. The acceptable profile depends on material thickness, slot width, optical direction and the threshold used by the reader.

Can an etched metal disc replace a glass encoder scale?

Sometimes, but not by default. Metal discs are robust and economical for many compact or moderate-resolution systems; glass or deposited scales may be better when very fine pitch, exceptional accuracy or thermal stability dominates.

Sources and Engineering References

Renishaw, The accuracy of rotary encoders — accessed October 2, 2026.

HEIDENHAIN, Modular angle encoders with optical scanning — accessed October 2, 2026.

Discuss Your Drawing with Metching

Send Metching the track diameter, slot geometry, material and thickness, functional datums, flatness zone and annual volume. We will review the photo etched encoder disc as a manufacturable flat-metal component and flag dimensions that deserve a tolerance discussion. Review our chemical etching capabilities before sending the drawing.