A Finish Schedule, Not an Adjective: Writing Cosmetic Requirements for Chinese CNC Parts

A finish callout is usually the last line added to a drawing and the first line disputed at goods-in. The pattern is familiar: a UK buyer writes 'black anodised' in a notes block, a supplier in Ningbo or Shenzhen quotes against it, and four weeks later two reasonable parties are looking at the same components and reaching opposite conclusions. Nothing has gone wrong in the machining. What has gone wrong is that a visual judgement has been asked to do the work of a specification.

The remedy is unglamorous. It is a finish schedule — a short annex attached to the purchase order that states process, standard, thickness, colour tolerance, gloss, defect limits, masking requirements and inspection conditions. Written properly, it takes an hour. Omitted, it can cost a batch.

Why 'Black Anodised' Is Not a Specification

The phrase describes a family of processes, not an outcome. Sulphuric anodising to BS EN ISO 7599 on 6082-T6 produces a neutral, slightly blue-black when dyed; the same dyed process on 7075 produces a warmer, bronze-tinged black because of the copper content in the alloy. Film thickness affects perceived darkness. Sealing method affects sheen. Two suppliers quoting 'black anodised' may both be entirely competent and still deliver parts that do not match each other, let alone the sample in your hand.

The same applies to powder coating. 'Paint to match RAL 9005' is a colour reference, not a tolerance. RAL is a collection of nominal colours; it does not define an acceptable colour difference. Nor does it say anything about gloss level, film thickness, texture, adhesion or which surfaces must remain coating-free for electrical bonding.

The Three Tiers of a Usable Callout

A finish callout that can be inspected has three tiers.

The process tier names the specification: sulphuric anodising to BS EN ISO 7599, Type II, film thickness 15 µm minimum measured by eddy current per BS EN ISO 2360 on a coupon processed with the batch; or polyester powder coating to a stated corrosivity category under ISO 12944, nominal film thickness 70 µm, adhesion by cross-cut per ISO 2409 with a result of class 1 or better.

The appearance tier fixes colour and texture numerically where possible: colour difference ΔE ≤ 2.0 against the approved reference plaque under D65 illumination, measured with a spectrophotometer at 45/0 geometry; 60° gloss per ISO 2813 within a stated band; texture matched to a retained physical sample rather than a photograph.

The defect tier defines what is not permitted, surface zone by surface zone. Scratch limits expressed in millimetres. No visible tool marks at a stated viewing distance. No dye pooling, no bare edges, no coating build-up in threaded holes, no colour variation between parts of the same batch beyond the ΔE limit.

Separating Visible from Functional Surfaces

The single most useful discipline is to zone the part. Mark the drawing with A, B and C surfaces.

A surfaces are customer-visible and receive the full cosmetic treatment: tight colour and gloss control, strict defect limits, higher surface finish before coating so that machining marks do not telegraph through a 60 µm film. B surfaces are semi-visible — internal faces, recesses, surfaces seen at an angle — and can carry a relaxed defect schedule. C surfaces are functional only: mounting faces, mating bores, datum features, earthing points. These should usually be masked, and the drawing must say so explicitly.

Masking deserves its own line. Anodising is an insulator; if an earthing boss is anodised, the assembly may fail a bonding test long after the parts have been accepted. Threads grow by roughly half the coating thickness per flank. Sealing faces and bearing bores should be identified as no-finish zones with a defined boundary, and the supplier should confirm in the first article report that masking was actually applied.

Colour, Gloss and Texture: Setting a Defensible Grey Area

Dyed anodising and powder coating both vary within a batch and between batches, and no specification will eliminate that. What a specification can do is define the acceptable envelope and the reference point.

For colour, nominate a physical reference — a plaque or a first-off component — and state the measurement method and the maximum ΔE. For texture, retain a sample rather than describing it; 'fine texture' means nothing across a language boundary, whereas a sealed sample does. For gloss, give a number and a tolerance band, because a matte coating and a satin coating of the same colour will read as different finishes to anyone looking at them side by side.

Where the finish is decorative and batch continuity matters — a product range where panels from different orders sit next to each other — say so. A supplier who knows that a second order must match the first will retain the reference and the process parameters. A supplier who is not told will optimise each batch independently.

Inspection Lighting and the Sample Boundary

Cosmetic inspection without stated conditions is an opinion. Set the conditions.

Specify a viewing distance, typically 300 to 500 mm for a handheld part. Specify illuminance, commonly 500 to 1,000 lux at the surface. Specify the light source — D65 is the sensible default for colour work — and specify that inspection takes place in a controlled booth rather than at a bench under whatever fluorescent tube happens to be overhead. UK winter daylight is a poor and inconsistent judge, and a part that looks acceptable in a north-facing office at 3pm may look distinctly purple under a factory LED.

Then set the sample boundary. A limited approval sample, signed and dated by both parties, photographed at 1:1 with a scale and a colour chart in frame, sealed and retained by each side. Consider retaining two samples: an acceptable limit and a reject limit, so that the argument is about where a part sits between them rather than whether it is 'bad'.

Give the reference an expiry. Dyed anodic films and some coatings shift over twelve to twenty-four months, and a three-year-old sample is not a fair comparator. Note the review date in the annex.

Putting It Into the Purchase Order

A finish annex is a table, not prose. One row per part number and surface zone, with columns for process, standard, thickness, colour reference, tolerance, gloss, defect class, masking notes and inspection method. Add a line requiring the supplier to confirm acceptance of the annex before the purchase order is acknowledged, and a line requiring finished first-off samples to be submitted for written approval before the balance of the batch is processed.

The cost of a finish annex is a small amount of engineering time. The cost of its absence is a batch that is technically compliant and commercially unusable — the worst of both outcomes.