A build-to-print mask plate for a European laser-systems OEM: 238 mm long, 24.5 mm thick, two surface finishes on one face, a hard-edged two-tone paint job and an engraved pattern field. This is the fourteen-operation route we ran — and the six drawing questions we settled before the first chip was cut.
The customer supplied a two-sheet drawing (revision C) plus the current STEP model. The part is a mask plate that sits in the optical path of a laser processing system: a 238 × 128 × 24.5 mm plate of cast PMMA, 125 cm³ of material removed, machined on all six sides.
It carries an engraved pattern field of several hundred small features on a ±0.1 mm position chain, 20 reamed H7 bores for dowel pins and optical mounts, two M8 threads, a 0.2 mm-deep identification engraving, polished windows at Ra 0.2 set inside ordinarily milled Ra 3.2 fields, and a white paint layer that stops on a hard, masked boundary. General tolerances are ISO 2768-mK with ±0.1 mm on coordinate dimensions; the GD&T follows ISO 8015.
Every requirement interacts with another. Paint adds thickness to a ±0.1 mm stack, polishing one window means protecting another, and every cut relieves stress in a material that is seven times more thermally expansive than aluminium.
| Item | Requirement |
|---|---|
| Envelope | 238 × 128 × 24.5 mm, 125 cm³ removed |
| Bores | 16 × Ø6.6 H7, 2 × Ø10 H7, 2 × Ø5.6 H7 — all Ø0.02 to A·B |
| Threads | 2 × M8, plus Ø7.4 and Ø135 counterbores |
| Form | Flatness 0.05, parallelism 0.05 to C |
| Finish | Ra 0.2 polished windows, Ra 3.2 general |
| Coating | White mask paint per customer spec 1008508, defined paint-free zones |
| Marking | 0.2 mm engraving, customer coding rule YYMMDD-##XX |
| Post-treatment | Tempering 80 °C, min. 8 h |
Cast PMMA expands about 70 × 10⁻⁶/K. Across 238 mm, one degree of shop-temperature drift is 0.017 mm — almost the entire Ø0.02 mm true-position tolerance. The part is measured at 20 ±1 °C after a 24-hour soak, or the CMM report is fiction.
Acrylic relieves internal stress as material is removed. Rough, rest, temper, semi-finish, finish — each stage separated by a stress-relief pause. Clamping is the other half: vacuum and low-pressure clamps only, never a vise jaw across a 238 mm span.
Ra 0.2 optical windows sit directly inside Ra 3.2 milled fields. Polishing has to stop on a defined edge without rounding it, and every subsequent step — masking, painting, engraving — has to leave that window untouched.
A 25–35 µm paint build sits inside a ±0.1 mm coordinate tolerance, and the paint boundary is a hard masked edge, not a fade-out. Mask registration, paint viscosity and de-masking timing all show up on the CMM report.
Every build-to-print job starts with a DFM review. Two notes contradicted each other between the German and Chinese annotation layers, and four features needed their function confirmed. All six were answered in writing within 48 hours, before programming.
| Query raised | Why it mattered | How it was closed |
|---|---|---|
| Engraving depth: 0.3 or 0.2 mm? | The Chinese note reads 0.3 mm, the German note directly above reads 0.2 mm. It changes cutter geometry, pass count and how the mark reads against the white paint. | Confirmed 0.2 mm ±0.03 into the substrate. Drawing corrected at rev C. |
| The 0.2 mm allowance — before paint or before polish? | Both readings appear in the notes. Wrong reading means a plate 0.2 mm under size after finishing. | Confirmed as polish stock on optical faces. Final dimensions gauged after paint. |
| Do paint-free zones include the 8 mm step and sealing land? | Paint on a sealing face is a leak path; masking it adds a manual operation and a visible edge line. | Confirmed masked. Step and land delivered paint-free with a clean de-masked edge. |
| Fit intent on 2 × Ø10 H7 and 2 × Ø5.6 H7 | Dowel-pin location versus clearance fastener decides whether we ream to the top of the band and add a lead-in chamfer. | Dowel location confirmed. Reamed to mid-upper band, 0.3 × 45° lead-in added. |
| Datum B definition for the Ø0.02 frames | With 20 bores on a 0.02 budget, the datum scheme must match how the part is located in the customer's fixture. | Datum scheme aligned to mounting face and pin pattern; CMM program mirrors the assembly. |
| Wall thickness at the 16 × Ø6.6 H7 edge row | Bores sit close to the 24.5 mm edge. Breakout would scrap the part at the last operation. | Checked against the model: sufficient wall, no breakout, drilling sequence fixed. |
This is where most overseas acrylic projects fail. Not in the machining — in an unanswered note. A written question costs two days; a misread note costs a 238 mm plate and a month.
One part per traveller card, every operation signed off. Temperatures, tools and inspection points below are the ones actually used on this part family.
Cast PMMA-GS sheet, batch certificate and CoC checked against the drawing. Sheet acclimatised 24 h before cutting.
STEP model compared feature by feature against both drawing sheets — 100 % match required at rev C.
0.6 mm rough stock, 0.15 mm semi-finish stock. Vacuum fixture on three-point support; no point loading across the span.
Uncoated two-flute carbide, high rake, air-oil mist only — emulsion coolant crazes cast acrylic.
Plate rests flat and unrestrained so roughing stress moves the part before any tolerance is cut.
Ramp-controlled oven, slow cool — done before finishing so paint solvents meet a stress-free surface.
Datum faces, the 8 mm step and sealing land to 0.15 mm stock. Datum A established and carried through.
All 20 H7 bores and the pattern cut in one fixturing so Ø0.02 is generated, not stacked. M8 hand-tapped.
Progressive diamond compound on felt, inside the windows only; gloss meter and profilometer checks.
Laser-cut vinyl masks on a light table, ±0.05 mm registration; acrylic-safe adhesive, no residue.
White paint per customer document, HVLP, two cross coats; film build 30 µm target inside ±0.1 mm.
Cross-hatch adhesion, colour against master chip, no bleed into polished windows.
Pattern field and serial cut last, after paint cure — fresh single-flute cutter at 30,000 rpm.
Full report at 20 ±1 °C, then PE film, double bag, foam-lined export crate.
| Requirement | Drawing spec | Method | Measured |
|---|---|---|---|
| Flatness | 0.05 | CMM, 25-point grid, 20 ±1 °C | 0.028 mm |
| Parallelism | 0.05 to C | CMM, two opposed planes | 0.031 mm |
| True position, 20 bores | Ø0.02 A B | CMM, three sections per bore | Ø0.014 mm |
| Ø6.6 H7 ×16 | +0.015 / 0 | Pin gauge, 100 % | all pass |
| Ø10 H7 ×2 / Ø5.6 H7 ×2 | +0.015 / 0 · +0.012 / 0 | Bore gauge + CMM | +0.006 … +0.011 |
| Optical windows | Ra 0.2 | Stylus profilometer + gloss meter | Ra 0.16–0.19 |
| Engraving depth | 0.2 ±0.03 | Depth gauge, 5 points per part | 0.20 mm |
| Paint film | 25–35 µm, white | DFT gauge, cross-hatch | 31 µm avg · 5B |
| Thickness after paint | 24.5 ±0.1 | CMM / micrometer, 8 points | 24.47 mm |
| Tempering | 80 °C, min. 8 h | Oven chart record | 8 h at 80 ±2 °C |
Delivered with every shipment: FAI report, CMM point data, material batch certificate, paint film and adhesion records, oven log, certificate of conformance, and a photo set per serial number.
For the customer, the value was not a cheap plate. It was a 238 mm optical part that arrives measurable, traceable and identical every time — on a component where a 20 µm error means the laser never finds its datum.
STEP + PDF, NDA on request. We come back with the questions we would ask before quoting — and a process route for your part.
