Can a Plated Zamak Case Hold a 0.03 mm Step Height?

Yes, a plated Zamak case can hold a 0.03 mm step height, if the callout sits on the machined seat and is read before the plating line. Copper, nickel and chrome add 18–27 µm and raise that face; post machining sets the seat, and the drawing has to name the stage.

Is the step height measured on the plated face or on the bare frame?

On the machined seat, on a surface plate with a feeler gauge, thin steel leaves that read the gap, and the callout names the stage. The mirror seat is the face the bezel lands on in a Zamak 5 case to ASTM B86 AC41A, and it is cut before the plating line.

Measuring after plating reads the coating as well as the seat. Because the coating is 18–27 µm and the tolerance is 0.03 mm, the stage decides the result: the same case can pass before the bath and fail after it.

The stage also decides who pays for an oversize case, and which operation is in the quote. A drawing silent on the stage hands that argument to whoever inspects first.

What do 18–27 µm of plating add to a 0.03 mm face?

The plating adds its full thickness to every plated face. Across a Ø1.5 mm bore it lands on both sides, 2 × 18–27 µm = 0.036–0.054 mm; on a flat seat the face rises by the same 18–27 µm.

Here the 0.03 mm is step height, not flatness: plating is conformal, so the shape does not change and the height does. That rise is 60–90% of the band, and a bezel fitted to the bare case no longer sits at the same height.

A dimension that moves by most of its own tolerance after the last operation is a drawing problem. Fixing it after plating means a second cut on a plated part.

Why can the mirror seat not be machined after plating?

Because cutting a plated face opens the coating. Chrome is 0.1–0.3 µm, nickel runs 10–15 µm, and a cut deep enough to correct step height goes through both and exposes copper or zinc.

The chain puts CNC before the plating line for that reason. Re-cutting after plating means stripping the part, machining it and plating it again; every strip puts the thin walls and edges back through the bath, and the size moves again. A strip bath also removes the evidence of what the seat measured, so the correction starts without a baseline.

That cycle is quoted as a second pass through the whole line, and that is why a case rejected for step height after plating is usually scrapped rather than reworked.

Which face sets the datum for the 0.03 mm seat?

The mirror step. It is the flat the bezel lands on, and it is machined in the same clamping as the inner cavity, so the 0.03 mm callout and the datum come from one setup.

Take the datum from the cast outer wall instead and the reading follows the clamp. The 30.0 mm inner cavity, upper deviation 0.05 mm and lower deviation 0, is machined against that same datum, so moving it moves two callouts at once.

A datum change is a drawing change, and a drawing change re-opens the quote and the fixture. The gauge that passed the first article no longer matches the part.

How should step height acceptance be written on the drawing?

Four lines: the seat at 0.03 mm, the stage it applies to, the gauge of a surface plate and feeler gauge, and the sampling ratio, set by batch size. With those written, an inspector accepts or rejects without a conversation.

The plating thickness belongs on the same sheet, because it is what moves the face. Copper 8–12 µm, nickel 10–15 µm and chrome 0.1–0.3 µm fix the total the seat has to absorb.

Acceptance written this way also fixes the rework clause. A case that fails the seat before plating goes back to the machine; one that fails after plating goes to scrap.

Send the drawing with the seat, the plating thickness and the measuring stage on one sheet.