Should a Watch Case Be Polished Before or After CNC?

Polish first, then cut: a 0.03 mm mirror step on a zinc watch case is produced by CNC machining after the polishing pass, because the wheel takes an uneven layer off and no later polish puts metal back. The chain fixes that order; the drawing has to match it.

How much does polishing remove from the 0.03 mm step?

The wheel takes off an uneven layer. A wheel pressed on a curved case changes contact pressure along the flank, so the amount removed at the bezel seat differs from the amount removed on the side.

The step measures 0.03 mm, and the whole callout fits inside that number. Plating alone adds 18–27 µm across copper, nickel and chrome, 60–90% of the 0.03 mm band, so a step polished to size lands outside nominal once the bath adds its layer; the cut has to come after the wheel.

That is why the chain runs die casting, degating, tumbling, polishing, CNC and only then the plating line. Polishing cleans the surface, and the cut sets the step. Removal is not a stock figure a quote can fix, because it follows the surface it meets.

Why can the mirror step not survive the polishing wheel?

A polishing wheel is a compliant tool: it bridges a step smaller than its contact patch and rounds the edge instead of cutting it square. Wheel pressure concentrates on the highest point it meets, which is exactly the corner the step needs, and the mirror step is a 0.03 mm change of plane.

Once the wheel has taken the high side down, the seat sits low against the bezel. The joint shows a gap, and the case measures under the step at the seat while the rim stays nominal.

No polishing pass puts metal back. The part goes back to the machine for a cut, and the cost lands on the second operation rather than on the first.

Which order holds the 0.03 mm step, polishing first or CNC first?

Polishing first holds it, because the cut is the last operation that can generate the step. Reverse the two and the wheel works on a finished feature, and the 0.03 mm is met by luck.

The reverse order also changes what the polishing pass is for. Polishing after the cut turns a surface operation into a sizing operation, and the wheel cannot hold the 0.03 mm step on a 38.0×38.0 mm Zamak 5 case made to ASTM B86 AC41A.

Two quotes for the same drawing look different here. One prices the chain above; the other prices a polish after the cut, and the second clamping it hides shows up as a cheaper number that cannot be held.

How many extra setups does polishing after CNC add?

One more clamping, which brings three things: a second fixture, a fresh datum, and the step cut back in. A polished face cannot be clamped without marking it, so the cut and the polish cannot share a setup once the step is finished.

Inspection puts the part on a surface plate and reads the gap with a feeler gauge, thin steel leaves, against the 0.03 mm target. A shop that polishes after the cut has to re-fixture the part to read it, and the reading follows the clamp.

Each setup is a line in the machining section of the quote and a chance to scrap a finished case. A quote that leaves the second setup out is not cheaper; it has not counted the operation.

What must a case drawing say about the 0.03 mm step?

Four lines: the step at 0.03 mm as a design target, the chain that produces it, the gauge, and whether the number applies before or after plating. The last decides who pays for a case that measures over.

Step and surface finish stay separate callouts. Polishing is judged on appearance, adhesion on the plating test, the step on a surface plate and a feeler gauge, and one folded number leaves the seat without a criterion. The 18–27 µm of plating belongs on the same sheet, because it moves the face the step is measured on.

A drawing written this way gives every quote the same basis: the supplier quotes the chain rather than a single operation, and rejects on the same criterion the order used.

Send the drawing with the step, the finish and the plating callout on one sheet.