Why Do Crown Notches Collapse During Polishing?

Crown notches collapse because a polishing wheel meets a 0.9–1.2 mm wall and rounds it. Milling the notch after polishing, and telling the finishing shop to leave the thin flank alone, keeps the corner square; tumbling and hand polishing both run before that cut.

How thin is the case wall beside the crown notch?

The wall beside the notch is a design target of 0.9–1.2 mm on a Zamak 5 case to ASTM B86 AC41A. The crown hole through it is Ø3.0 mm with an upper deviation of 0.05 mm and a lower deviation of 0. The notch opens at 3 o'clock, where the crown tube enters the case.

At that thickness the wall moves when the polishing wheel presses on it. The wheel straddles the opening, the edge under it has nothing behind it, and the corner gives before the metal is cut.

Reworking the die is not the only fix: leave the notch to be milled after polishing, and the collapse comes from hand-polishing round the thin edge. Moving the notch or thickening the wall is the other route, paid once, but it need not be paid to stop a collapse.

Why does a polishing wheel round the crown notch edge?

A polishing wheel is soft and follows the shape it is given, so it conforms to the corner instead of cutting it square. Pressure concentrates on the highest point, which is the notch corner, and a 0.9 mm flank gives the wheel nothing to resist it.

What the drawing called a square corner becomes a radius set by the wheel. The crown tube then sits against a face that no longer matches it, and the gap shows when the crown is pressed home.

Rework at this stage means polishing a finished case, and a case that has been through the plating line cannot be polished without stripping it. The strip is the cost of catching the notch late.

Why must the crown notch be milled after polishing?

Because the cut is the only operation that restores the corner. The notch is milled in the CNC step, which follows polishing, so the geometry is generated after the wheel has passed over it.

Overflow sits at 3 o'clock on both sides of the crown notch, wells that hold the cold end of the shot, so the as-cast edge carries extra metal there. The mill takes that metal off, and the corner it leaves is cut rather than wheel-formed.

The extra is the post-polish CNC clamping, with the seat, both lug bores and the notch still in one setup, not a second setup for the notch; that clamping is why a notched case carries a longer route than a plain one. A quote that skips the notch cut is pricing a different part.

How is the crown notch corner gauged against a master?

The master goes on a profile projector, an optical instrument that traces the corner outline, and is read at the notch and at the wall beside it. The master is the signed first article, not the drawing.

A visual pass under the D65 light box catches colour and polish, not a corner radius. The projector turns a looks-right judgement into a number that can be accepted or rejected.

Whoever signs the first article owns the appearance risk, and that signature is what the rest of the order is judged against. A sign-off without the projector reading leaves the corner to opinion.

What should the finisher be told about the notch?

Tell the finishing shop which edge to stay off, and put that instruction in the enquiry rather than in a phone call. The notch flanks are the thin section, and a wheel that crosses them rounds the corner the mill is about to cut.

The same note should name the appearance face and the polish level, because tumbling and hand polishing are judged there. A shop told to polish the case will treat the notch like any other edge.

Writing 'do not touch the thin flank' into the RFQ costs nothing; after the first lot comes back, it is the outside shop's second pass, and the schedule carries it.

Send the notch location, the master sample and the instruction to stay off the thin flank with the enquiry.