Why Does a Plated Bezel Ring Fail to Press Into the Case?

Because 18–27 µm of copper, nickel and chrome lands on every face of a Zamak 5 ring cast to ASTM B86 AC41A, and its interference fit is only 0.02–0.05 mm. A ring that pressed in before plating can arrive oversized by more than its whole allowance, which is why the press fit is checked at 200–600 N on a trial case rather than by hand.

Interference is the overlap between the ring and the case opening before assembly, set at the post-machining stage: the ring is made slightly larger than the bore it enters, and the squeeze holds it in place with adhesive. Plating adds to that overlap on the outside of the ring and subtracts from it on the inside, so the same drawing produces two different fits before and after the bath.

Why does a plated ring no longer press into the case?

The bore of the ring closes. A Ø32.0 mm opening with +0.05/0 mm is coated on its wall, and the coating grows inward from both sides, so the opening loses 0.036–0.054 mm across the diameter.

Outside the ring, the coating grows at the same time. Both changes push the fit tighter, and the interference that was designed as 0.02–0.05 mm can exceed its upper limit before any force is applied.

The plating line can be in control and the ring still on drawing; the drawing simply did not say which stage the interference belongs to.

How much does 18–27 µm close the interference fit?

Across a diameter, 18–27 µm per face becomes 0.036–0.054 mm, which is larger than the entire 0.02–0.05 mm interference band. A ring built at the tight end of the band is already beyond its fit before plating.

Written on the drawing, the arithmetic turns an argument into a number: the plating specification fixes the total, and the bore has to absorb it.

That is why a plated ring can pass a bore gauge at the machine and still fail at the press. The gauge reads metal; the press reads metal plus coating.

What press force proves the fit?

A trial press with the mating case, read as force, proves it. The band is 200–600 N: below it the ring is loose enough to lift or rotate, above it the ring distorts or the coating cracks.

Force beats feel because it repeats across operators and shifts; a fit that needs a hammer and one that drops in both pass a visual check.

The trial belongs in the first-article report with the force and the case it was pressed into. Once the band is recorded, a new lot can be compared against a number instead of against the last lot.

Should interference be specified before or after plating?

Before plating, with the plating thickness called out separately. The bore is cut to the pre-plate size on CNC turning, the coating brings it to the drawing size, and the interference is defined on the coated part.

The alternative, specifying the finished bore and letting the machine guess the allowance, pushes the calculation into the shop. Two suppliers then use two allowances, and the second one is wrong at the press.

Both numbers belong on the drawing: one is what the machine cuts, the other is what the customer measures.

Which drawing callouts prevent a press-fit failure?

Four: the interference band, the pre-plate bore, the plating thickness by layer, and the press-force band with its trial case. Add the stage, and the drawing can be quoted and inspected without a conversation.

Thickness by layer matters because the total closes the fit, and a supplier who only lists chrome misses the two layers underneath. The 0.1–0.3 µm of chrome is not the problem; the copper and nickel below it are.

The press-force band needs the case it is measured against, since force depends on both parts. A ring checked against a worn trial case passes on a number that means nothing.

Send the interference band with the pre-plate bore and the trial case in the enquiry. The press fit can then be quoted against a number instead of a hope, and the post-machining and plating steps can be priced against the same stage.