Why Does a Plated Retaining Ring No Longer Fit the Case?

A plated retaining ring stops fitting the case when coating pushes the outer diameter past its limit, and the Ø26.0 mm callout at 0/-0.05 mm allows nothing above nominal. Nickel at 3–5 µm per side adds 0.006–0.010 mm; a Cu/Ni/Cr stack of 13.1–20.3 µm adds 0.026–0.041 mm, more than half the 0.05 mm band. Either figure can turn a ring that gauged true before plating into one that will not enter the bore.

The part is a Ø26 × 2.0 mm ring in Zamak 5 to ASTM B86 AC41A, at 1.8±0.2 g, with a 0.8 mm wall and three claws at 120°. The die casting reaches final diameter before the rack; trimming and polishing come first.

How much does plating grow a 26 mm ring?

Diameter growth equals twice the deposit thickness. Nickel at 3–5 µm measures 0.006–0.010 mm, and a Cu 8–12 µm, Ni 5–8 µm and Cr 0.1–0.3 µm stack totals 13.1–20.3 µm, which measures 0.026–0.041 mm on the diameter. Both are example calculations.

Against the 0.05 mm band, the three-layer case spends more than half the allowance. Out-of-round from the die and gauge wear draw on the same 0.05 mm, so a finish picked for colour can close the window.

Why is the outer diameter tolerance one-sided?

Ø26.0 0/-0.05 mm is a shaft fit written for a bore: the ring may sit at nominal or below, never above. A ring over nominal enters with interference, and the press climbs past the 80–200 N window the claw roots were sized for. A ring inside the band still centres on three claws.

The one-sided callout also gives the plater direction. A tolerance with a plus side would open allowance above nominal, where the coating cannot go. Writing 0/-0.05 mm states that the blank runs undersized and the deposit closes the gap.

Should the drawing be pre-plate or post-plate?

Two numbers belong on the drawing: the blank at 0/-0.05 mm on Ø26.0 mm and the deposit at 3–5 µm nickel or 13.1–20.3 µm for the stack. The die caster holds the blank, the plater the layer, the first article reads both. A post-plate callout makes the plater work back to an unwritten blank size.

Either choice changes the quote. Pre-plate sizing adds a gauge step before the rack and a reading after it; post-plate sizing adds a blank target and a second inspection. The drawing decides which lines a supplier prices, and who carries an oversize batch.

Which gauge settles a fit argument?

A Ø26.0 mm ring gauge, go and no-go, settles the diameter without the mating case. The go side proves the ring enters the bore, the no-go side proves it does not sit loose at the top of the band. A ring that fails the no-go side is over nominal whatever a sample case does.

The gauge reads size, not seating force. Press force is measured on a bench with the mating case and has to land between 80 N and 200 N. Put both readings and the X-ray film at 3 µm or more in the first-article report.

What happens to a ring that is already too large?

A ring reading over Ø26.0 mm has no cheap route back. Stripping and plating again removes the finish, returns the diameter to a blank that may sit high, and adds a second pass of variation. The purchase order has to say who pays for the rework, and at what reading the ring is scrapped.

The better answer sits upstream. A pre-plate target, a stated stack and a ring gauge at goods-in catch the drift while the blank can still move. Those three items turn a fit complaint into a drawing note a second plater can price.

Send the drawing with the pre-plate diameter, the deposit stack and the ring-gauge callout on it, plus the case the press force is read against. Those inputs let a plater price the finish and an inspector reject an oversize ring on a reading.