Can a 1.8 mm Zamak Bezel Ring Hold 0.05 mm Roundness?

Yes: a 1.8 mm Zamak 5 bezel ring, cast to ASTM B86 AC41A, holds 0.05 mm roundness when the ring is turned after polishing, the chucking load stays low, and the gauge is named on the drawing. Roundness means the outer circle stays within 0.05 mm of a true circle, not smoothness; the bore and the outside are cut in one clamping on CNC turning.

An oval ring leaks at one side and binds at the other, so roundness is a fit dimension before a cosmetic one.

Why does a 1.8 mm ring lose roundness after polishing?

It does not lose roundness at the polishing wheel. The ovality is already in the ring when it leaves the die and the chuck, and polishing only removes the high side far enough to make it visible.

Two variables set it. A thin ring cools unevenly in the die, because the wall runs 1.5–1.8 mm and the ring has no thick section to hold heat. Then the chuck closes on that same thin wall and pushes it out of round; the cut is made in the deformed state, and the error comes back when the jaws open.

Polishing a ring to make it round is the wrong order: the wheel removes metal where it can reach, not where the circle is high.

Which step actually holds the 0.05 mm roundness?

Turning holds it, and only turning. The boring and turning pass cuts the inner step, the bore and the outer diameter against one axis, so the circle is generated by the machine rather than copied from the casting.

That pass sits after deburring and polishing in the process chain, and before plating. Cutting before polishing leaves the polished skin as the final surface but loses the circle; cutting after polishing puts the circle back and leaves a machined face where the wheel had been.

A ring turned before polishing needs a second setup to restore the circle, and the second setup is where chucking error enters.

How does chucking load deform a thin ring during turning?

Three jaws closing on a 1.5–1.8 mm wall bend the ring into a triangle before the tool touches it. The tool then cuts a circle in the deformed body, and the ring springs back to a different shape when the jaws release.

The error repeats with the jaw positions: three high points at the bench, none in a fixture. That pattern is the signature of chucking, not of the die.

Control is mechanical rather than procedural: a softer clamping method, a support ring or a low-pressure chuck, and a spring pass that removes the last few hundredths without adding load. None of these change the drawing; they change what the machine does with the same drawing.

How is roundness gauged on a production ring?

A roundness instrument reads the circle directly; a three-point measurement with a micrometer reads it well enough for a shop floor check. A caliper across the outer diameter does not read roundness at all, because it averages the error it is meant to find.

The gauge belongs in the first-article report with the reading and the setup it came from. A ring that measures 0.05 in the fixture and 0.08 free is not a 0.05 ring, and the report should say which condition was recorded.

Sampling follows the lot, not the day: a first article plus a check at the start and end of each run catches a chuck that has drifted.

What must the drawing say before a ring is quoted?

Five values, one gauge and one stage. The five are the outer diameter, the bore, the height, roundness and flatness; the gauge is the roundness instrument or the three-point method; the stage says whether the values apply before or after the 18–27 µm plating.

Leave the stage out and two suppliers quote two different parts: a bore signed before plating arrives undersize once 18–27 µm lands on every face.

A drawing that carries all seven items can be quoted against a gauge and a stage rather than against a promise. Send the drawing with the five values, the gauge and the stage, and the quote can be checked line by line.

The same ring must also survive the press fit after plating, checked at the post-machining stage against the roundness above.