Why outsourced CAD files fail at the bench

Most jewellers who have outsourced CAD have at least one story about a file that rendered beautifully and then failed somewhere between the printer and the setter. The frustrating part is that these failures are almost never visible in the preview images you approve. They live in the geometry.

Here is what actually goes wrong, in roughly the order you will encounter it, and what to check before a file goes anywhere near metal.

It will not print or slice cleanly

A mesh has to be watertight to be manufacturable: no open edges, no flipped normals, no surfaces intersecting themselves. A model can look perfect in a render and still be full of holes the renderer politely ignores. Your printer or bureau will usually catch this, but they catch it after you have paid for the file and lost the day.

Sections that starve or shrink

Two casting problems, both decided in the model long before the metal is melted.

• Too thin and the metal solidifies before it fills the cavity. You get an incomplete prong or a gap in a fine gallery, and the piece is scrap.
• Too thick, or an abrupt jump from thick to thin, and the section cools unevenly and traps shrinkage porosity inside the metal. This one is worse, because it often appears only at final polishing, after the setting work has been done.

Minimum wall thickness is not a universal number. It depends on the alloy, and a model built to silver tolerances and then cast in platinum, or the reverse, is a model built for a different piece.

Pavé that cannot be set

This is the most common expensive failure. Seats are laid out to look evenly spaced on screen without enough metal left between them for a setter to raise beads. The file is geometrically fine and practically unusable. Your setter finds out first, usually with the stones already bought.

The fix is that pavé should be modelled against the actual stone sizes you hold, not against a pattern that assumes them.

Seats cut to a carat weight instead of a stone

A nominal carat weight is not a measurement. Two stones sold at the same weight can differ enough in diameter and depth that a seat cut for one will not accept the other, or will sit it too high. If your supplier did not ask for millimetre dimensions of every stone, including the melee, they have guessed.

Size expressed in the wrong system

UK, US, European and Japanese sizing scales do not align, and a conversion applied from the wrong starting scale puts you out by a full size. For an eternity band or a full-circumference pattern, that is not an adjustment at the bench, it is a remodel. Internal diameter in millimetres removes the ambiguity entirely, and it is what a model is built to anyway.

Resolution and format mismatches

An export too coarse shows faint terracing on what should be a polished curve, and filing it out takes metal and detail with it. Millegrain and fine engraving are the first things lost. Too fine and you have an enormous file that slows every step for no visible gain. And a manufacturer who needs to adjust the geometry cannot usefully do it in a mesh at all, which is what STEP is for.

A checklist before you commit metal

• Confirm the mesh is watertight and manifold, and that your bureau has accepted it rather than merely received it.
• Confirm minimum wall thicknesses were built for the alloy you are actually casting.
• Confirm every seat, including melee, was modelled to measured stone dimensions you supplied.
• Confirm pavé spacing has been checked for settability, ideally by whoever will set it.
• Confirm ring size as an internal diameter in millimetres, not a letter or a number in an unnamed system.
• Confirm the export resolution and format match what your caster asked for.
• Get the render approved by your customer in writing before anything is grown or cast.

What prevents most of it

Almost every failure above traces back to information the modeller did not have. Tell your supplier the alloy, the caster and their requirements, the real stone dimensions, the intended finish, and whether the piece is a one-off or the first of a repeated line. A model built to those constraints from the first pull is a different object from one adapted to them afterwards.