Six Features That Look Perfect in CAD but Fail on the Bench

CAD renders everything as possible. The bench disagrees with some of it. The features below are not rare design mistakes — they are the ones that arrive in drawings week after week, look perfect on screen, and fail the moment someone tries to make them in metal.

1. Knife-edge walls

A wall modelled at the thinnest dimension that still looks solid on screen becomes a knife after polishing — and a knife edge bends, rolls and wears through. There is a workable minimum that depends on the metal and the span, and it is a finishing question, not a rendering question. The wall thickness decision belongs at the CAD stage, because it cannot be fixed later.

2. Prongs thinner than the stone they hold

A large stone on thin prongs is a lever waiting for a moment. The prong has to be proportional to the stone's height and the load direction, and it has to survive finishing with usable metal left. If the drawing shows a 6 mm stone on prongs modelled to match it visually, the first thing the bench will ask is what the piece is for.

3. Closed cavities

A hollow form with no opening cannot be finished inside. Polishing compound and rinse water go in and do not come out; the interior stays rough, traps residue, and eventually shows at the surface as a stain or a smell. Hollow construction is legitimate — the hollow vs solid discussion covers when it earns its keep — but a closed void with no service access is not a construction choice, it is a defect designed in.

4. Blind angles and deep undercuts

Finishing is not decoration; it is where the piece gets its surface, and surface is what the customer touches. A geometry that a polishing wheel, a file or a brush cannot physically reach does not get finished — it gets abandoned at whatever surface it had. Deep undercuts and internal corners drawn without tool access are the most common reason a "simple" design takes three times the quoted bench hours.

5. Dissimilar-metal joints at stress points

Joining platinum to gold at a hinge or clasp puts two different working behaviours into one small volume: different expansion under heat, different hardness under load. The joint becomes the weak link of the piece by design. It can be done — the metal comparison covers where each belongs — but it belongs away from the stress point, not at it.

6. Stone diameter against wall thickness

A large stone on a thin shank turns the shank into a lever: every impact on the stone loads the wall in bending. The ratio between stone size and the section that carries it decides whether the piece survives daily wear or fails in the first month. This ratio is invisible in a render and obvious on a bench.

What a good manufacturer does with an unmakeable design

Tells you early, with the reason and the alternative — not silently changes your design, and not accepts it and fails later. A workshop that returns a drawing marked with bench objections is doing you a favour that a workshop answering "no problem" is not. The mark-up is where the manufacturing conversation should happen, before the first gram of metal is committed.

What to send with the design

The intended wear, the metal, the stone sizes, and the quantity you expect. These four change the manufacturability answer: a display piece and a daily-wear ring with identical drawings have different workable minimums. What to send for an accurate quote covers the full list.