A brand in Seoul approved a CAD render of a signet ring, received the cast piece, and found the shank 0.4 mm thinner than the render. The CAD file was correct. The wax pattern had been printed on a resin printer at a scale factor that did not account for burnout shrinkage, and the casting had pulled the final gold piece below spec. CAD-to-casting is not a straight line: the digital file has to be compensated for wax print shrinkage, investment expansion, and alloy contraction. This guide covers the scale factors and the checkpoints between CAD file and cast ring.

Modern jewelry CAD (Rhino, MatrixGold, ZBrush exported to .stl) produces a digital model. The model is 3D-printed in castable wax or resin, then invested and burned out. Each step changes dimensions. A buyer who approves a render without asking about compensation will receive pieces that drift from the approved size by 0.5-1.5% over the whole part. For most rings that is invisible; for a half-eternity band where stone seats are milled, it is the difference between stones dropping in and stones not fitting.

The three shrinkage steps

- Wax/resin printing: photopolymer patterns shrink slightly as they cure, typically 0.3-0.7% depending on printer and wax brand.
- Investment expansion: the plaster-based investment expands during burnout, compensating partly for alloy shrinkage.
- Alloy solidification: gold contracts 4-5% by volume, roughly 1.5-2% linearly; silver contracts slightly less.

Experienced CAD setups apply a combined scale factor of about 1.010-1.020 (1.0-2% enlargement) to the .stl before printing, tuned per alloy and per flask size. A ring designed to be 16.5 mm inside diameter may be printed at 16.7-16.8 mm so the cast gold lands at 16.5 mm. For karat reference, see karat fineness.

Wax pattern checkpoints before investing

Before a wax tree goes into the investment flask, the factory should check:

- Calipers on the printed pattern: ring size, shank width, stone seat diameter. Compare against the approved CAD dimension.
- Surface finish: printed wax carries layer lines (typically 25-50 micron layers). For a high-polish piece these are fine; for a brushed or matte finish they will show. A vapour smoothing or hand touch-up step may be needed.
- Stone seats on the printed wax: test-fit a sample stone. If the seat on the wax is already too tight, the cast seat will be tighter after shrinkage.

Buyers should ask for photos of the wax pattern next to calipers, not just the render. The wax is the first physical object that will become the jewelry, and it is cheap to correct at that stage.

When CAD-to-casting drifts

The most common drift is ring size. A CAD file at size 7 (17.3 mm ID) cast in 14k gold without compensation can come out at size 6.5-6.75, and the buyer only discovers this after stones are set. The fix is a documented scale factor per alloy, plus a first-article ring measured and approved in writing before the full run. A buyer who skips the first-article step is accepting the factory average, not the approved design. That average drifts between operators, printers, and wax batches, which is why two reorders of the same CAD file can feel slightly different on the finger. The fix is a documented scale factor per alloy, plus a first-article ring measured and approved in writing before the full run. See also investment ratios and spruing practice.

Why is my cast ring smaller than the CAD render?

Three shrinkage steps: wax cure, investment expansion, and alloy solidification. The .stl needs a 1.010-1.020 scale factor before printing to compensate.

Should I approve the render or the wax?

Approve both. The render shows design; the wax pattern, measured with calipers, is the first physical check before investing.

Do layer lines on printed wax show in the final piece?

On high-polish pieces they polish out. On matte or brushed finishes they can remain; vapour smoothing or hand finishing is needed.

How much does ring size drift without compensation?

About 0.5-1.5%, typically half a size to a full size on a 14k gold ring. A first-article measurement in writing prevents this on the full run.

CAD speed is the selling point, but CAD-to-casting accuracy is what protects the reor

Who owns the CAD file on a private label

When a buyer commissions a CAD design, the file itself is often held by the factory, not the buyer. That is fine for a single-supplier relationship, but it becomes a problem if the buyer wants to move production or run a second factory. Before the first export, ask whether the .stl and .3dm files are delivered with the order or stored by the factory. A factory that keeps the files is not unusual, but the buyer should know that the design cannot be reproduced elsewhere without starting the CAD process over.

der. We measure every wax first-article against the approved file and share caliper photos before the flask goes in. If you are launching a custom line, plan the collection and structure the private label before the first CAD export.