Dimensional accuracy/consistency

Hi,
I have a business making precision CNC machined wedding bands. My pieces are complex and are not re-sizable, so I furnish clients with 3 dummy rings in 1/4 size gradations to really hit their perfect size BEFORE I start making their actual ring. I'd been machining dummy rings on my lathe but recently had Shapeways print a couple of batches in strong flexible plastic. I'd like to make all of my designs available in all sizes (a lot of CAD models) and ideally have the client order them directly. Unfortunately my first batch results were quite mixed- the black strong flexible plastic were spot on dimensionally and looked quite acceptable, but the white strong flexible were oversize with a rather poor outer surface. Because these are just for sizing, looks are not that important, but dimensional accuracy and consistency are critical. Do you have any suggestions to consistently achieve better results?
Thanks,
Jeff

www.mcwhinneydesigns.com

The design guidelines do specify accuracy tolerances for the various materials.

Strong and Flexible: Accuracy: ± 0.15mm, then ± 0.15% of longest axis

Alumide: Accuracy: ± 0.15mm, then ± 0.15% of longest axis

Detail Plastic: Accuracy: ± 0.1mm

Frosted Detail Plastic: Accuracy: ± 0.025-0.05mm for every 25.4mm



Perhaps you could make reference parts using detail plastic or frosted detail plastic instead. These options are pricier but they are likely more consistent. Changing the size of a ring by an increment of 1/4 only changes the inside diameter by about 0.2 mm so it's not unreasonable for any printed ring to come back closer to the next higher or lower 1/4 ring size based on stated accuracy for strong and flexible or alumide.

I don't know if these tolerances are derived from statistics based on actual Shapeways production history or based on numbers coming from the 3D printer manufacturers. I would assume the latter. There are also some additional factors which affect variations such as the orientation of the objects in the printer which you cannot specify. I think the x,y printer dimensions are more accurate than the y dimension, and in theory the operators packing together build files orient your rings consistently and for the most accurate ring fit.

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I have a similar problem, but it involves rings made in premium silver. When made in Strong & Flexible the sizes are OK, that is, the finished product and design dimensions are reasonable close. When I ordered a 6-1/2 ring (ID=16.93 mm) made in premium silver it had an ID=17.15 mm which between a 6-3/4 and 7 size. The ring was too big for the customer.

Does anyone have any experience with how much material the premium silver polishing takes off? If I knew this, I could adjust my STL files to compensate.

Is there a way to specify that the polishing not be done on the interior of the ring?