By Arthur Skuratowicz
Whether you need to ensure a fancy-cut gem sits securely in its prongs, a ring fits snugly on a customer’s finger, or an appraisal adds up, measurements matter. In this Tech Sheet, Arthur Skuratowicz offers a few tips for ensuring your tools are up to par and your readings accurate. He is the director of training for Choice Jewelers Trade Shop and the co-founder of Anton Nash LLC Independent Jewelry Appraisers, both in Colorado Springs, Colorado,
This should be done on a regular basis. Contact the original manufacturer of the device and ask if they have a calibration service. You can also check for local businesses that offer these services.
You can easily perform a couple of "field tests" to verify a gauge’s accuracy.
First, for a general check, measure a drill bit. Bits work well because they tend to be cut to specific sizes, and they are also affordable and easy to carry. In the photo above, a 1.2 mm drill bit was inserted between the gauge’s jaws; as you can see, the measurement is a little off.
For a more accurate check, try using metric gauge blocks (also known as "gage blocks"). The blocks can be made of either metal or ceramic, and each is precisely ground to a specific thickness (which is usually certified by a standards lab). While the blocks can be expensive-a set of 10 can run several hundred dollars-they do provide greater accuracy.
In the photo above, a 1.1 mm block was tested, and the reading is slightly off. If this occurs, don’t rely on just the one measurement; test a range of thicknesses. It could be that the two jaws are not aligned properly, and a skilled technician or jeweler could bend or file them as needed. However, if the inner workings need to be realigned, then it’s time to send the gauge out for calibration-or to simply replace it.
Measuring a customer’s finger for a ring should be simple, yet many jewelers have to re-size those rings after they’re finished. What gives?
For one thing, ring mandrels and finger sizers typically don’t match, even when they’re from the same manufacturer. While that could be solved with a little bit of home-grown calibration (e.g., noting where each sizer actually falls on your mandrel), it’s not the only issue. You also need to ask whether the ring opening is being sized correctly. Many jewelers are taught that you must measure all rings in the center of the band. However, in my experience, that’s not always the case.
The photos above show two finger sizers, a flat band (left) and a ring with a domed interior (right), also known as a "comfort fIt." Both sit in a similar area on the ring mandrel, centered over the 7. However, the interiors are not the same size: While measuring the comfort-fit from the center will work (since the interior is narrowest at that point), doing the same for the flat band may produce a loose fit. Instead, try measuring the flat band at the edge, much as a plumber would measure the inside dimension of a pipe.
The same holds true when measuring actual rings. The rings in the photos above both have flat interiors. However, their different widths will produce two different sizes if measured at the center: The wide band might be called a 13.75 and the narrow band might be called a straight 14 or a little less. Typically a wider band needs a slightly larger size to clear the wearer’s knuckle, but here we see the reverse happening-and the likelihood of a resize growing. To avoid this, measure both rings at the bottom edge.