Machine Design

How dazzling are diamonds? Leadscrew helps say for sure

A precision leadscrew helps an imaging spectrophotometer measure how well diamonds refract and focus light.

How dazzling are diamonds? Leadscrew helps say for sure

The GemEx Systems BrillianceScope Analyzer is an imaging spectrophotometer that measures the ability of a diamond to refract and focus light. A Kerk Motion Products RGS Slide positions a ring light in the instrument with minimal torsional play and high rigidity to ensure light shining through diamonds under test remains absolutely still.


The instrument, called a BrillianceScope Analyzer, beams a light at a diamond and measures both the white and colored light and the scintillation (or sparkle) reflected back. A stepmotor powers the leadscrew and positions a ring light at preset positions to get reflections at several precisely set angles for the measurement. Measurement accuracy relies heavily on precisely positioning the light. A small inaccuracy in positioning can affect the angle of light incidence and, as a result, the final analysis of the diamond.

The leadscrew, from Kerk Motion Products, Hollis, N.H., helps the BrillianceScope hit ±5% repeatability, more than enough for valid results.

The Kerk leadscrew replaced a homegrown drive screw and power nut on the BrillianceScope that, among other things, suffered from backlash. The Kerk positioning equipment now consists of a ScrewRail comprised of a drive screw, concentric rail, a combination nut and bushing, and a support on the end. The Kerk products are wear compensating with low drag torque and high axial stiffness.

The system underwent a slight redesign to correct minor issues caused by the light cantilevering from the ScrewRail — the drive nut/bushing would rock on the screw. Now an RGS-type slide with an extrusion prevents rocking, making the instrument more stable and accurate.

There are currently about 160 BrillianceScopes in use at diamond-cutting facilities worldwide. Analysis results are put into a report that goes to diamond retailers. These reports are the actual product sold by BrillianceScope developer GemEx Systems Inc., Mequon, Wis. They help retailers maintain the standards of their benchmark diamonds.

Kerk Motion Products,

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