Correcting Changes in Part Shape

Q. I frequently need to measure the thickness of anodize, paint and powder coatings on many differently shaped aluminum parts. I see the part shape and sometimes the aluminum type can affect the measurement value. What’s the best way to correct changes in part shape and different grades of aluminum?

A. You are correct in your observation that the shape and grade of aluminum can affect the measurement value when using electronic dry film thickness (DFT) gages. The effect comes from the technology with typical gages used to measure coatings on aluminum.

Typical electronic gages use the eddy current test method to measure the thickness of electrically non-conductive (insulating) coatings applied to aluminum. The gage’s probe generates a high-frequency magnetic field, which induces eddy currents in the base material. Magnetic fields, created by the eddy currents, oppose the original field. As the coating thickness increases, the changing interaction between the original eddy currents and the opposing magnetic fields correspond to the distance from the measurement probe to the base material. This is measured, and the gage interprets the signal as a coating thickness.

Working with electrical fields, the electrical conductivity—or inversely the electrical resistivity—can affect gage readings. If you calibrate on a pure aluminum base and then measure on a 2024-T3 aircraft aluminum part, you may see a difference due to the differences in electrical conductivity between the calibration base and the part’s base.

The shape of the part also affects the strength of the eddy currents and therefore affects the gage’s readings. Calibrate the gage on a flat part and then measure on a small, round part; the eddy currents are different from flat to round and you should be aware that those differences can influence the reported thickness readings.

Be sure to calibrate the gage to the same shape and material as the coated part.

Some electronic DFT gage manufacturers offer gages with conductivity compensation circuitry, minimizing the effects of the base material’s electrical conductivity. There are also probes available with curvature compensation circuitry, which greatly minimizes the effects of part shape.

If working with aluminum of varying base alloys and shapes, it’s best to consult the gage manufacturers to compensate for changes in part shape and base material conductivity.

Rob Weber is an applications and product support engineer at Fischer Technology, fischer-technology.com.