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Assessing the Accuracy of Thickness-Measurement Tools

(April 2004) posted on Tue Apr 13, 2004

Discover what tools and procedures lead to the most accurate measurements for screen printing.

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By Fernando Zicarelli

However, if we compare the thickness readings from the Heidenhain MT-60 motorized gauge after it was set up to achieve a force of 1.25 N (Table 3) with the reading from the Mitutoyo model #543-252 (Table 2B), in which applied force was 1.2 N, we see that the thick-ness readings are within 1 micron of each other. The close similarity between these results is due to the fact that both gauges used nearly identical probe shapes and activation forces. This is further evidence that gauge type has little bearing on measurement results; probe shape/size and measuring force have a much more significant influence.

To complete my assessment of gauge types, I also decided to look at electromagnetic gauges, which are frequently used in the screen-printing industry for measuring dry ink-film thickness, but are increasingly being used to measure mesh and stencil thickness. Magnetic thickness gauges are lightweight devices that you can take anywhere. They operate by measuring the distance between a probe and a steel surface and can measure the thickness of any non-conductive materials placed between these two points.

For this portion of the study, I used a DeFelsko Positector 6000 SPFS magnetic thickness gauge. The device contains a magnet and a hall sensor. The hall sensor measures how the magnet reacts as it is brought closer to the steel base; the closer to the steel plate, the stronger the magnetic attraction. The gauge's mi-croprocessor converts that magnetic value into a thickness measurement. Assessing the Accuracy of Thickness-Measurement Tools

Assessing the Accuracy of Thickness-Measurement Tools

Tables 2A and 2B the Impact of Gauge Type and Brand When the same screen fabric was measured with two different brands of spring-activated gauges, the values returned varied substantially for all probes except the 4.8-mm flat type. The discrepancies were primarily the result of the different probe forces applied by the devices--the gauge that produced the values shown in 2A used a pressure between 0.6-0.85 N, while the gauge represented by 2B applied a force of 1.2 N.

Magnetic-gauge accuracy is impacted by several conditions. First, if the steel base is dirty or scratched, this can affect the readings. Second, if the probe position is not held correctly, inaccurate readings again can result. Finally, the operator must take several readings to obtain an average measurement, which will be more accurate and repeatable. This gauge is designed to take a reading every 2 sec if the probe is left on the mesh, so it is best to discard the first measurement and use the second or third measurement.

I took 16 readings from the 8 x 8-in. image area of eight separate screens, again featuring NBC 380 thread/in. mesh with 33-micron thread diameter that had been stretched to 18 N/cm2. The probe was zeroed before each screen was measured, and I took three readings from each screen before lifting the probe (remember, this device takes a reading every 2 sec).

The averaging feature on this thickness gauge allowed me to record an average thickness reading of 45 microns with a tolerance of


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