Look into My Styli: Care of Surface Finish Gauge Contacts
Proper care of contact points is one of the basic considerations in gauging. Whether you are using a simple indicator gauge or a sophisticated surface finish instrument. Much depends upon the condition of the sensitive contact point, which is the interface between the gauge and the workpiece.
Dial indicator contacts are easy to inspect. They are big enough to get your fingers around and you can check them visually for wear, damage or contamination.
The tiny contacts on surface finish gauges are quite another matter. Compared to blunt dial indicator contacts, a surface finish gauge has a fine “stylus” point, enabling it to follow the texture of the surface but being of a form and dimension that avoids scratching the part. Gauging force from a dial indicator is typically 120 grams/1.2 N; in contrast, force from a surface finish gauge is light, usually between 100 mg and 1,500 mg (1 mN to 15 mN). However, because the stylus is “dragged” across the surface of the workpiece, it is just as subject to wear and damage as a dial indicator contact.
Surface finish contacts are made of diamond and are conical in form, having a 60° or 90° included angle and a spherical vertex with a radius of 0.0004″, 0.0002″, or 80 micro inches (10 micrometers, 5 micrometers or 2 micrometers). Criteria for choosing between these options is outlined in ASME B46.1 – 1995 paragraph 4.4.5.1.
Even with its small radius, a stylus in perfect condition may be too broad to reach the bottom of the small “valleys” on the part surface, as shown in “A” in the diagram. However, this inherent “error” is accounted for by calibration.
Wear or damage to the stylus will affect measurements. An evenly worn contact will typically under-report the distance between surface peaks and valleys, as shown in “B.” A broken stylus may under-report surface variation, or may over-report it, as shown in “C,” depending on the nature of the break and the part surface. On occasion, the break may be so sharp that the stylus could scratch the part: a clear indication that the stylus needs replacement.
Paragraph 11.7.2 of the B46.1 standard describes several more methods that are practical to check the stylus condition. Because the stylus radius is too small to see with the naked eye, a handheld magnifier or a microscope is required for visual inspection. If it is necessary to assess the stylus condition numerically however, a stylus check patch or reference specimen, rated for about 20 micro inches, is required.
To use a stylus check patch, a benchmark reading must first be established when the stylus is in new, unused condition. First, the gauge is calibrated to a certified specimen, rated nominally at 125 micro inches (3.2 micrometers). Then it is tested against the 20 micro inch patch, and the reading recorded for future reference. The test need not read exactly 20 micro inches, because a specimen contains considerable inherent error (e.g., the inability of the probe to reach the valley bottom). For this reason, the 20 micro inch patch should not be used to calibrate the gauge, but it is nevertheless useful for the purpose of stylus inspection.
When it is necessary to check the condition of the stylus, the gauge is again calibrated against the 125 micro inch specimen. Then the 20 micro inch specimen is measured, and the reading compared against the when-new results. If the reading has changed by more than 25 percent, the stylus must be considered worn or damaged.
As noted above, a broken stylus may be sheared off so that it is either too blunt or too sharp. Conceivably, it could reach exactly as deep into the valleys as a new stylus, and rise exactly as high on the peaks, but the surface pattern that it reports will likely be distorted. Therefore, any time readings become non-repeatable, or reflect a sudden change in surface finish with no known change in the manufacturing process, a broken stylus is a likely suspect.
Should damage or wear be discovered, there is only one option available: replacement. Some gauges feature an easy-on/easy-off contact mounting, while others require a more involved procedure. Users should be aware of the variety of contact shapes available from some gauge suppliers. These may include extra-long lengths, smaller diameters, or special shapes that provide access to features that are otherwise hard to reach. Nevertheless, no matter what contact shape is used, it should be inspected regularly to ensure accurate surface finish measurements.
George Schuetz, Mahr Federal Inc.
