Gauge calibration is a routine process followed by most users of precision gauges. How regular a gauge needs to be checked and its performance documented is usually defined step-by-step in a documentation procedure.
Surface finish roughness instruments use a roughness specimen, or “patch,” that is certified for a known wave form to provide a specific surface parameter result. With this certified specimen in hand, it is easy to verify that the surface finish measuring instrument is reading properly. Because of the nature of their sensitive probes, and because they are often used in harsh environments, it is not uncommon for users to check the performance of surface finish gauges at the start of each shift or even between each measurement.
What’s really being checked during this process is the gain or magnification of the probe as it goes through its measuring range. For most applications, a surface finish standard with a Ra value of 118 µin (3 µm) is typical. This will ensure that when measurements are being performed, the mechanical range of the probe goes through sufficient travel to ensure proper magnification for the results.
Since surface finish measurements have moved out to the shop floor it is important that the reference specimen be monitored closely. The calibration artifact should always be kept clean and dry to ensure that dirt and debris do not affect the measurement results.
Over time it is not uncommon to see the surface finish instrument’s displayed results fail to coincide with the certified values of the roughness standard during calibration. Erroneous results could be interpreted as a fault with the measurement amplifier or the measurement probe (heaven forbid), but this is rarely the case. The most common cause is a “dirty” roughness patch.
Since most roughness standards used in industry have a triangular wave form pattern with very sharp corners, it is very easy for debris to collect on the valley areas. Contaminants such as dust, powdered metal, oil in the atmosphere, dried coolant, or other things can greatly affect the measurement results. Even though present in tiny amounts, such debris can prevent the measurement probe from getting down into the valleys on the roughness standard. Debris also can affect the measurement on the peaks as well.
Figures 1 and 2 are printed profiles of certified triangular roughness specimens. Figure 1 is a new clean roughness specimen with a high roughness value of Ra 2.993 µm. Notice the perfect sinusoidal type wave form over the whole length of the profile.
Figure 2 shows a measurement using the same surface roughness gage but on a patch that has been out on the shop floor for a number of months. Notice the inconsistency of the measured values, and that the peaks and valleys are not consistent across the profile length. This is particularly noticeable on valleys at the beginning of the profile and is typical of debris in the valleys of the roughness standard preventing the measurement probe from reaching the bottom of the profile.
Roughness standards may look clean to the naked eye, but the dirt and debris present is typically in the 2-10 micron level in size. Therefore, most of it cannot even be seen but it is present nonetheless.
To clean a roughness artifact, use cotton swabs, acetone (denatured alcohol for plastic roughness standards), and the surface finish measurement instrument itself with profile printout capability. This last is just as important as the acetone and swabs since its magnification capability is significantly greater than the human eye and since measurement results can be demonstrated with a printed profile of the measured surface.
Handling of roughness standards should be done with care to prevent scratches on the surfaces. Here’s a step-by-step procedure:
1) Take a measurement on the roughness artifact and print out the measured profile. If the profile looks similar to Figure 2, chances are it has dirt in the valley areas of the roughness standard.
2) Saturate the end of the swab with acetone. Using medium force, wipe the surface parallel to the direction of the lay pattern on the standard—not perpendicular. Measurements are performed perpendicular to the lay pattern; cleaning should be done in a parallel direction
3) Take a second measurement and print out the measured profile again. The measurement results will demonstrate a more consistent wave form in the profile. Again, the profile should show a consistent profile through the whole length of the trace.
It may be necessary to perform these steps repeatedly since dirt can be stubborn. When cleaning, make sure that nothing other than the swab tip comes in contact with the roughness standard.
– Courtesy of Mahr Federal Inc.
