When two different dimensional measuring systems are used on the same part, one would expect to get pretty much the same results. But in certain cases, this may not be true. Take, for example, a contact gage and a non-contact measuring system, such as an air gage. Even though both are measuring the same dimension, there is a significant chance that different results will be obtained because of the way the part’s surface interacts with the different sensor types.
The same problem can occur with surface finish gaging itself. Two measuring systems, using what appear to be similar techniques (whether or not they are from the same manufacturer) can sometimes produce very different results. When this happens, you need to conduct a comprehensive regimen of process-matching to achieve similar results.
First, make sure that the two measuring systems are individually producing repeatable readings on the part they are measuring. Make numerous traces in the same area on the part with both gages to check this. The results of each gage should be consistent.
If each gage is producing its best results but they still don’t match up, the next thing to do is a settings check. While surface gaging may appear to be pretty simple—drag a probe over a surface and get some results—it really isn’t that easy. A complex combination of various measurement settings and data calculations produces each result. Change any of them and you can get significantly different outcomes.
There are basically two reasons surface finish gages might not correlate. The first involves the gage settings. On most gages, there are ways to select cutoff length, the number of cutoffs, the filter to be used, traverse length and speed, or even what type of probe should be used (skid or skidless). To correlate these, make a list of the switch and dial position settings of one gage—almost like a checklist—and compare it to the settings of the other gage. (On newer gages, you may be able to print out all of the settings.) Go down the list setting by setting to make sure they are all the same.
The second reason involves the condition and configuration of each gage. This includes the tip radius (there are a number of standard tip radii); the tip condition (if it is worn or cracked); whether or not the gage is calibrated; and the gage’s measuring force.
Environmental conditions need to be examined as well. Is there a source of vibration entering into the measurement? Run the measurement without moving the probe: Do you see some results? This could be the measurement of vibration coming through the base granite and into the gage.
After you have checked the settings of the gage and its physical condition, there are a couple of “hidden” influences that may also affect the results from each gage. Newer gages have updated filtering capabilities compared to older versions. These filters can separate out high frequency noise from the measurement. Check to see if these (lambda) filters are part of each measuring system.
Finally, in the world of surface finish parameters, we know that different parameters sometimes have the same name. Rz is a perfect example. This parameter has a number of different meanings (and algorithms) in different countries and standards. Always check to see that the parameters in each gage are using the same method of analysis.
Once there is confidence that each gage is set identically, there should be fairly good correlation between the systems. But remember, there will always be some slight variation, especially when analyzing very complex surfaces.