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Finding Common Ground for Measurement Disagreement
When we think about making a simple measurement, such as an outside diameter, many methods can be used. These would include calipers, micrometers, mechanical snap gauges, air snaps, CMMs, and special fixture gages, just to mention a few.
By George Schuetz
The problem with all these methods is that while each is good in its
own right, there may be a bias seen among them. Each method has some
slight variations that can cause differences in result. It may be
different gaging force, different process methods (such as the
difference between contact and air gauging), or different measuring
techniques such as a coordinate measuring machine (CMM) doing average diameters as opposed to a
caliper using two point measurements.
So with all these techniques and potential pitfalls, how do you account
for multiple measurements to be made by different personnel, in either
the same or different locations, and then agree upon the results? The
bottom line is, you don't. The only correct way to do this is to
determine an agreed upon method for making the measurement and ensure
that each location follows the same process.
The good news is, these methods have already been fairly well defined
in documents we're all aware of, called Standards. One of these
Standards is ASME B46.1-2002 for Surface Texture (Surface Roughness,
Waviness, and Lay). When there is a disagreement regarding the
interpretation of texture measurements, this standard provides common
ground for making the measurement. This includes defining the type of
instrument and the parameters it is set to. With these defined, all
users are at a common starting point to eventually agree on a result.
Let's walk through the process and see how this works out:
The first recommendation of the ASME Standard is for the type of
instrument to be used. The choice in this caseas seen in Section 2.3,
Classification Schemesis a Type I Skidless Contact profiling
instrument. This is the most common analytical tool for surface
measurement that includes the ability to:
Measure smooth and rough surfaces
Measure roughness and waviness compared to an outside surface
Utilize a selection of filters and parameters for data analysis
Once the particular instrument is chosen, the next step is to agree on
the same measurement characteristics. Section 9 of the Standard
specifies that the instrument must have Gaussian (50%) filtering. These
are specified because they tend to separate roughness and waviness more
effectively than the 2RC filters often found on older systems. Also,
the recommended bandwidth, stylus tip and radius, and sampling interval
are to be determined using Table 9.2 in Section 9 based on the desired
roughness cutoff (λc).
Finally, the last setting to select is the stylus force. This can be
found in Section 3, Paragraph 22.214.171.124 and is based on the desired
radius tip. The stylus force chosen is sufficient to maintain
top-surface contact but not so large that the stylus will cause damage
to the surface. The maximum recommended values are always determined by
the stylus radius.
These are all fairly common settings selected for the skidless
measuring system. Those familiar with these systems are no doubt
proficient with making this setup and would be satisfied with choosing
this common format. So for most common measurements, all parties are
apt to be in agreement.
However, there may be some unusual measuring conditions which require
further definitions. These may occur, for example, if the surface
structure being assessed requires a short wavelength cutoff smaller
than the 2.5µm specified in Section 9.2. In this case, Section 7 in the
specification deals with measuring Nano Surface Texture and Step Height
measurements with Skidless Profiling Systems. It talks about the
measurement of very small (nano-meter) sized features on surfaces that
may have irregular roughness and waviness features or a depth
measurement such as a groove. Following the recommendations in this
section will reduce the uncertainty of the measurement of very small
features and will improve the comparison results from different
Finally, there are some particular cases where the above
recommendations may not apply. These usually involve soft surfaces
where significant damage can occur to the surface using a contact
stylus system. In these cases, the parties involved have to devise and
jointly agree on a measuring process and its characteristics.
In the end, it's always satisfying to have various measurements made in
various locations arrive at nearly the same results. The only effective
way of doing this is to agree on a standard measuring technique and
adhering to it.
Tech Tip courtesy of Mahr Federal Inc.