A typical surface finish callout consists of many elements, all of which may be required because of the
complexity of surface finish metrology. Under the old ANSI B46.1-1985 standard, a default value of .030"
(0.8 mm) applied to any surface finish callout that did not specify a cutoff. This is an effective cutoff value
for many, but not all, roughness measurements. To avoid confusion and lax specifying, the new ASME
B46.1-1995 standard abolishes the default and insists that all cutoffs be specified.
It is standard procedure to measure five cutoffs within a single stroke and average the results, although
fewer may be used if the surface is too short to include five. Where a manufacturing process is known to
be consistent, less than five cutoffs may be sufficient to generate reliable results. When reporting results,
the number of cutoffs should be indicated if it is not five. To report the use of four cutoffs, the form shown
in the following example would be followed: Ra4 = 25μ".
4.Choosing the wrong signal-processing filter The signal generated by the gage head must be
filtered electronically before the data can be converted to a numerical result. The new 50 percent Gaussian
filter, which is the default according to ISO, is a better, more accurate filter in metrological terms than the
older 2RC filter. Many part prints, however, still specify (or assume) the use of a 2RC filter, because it is
more familiar, and because it is available on most gages, including those with old analog technology.
Because the two filters will generate different results, design engineers should be sure to specify which
filter is intended, and the inspector should follow the specification, or seek clarification if it is absent. For
evaluation of some specific surfaces, such as plateau-honed cylinder liners and porous materials, another
filter, known as the "Valley Suppression" filter, is used.
Modern skidded roughness gages can be compact, economical, and well-suited to shop-floor inspection
tasks. In addition to the possibility that the part might be ruined by scratching it, the measurement could
be inaccurate, because the scratch representsa modification of the surface. Plastics, rubber, graphite,
copper, aluminum, and many alloys are also subject to deformation at fairly low levels of force, which
could also influence profile measurement results. These materials therefore require the use of a probe
with the lowest force possible.
5.Using a damaged stylus As with a stylus that is too large, a stylus that is worn or chipped will not
penetrate as deeply into fine surface irregularities as one that is in good condition. (Figure 1) The first
indication of a worn or broken stylus may be a change in gage readings, or may be found during the
calibration process (see Common Error #11). If a problem is suspected, the stylus may be examined with
a special test patch or a low-power microscope.
6. Ignoring the lay (measuring in the wrong direction) Most machining processes create tool marks that
run in a particular direction across the part surface. When measuring roughness parameters, it is essential
that the gage traverse at right angles to the "lay," or pattern of toolmarks, to capture worst-case
conditions.
Fig. 1 -Styluses that are worn or broken, or too large for the surface, may generate inaccurate results
because they can't reach the bottom of the surface valleys. Some processes, however, such as electrical-
discharge machining (EDM) and plastics
molding, do not create unidirectional tool marks. When this is the case, it is advisable to take two
measurements at 90 deg. To each other, to check whether there is a distinct lay to the surface. If there is,
then the rougher of the two measurements must be used.
7. Including flaws in the measurement Parts may include flaws that are unrelated to the manufacturing
process. Parts may also include other features that interrupt the surface, such as a pore in a porous
metal. Though surface finish gaging is intended to monitor and ensure the stability of the manufacturing
process, it is essential that these irregularities be excluded from the measurement. This can be easily
done post-measurement with some computer-driven gages, where software enables the user
to visually identify irregularities on the trace, and exclude them from the calculation. For gages that report
roughness only as a numerical result and do not generate a trace, care must be taken to avoid
irregularities in the first place. On the other hand, there are some cases where it is desirable to include
these irregularities in the measurement: for example, to check if a part meets specifications in spite of a
scratch. So irregularities should not be automatically excluded. Each instance must be considered
separately.
8.Inattention to leveling This problem applies to skidless gages, where the gage must be level relative
to the workpiece. To measure fine surface characteristics at high magnification, the gage must be
mechanically pre-leveled, before final electronic leveling can be performed by the computer. To pre-level
the gage, a test trace is performed, then software indicates how many turns are required on the leveling
knob on the drive mechanism. Once the gage's attitude is within an acceptable range, the
computer performs the final leveling, compensating for any remaining out-of-level condition after the
measurement is performed.
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