Gaging Tips

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Get the Most From Your Form Measuring System: Part II
George Schuetz, Mahr Federal Inc.
http://www.mahrfederal.com/  

Last issue we looked at some of the setup and analysis issues that can influence the results of your form measurement, including: the way the parts are staged; the influence of the stylus tip; using the wrong filter; confusion over the look of the graph; measuring the wrong parameter; and the influence of poor part tilt/centering. These are some of the pitfalls that can rob accuracy from your measurement, but there are others that also need to be addressed.
Mechanical filtering
We mentioned that sometimes the stylus tip can inadvertently act as a filter (too large a contact radius may miss high or low spots), but sometimes the response characteristics of the electronic transducer can also influence the reading. Most transducers have a response time of around 40 cycles per second. There are some form systems that allow the user to "spin" the table manually, rather then use a fixed rotational speed controlled by an electronic motor. If the part is spun too fast for the transducer, the mechanical/electronic lack of response can filter out critical information. Plus, inconsistent speed may filter out information at the beginning of the rotation, but let it through in the latter portion of the rotation.
Datum issues
Some parameters do not require a datum be established on the part—for example, when a part's profile is compared to a gage spindle's axis of rotation. Other parameters, however, measure relative to a datum surface, or to an axis representing another portion of the part. Understanding the parameter and whether or not you need to establish a datum is the first step in making the measurement.
Relying too heavily on default settings
All form measurement exhibits variation that results from several influences. The true form of the part is influenced by variables in the manufacturing process, such as errors found in the ways of the machine and the right angle relationships of those ways. Also, internal vibrations from motors and tool chatter, along with clamping influences, all pass their variation along to the part and can be picked up by the measuring process.
Each type of variation produces a pattern of undulations on the trace of the part's surface that the gage generates. For example, bad leveling will make the part appear to have a two-lobed condition. The dynamics of centerless grinding typically impose an odd number of undulations per revolution of the part—most likely five or seven. The influence of bearing vibration in the machine tool spindle might add a larger number of undulations.
Form machines incorporate electronic filters to simplify the trace by eliminating undulations that appear outside of certain desired frequency bands. For example, an operator can choose to show only undulations that occur between zero and 15 times per revolution. This type of trace reveals low frequency errors due mostly to clamping and setup factors. Or the operator can choose to filter out only frequencies above 125 undulations per revolution to include the results of more dynamic factors in the analysis.
 The ANSI standard establishes 50 undulations per revolution as the default value for measurements of out-of-roundness. Some rotating parts may produce undesirable noise if higher frequency undulations exceed certain amplitudes, so it may be necessary to filter gage data at higher undulations per revolution. A part designer should define the frequency filter to be used based on the functional needs of the application.
Using the right reference circle
Out-of-roundness is measured by comparing the profile irregularities to a gage spindle's axis of rotation by means of one of four reference circles. As described in the ANSI specifications, these are: Maximum Inscribed circle, Minimum Inscribed circle, Least Squares circle and the Minimal Radial Separation method. Results generated by the four approaches can differ by as much as 10-15 percent when evaluating the same profile.
The minimal radial separation method was used by most early form machines. But today's machines offer all the alternatives and it is important for a part designer to specify the method best suited for the part.
Not using the advanced features of today's form machines
Advanced metrology software can tell a user much more than whether a part is within tolerance. Sometimes simply sorting good from bad parts is all that is required. But by subjecting the part to more in-depth analysis, the user can often find out what is wrong with a production process and potentially find ways to correct it. He may also be able to predict the performance of a "bad" part if it were to be installed.
Harmonic analysis involves the analysis of individual predominant numbers of undulations per revolution. While filtering techniques may allow the user to view the effect of out-of-roundness of several undulations within a broad frequency, harmonic analysis allows the user to focus on a single harmonic frequency.
Slope analysis allows the user to look at the "slope" rate, or change of radius with respect to the angle of rotation. The maximum difference between the longest and shortest radius on a form trace might be a critical factor for certain parts.
Form gages today are very powerful tools that can help manufacturers improve the quality and functionality of their designs. But as with any measurement tool, an investment is also needed in training and application usage to get the most from the equipment.