Modern Machine Shop

JUN 2018

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Page 34 of 204

MMS JUNE 2018 32 QUALITY GAGING TIPS Measurement Tools Steps for Getting Form Measurement Right, Part II 1. Placing too much reliance on filter default values. Geometry measurement exhibits variation from several inf luences. A part's true form is inf luenced by variations in the manu- facturing process, including clamping and tool chatter. There are also variables in the measuring process, including setup accuracy, part clamping and environmental inf luences. Each influence produces a pattern of undula- tions generated by part trace. Bad leveling will make a part appear to have a two-lobed condition. The dynamics of the centerless grinding process typically impose an odd number of undulations per revolution (UPR). Bearing vibration in the machine tool spin- dle might also add a larger number of undulations. Geometry gages incorporate elec- tronic filters to sim- plify the trace by eliminating undu- lations that appear outside of certain desired frequency bands. A trace can be generated to show only undulations that occur between zero and 15 times per revolution. This trace reveals low-frequency errors, mostly due to clamping and setup factors. Or, frequencies above 125 UPR can be filtered to include the results of more dynamic factors in the analysis. Standards establish 50 UPR as a default value for out-of-roundness measurements. With the 50-UPR filter, undulations that occur at frequen- cies above 50 UPR are filtered out. Use of this filter is appropriate for many, but not all, applications. Some rotating parts may produce undesirable noise if higher-frequency undulations exceed cer- tain amplitudes, so it may be necessary to filter gaged data for up to 150 or 500 UPR, or even to analyze gage data without electronic filtering. A part designer should define the frequency filter to be used based on the needs of the application. 2. Using the wrong reference circle. Out-of- roundness is measured by comparing profile irregu- larities to a gage spindle's axis of rotation by means of one of the following four reference circles: maximum inscribed circle, minimum circumscribed circle, least squares circle and the minimal radial separation methods. Results generated by these four approaches can differ by as much as 10 to 15 percent when evaluating the same profile. Today, PC-driven geometry gages offer all four methods. Part designers should specify the method best suited to the application. 3. Being confused by a scale effect. The trace of a roundness measurement, as shown on a gage's computer screen, rarely looks "round." It usually looks like a mass of sharp peaks and valleys. This can distress users who believe that the chart shows terrible part geometry. The solution provided by gage manufacturers has been to normalize the part dimension to a fixed-size circle on a screen. This may involve a low level of magnification for small parts or even a reduction in scale for large parts. Deviation is highly magnified, from 1,000× to 20,000× or more. Geometry gages allow a user to select a magni- fication level. Even though the computer's trace does not accurately depict the part's actual profile, the relationship between peak heights and valley depths remains consistent with the level of magni- fication chosen. Peaks and valleys also retain accu- rate angular relationships to one another around the circumference. 4. Choosing the wrong signal processing filter. The original filters used in geometry mea- surements—known as double resistor-capacitor (2RC) filters—were designed before the age of GEORGE SCHUETZ | COLUMNIST Avoid these five form-measurement mistakes. The five setup processes that should be considered to get the best perfor- mance out of a gage can be found at: FORM MEASUREMENT, PART I

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