Modern Machine Shop

MAY 2018

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Page 33 of 179

QUALITY GAGING TIPS MMS MAY 2018 32 Measurement Tools Datum problems. Some form parameters do not require that a datum be established on a part (when a part's profile is compared to a gage spindle's axis of rotation). Other parameters are measured relative to a datum surface or to an axis representing another element on a part. In these cases, establishing the datum is the first step in taking the measurement. The part print should call this out. If not, it may be time for a print revision. Too much reliance on filter default values. Form measurement will exhibit variation result- ing from several inf luences. A part's true form is inf luenced by variables in the manufacturing process, including clamping and tool chatter. Added to these are variables introduced by the measuring process itself, including setup accu- racy, part clamping on the turntable and environ- mental inf luences. Each inf luence produces a pattern of undu- lations on the trace of the part surface that the gage generates. Form gages incorporate elec- tronic filters to simplify the trace by eliminating undulations that appear outside certain desired frequency bands. One can choose to generate a trace that only shows undulations that occur between zero and 15 times per revolution. This trace reveals low-frequency errors due mostly to clamping and setup factors. Or, one might choose to filter out only frequencies above 125 undulations per rotation (UPR) to include the results of more dynamic factors in the analysis. The standards have established 50 UPR as a default value for measurements of out-of-round- ness. When a 50-UPR filter is engaged, undula- tions that occur at frequencies above 50 UPR are filtered out. This filter is appropriate for many, but not all, applications. Still, a part's design should define the frequency filter to be used. AUTHOR | George Schuetz Director of Precision Gages, Mahr Inc. Come visit us at BOOTH 432217 Visit or call 330.686.5700 to receive more information.

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