Modern Machine Shop

MAY 2018

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Page 31 of 179

QUALITY GAGING TIPS MMS MAY 2018 30 Measurement Tools Steps for Getting Form Measurements Right Form systems are becoming standard tools on the shop floor as their cost decreases. Instead of being operated by a typical quality specialist, these sys- tems are more likely to be used by a machine tool operator. As such, the user may not thoroughly understand part form as it relates to the part's application or the manufacturing process. While today's most basic form-measuring systems are greatly simplified operationally and much more powerful than they were just a decade ago, form measurement is still a complex process. As a result, operators should have a good under- standing of the meaning of the parameters they are measuring and be familiar with how they relate to the parts being manufactured. It is crit- ical for the operator to know how to properly set up the form-measuring system and the part for the various form functions that are available. Numerous setup errors can ultimately inf lu- ence measurement results. By recognizing the sources of errors, more reliable results will be obtained. Here are few errors that come to mind: Staging. Like any other measuring instrument, if you cannot stage it, you cannot gage it. The same is true with form-measuring systems. How a part is staged on the table will inf luence the measure- ments. For instance, a three-jaw chuck can distort a thin-walled part; tall, narrow parts may require special clamping for stability; and tight-toler- ance parts need to have fixturing that repeatably places the part in the same orientation. Using the incorrect stylus tip. It is common to rely on a single stylus tip for measuring all parts, regardless of the size of the part being measured. In reality, the tip itself represents a mechanical filter that should be selected according to the part size and the maximum number of undulations per revolution that can be measured. Measuring the incorrect parameter. Many geometry parameters that seem self-evident are not. In the world of simple indicator gaging, the terms "runout" and "out-of-roundness" are occasionally used interchangeably, but they are not the same and must be measured differently. Similarly, when gaging perpendicularity, paral- lelism and some other parameters involving the relationship between two surfaces, the user must understand which surface to measure first. For example, in geometry specifications that appear on part prints, the statement "B is parallel to A" is not equivalent to "A is parallel to B." By definition, the second surface in the statement is considered to be the reference, or datum. Out- of-straightness, or circular out-of-f latness, are ignored on that datum surface but are relevant to the other surface. All of these parameters, in addition to circular- ity, concentricity, coaxiality and others, are defined in industry standards, which also have standard- ized the symbols used with the various callouts. Not paying close enough attention to centering and leveling. Centering and leveling are critical elements of a gage setup procedure. A round part that is not level will produce an oval measure- ment trace, while one that is significantly off-cen- ter on a spindle will produce a vaguely heart- shaped trace. Even if a gage turntable is equipped with a holding fixture for a particular part, an operator should use the centering and leveling capabilities of the system before measuring. GEORGE SCHUETZ | COLUMNIST Recognize the source of setup errors.

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