Measuring workpiece dimensions is relatively simple for machine operators but measuring workpiece geometry which involves more complex comparisons of part shape to an ideal shape--is now also practical on the shop floor. The gaging equipment for doing this is coming down in price while becoming easier to use.
Geometry measurements, which were confined to laboratory settings just a few years ago, are rapidly becoming common in many high-precision metalworking applications. As designers increasingly specify sub-micron or microinch dimensional tolerances, they learn that small variations in part geometry can have a significant influence on functionality, and consequently, many have begun to specify geometric tolerances on part prints. To meet these specifications efficiently, and to obtain measurement results quickly, some shops now feel a need to measure geometry on the shop floor.
Gages that measure various parameters of circular geometry--including roundness, circular flatness, circular parallelism, cylindricity, concentricity, and othersare steadily proliferating. There was a time when these gages were largely confined to Quality Assurance or development lab settings, but now, many are used by machine operators out in the shop.
Two trends support this migration from the lab to the shop floor. First, geometry gages are becoming easier to use, and second, they are rapidly becoming more affordable. Over the past few years, gages have begun to incorporate user-friendly control interfaces and setup aids, including "Windows-style" software and touch-screen controllers.
And while ease of use has improved, costs have declined. As recently as 1990, a basic roundness gage cost about $25,000. By 1995, the lowest price roundness gage could be had for $18,000, and new versions just now being introduced will sell for under $13,000--with greatly expanded functionality. Likewise, a typical cylindricity gage cost $90,000 in 1990. Now gages capable of measuring cylindricity of smaller parts are available for $40,000, and the trend is for further price reductions. It is now easier to cost-justify the acquisition of several geometry gages, for use beside every machine tool that requires frequent monitoring for geometry.
There is a logical connection between part geometry and dimensional accuracy. If a nominally cylindrical part exhibits some taper or out-of-roundness, then diameters will vary at different locations on the part. Years ago, when manufacturing tolerances were typically specified in thousandths of an inch, slight geometric inaccuracy could be safely ignored, because the dimensional variation that it caused was also slightoften on the order of a few millionths of an inch. But, with much tighter dimensional tolerances now common, such geometric inaccuracy can no longer be overlooked.
A simple example will show why geometry is so important. Assume that a shaft must be assembled in a bore, and that the bore is perfectly round. If the shaft diameter is measured by conventional methods (a snap gage, for example), it might appear to be within tolerances. Because of out-of-roundness, however, the shaft's effective diameter could be larger than the measurement, and the two parts then could not be assembled.
Several other aspects of functionality may be influenced by geometric error. If, for example, the mating cylindrical components were parts in a valve, fuel injector, or other fluid-handling device, their geometric accuracy might affect the device's ability to contain fluid without leakage and generate fluid pressure as designed. For parts in relative rotational motion, geometry errors might result in noise, vibration, and excessive wear. If the assembly were part of a precision positioning device, geometry error might affect accuracy of operation.
The geometry of circular workpiece features is too complex to assess with conventional dimensional gaging equipment, regardless of their level of accuracy. Two nominally round workpieces may exhibit identical effective maximum and minimum diameters, and yet, the parts have very different functional attributes. Depending upon the application, one of these parts might be acceptable and the other not. The capabilities of a geometry gage are required to measure geometric parameters and perform complex form analyses.
Geometry measurements can be divided into two major categories, based on the type of gaging instrument required. Most modern geometry gages stage the workpiece on a turntable, and provide a means to position a gage head against the part. As the turntable rotates, the gage head measures deviation from the true circle. Those gages where the gage head is supported by a simple, rigid, manual or motorized stand that does not provide precise control over positioning, are capable of performing the following measurements: roundness, concentricity, circular runout, circular flatness, perpendicularity, plane runout, top and bottom face runout, circular parallelism, and coaxiality.
On other gages, the gage head is supported by a precision vertical slide that serves as a vertical reference with a known degree of straightness and linear positioning accuracy. In addition to all of the parameters listed above, these more sophisticated gages can measure cylindricity, total runout, vertical straightness, and vertical parallelism. No clearly established terminology exists to distinguish between these two types of gages, but they are informally referred to as roundness gages, and cylindricity gages, respectively.
Geometry measurements can also be divided into two categories according to their datum requirements. These categories might be called self-referenced, and datum-referenced measurements. Self-referenced measurements, such as roundness and flatness, require only one measurement, and do not require that a datum be established on the part independent from the feature being checked. In datum-referenced measurements, such as concentricity and coaxiality, one or more initial measurements are required to establish a datum on the part, before the feature in question can be measured relative to that datum.
Geometry measurement often involves making judgment calls. For example, a shaft may be specified to be round to within a certain tolerance, but where should the part be measured: at the middle? near one end? at both ends and the middle? The designer probably had a functional rationale behind the specification, and has probably made certain assumptions about consistency of form on the part. Unfortunately, this information is rarely communicated to the machinist, who usually has only the callouts on the part print to rely upon. However, a familiarity with machine tool performance often provides useful guidance: One may learn from experience that, if the part is round within tolerances in the middle, it is unnecessary to measure roundness at the ends. (The opposite, of course, might be true too: A part that is round at one location is not necessarily so at all others.)
Aside from understanding the stability of their process across a single workpiece, machine operators who do their own gaging must be able to analyze and apply the parametric requirements of geometry measurement. Although we can't describe all of the numerous parameters here, we can give a few examples.
As noted above, roundness (also called out-of-roundness or circularity) is a self-referenced parameter. After the workpiece has been centered on the turntable, the gage generates a centered polar trace, creates a reference circle that represents "ideal roundness," and then calculates roundness as deviation from that circle. There are four different methods by which reference circles can be established mathematically, and these are described in the ANSI B89.3.1 standard. (They are: Minimum Radial Separation, Least Squares Circle, Maximum Inscribed Circle, and Minimum Circumscribed Circle.)
Without going into detail here, we can state that results may vary by as much as 10 to 15 percent between the four methods. Minimum Radial Separation is a default method according to standards, but many geometry gages allow the user to select any one of the four, so it is important that the QA/QC manager study the practical applications of each method and understand their strengths and weaknesses. Gage operators who do not have sufficient expertise to make these determinations should be instructed in the appropriate method for each roundness measurement task.
Concentricity specifications may apply to part features that lie within the same plane (for example, the ID and OD surfaces of a bearing ring), or to features that lie on the same axis in separate planes (for example, two journals on a shaft). Concentricity is a datum-referenced parameter; calculating it requires a minimum of two or three separate measurements. To gage the single-plane concentricity of a bearing ring, the ID may be measured first to establish a center datum. Then the OD is measured and its center established. Eccentricity is the distance between those two centers: Concentricity is twice that value.
Concentricity in separate planes requires more planning, because an axis (as opposed to a center) must first be established as a datum. At least two roundness measurements are required to establish the reference axis, or datum. Then a third roundness measurement is performed on the part feature in question, to establish the location of its center relative to the datum.
The question arises, then, how to establish the datum? If the shaft was turned on center holes, it may be tempting to use these centers to establish the axis. This might be unwise, because the holes are probably mere artifacts of the manufacturing process: they are unlikely to play any functional role in the part's actual application. A second option is to establish the axis between the centers of two end journals. This method allows concentricity of the center journal to be established relative to the axis, but it relies upon the questionable assumption that the end journals are coaxial to each other. A third option establishes the datum axis based on two planes on the same end journal, thus enabling both the center journal and the other end journal to be measured relative to the first. While theoretically attractive, this method has practical limitations, because any angular error in the setup will be magnified by the length of the part.
Although software-guided geometry gages can help in this process, they only go so far. After the user initializes the "concentricity" sub-program, software may instruct: "measure plane No. 1; measure plane No. 2; (wait for computer to calculate datum); measure plane No. 3; (wait for final result)." But the gage cannot tell the user where to select the reference planes. This decision may require a combination of shop-floor know-how and engineering analysis. Similar value judgments have to be made when measuring other complex parameters, such as total runout and cylindricity.
Examples of shopfloor geometry gaging are multiplying rapidly. As ease of use has improved, machinists are accepting geometry gages as a tool to monitor process stability. Here are some notable examples.
As gages are becoming more economical, more manufacturers are installing them in production environments. As customers become aware of this trend, they will surely increase their use of geometry specifications to improve the functionality of their designs: The process will feed on itself.
Meanwhile, the introduction of multilevel artificial intelligence in gage software will help gage operators make the right gaging decisions based on the knowledge of metrology experts. The gage will signal the operator when a part fails to meet specifications. It could then use several analytical approaches to help in solving manufacturing and gaging problems.
For example, if a workpiece is excessively out-of-round, the software may perform additional analysis (such as checking the three largest harmonics; checking part centering and leveling; or analyzing maximum peak height and valley depth), which could help to identify the source of error. A workpiece might be geometrically in tolerance with the exception of a single scratch or a bit of dirt, creating an asperity that puts it out of tolerance. This kind of analysis might indicate that, in spite of the out-of roundness reading, no processing changes are required.
Yet another major advance in geometry gaging is in the area of international standards, which are under development by the ISO, the international standards-setting body. When they are published, probably within a year or two, they are expected to influence new U.S. standards now being developed by the American National Standards Institute (ANSI).
In the new ISO standards, paths for signal processing will be specified, as will the use of new phase-correct, distortion-free Gaussian filters (as opposed to the filters based on original analog concepts, which are in use in most existing instruments). The standard will also specify the number of data points that the gage must acquire per revolution, a ratio between the radius of the gage stylus and the diameter of the part, and accordingly, the maximum number of undulations per revolution that can be evaluated.
The purpose of specifying all these functional and structural details is to support consistency and reliability of results across different gaging instruments. Adherence to these specifications will help to resolve disputes between suppliers and customers over whose geometry measurements are correct.
A separate ISO standard is also under development specifically for cylindricity measurement methods. This standard will define the parameter, identify sources of error, and specify patterns of data collection and the minimum number of data points.
Geometry gaging is rapidly becoming more practical, more affordable, and necessary. With advances in software design, even complex parameters can be measured and analyzed by operators without special training in metrology. For most companies involved in precision metal cutting, geometry gages present a clear route to quality improvement.blog comments powered by Disqus