Measuring Part Geometry On The Shop Floor

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 others are 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.

Why Measure 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 slight, often 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.

What To Measure

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.

Roundness

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 In One Plane

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 Relative To An Axis

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.

Onto The Shop Floor

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.

• Photocopier drums. One geometry gage user produces aluminum drums for manufacturers of photocopiers. Tight tolerances must be maintained for both cylindricity and concentricity, plus separate roundness and straightness parameters. Parts are measured on an audit basis, which has proven sufficient for maintaining control over the process.
  A shopfloor geometry gage, like the Mahr Federal, Inc. Formscan is centrally located on the shop floor for accessibility by several lathe operators. A fixture on the turntable speeds setup of the 10-inch-long drums, making the gage more efficient for shared use. Because every part undergoes the same series of measurements, the gage has been programmed to perform all four measurements sequentially and automatically. Each lathe operator initiates the measurement sequence by pressing a single "button" on the controller's touch-screen. The gage automatically measures the cylinder at ten pre-set intervals along its length, then calculates cylindricity from the data. The entire procedure takes less than five minutes, and operators use the measurement data in real time to adjust their lathes. All measurement data, which is stored by the operator, is batched, summarized, and exported weekly to customers.

• Precision bearings and gimbals. A manufacturer of precision miniature bearings and gimbals for aerospace, computer, and medical applications has to perform a variety of roundness-related measurements on a range of part types and sizes. Geometry parameters vary with each component, and include: roundness, concentricity, face squareness, and coaxiality. A Formscan gage on the shop floor is accessible to operators of centerless grinders, lathes, and machining centers. "One-button" operation, as used by the photocopier drum maker, is unfeasible for this manufacturer. Nevertheless, the "Windows-style" software helps simplify operation. Operators retrieve the appropriate program sequence for each part from the system's memory. The instrument then performs a setup sequence: The turntable makes one revolution while the gage head determines the precise position of the part relative to the spindle axis. The computer tells the operator what adjustments to make on the tilt/centering table on the spindle. Once the axis has been trued, the rest of the gaging cycle is initiated by pressing the "start" icon. The manufacturer reports that machinists find the touch-screen controlled, icon-based software less intimidating than a conventional computer keyboard interface. Ease of use translates into operational efficiency: gaging cycle times were cut by more than 75 percent with this gage, compared to the metrology instruments used previously.

• Spool valves for automatic transmissions. A major U.S. auto maker uses a geometry gage on the shop floor to support the production of valve spools for automatic transmissions. A line of 24 centerless grinders, equipped with automated parts handling, produces several thousand parts per hour. Random-sample parts are gaged for roundness tolerances of ±0.0001 inch, then the results are subjected to harmonic analysis using a sub-program in the software.
  
  A typical out-of-roundness trace exhibits a complex profile, which is the result of numerous, overlapping lobing conditions with different numbers of undulations per revolution. For instance, the use of a three-jaw chuck might impose a three-lobed condition to the part, while motor vibration in the machine tool might produce a 50-lobed condition, and chatter from the cutting tool might generate yet another lobing pattern. Harmonic analysis enables the gage user to "filter out" and separate different frequencies, to analyze those lobing conditions relevant to the part application or processing problem.

• Hydraulic pumps and valves. A supplier of spool-type valves and pumps made a commitment toward improving the performance of its products, and quickly found that its gaging practices also had to be upgraded. The company had been using air gaging to monitor bore IDs, inserting an air plug tool into the bore and rotating it, while watching the minimum and maximum readings on the gage to get a "feel" for bore roundness. In-house design engineers wrote new specifications for cylindricity, which is a sophisticated parameter incorporating roundness, straightness and taper in a single measurement. The company subsequently installed a geometry gage in a lab adjacent to the shop floor.
  With the new gage, valve body bores could be analyzed more thoroughly. In one component, bore entries were observed to be consistently bell-mouthed, while in another, bores exhibited helical ridging. Both problems were traced back to worn components in machine tools, and were resolved by appropriate maintenance.

• Machine spindles. A manufacturer of spindles for machine tools also installed a Formscan cylindricity gage on the shop floor to support efforts at product-line improvement. When spindles for higher speeds and accuracy were designed, the company wrote new geometry specifications. A runout tolerance of 70 microinches was established for some high-speed spindle shafts. On one 20-inch-long spindle body, the tolerance for concentricity between the front and rear bearing pockets was specified at 0.0002 inch. Equally demanding specs were written for roundness and cylindricity of various components.
  
  Although the company had just installed a new, ultra-precise vertical machining center, machinists found it difficult to meet the new specifications at first. Operators began tweaking the process with adjustments in speeds and feeds, fixturing and setup practice, and tooling selection, while test parts were analyzed on the geometry gage. Through intelligent trial and error, operators were able to identify those changes that improved their process, and the formerly demanding geometry requirements were brought under control.

 

What's Next?

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.