ISO 9000 and QS 9000 have the whole world buzzing. If you're inspecting any round surfaces, your customer callouts probably dictate that you measure ISO/QS-specific features related to contours, roundness, and surface finish. And, if you're specifically servicing the automotive industry, you're familiar with the Big Three's high expectations for compliance with the newer standards. The question then becomes how to select instruments for curved surfaces that provide the most accurate readings and the fastest, most efficient process control at a reasonable cost. This question is especially timely today due to the resurgence of match-fitting practices, which depend on more precise and finite measurement data. If you don't get accurate readings on the first round part you sample, you may run into too many rejects by the time you audit down the line. In the case of engines, if the inaccuracy goes undetected, it may impair operational performance and durability. The bigger the engine, the higher the stakes.		Add to the picture the range of inspection products now available to help you handle curved parts better. So, how do you choose the measuring system which meets your specific requirements for curved surfaces, and where is the return? If you haven't already confronted these issues, you'll probably need to make some decisions sooner or later. Take heart. Today's automated measurement instruments simplify inspection of round parts, because they are equipped with software capable of compensating and removing curvature, a necessity in analyzing curved surfaces. Some manufacturers offer sophisticated analytical tools and mechanical accessories for curved surfaces as standard to improve measurement throughput, accuracy, and process control. The higher quality instruments also have larger measuring ranges and more powerful resolution throughout those ranges with millionths of an inch accuracy to satisfy ISO and QS requirements. Here's what a switch from a manual flat surface tester to an automated surface finish tester did for an engine parts OEM, who was gearing up for ISO certification.
Case One: Surface Finish Inspection
Welles Manufacturing (Northvale, NJ) measures mostly curved surfaces with surface finish specs in the 8 to 18 micro-inch range. They manufacture 150,000 parts/week, mostly engine shafts, followers and rocker arms, and in 2,000 varieties. Inspection frequency amounts to about twice per shift, and inspection is done by any of seven people: machine operators, engineers or QA personnel. With the old system, all readings were transcribed and recorded manually, which was slow and error prone. Now, the new system has a built-in round surface compensation function which automatically calculates the roughness parameters and suppresses part form. Without this function, roughness parameters like Ra may be artificially inflated in parts with even a small amount of curvature. With this function, Welles is confident that any surface roughness measurement are well within customer specs and ISO 9000 standards, regardless of		the curvature or who performs the inspection. Although a surface finish tester solved Welles' problems, it may not be the right answer for everyone. It's a matter of defining your requirements and relating them to the basic functions and limitations of available systems made specifically for curved surfaces. The four curved surface needs you'll typically run into are roughness, waviness, contour geometry and roundness. To cover these needs, we'll briefly discuss four generic systems, including a newcomer to the market. Then, we'll guide you through a decision-tree to help determine which system will best serve you.
Four Options Now
Included in the universe of curved surface measurement systems are contour and roundness measuring instruments, as well as surface finish testers. All three types are equipped with built-in curve compensation functions and algorithms, as well as mechanical accessories specially designed for curved surfaces. Special hardware provides larger and more powerful fields of vision and sophisticated probes to produce more accurate readings faster.		These instruments are usually hooked up to a conventional PC and are pre-programmed to plot results in real time. They can also print out ISO/QS-ready customer reports at the touch of a button. These special capabilities save time and reduce the risk of errors vs. earlier systems, where this level of automation did not exist. Furthermore, results are expressed in curved part terms, e.g. radius, polar coordinates, concentricity, etc.
Contour Measurement Systems
Contour measurement systems measure linear distances and angles on curved parts, such as radii, angles, step heights, and deviations from reference values. A high resolution laser holoscale is incorporated into the Z-axis detecting unit, so you can measure with 2 µ in., or better, resolution over the entire measuring range. A standard bench-top system, such as Mitutoyo's Contracer, has a wide measuring range of 4 in. (100 mm) for the X axis and 1.57 in. (40 mm) for the Z axis. Magnification is up to 200X.		A new development in contour measurement is a Mitutoyo economy portable model with digital data output for measuring sections of large workpieces right on the production floor. It measures approximately 21 W x 5 D x 7 in. H and weighs 9.24 lbs. An electronic accessory with LCD readout measures 10 W x 8 D x 3 in. H and weighs 4.40 lbs. Price is about half of a standard contour measurement machine. Measuring range is 2 in. (50 mm) for the X axis and .7 in. (18 mm) for the Z axis. Magnification is up to 100X.
Roundness Measuring Machines
Roundness measuring machines measure at least 12 key relative dimensions associated with round parts or round sections of non-round parts. Upscale models measure as many as 18 roundness parameters, many of which are required by ISO/QS compliant companies. A standard unit, such as Mitutoyo's Roundtest RA400 series, measures deviations from a geometric true circle based on all four roundness references. It measures parameters according to least squares mean circle (LSC), maximum and minimum inscribed circle (MIC, MCC) and		minimum zone circles (MZC). The remaining eight parameters include diameter, concentricity, coaxiality, circular runout, thickness deviation, flatness, parallelism and perpendicularity. Higher-end models measure an additional six parameters: total runout, cylindricity, spiral scanning, radius deviation, straightness and symmetry. Centering and leveling of the workpiece, normally a skill-intensive and finicky manual operation, is now totally automated via a digital adjustment table (DAT).
Case Two: Roundness Inspection
Here's how a contract manufacturer of electric motor cores dramatically improved process and quality control of cylindrical parts by implementing 3D roundness testing with 20 millionths resolution. Danco Precision Inc. (Phoenixville, PA) switched from a general purpose coordinate measurement machine to one specifically designed for roundness inspection. The motor cores are assembled from 13 to 1,000 precisely matched stampings. While it is within stamping capability to hold		.001 in. on an individual lamination, holding it on a finished stacked motor core is another matter altogether. Cumulative error becomes a big problem. With the Mitutoyo Roundtest RA-424 roundness machine, Danco measures 12 parameters and satisfies modern GD&T drawing requirements for +/- 0.0002 in. tolerance of several difficult-to-measure features. Now, their confidence level on accuracy is between 95-100%.
Surface Finish Testers
Surface finish testers, aided by special software and hardware, measure curved surface parameters such as "R" (roughness), "P" (profile), "W" (waviness), "S" (spacing), etc. They rely on software with powerful curve removal algorithms to compensate for simple radii, multiradii, ellipses, parabola, hyperbola or complex curves. On the hardware side, special probes and skids measure induced waviness or chatter on the curved surface to ensure proper mating of parts, as well as part wear over time. Recently, many customers expect you to evaluate surface finish not only in the Z-axis, but also in the horizontal (X-axis). Some standard machines, such as the		Mitutoyo SV-500 series Type 2 and SV-600 series Surftest surface finish testers, provide X-axis evaluation, because of the addition of an accurate X-axis scale incorporated into this equipment. These systems directly address 48 of the most sophisticated surface parameters more and more customers are requiring today. Now that we've covered four options in curved surface measurement equipment, let's pull the pieces together. Here are some of the questions you should ask yourself to determine whether you need a more sophisticated system, and, if so, which type.
Look Out for the Curves
What percentage of your workpieces have curved surfaces? Are you measuring many geometrically challenging and multiple features on those parts? The higher these numbers, the more you'll get the benefits of lower inspection costs and accuracy with instruments designed specifically for your curved surface parameters. Are your customers' specifications getting more demanding with regards to curved surfaces? If they are, you have no choice but to upgrade to specialized instruments. What are the specified tolerances on the curved part print or customer specification? The rule of thumb you can apply here is that if tolerances are less than +/- 0.0005 in., a system for curved surfaces will be favored. How many of your people are measuring and inspecting round parts? Chances are that if you have multiusers on manual instruments, they are burning up time in computation and transcription and not always achieving consistent results. What is the skill level of your people? The lower the skill, especially as it relates to manipulating polar measurements, the more you'll need a system that will read, compute and transcribe automatically. If you are measuring surface finish: 6.1 How many surface parameters does your part print specify in curved parts? Are these parameters tied to ISO, QS, JIS, DIN, or NF guidelines? If you have a variety of parameters to measure, you need and instrument with the ability to handle all of them.		6.2 Does your hardware provide long enough range, appropriate stylus, proper skid configuration to measure various-shaped workpieces? There are a wide variety of styli, both standard and optional, you can select according to the workpiece shape. 6.3 Are your system's curve compensation functions powerful enough to measure surface parameters without any manual intervention? Make sure you select a system which automatically detects the peak and base points of the curved surface and sets the stylus range in start position automatically. If you are measuring features on contours: 7.1 Does your instrument have a large enough measuring range and 0.1 µm resolution throughout the range? 7.2 Do you have to measure parts on the production line? In that case, the portable model may be appropriate. 7.3 Do you measure many highly-critical parts, with tight tolerances essential for their proper functioning? If you do, you may need to consider a bench-type contour measuring instrument. If you are taking 3D roundness measurements: 8.1 Do you have multiple roundness call-outs on your specification? If so, a roundness measuring system will be more cost-effective and will measure 12 to 18 roundness parameters more efficiently than a rectilinear system. 8.2 Do you have any round parts that will operate in a wear or friction environment? To ascertain the part is truly round, you'll need a roundness measurement system based on polar coordinates.
Selecting Vendors
Once you have decided on the generic type of system requirements, it's time to compare vendors. Main point: make sure you have a true apples-to-apples comparison. For example, ask yourself: What is the true range of measurement and resolution? How big an area of the part can I inspect without having to move it? Each time you have to move a piece, you risk inaccuracies and increase throughput. Does the standard package provide readings of all the parameters specified by ISO and QS standards? You will find wide variations among vendors on what constitutes standard. What accuracy standards are you using in your accuracy claims on both X and Z axis?		What's included as standard? Optional? Does the standard package include powerful curve compensation software, automatic centering and leveling (roundness), probes with sufficiently long reaches, laser holoscale (contour testing)? You'll find wide differences among vendors on those scores. Compare warranties and after-support. Are installation and training included in the quoted price? Are hardware and software upgrades offered, and at what cost? Does the software offer SPC data collection or word in conjunction with an SPC software? How user-friendly is the equipment and documentation? Ease of use can affect skill requirements and accuracy.
Conclusions
Process control simplification and quality are important in any business, if you want to remain competitive. The reality is that curved parts now, and in the foreseeable future, will only become geometrically more complex and demand tighter tolerances. The cost of not modernizing will show up in low throughput and high inspection costs, scrap, returns and failure rates. If you have to be ISO and QS compliant, you may have no choice but to invest in more sophisticated equipment. If, for example, you're overfinishing the part in order to make your quality specs, it's clear that your requirements have surpassed your capabilities.		When you're ready to take the first step, analyze your requirements, then compare throughput, inspection costs and scrap among your old and new system. Don't overbuy. Select standard curved surface equipment that has accurate software, drive units and probing, appropriate skid configurations, and sufficient enough magnification and resolution to do the job. Also ascertain the manufacturer supplies optional accessories to expand your applications, if you need to go that route in the future.