CMMs Are Key to Auto Quality
Measuring for success
By Thomas R. Kurfess
Professor and BMW Chair of Manufacturing
Department of Mechanical Engineering
Director Carroll A. Campbell Jr.
Graduate Engineering Center
International Center for Automotive Research
Quality requires inspection, and automakers are relying more on CMMs to fill their metrology needs. To handle these automotive assignments CMM suppliers have made strong developments in the areas of shop hardening, throughput, and particularly, software.
Initially, CMMs were slow and too expensive to measure parts and provide feedback in a timely fashion. Lower cost and faster CMMs have addressed these issues. Furthermore, as CMMs are already computerized they are easily interfaced with automated production systems. In the future, more and more CMMs will be functionally interfaced with production lines.
The second reason for the CMM's early lack of acceptance is the environment. Typically, the CMM is considered as being found in a metrology lab with very tight environmental control. However, the environment on many production floors can vary greatly, often in a short period of time. For example, when starting-up on a cold winter morning, a building temperature could be 65°F (18.3°C), and easily heat-up to 75°F (23.9°C) or higher during the day. When the temperature shifts, three undesirable things can happen: the part changes size, the CMM changes size, or both. And these variations can be significant. There are some very odd results from temperature variations. But, in general, CMM variations due to temperature changes have been mitigated via proper machine design, thermally stable materials, and temperature compensation.
The big problem is part-size variation due to thermal expansion that is made more complex by the fact that different materials have different thermal coefficients of expansion.
A well-designed CMM will not have much thermal change from a 10°F variation in the environment's temperature. However, a part may have a significant change in size. Furthermore, a part made from aluminum will have a larger change in size than an equivalent part made from steel as the thermal coefficient of expansion for aluminum is greater than that of steel (about 30% higher). To overcome this, the software in the CMM can do some rudimentary thermal compensation of the part.
Basically, a temperature probe is put on the part to provide temperature information to the CMM computer. The CMM computer also must be informed as to the material of the part being measured (e.g., steel, aluminum, etc.). Using the thermal coefficient of expansion for that material in conjunction with the measured temperature deviation from the standard temperature 68°F (20°C), the CMM corrects its metrology results for the part not being at standard temperature.
This approach works fairly well, as long as the temperature of the part is constant during the measurement. But this is not a given. A part that just finished an operation that has increased its temperature (e.g., machining or some forming operations) may be warmer than the environment, so the part cools down during the measurement.
One of the most significant ways CMM thermal sensitivity has been reduced is via the use of glass scales as the primary means of position measurement. In particular, scales made of Zerodure, a ceramic whose thermal coefficient of expansion is close to zero at room temperature, are used regularly in most CMMs.
Again, many factors come into play in terms of the changing of a part's temperature. However, the two critical elements are the difference between part temperature and room temperature, and the time it takes to inspect the part. If the part is only above room temperature by a few degrees, and the inspection takes only a minute or so, then the variation of the part temperature during the inspection will not be significant, and the CMM software will be able to thermally correct the results. However, if there is a significant difference, for example, a part temperature of 100°F (37.8°C) and room temperature of 70°F (21.1°C), and the inspection of the part takes long enough that its temperature changes significantly, then the accuracy of the inspection results will be compromised. Again, the CMM computer is programmed to check the part temperature at the start of, during, and at the end of inspection. If too much variation occurs, the CMM will inform the user that the results may not be to the level of certainty required.
If you take proper precautions, these thermal errors can be easily reduced to 1% of their original value. If proper thermal control is executed in the area of the CMM, you can virtually eliminate thermal issues.
With thermal compensation, you can do pretty well. However, if the part's temperature changes during the measurement, then thermal compensation runs into problems. As long as the part's temperature is fairly constant during the measurement, you are set. You really only run into problems when you are trying to measure a very large part with significant accuracy. If the part is still hot from the previous operation and the inspection on the CMM takes a significant amount of time (allowing the part to cool), then your results will be inaccurate, and typical CMM software will not be able to compensate for thermal errors.
Competitors to the CMM are dedicated or hard gaging and laser/optical sensors. A big advantage of the CMM is its flexibility, which permits variations on the production line that hard gaging doesn't.
For traditional applications, the automotive sector is close to saturation with respect to CMMs. Thus, a good portion of the market is involved in replacing old systems or identifying new possibilities for employing CMMs in production.
One of the CMM's big pluses is that it is an excellent source for SPC (statistical process control) data. This is something that hard gages do not lend themselves to very well. CMMs also enable automation, reducing labor costs.
Software is a big issue with CMMs. They are being adapted for operations on the production floor, as opposed to just operating in the metrology lab. Throughput is probably the most limiting factor for the CMM. As they get faster, they will be used in more applications.
CMMs have really been coming into use on automotive production lines over the past 10–15 years. In the past, they have not been integrated directly into production lines. They are off on the side doing audit work for two major reasons. First is throughput. It's the bottom line for many high-rate production situations.
Measuring powertrain components is a major use of CMMs in the auto industry. This is because powertrain components typically have higher precision requirements (1 µm for powertrain components vs. 0.1 mm [100µm]) for frame/sheetmetal components).
Powertrain components tend to be smaller than overall car frames and bodies. When inspecting a car, optical inspection devices or laser scanners may have the necessary accuracy capabilities, and they are faster than CMMs. Also, CMMs that are large enough to inspect entire automotive bodies are quite costly. CMMs are used to inspect the entire car body, or large sheet metal parts, but typically this is done during setup or on an audit basis, where speed and cost of the machine are not as critical, but accuracy is.
CMMs are capable of running "repeatability and reproducibility" studies in the same fashion as dedicated gages. This capability helps to link the capabilities and results of the CMM to those of the familiar dedicated gages.
A good example of flexibility might be in cylinder-diameter inspection of an engine block. With the high price of gasoline, four-cylinder engines are now more popular. However, down the road, producing more six and eight-cylinder engines may again become popular.
CMMs can be reprogrammed to inspect all of these types of engines. With hard gaging, new gaging has to be developed for the different engine types. And it has to be stored if you are not using it. With a CMM, you just store the program. CMMs are not completely flexible in nature, as they do need different fixturing for different parts. However, they are significantly more flexible than dedicated gages.
Computers run CMMs, so the data generated by the CMM (raw or processed) are already on a computer. Thus, the CMM can be used to provide reliable and automatic information for SPC. This is just starting to happen, and little SPC information is currently being directly garnered from CMMs. This is mainly due to the fact that this capability is rather new. However, as time progresses, manufacturers will find that SPC can be readily applied to CMM data, and this application area will take off. In fact, there is probably much more information in the CMM data than most manufacturers realize. With increased bandwidth and data storage capabilities in factories and in manufacturing global networks, there is no doubt that more and more of the data generated by CMMs will be put to good use.
Another benefit CMMs have is that they permit automation and reduce labor costs. They do need to be maintained, programmed, and monitored. So while some jobs are being lost, others are being created. However, the new jobs will require higher skill levels and they will be higher paying. This does remove lower-end jobs from the production system, and that tends to level the playing field with respect to production costs.
Automatic CMMs are most prevalent in the automotive sector. It is not often that auto manufacturers use a manual CMM, as the big advantage of the CMM is automation and getting the operator off the line into a more skilled position.
Software and control are the major areas of development. First, software makes that CMM flexible. When switching from one product line to another (e.g., four to six-cylinder engines) programs must be changed. Of course, there may be fixturing issues and probe issues as well, but the software really gives significant flexibility. However the software can be a limiting factor. CMM manufacturers have proprietary software, and many companies spend a significant amount of resources training on this software and developing for this software. So in many instances, companies opt to replace CMMs with ones that are produced by the same manufacturer so they can use the software that they have in place, and not have to retrain personnel and reprogram all of their systems.
Software is often a major selling point for a CMM. User interfaces are continuously improving, and in many instances the decision point in purchasing a new CMM is related to the ease of use, rather than accuracy or other features. That is, the capabilities of many CMMs in a given price range may be similar, and their major distinguishing characteristic is the user interface.
In many instances, CMM manufacturers are being asked to integrate their product into production systems. CMM suppliers are now providing more and more turnkey systems. In fact, a manufacturer may order an entire flexible manufacturing cell with a CMM included. The cell manufacturer will then contract out to the CMM manufacturer to supply a turnkey CMM solution.
That CMM solution may be used in the validation of the overall cell. That is, the cell will be run to produce parts, and the CMM will be used to validate the parts that are being run off the cell. Not only are the CMMs being used as part of the cell, but they are also being used to qualify the cell and the cell's programming.
CMM programming is typically done off-line and may actually be done by the CMM manufacturer or a second party. However, even though there are a variety of excellent offline programming tools, the CMM program is almost always fine-tuned on-line once it is integrated into the production systems. This is necessitated by the fact that there are always a few unknowns or inaccuracies in the overall production system such as minor variations in part or fixturing geometry, or variations in part-loading systems. It is important to pay close attention to those. For example, when inspecting a cylinder bore right at the top of the engine block. If the block is fixtured a few millimeters too low, then the CMM probe might completely miss the top of the cylinder.
A highly accurate and robust sensing head provides a major part of a CMM's capabilities. Renishaw (Hoffman Estates, IL) a major provider of these units recently offered the Revo measuring head and probe systems. Unlike conventional scanning methods, which rely on speeding up CMM axis speed to scan quickly, this head uses synchronized motion and Renscan5 five-axis scanning technology to minimize the dynamic errors. Speeds of 500 mm/sec are possible without accuracy penalties. The Revo head has spherical air bearings on two of its axes, providing a stiff metrology platform. These axes are driven by brushless motors linked to 0.08 arc-sec high-resolution encoders.
"There is a lot of activity in the automotive industry with a growing need to measure complex parts with compound angles and tight tolerance," says Kevin Santilli, Carl Zeiss (Maple Grove, IL) strategic business manager. "This is particularly true with drive-train components."
One of the Zeiss products recently developed for this type of work is the Vast XXT probe head. It has continuous-scanning capability and can be used with stylus lengths from 50 to 250 mm. This unit is often used with the rotating dynamic sensor probing system that can be indexed in increments of 2.5°.
Another trend is the increasing importance of part surface condition. So there is an emphasis on measuring surface features such as form and flatness. For example, in an effort to improve vehicle mileage the roundness of cylinders and bores has become more critical. It takes a very accurate CMM to handle this job.
Another trend is integrating the CMM to the production line. The Zeiss CenterMax and GageMax are designed for this application. They are entirely automated systems—including loading and unloading—that provide real-time correction to the manufacturing equipment.
Portable CMM arms from Faro (Lake Mary, FL) solve a lot of problems for the auto industry. According to company engineers, they focus on making their products durable and accurate enough to hit tolerances in any area of the facility, but they also work with more than 30 different software platforms—many that are embedded in auto manufacturing operations—so users can choose software with which they're already familiar—or that's best for their application.
The operating envelope varies with arm length, which ranges from 4 to 12' (1.2–3.7 m). The accuracy provided is also matched to the job. For example, a transmission gear has a tighter tolerance than, say, a mold for a car seat.
The most precise unit is the Faro PowerGAGE. Its CAD-to-part analysis capability enables manufacturers to verify that a part meets the CAD file's specs to within 0.0002" (0.005 mm)– right on the machine that's producing the part. The PowerGAGE runs exclusively on a version of Delcam's PowerINSPECT, a software already used by the majority of the OMI market.
In operation, the user traces the arm's tip over the part's entire surface, then the system's laptop computer verifies all of the part's 3-D measurements against the original CAD file.
Romer (Farmington Hills, MI) equipment is used by manufacturing engineers on the shop floor, often to resolve bottlenecks. In one case a large company has a kind of one- man "swat team" that consists of one person and a Romer arm in a golf cart. He goes around resolving problems, usually with very large parts. That is, those that can't be brought to the measuring machine. In one case he discovered that the fixture used to join two massive truck parts was in error by checking the dimension of part elements no one could easily reach.
Software Runs the Show
According to Jim Clark, Metris (Rochester Hills, MI) VP, software and controllers are major issues with CMMs. Software takes the error out of the system. The software "knows" the errors in the CMM and stores these errors in the form of a 3-D map and thus automatically compensates for them while taking measurements.
The range and accuracy have improved dramatically. Metris offers units with from 1 to 300µm accuracy at ranges up to 100 m. For example, we can find a single screw hole in an aircraft fuselage at 60 m.
Data can be collected in three ways: touch probe, which can pick up a single point; analog probe, where a probe is dragged across a surface, producing a continuous linear stream of points; and laser scanner, which sweeps a laser across the surface, measuring surface points at rates up to 200,000 points per second. Reflectivity influences this number, so the laser sensor automatically adjusts the contrast for maximum input.
Although robots exhibit good repeatability their overall accuracy is often limited, thus any scanner mounted on a robot that relies on the robots' encoders will suffer in terms of accuracy.
To achieve near 100% inspection, the point cloud data can be compared to the CAD data. The resulting output is a color image where the colors represent distances above and below the nominal. Additionally, a mouse can be used as a virtual CMM. Click on the model and the error at that point is reported. Reports can be customized and automated.
CMMs on the Job
Mitutoyo (Aurora, IL) has developed a CMM-like device that measures surfaces instead of dimensions. A machine, designed much like a CNC machine tool, carries a head with an LVDT-type sensor. The diamond stylus is moved over the surface to be checked and stylus oscillations are converted into any number of standard surface readings.
"This unit does jobs on the shop floor that were formerly done only in a lab, and the work is done automatically, explains Bob Sand, general manager. "For example, a cylinder head has to be checked in several planes. The CNC automatically positions the head to make these tests.
"Most of our work has been done on power-train elements, chiefly the 'Five Cs'–camshaft, crankshaft, cylinder head, cylinder block, and connecting rods. We are also working with transmissions."
It's a stationary unit that interfaces with the line and the operator. The unit is mounted "near line" and is fed a statistically determined sampling of parts.
In some jobs, the surface sensor is mounted next to a high-speed dimensional CMM called the Mach V. It's a very robust robot that is embedded into the production line.
This article was first published in the September 2006 edition of Manufacturing Engineering magazine.