What Can CMMs Do?
They can measure almost anything
By Thomas R. Kurfess, P.E.
Professor and BMW Chair of Manufacturing
Department of Mechanical Engineering
Ph: (864) 656-6339
A coordinate measuring machine (CMM) is typically used to generate 3-D points from the surface of a part. It’s digitizing a part in three dimensions. However, it is often used to make 2-D measurements such as measuring the center and radius of a circle in a plane, or even one-dimensional measurements such as determining the distance between two points. Typically, CMMs are configured to measure in Cartesian coordinates. There are also CMMs that measure in cylindrical or spherical coordinates. They can measure any part surface they can reach.
CMMs typically generate points in two ways: point-to-point mode, where the CMM taps or touches the part and generates a single point per tap, or scanning, where the CMM moves over a part, generating data as it moves. Scanning generates significantly more data than tapping, but is typically not as accurate.
CMMs are manual or automatic. In manual mode, the CMM is moved by the user. An automatic CMM is typically actuated by electric drives (using ballscrews or linear motors). Articulated-joint CMMs look very much like six-degree-of-freedom robots, and are almost always manually driven. Hybrid CMMs are a cross between articulated-arm systems and traditional CMMs. They may have servo assist for making measurements.
While the CMM hardware generates the coordinate data, the software bundled with the CMM (or in many instances sold separately) analyzes the data and presents the results to the user in a form that permits an understanding of part quality, and conformance to specified geometry.
The most important advancement in CMM technology over the past several years is error mapping of the CMM. A machine is precisely measured and significant errors are corrected mathematically via software. As a result, looser tolerances can be used on the system hardware, and the resulting errors (as long as they are highly repeatable) are eliminated in software. This results in lower manufacturing costs, while retaining or even improving the capabilities of the CMM. Other major design innovations in the past were linear air bearings and linear scales for improved repeatability and accuracy.
New user-friendly software that allows the CMM and probe to be accurately, quickly, and easily calibrated have also made the CMM more accurate and easier to use.
Two types of probes dominate CMM operation: trigger probes and scanning probes. Trigger probes send a signal to the CMM when contact has been made with a surface. These probes operate in a point-to-point modes, generating a single point of data every time contact with the part is made.
Scanning probes are servoed over the part surface, generating points as they move across the part and can be contacting as well as noncontacting (laser triangulation-based probes, capacitance-based probes, and some probes based on laser intensity). Most scanning probes are passive, they measure deflection and report it back to the system. Some probes are active, and provide a higher-bandwidth scanning capability.
Examples of geometries that are difficult to measure include very deep holes, where a probe must be inserted down the length of the hole. If the hole diameter is small, such as cooling holes on turbine blades, the task becomes even more formidable.
Scanning CMMs already have dynamic compensation. However, scanning CMMs have difficulties at higher speeds, because inspecting a part faster increases vibration due to the higher acceleration and lower stiffness of the system. Much of this can be eliminated by “input shaping,” which is the same technology that is used to reduce vibration on low-stiffness robot arms.
A controlled environment is important for efficient CMM operation. CMMs can operate well on the shop floor if they are equipped with thermal compensation capabilities that correct for temperature changes from standard temperature (20ºC, 68ºF). In any case, the CMM should be kept in a relatively clean environment and located in a space that is isolated from vibration.
Accuracy of a stationary bridge-type CMM is usually better than that of a mobile articulated-arm CMM. However, recent advances in the articulated arm area, in particular related to error mapping, have yielded significant advances in the capabilities of the articulated arm.
But for many operations, the accuracy of articulated arm CMMs is sufficient for a variety of processes. The advantage of articulated-arm CMMs is that they generally have a larger work volume than bridge CMMs, and can reach areas that are not easy to access with typical CMMs. Thus, if quoted accuracies for articulated arm CMMs are sufficient for a particular application, it should be seriously considered as an alternative. Also, articulated arm CMMs are more portable. Typically, they can be set up for measurement quickly.
On the downside, articulated-arm CMMs are manually driven while gantry-type CMMs are both manual and servo-driven. Thus, articulated arm CMMs do not lend themselves as well to automation as servo-driven gantry-type CMMs.
The size range of a CMM can span about four orders of magnitude with respect to part size. There are a variety of enormous CMMs that are used for measuring entire car bodies, the bodies of earth moving equipment, and even large aircraft elements (e.g., wings that are 10-m long). There are other CMMs that measure parts that have features on the order of 1 mm. This capability can offer significant advances in micromanufacturing.
One of the greatest problems in micromanufacturing is quality control. For example, with a bearing a few millimeters in diam and ball tracks that are 100 µm wide, how do you measure critical features like the ball track’s major and minor diameters, and its orientation with respect to the bearing’s mounting face? Without precise measurements, essential feedback process improvement isn’t possible.
In the future, higher-speed measurements facilitated by linear drives and more advanced controls, in conjunction with thermal compensation, will be making further inroads in the CMM area. Other big changes may be expected in the software for CMMs as it becomes more user-friendly, and flexible. This will allow for much easier integration of the CMM into automated production facilities, and automation is a very big issue.
CMM Product Summary
The Phoenix Dcc CMM is the latest unit from Helmel Engineering Products Inc, (Niagara Falls, NY). The heavy-duty unit is designed for harsh environments. Model 216-142 has a measuring capacity of 20 × 16 × 14" (500 × 400 × 350 mm) with a resolution of 0.00002" (0.5µm). Repeatability is 0.00012 (3 µm). The design, patterned after an earlier Checkmaster, features precision ground ways, mechanical bearings, and dual-beam bridge.
It has a Delta Tau PMAC controller and Helmel’s Geomet software, which is available in four versions: Junior, 101, 301, and GeoCAD. The CAD interface supports surface modeling and data point-cloud mapping, which makes it suitable for reverse engineering. The designs do not rely on volumetric correction during operation. Accuracy is mechanically intrinsic.
Romer CimCore (Wixom, MI) now offers the Infinite CMM with wireless connectivity and battery power for portability. Unit opeates with no cables attached. Features include high strength carbon-fiber tubes, Heinhain encoders, and a built in digital camera to document set ups.
To verify and document performance as well as help establish ISO requirements, there is a NIST-traceable, calibrated length standard. Unit is available with seventh axis, combined laser scanning and probe assembly for real-time laser inspection and reverse-engineering applications.
LK Meterology Systems (Brighton, MI), now in the Metris group, was known as a supplier of large, specialized CMMS to the OEM and Tier One market. Now they are offering a group of entry-level CMMs known as the Ascent family. These units are intended for smaller job shops and Second Third, and Fourth Tier suppliers. The designs stress small footprints, rugged construction, and low cost.
Work envelopes for the three are 24 × 20 × 16" (610 × 508 × 406 mm), 32 × 28 × 24" (813 × 711 × 610 mm) and 40 × 28 × 24" (1016 × 711 × 610 mm).These machines, which are available with three probing options are driven by machine-specific software. The Ascent units draw on key modules from LK’s Camio Studio software platform.
Hexagon Metrology, (North Kingstown, RI) is a company that has undergone massive expansion by adding a number of new companies to its operation; the group now encompasses Brown & Sharpe, DEA, Leitz, Romer, Sheffield, TESA, and Leica Geosystems, among others. They offer a wide range of units from large fixed-bridge machines and portable arms, to small portable units. Their operating philosophy is that they do not want to try to force customers into buying from a narrow product line, but rather, offer a wide variety of machinery types to suit most any application.
Software strategy is to provide one software platform, PC-DMIS, for the entire product line. Company planners took this route because they found that many customers had a lot of different software systems on their measuring devices. The trouble with that is you can’t share programs or reports easily and there is no common user interface.
One company-wide trend is a rapidly expanding service business. With increased business, along with the move to outsourcing, many companies are overwhelmed with inspection work, particularly during product launches. The company provides help on several levels that includes advice, temporary staffing, even performing off-site contract inspection.
Optical Gaging Products – OGP (Rochester, NY) offers systems with multiple sensors from a different perspective. Many of its SmartScope video measuring systems are available with one or more additional sensors. Such a video-based multisensor machine can include laser, touch probes, and even white light scanning. The touch probes can be single point touch trigger type, or continuous scanning with even the PH-10 articulating head available on some models.
Use of multiple sensors allows a user to get more detail about a part from a single setup on a single machine. For example, video measurement excels at edge measurements while scanning laser or white light sensors provide surface contours. Touch probes can access walls and surfaces that cannot be imaged optically, or accessed with laser or white light. Scanning contact probes are also available. Collecting and analyzing diverse sensor data in the system metrology software uses a single datum reference, simplifying construction of measured features from any or all of the acquired data points.
In addition to a variety of sensors, single or compound rotary indexers can bring important features to positions where they can be measured with each sensor. Doing this probing and positioning under program control allows for totally hands-free measurement.
FARO Technologies Inc. (Lake Mary, FL) initially developed equipment for diagnostic and surgical work. Later they changed to computer-aided manufacturing measurement. With the acquisition of German iQvolution, FARO broadened its customer base to include all computer-aided measurement.
FARO’s portable CMMs have what’s known as an “open architecture,” meaning that their 3-D measurement devices are compatible with many varieties of software, although the company has its own: CAM2 Measure X. FARO supports more than 60 different software platforms and can provide customers with the best possible alternatives.
One of the company’s latest instruments, which is portable enough to set up almost anywhere, is the Scanner. It captures more than seven million points in less than a minute. With a few setups, an entire 3-D image is created with the clarity of a digital photograph, complete with a color overlay for a true real-world scan.
The unit is currently used for factory planning, facility life-cycle management, quality control, performing forensic analysis and, in general, processing large volumes of 3-D data. Laser-scanning technology simplifies modeling, reduces time requirements, and maintains or increases the accuracy of the image. The resulting data are used with major CAD systems or the company’s proprietary software, FaroScene, for modeling and designing new factories or redesigning existing layouts.
From Zeiss (Maple Grove, IL), the latest development is the F25, a machine designed specifically for the measurement of micro components. Measuring volume of the F25 is one decimeter, with a measuring uncertainty for this volume of 250 nm at a resolution of 7.5 nm. It can measure bores less than 1 mm in diam. Key element of the new design is a tactile sensor that uses a silicon-membrane spring and integrated piezoresistive elements. The stylus permits measurements in both single-point and scanning mode.
The 3-D microstylus is designed for stylus diameters of 50–500 µm and stylus-tip diams of 100–700 µm. With a free shaft length of up to 4 mm, it can measure structures with probe forces of less than 0.5 mN/ µm.
The F25 also has a ViScan camera sensor for 2-D measurements. It is optimized for depth of field and has minimized distortion, and ensures accurate measuring results with maximum resolution in reflected and transmitted light.
The combination of touch and optical sensors enables measurements of 2-D and 3-D structures in the same coordinate system. The F25 uses the established Calypso software.
Geodetic Systems (Melbourne, FL) offers its V-STARS family of 3-D coordinate measurement systems which use this technique. V-STARS photogrammetric systems are predominantly employed for in-place measurement of large stationary objects in unstable environments, where conditions like vibration, movement, and extreme temperatures present problems for other types of metrology devices.
V-STARS digital photogrammetry offers the ability to measure objects in dynamic motion, to capture precision data in unsteady environments, and to accomplish assignments requiring simultaneous multiple point coordinate acquisition. For example, in one application the system was used to evaluate the surface integrity of antenna panels in Antarctica.
Leica Geosystems (Lawrenceville, GA) now offers its T-SCAN hand scanner which is a high-speed optical system designed for measuring large complex surfaces. It can acquire 7000 points per sec.
The operator can use the handheld scanner to digitize both small and large objects, and gather millions of 3-D points in minutes with an accuracy of better than 100µm. Leica’s CMM combines laser tracking functionality with probing and scanning capabilities. The scanner’s four faces provide multiple angles of access to a workpiece face.
This article was first published in the March 2006 edition of Manufacturing Engineering magazine.