Probing Technology Moves Ahead
Scanning offers opportunities for speeding part inspection
By Denis Zayia
CMM Products Manager
Hoffman Estates, IL
Rigorous part inspection becomes increasingly essential as industries comply with ISO and QS9000 requirements and adopt Six Sigma disciplines. Parts must be produced to ever-tighter tolerances, then validated and documented before shipping. Rather than sample parts to a set schedule, producers are now looking to verify each critical feature on every part. Sandwiched between JIT delivery schedules and pressures to cut work in process, manufacturing planners are looking for inspection technologies that can keep pace with manufacturing flow, while providing prompt feedback for process control.
The overlay of high-speed compensated scanning with low-speed (10 mm/sec) scanning above shows how dynamic errors are effectively eliminated by compensation.
Scanning at high speed (150 mm/sec) produces the plot above with a form error of 8 µm.
The automotive, construction, and agricultural machinery industries--historically, drivers of innovation in part inspection technology--are again leading the way in seeking faster, leaner part inspection and qualification, especially on high-value, complex geometric parts. Large OEMs cannot afford drift in process performance while parts undergo lengthy inspection routines, or bottlenecks in production due to part qualification. They want lean solutions that avoid multiple CMMs taking up valuable plant-floor space and adding material-transfer complexity.
The metrology industry has responded with advances in part-inspection technology, particularly in probing capabilities that enable automated, programmable, part checking to high levels of accuracy and repeatability.
Touch-trigger and contact-scanning probes are similar in using physical contact with the part surface to provide dimensional measurement to very high levels of precision. They are capable of submicron accuracies to 0.00004" (1 µm), compared to 0.001" (0.25 mm) for many noncontact, video-scanning probes.
Touch-trigger probes take their measurements by intermittent single-point touches, while scanning probes maintain continuous moving contact with the part surface to capture 2-D and 3-D data. With the proper strategic use of both kinds of probes, manufacturers can achieve automated, high-throughput qualification of various part features.
Touch-trigger probes are very well-suited for high-speed discrete point measurement. They excel at checking part features where clearance, location, or a single particular dimension is critical. Touch-trigger probes are well-proven, robust devices capable of very-high-precision inspection. A wide selection of probe configurations and motorized probe heads provide optimal part access and approach angles.
Discrete point measurement is done at constant velocity with zero acceleration during the point of contact. The touch displaces the probe's stylus tip, triggering the measurement.
This intermittent go/touch/retract motion, and the need for constant velocity at touch, means that the fastest touch-trigger probes can register up to 12 measurements per second. In practice, a much slower rate is normally used. Point-at-a-time measurement makes touch-trigger probing inefficient for most form inspection, where hundreds of measurements may be required to fully define and qualify a part feature for close-tolerance form or fit.
Scanning probes provide the faster data acquisition needed for form measurement. A continuous-measurement process, scanning rapidly records the equivalent of many data points to measure and control a form. Scanning probes are particularly suited to determining functional fits--cylinders, bores, valve seats, gears, etc.--where variation and deviation can be critical.
While modern CMMs are capable of moving at high speeds--several hundred mm/sec--scanning methods have traditionally been restricted to slower measuring speeds to minimize acceleration and dynamic effects during the collection process. Unlike touch-trigger probing, where the probe moves along a single vector at constant velocity, scanning takes measurements at continuously changing velocity vectors.
To get acceptable accuracy on tight-tolerance parts, conventional scanning systems generally take measurements at less than 20 mm/sec. For example, automotive engine plants in North America presently scan at speeds ranging from 5 to 15 mm/sec.
A new dynamic compensation method, however, developed and patented by our company, enables CMM users to measure at extremely high speeds, with accuracies that previously could only be achieved at low speeds. The method, called RenscanDC, is based upon feature correction. Dynamic errors are normally repeatable. Scanning a feature at high and low speeds allows dynamic errors to be measured and a dynamic compensation map to be computed. Only one sample component from a production line needs to be scanned at the slow speed to generate dynamic maps for most of the required features. The correction data are saved and automatically applied to subsequent parts and batches, which can now be measured solely at the fast speed.
RenscanDC is only available on our UCC universal CMM controllers. Universal design permits use on a wide range of CMMs, including many machines not previously capable of carrying out high-speed scanning. The UCC can be retrofitted on existing machines, and RenscanDC improves measurement throughput without degradation of position and size measurement accuracy, and minimal degradation of form.
High-speed scanning provides a variety of advantages in meeting the objectives of lean manufacturing:
- Greatly increases inspection throughput, especially on complex parts with many critical features to be measured, such as engine blocks.
- Makes it easier to synchronize and integrate inspection with manufacturing flow.
- Offers significant cost and space savings by reducing the number of CMMs required to process output from machining lines.
- Checks parts faster--many times by a factor of 10 or more--to generate timely process feedback, enabling correction or compensation to achieve best fits and avoid rework or rejects. This feedback can be especially effective on parts like engine blocks or cylinder heads, where at present a sizeable number of valuable units can be machined before inspection results are available to reveal unacceptable process shifts.
As an example of the gains in throughput that are possible with high-speed scanning, comparative measurement tests were conducted on two cylinder bores (each measured at six sections) and crankshaft bores with a single scan in each journal. The measurements were first made at 5 mm/sec, a typical scanning speed today. The measurements were repeated with RenscanDC applied at a scanning speed of 100 mm/sec.
- Inspection time at slow speed: 27 min, 27 seconds,
- Inspection time at fast speed: 4 min, 34 seconds,
- Reduction in inspection time: 83%.
All of our company's passive scanning probes can make use of RenscanDC, enabling manufacturers to cover a variety of part applications. Engineered for high-speed scanning, the passive designs eliminate the motor drives of active sensor designs to avoid vibration during point measurement, remove heat sources (to improve stability), and (especially) minimize moving weight and size. The passive probes use the CMM drives to provide the X, Y, Z deflections.
Smaller mass reduces dynamic effects at high scanning speeds. The compactness of the passive probes allows greater part accessibility for tight areas and small features, as well as greater available Z axis for longer stylus lengths. There's no need to counterbalance a cranked stylus, which contributes to greater accessibility in tight situations. Lightweight spring-motion system design allows low inertia for rapid dynamic response to surface contour changes. A high-natural-frequency suspension system allows the stylus to remain in contact with the surface at higher speeds.
Isolated optical metrology measures deflection of the whole mechanism, relative to an optical reference frame. This feature prevents inter-axis errors, while capturing thermal and dynamic effects. Our SP80 readheads, as an example, have precision gratings that provide 0.02 µm resolution. Accuracy is defined by straightness of the scale lines and calibrated squareness of gratings, not limited by probe mechanical design.
Design simplicity makes passive probes cost-effective to purchase and repair, as well as highly reliable and robust, delivering 50,000+ hours of operating life. In fact, our passive probes will still work even after the stylus is bent or broken. In comparison, the complex drives and electronics of active sensors increase costs and can be susceptible to crash damage.
Some scanning probes such as our SP25M can be fitted to articulating heads, optimizing probe access on complex part geometries. An indexing head rapidly advances the stylus in the Z axis to give flexible access to recessed part features with no impact on cycle times. Some articulating heads provide interchangeability between scanning probes and touch-trigger probes, enabling a complete automated part inspection routine without delay for physical change of probe heads.
Effective high-speed scanning demands close integration between machine and sensor controls. Our company designed the UCC specifically to perform error mapping and thermal compensation, and to deliver the fast data capture and contour following required for high-performance scanning. The UCC provides the capability for adaptive scanning, using scanning algorithms that adjust to the actual surface rather than just running along a known path. This capability reduces the need for precise positioning or fixturing, and allows automated inspection as another step in a production or transfer line--critical to high-throughput part processing.
Unlike traditional CMM controllers that use several interconnected interfaces, the one-box UCC integrates all functions for high-speed processing, eliminates external interfaces, and reduces installation and ownership costs. It provides the capability for dual-scale, dual-drive, and rotary-table applications.
At 25 mm in diam, Renishaw's SP25M employs a patented X - Y pivot mechanism featuring a compact planar spring. The probe uses three scanning modules optimized for different stylus lengths, and the longest version permits a total reach of nearly 400 mm.
We have done extensive testing that has identified three phenomena from this interaction that can affect scanning accuracy:
The UCC design allows it to be installed on different makes and models of CMMs to standardize high-speed scanning across multiple machines. Its open interface permits use of a wide selection of CMM application software.
Stylus choice is critical to scanning probe performance. The stylus affects feature access (depending on effective working length and configuration), speed (stylus weight affects dynamic response), repeatability (affected by stiffness, joint flex, etc.), material to be scanned, and accuracy over time (influenced by wear, and material pick-up on stylus).
The first three factors apply equally to touch probing. The final two, however, are especially critical in scanning. While touch probing involves temporary static contact with the measured surface, scanning requires continuous and more aggressive contact between the stylus and the workpiece.
Debris: Any contamination present on the scanning path (metal particles, coolant mist, dust, dirt, etc.) will collect on the stylus ball as it passes over the surface. Debris is practically unavoidable with any contact scanning, regardless of stylus ball or surface material unless scrupulously clean surfaces are being scanned in a clean room environment. Debris can be removed by regularly wiping the ball with a clean, lint-free cloth.
Adhesive wear: In some extreme scanning applications, surface material from the part can build up as a patch on a stylus ball, typically when there is a single point of contact over a repeated path during scanning routines. This buildup is more likely to occur where there is atomic attraction between the ball material and workpiece material. In most cases, the buildup will be minimal and have negligible impact on the ball form. Balls should be checked periodically for buildup.
Abrasive wear: Continuous contact between two hard materials will remove material from both the ball and the workpiece. The consequences of this are more severe on the ball, as contact takes place over a much smaller surface area. Balls should be checked periodically for wear. A change in the ball form can degrade measurement accuracy.
Based on extensive ball material testing, we offer these conclusions and recommendations:
- Ruby can suffer adhesive wear (pick-up) on aluminum under extreme conditions, but performs well in most applications. It's the best material to use on stainless parts.
- Silicon nitride is a good substitute for ruby in extreme aluminum applications, but suffers from abrasive wear on stainless and cast iron components.
- Zirconia is the optimal choice for scanning cast iron parts, although tungsten carbide also performs well.
The extent of adhesive wear and/or abrasive is directly proportional to scanning force and distance, so try to minimize contact force and check stylus balls regularly.
This article was first published in the May 2004 edition of Manufacturing Engineering magazine.
Published Date : 5/1/2004