[Gary Hobart and Steve Logee]

Miniaturization is the "wave of the present" and new CNC micromachining systems are taking this wave beyond the simple 2-D geometries of electronic component manufacturing to the complex world of 3-D milled, turned, EDMed and even molded components. CNC equipment makers are dazzling their customers and prospects with how well they can make these tiny parts. However, once you have signed on to these technologies, you are left with the question of how to measure what you make.

If you are enthusiastic about micromachining, but dismayed about the conundrum of measuring very small parts accurately and efficiently in production and prototyping manufacturing situations, you can take a deep breath and relax. Metrology systems, sensors and software suppliers are at least a few steps ahead of you. They have developed a range of micro parts measurement options that are flexible enough to match up with the validation requirements of almost any micromachining application.

Think Multiple Sensors

The first question you need to ask yourself is not “What kind of measurement system do I need?” but “What type or types of sensors will provide the best solution for my requirements both in terms of accuracy and productivity?” Once you have settled on a sensing strategy, the rest—in terms of measurement equipment and software—will fall into place pretty quickly.

Here are some of your choices.

Tactile Probes. Everyone involved with traditional manufacturing and measurement is intimately familiar with CMMs and touch trigger probes. In the past, this was all you needed to measure most 3-D prismatic and contoured components. However, tactile probes get into trouble with micro-part geometries when features are too small to allow probes access for measurement.

Cameras. Until recently stage-type vision systems were used almost exclusively to measure flat objects like 2-D electronic circuitry. Within the past few years, advances in CAD-based metrology software have made it possible for cameras to be employed for true 3-D measurements. One drawback to 3-D measurement using a stage system, however, is that the sensor must be normal to the feature that it is measuring (at least within a few degrees). So the measurement positioning of small sculptured parts like surgical tools and implants can be a real drag on both the product development cycle and production manufacturing. One way to ameliorate this problem is to use a stage system with tilt and rotary axes.

This vision probe (CMM-V, Hexagon Metrology) mounted on an articulated wrist allows the software to orient the probe to the part’s surface for precise measurement.

Vision Probes on CMMs. Another way to check features that don’t lend themselves to measurement with a tactile probe is to equip a conventional CMM with an articulated wrist and a vision probe. As is the case with a touch trigger probe, the measurement software allows the programmer to orient the probe normal to the feature it is measuring. Even better, this approach makes it possible to use conventional touch trigger probing and vision probing on the same machine within the same measurement program. Since many shops already have the CMM, they only need to buy the vision probe.

CMM-based vision probes will continue to improve but they are currently limited by the amount of magnification they can provide. Therefore, they are not yet appropriate for very small features requiring ultra-high resolution measurement.

Multi-Sensor Vision Systems. The most common multiple sensor approaches combine high-accuracy, camera-based measurement with tactile probing, theoretically providing the best of both worlds. However, early, multi-sensor system designs, which added a touch probe to the Z-axis holding the camera, had two problems. First, measurements taken with the probe did not always correlate well with measurements taken by the camera. Second, the length of the probe often prevented the camera from getting in close enough for high magnification measurements.

Recent advances in multi-sensor system hardware and software designs have eliminated these issues. To solve the clearance problems, the latest machines provide a second, retractable Z-axis for carrying tactile and other types of probes. To solve the correlation problems, software vendors developed advanced, cross-calibration algorithms that virtually eliminated discrepancies between measurements taken with different probe types. Now, these accurately relate measurements, regardless of the data source, to the CAD model.

Choosing the right vision or multisensor system While the manufacturers of measurement systems, probes and software are excited about new technologies for measuring 3-D micromachined components, you need to determine how those technologies apply to your individual applications. Vendors must prove to you that their solutions measure up both to your accuracy and productivity requirements. Consider the points below when talking to measuring system suppliers. Focus on your application. Challenge their applications experts to do some creative thinking about the micromanufactured parts you intend to measure. Ask them specifically about your parts and what equipment and techniques they would use to measure them. Think total package. Is the vendor able to integrate its best solutions into a total package consisting of software, hardware and ongoing product improvements that will give you a sustainable competitive advantage? Leverage software for productivity. In terms of software, look for an open approach that provides such productivity-enhancing features as: intuitive, CAD-based programming environment; cross-platform applicability; automatic setting adjustments; offline programming; and parametric programming. Look to the future. Find out how the vendor plans to bring emerging sensor technologies to bear on future measurement challenges. Ask about what’s coming in the product pipeline that’s going to help you in your specific applications. Find out what the problems are now and what the vendor is doing to overcome them. As parts and features shrink, the challenges faced by the manufacturers who must measure them productively expand. The benefits of multisensory measurement solutions will only multiply in the future. — G. Hobart and S. Logee

Laser Probes. Camera-based scans of sculptured surfaces are inefficient because of the continual need to steady and refocus the camera. In contrast, laser sensors, in this same application, can be exceptionally fast and are capable of capturing thousands of data points per second. Problems can arise, however, if changes in the parts reflectivity or in the laser’s angle of incidence come into play. This is because laser probes require a streamed feedback of light with a fairly constant return of light amplification and power to the sensing array. If these vary too much, the results become suspect. So, users must exercise great care in measuring extremely small geometries to ensure that neither of these factors distorts the results.

White Light Sensors. White light sensing, a relative newcomer on the micromachined part measurement scene, can capture data as quickly as a laser and with greater accuracy. Since a white light probe depends only on the evidence of the signal (rather than its power and amplification), there are no significant issues relating to angle of incidence or surface reflectivity. Multi-sensor measurement users are quickly converting to this new approach for collecting clouds of data. About half of the new purchasers are selecting the white light probe option when it is available.

Micro-Optical Probing. Tactile probing is still one of the most accurate and reliable methods for capturing data from 3-D parts. Unfortunately, even the smallest touch trigger styli are too large to fit or be maneuvered accurately within the tiny holes, slots and other features that typify micromachining. Recently though, probe manufacturers have come up with a method for shrinking a tactile probe so that they can use it on these microgeometries, the micro-optical probe. Instead of using an electronic switch to sense probe deflection, this type of probe uses a camera. This eliminates the circuitry associated with traditional touch trigger probes and allows for significant reductions in styli size.

The camera, which is mounted at a fixed distance from the probe tip, monitors the probe tip’s position. When the probe makes contact with the part, the tip moves, the camera sees it and the software reads and records the position of the machine’s counters. This marriage of optical and tactile probe technologies has significantly extended the usefulness of tactile probes into a wide range of micromanufactured part measurement applications.

These are some of the most viable, commercially available multisensor probing options. Many more are under development.

Systems and Software Selection

The good news is that there is a range of measurement systems capable of supporting some or all of the probing strategies indicated above. These include conventional CMMs, stage type vision systems and multi-sensor CMMs incorporating a camera and one or more additional sensors. Microparts manufacturers have a wide range of measurement tools available that they can tailor to their specific needs.

There is even better news concerning software. Modern, 3-D multi-sensor metrology software has come a long way from the esoteric, code-based programming environments that were prevalent only a few years ago. Here are some software capabilities to look for:

• Intuitive, CAD-based programming environment. Similar, and in some instances, identical to those associated with your CMM.
• Cross platform capable. Usable on different brands and types of 3-D measurement equipment. This shortens the learning curve and reduces training costs. It also makes it possible to exchange programs and data between systems. In many cases it is possible to retrofit your existing equipment with some of this software allowing you to run the same software on all of your measurement systems.
• Automatic setting adjustments. Saves time by automatically controlling such parameters as lighting, focus and image capture settings in accordance with changes in sensors, lighting or part surface conditions. The system user only needs to make minor adjustments when a part is changed or a new type of sensor is employed rather than doing everything from scratch.
• Off-line programming. Provides a graphics-based environment that allows for off-line programming. This is particularly important in prototyping and short-run manufacturing settings where time spent programming parts takes away from the time spent measuring them.
• Parametric programming. Allows for the rapid programming of families of similar parts simply by changing values in parametric tables. This type of programming is particularly important for products where there are many variations to meet specific individual requirements.

Focus On Your Application

While the manufacturers of measurement systems, probes and software are very excited about the technologies they are bringing to the marketplace for measuring 3-D micromachined components, you need to focus not on them, but on your particular applications. Your vendors need to prove to you that their solutions measure up both to your accuracy and productivity requirements.

Challenge their applications experts to do some creative thinking about the micromanufactured parts you intend to measure. Are these vendors able to integrate their best solutions into a total package that gives you a sustainable competitive advantage today? In addition, how do they plan to bring emerging sensor technologies to bear on the measurement challenges in the future? As parts and features shrink, the quality of the multi-sensor solution you choose today must be constantly improving as these rapidly developing technologies move forward.
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About the authors of this article: Gary Hobart is the global product manager for Hexagon Metrology, and Steve Logee is director of business development at Wilcox Associates, a Hexagon company.