Measuring Micro Parts
Sensor technology holds the key
By Jim Lorincz
When it comes to inspecting and measuring the very smallest parts, there are limits that are being pushed back by developments in sensor technology.
Parts for critical applications in the automotive, aerospace, medical, and electronics industries are getting smaller, and have more complicated features that can prove elusive to the best sensing efforts of traditional measuring technologies.
The familiar and widely-used touch-trigger probe has proven itself in countless applications on CMMs. Generally, the smallest probe tip available for tactile probing is 0.3-mm diam with a stylus length in the 2 - 3-mm range. Size can be a limiting factor when it comes to measuring the smallest parts and their features.
However, touch-trigger probes in combination with optical image processing sensors, analog scanning probes, and lasers have created a whole class of versatile multisensor machines that can be configured to tap the respective strengths of each technology for shop-floor or quality-control laboratory measurement and inspection.
What is considered small, of course, is relative. For a long time, medical devices and other Swiss-turned precision parts offered the most serious challenge to production measurement and inspection. Rowan Precision (Birmingham, England, UK), for example, adopted two noncontact measuring systems as effective ways of providing gaging of "first-off" parts, and automating measuring systems and programming routines for part traceability.
Rowan Precision's products tend to be small, precision-turned components with a number of complex internal and external features. They range in size from 0.5 to 85-mm diam, and are machined from aluminum alloys, stainless, and plastics, such as PTFE, for the medical and defense industries.
A typical application involves connectors that must be manufactured to a defense specification that is backed up by component traceability. These connectors have a number of key broached features that ensure that the finished component will only be located in the correct socket.
Accurate gaging of these broached features for Rowan Precision relative to the datum proved to be an extremely difficult and lengthy operation. Using a shadowgraph required sectioning the product before a measurement could be made. Besides destroying the product, the procedure was time-consuming for the operator.
To confirm that products were produced to the tight tolerances demanded by its customers, Rowan Precision chose Vision Engineering Inc.'s (New Milford, CT) Kestrel two-axis gaging system for the shop floor, and its Hawk three-axis automatic measuring system for the QC laboratory.
The Kestrel gaging system is located alongside the CNC machine, allowing first-off components to be taken directly from the machine for immediate inspection and measurement. In the QC lab, the Hawk motorized three-axis system with automatic Video Edge Detection (VED) runs alongside QC5000 PC-based software that allows measurement routines to be programmed and stored for future use.
When frequent or multiple checks are needed on a particular component, the quality engineer recalls a previously configured program, locates the component on the Hawk's stage, and starts an inspect cycle, which runs automatically. Once the routine is completed, a data report can be stored and printed, forming part of the data trail that is vital for quality systems and component traceability.
Parts are smaller, and more complex. The trend toward more sophisticated combinations of multisensor machines to meet specialized measuring requirements is not new. What is new are the choices in sensors that are available to characterize the form and fit of smaller and smaller, even microsized, precision parts.
According to William Gilman, vice president, Optical Gaging Products Inc., (OGP, Rochester, NY), today's multisensor measurement machines go beyond vision, laser, and touch-trigger probes. As much as the combination of these sensors on a single platform improves productivity, each has its limitation, especially when applied to the latest manufacturing processes. These requirements have led to the development of "a new breed of microsensor technologies" that extend the capabilities of multisensor systems.
"The power of today's CAD software allows companies to design parts that have complex surface shapes that can be difficult to define geometrically. These surfaces may have intricate features with important distance or spatial requirements for proper fit or finish," Gilman explains.
What lies beyond the reach of conventional tactile probes is often a new twist on probing technology. For example, OGP's Feather Probe is a microprobe technology with a probe tip diam as small as 0.125 mm, making it well-suited for measuring small slots, holes, grooves, or bore draft angles. Feather Probe is not a scaled-down touch-trigger probe. Instead, its miniature probe is in constant micromotion. Proximity to a surface causes a change in the micromotion, and it is this change that is registered as a measurement. Unlike touch triggering, the Feather Probe does not deflect at all, and it can trigger from a surface no matter the angle of approach. This feature makes it possible to measure the walls and bottoms of narrow slots or small bores. The technique requires less than 10 micro-Newtons/micrometer of force.
Another sensor is OGP's noncontact Rainbow Probe, which is a microprobe that uses a white light source and spectral analysis of reflected light for accurate surface measurement. The underlying technology of the Rainbow Probe is the extended axial chromatic dispersion of light through a lens where each wavelength of light focuses at a different point on its optical axis. The technique has a resolution of 10 nm.
Chromatic analysis makes the Rainbow Probe insensitive to surface reflectivity, color, and roughness variations, so submicron Z-axis resolution is easily obtained. In a multisensor measurement system, the 7-µm spot size of the Rainbow Probe can scan a surface to provide noncontact measurement of high-frequency surface detail along a contour of virtually any shape. The integration of sensors in OGP SmartScope systems allows sensors such as Rainbow Probe to be scanned across large surface departures by moving the entire probe assembly in the system's Z-axis. OGP metrology software allows any sensor to be used at any time within a measurement routine--all data are referenced to a common datum.
Multisensor technology offers manufacturers a way to consolidate multiple, unique applications on the same measuring platform and distribute capital purchase funds by combining measurement and inspection capabilities that satisfy the requirements of diverse departments such as R&D, engineering, and manufacturing on one machine.
Michael L. Majlak, vice president-sales and marketing, Werth Inc. (Old Saybrook, CT), explains: "For example, a customer can configure a machine with a laser sensor to reverse-engineer a competitor's products; a patented Werth Fiber Probe sensor for small feature characterization; a motorized Werth Zoom optical sensor for high-speed, flexible measurement of many parts with different heights; and a touch-trigger sensor to access features or datums on fixtures or workpieces that cannot be accessed optically."
Werth Messtechnik GmbH (Giessen, Germany) has developed many sensor technologies including fiber optic, chromatic focus, contour, and computerized tomography (CT) to address measurement and inspection of parts and features that are often invisible or otherwise inaccessible visually or using tactile means. Tiny holes like those found in turbine blades and diesel fuel injectors are well beyond the reach of traditional probing techniques.
Werth's 3-D WFP fiber probe sensor features a tip with a diam as small as 25 µm, long stylus length (often in excess of 50 mm), and extremely low gaging force (typically in the µN regime) for the nondestructive measurement of optical-quality surfaces, deep bores, and extraordinarily small holes and elements. In addition, the fiber probe tip can be illuminated to measure blind or closed features, counterbores, and geometries/profiles that cannot be measured by transmitted light.
Typical applications of this sensor include tiny turbine blade cooling holes, microscopic-diam diesel fuel injector spray holes, and infinitesimal cranio-facial surgical screws, often with diam of 0.3 mm and less. Additional applications include the measurement of long, small-diam holes in high-strength polymer extrusion dies that are 0.2 mm in diam and up to 75-mm deep, and the characterization of tiny profiles found in vascular stents.
For nondestructive testing, Werth has harnessed CT scanning capability in its TomoScope sensor that utilizes X-rays to capture, process, and reconstruct 3-D workpieces from multiple views, creating high-resolution images of internal and external geometries. Typical applications include visualization of the quality of internal elements of electronic devices, and the quantified 3-D measurement of porosity in advanced castings.
Microsystems and MEMS (Micro Electrical Mechanical Systems) manufacturing are presenting challenges to measuring systems that clearly fall outside the lines of traditional industrial metrology--both in the sensors required and the workholding forces that can be tolerated by often extremely delicate parts.
"Some of these small parts are so precise that even the temperature of the human finger can affect both their size and the attempts to accurately measure them," explains Gerrit deGlee, product manager, Carl Zeiss IMT Corp., (Maple Grove, MN). They may be gears and components for such precision-engineered products as hard disk drives, ink-jet print heads, heart pacemakers, fiber-optic switches, and sensors of all types.
Zeiss' F25 CMM is based on a 3-D microprobe that exerts measuring forces that are about 1/400th of the probing force of a large industrial machine. The F25 combines the microprobe with an optical sensor based on Zeiss microscope lens technology for 2-D measurement.
"The requirements on a measuring machine for microsystem technology are completely different from those on larger versions for automotive or tool making," says deGlee. "Both the workholding of the machine and the sensor must work with extremely low forces so that microparts are not deformed or moved when measured."
The F25 CMM's 3-D microprobe features stylus diameters of 20 - 500 µm, and stylus tip diameters of 120 - 700 µm, with a free shaft length as long as 4 mm. Forces exerted in measuring are less than 0.5 micro-Newtons/micrometer of force. Zeiss has fully implemented scanning with the F25 tactile sensor, making form measurement practical for the micro-geometry being inspected.
Designed to be used in a controlled laboratory environment, the F25 uses minimal probing forces, high-resolution capability, and control of linear drives for touch measurement of bores that can be less than 1 mm in diam. Measuring uncertainty of the F25 is 250 nm at a resolution of 7.5 nm within its 1 cubic decimeter composite (combined tactile and optical sensors) measuring volume.
Programming for multidimensional measurement of workpieces with features that are practically invisible to the human eye is handled by Zeiss' CAD-based Calypso software. Using the Calypso Planner and Calypso Simulation tools, the user can create inspection programs directly from the CAD model, ensuring that features can be reached with the tiny stylus or optical sensor without the risk of an inadvertent collision.
The F25's microprobe features a touch sensor based on complex, silicon-membrane spring technology developed in a joint effort with the National Metrology Institute and the Institute of Microtechnology (IMT) (both located in Brunswick, Germany). Both the F25 CMM and the associated sensors to measure microsystem components and modules were developed as part of the MiMiKri project sponsored by the German Federal Ministry for Education and Research (BMBF).
For measuring the mechanical properties of micron-sized devices and materials, typical of those being produced by MEMS-based manufacturing, Admet Inc. (Norwood, MA), has introduced a commercially-available testing machine called the nmTester.
Equipped with a needle-type probe capable of indenting, scratching, pushing, pulling, and bending in specific locations, the nmTester measures with forces of less than 10 µN, and displacements of less than a millimeter with nanometer resolution. The Admet nmTester is designed to work with visible light and scanning electron microscopes, for precise positioning of the needle probe and analysis during specimen testing.
This article was first published in the November 2005 edition of Manufacturing Engineering magazine.