Finding the Flaws
Where no one has measured before
By Robert Aronson
The emphasis on miniaturization and precision has stirred a strong interest in metrology. You need to know your product is the size it's supposed to be. This requirement in turn has spurred the development of new measuring systems and improvements in the performance of existing metrology techniques. Here's a look at some of the more prominent ideas.
The L.S. Starrett Co. (Athol, MA) now offers the "Galileo EZ," compact manual vision system. It has live video-camera feed and automatic edge detection through a Metronics QC300 Digital Readout and special optics. It's designed for videobased measurement in general-purpose, noncontact inspection tasks.
The Galileo EZ features a precision mechanical bearing with an X-Y-Z measuring stage of 8 x 4 x 6" (200 x 100 x 150 mm) or 12 x 6 x 5" (300 x 150 x 125 mm), 6:1 zoom video optics, a dual-output 150-W fiber-optic illumination for the fiber-optic ring light and a collimated substage light.
A key component of the EZ system is the new Metronics QC300 Digital Readout. Simon Cosham, Metrology Systems Division Manager, explains, "The QC300 clearly shows the X-Y-Z digital readout display and measurement data, while a built-in video card supports a live video camera image. It also allows video edge detection that automatically detects and targets edge points by scanning the image area within a circle around the crosshairs on the display, eliminating operator subjectivity."
The system has a resolution of 0.00002" (0.0005 mm). Maximum workload is 20 lb (9 kg).
Mitutoyo (Aurora, IL) offers several innovative metrology systems. "Vision systems along with other noncontacting metrology systems are gaining prominence, particularly in small-part manufacture," according to Frank Demski, Mitutoyo America Vision Product Group Manager.
The company's most recent instrument for this market is the Quick Vision Stream instrument. It uses a CCD camera for noncontact part inspection. In earlier designs, a camera moved to the measurement locations, scanned the product, then generated an image. It is an off-line or near-line system. Parts to be inspected are placed on a pallet, viewed, and returned.
With this unit, the camera is in constant motion, traveling above the products. Once an entire stage, or pallet, of products has been scanned, the unit processes all of the image data collected. Parts can range in size from less than 1 mm up to 2.5 m. Measurement resolution is 0.01 to 0.1 µin. The system is designed for inspecting parts that have a large number of features in a small area. Most applications are for inspection of components before assembly.
According to Ron Weller, Mitutoyo America Form Products Specialist, the new Mitutoyo CS-3100 and CS-5000 Series instruments are unique in three ways:
- The CS-3100 and CS-5000 combine contour and surface roughness in one system without the need to switch from one type of sensor to the other. Both measurements can be made from a single trace.
- The machines are fast with drives positioning at up to 80 mm/sec (X axis) and 20 mm/sec (Z axis), while the contour is measured at 0.02 to 2 mm/sec, and surface roughness at 0.02 to 0.2 mm/sec.
- Both are suitable for systems integration.
Advances have been made in the less complex instruments.
The same technology that brought us the medical endoscope was applied to industry to give us the borescope. It enables inspectors to check the interior of complex products and mechanisms.
There are two basic types: Rigid and flexible. The rigid version uses a series of lenses while the flexible version carries the image through fiber-optic strands. With both it's possible to view and photograph a part or assembly interior.
Gradient Lens Corporation (Rochester, NY) manufactures both rigid and flexible borescopes. Their Hawkeye rigid borescopes range from 0.073 to 0.250" (2–6.4 mm) in diam. Hawkeye Pro and Classic borescopes use a gradient index relay lens which is a rod of specially treated glass instead of the more common stack of individual lenses. A 7" long (178-mm) borescope replaces 24 conventional lenses with no loss of image quality, and at a much lower cost.
Field-of-view can be up to 100°. The FOV needed depends on the area to be viewed and the space available.
The Hawkeye Flexible borescope diameters range from 0.098 to 0.314" (2.5–8 mm) and lengths up to 1.8 m.
In general, the rigid scopes are less costly and give higher quality images. The flexible units can get into more places and can be steered using built-in guide wires.
Getting a 3-D look inside a product is now possible. The process that gave us the CAT scan is now available for industrial applications. Called Computed Tomography, it's a technique that takes multiple X-ray slices of an object and combines them into a 3-D image. It provides the ability to see inside a solid component, thereby opening some major new avenues of inspection.
In the system now offered by Zeiss, the object being investigated is placed on a turntable and rotated. An X-ray beam in a fan or cone shape, depending on the type of test, scans the object. From the resulting data, the user evaluates a single "slice" of the product or a series of slices that when combined produce a 3-D image.
With the system, a user can actually "drive" through products and look for flaws, or shrinkage, or make critical measurements.
The system can be easily set up to scan a single slice, a region of interest or the entire part. The resulting data are then analyzed, with the images color-coded to show critical areas such as porosity and wall thickness.
The Zeiss system is available with radiation sources from 90,000 V (for small plastic parts) to 8,000,000 V (for large assemblies). Resolution achieved depends on beam power and object thickness. From the resulting images, the operator can check part dimensional characteristics (just like on a CMM), and check for porosity, delamination, cracks, and inclusions as well as do reverse engineering.
A unique new way to evaluate a surface comes from Coherix (Ann Arbor, MI). The company's "Surface Detective" provides high-definition 3-D images of machined surfaces. Features include local waviness, parallelism, or hole patterns with Z-axis repeatability in the micron range.
In the process, the unit scans a 300 x 300-mm area with a laser sweep in one minute or less. Interferometry algorithms are applied to the readings, resulting in a detailed, 3-D color-coded hologram with more than four million data points.
With the dense 3-D color-contour map of points across the entire surface, it's possible to see surface variation at a glance. Because of the color variations any data points can be called up to obtain specific dimensions.
Rather than relying on measurement of only small areas on a small percentage of the manufactured parts on a manufacturing line, the system measures the entire surface of every part, quickly detecting any deviations from CAD data or part documentation. This allows continual adjustment to enable closed-loop process control.
"One of the newest units from Mahr Federal," explains Pat Nugent, Vice President for the company's Metrology Systems in Providence , RI, "is the MarForm MFU 100. It was developed in conjunction with Bosch for measuring fuel injection components within a measuring volume of approximately one liter. We currently work with fuel injectors in the micron range, but are looking at diesel fuel injectors with nanometer tolerances. The reason for the tight tolerances is that our customers' fuel injectors must perform at pressures like 40,000 psi [280 MPa]."
The MarForm unit features a small measuring volume and the ability to generate high-quality angle measurements. It measures things like straightness on sealing seat surfaces with a low degree of uncertainty. It also makes traditional form and position measurements.
At the highest end of metrology research is the work being done by the National Institute of Standards and Technology (NIST). Located in Gaithersburg, MD, it has several programs directed specifically toward solving manufacturing problems.
One recent achievement was the development of an optical configuration for high-speed infrared and visible video cameras involving a reflective lens with a larger depth of field. This configuration is a significant improvement in depth of field over the prior configuration, with sharper images of target objects with rough surfaces, allowing imaging of the rough side of the chip at the toolworkpiece interface.
One major NIST effort is the "Smart Machining Systems" program in the agency's Manufacturing Engineering Laboratory.
Program Manager Alkan Donmez explains that a smart machining system will:
- Know its capabilities and condition and will communicate this information.
- Monitor and optimize its operations autonomously.
- Assess the quality of its work/output.
- Learn and improve itself over time.
On the research side NIST is working to develop new technologies.
"The criteria for this type of research are projects that both improve the competitiveness of US industry and match the equipment and talents available at NIST," explains Steven Phillips, Dimensional Metrology Program Manager, of the agency's Manufacturing Engineering Laboratory.
One such area is a micro-feature probe for CMMs. Currently these instruments are usually limited to an internal feature, e.g. bore, size of 0.5 mm because of the probe stylus size and the force that the probe places on the test object.
An indirect way to evaluate machine-tool performance is lubricant analysis. It doesn't give data on the machine's accuracy, but tells what's going on within the machine that might influence performance.
At ExxonMobil (Fairfax, VA ), oil analysis is offered as a service to customers through a program called Signum. Customers send in samples on a regular basis, and if a problem is discovered, there is an investigation. Most customers realize that inadequate lubrication practices, for whatever reason, can cause machine failure.
"Oil analysis has two benefits. It tells how the oil is performing and gives some important clues as to how the machine is performing," explains Tim Nadasdi, Industrial Technical Advisor, ExxonMobil Lubricants & Specialties. "In a normal used-oil analysis you first look for a viscosity change. If it's lower than required, that means something in the machine operation is sending unwanted fluid, often water, into the lubricant. If it's too high, then something unwanted, such as sludge, is being added."
Analysis also looks at additives to be sure they are not being depleted.
The lubricant carries the history of what is going on in the machine. If there are traces of metals, usually iron, copper, zinc, or tin, it could mean that lubricant is not getting to the moving part.
"Filtration is of greater interest to the users," says Nadasdi. "But remember, it's not what size you can filter, but what filter media to use. You don't want a filter so fine it pulls out critical additives."
This article was first published in the November 2006 edition of Manufacturing Engineering magazine.