Robert Sanville
Published: Monday, July 13, 2009 - 04:00
There are several different tools available for the measurement and inspection of parts and products. The specific application often determines the best choice as each tool has its own benefits and drawbacks. Over the years, these tools have become more advanced to keep up with improved quality standards. In this column, I’ll briefly discuss various measurement tools and how they are used, focusing on the advantages of portable CMMs, and why they are the preferred tool in many instances.
Manufacturers are increasingly implementing quality methods such as Six Sigma and working toward compliance to quality standards such as ISO 9001 to continuously improve their products and processes. In addition to reducing or eliminating product defects, these measures strive to detect problems in the manufacturing process. This allows companies to prevent adding value to (trying to improve) already-defective works in progress. To be successful with this approach, manufacturers need to measure every step of their processes, including the various stages of product assembly that may never have been measured before.
Most of you are familiar with hand measurement tools such as calipers and micrometers—they have been around for more than 100 years. These tools are often appropriate for simple geometric measurements, including hole diameters or for the length and width of a rectangular component.
Their use for complex part measurements, however, is very limited. They also do not allow for measurements to be compared directly to computer aided design (CAD) models of the part, are very susceptible to human error, and do not provide results that are rich in data. For example, calipers can return a result quickly for a hole diameter at the two points used to take the measurement, but do not give any information about how circular the hole is (this is called the circle’s form) or tell you if the diameter differs greatly at other points of the circle. To gather this data using calipers is a time consuming process whereby you must take multiple measurements at various points along the circle and then compare them to each other.
Another tool that has been around for a while but is more expensive is the optical comparator. Optical comparators are a mature technology that use magnified profiles of back-lit parts that are then compared to a scale on the viewing screen, or use a combination of stage movement and linear encoders to measure the projected image. In this way, 2-D features can be measured relatively quickly although the accuracy is limited and the process is manual. Machine vision systems are similar to optical comparators except that they utilize cameras and imaging software to perform the inspection analysis. Machine vision systems are very fast, may be automated, but can be expensive. They are typically limited to specific tasks, and are only useful for inspections of small parts.
Over the last few decades, the term “CMM” has become common in the world of quality control. For those of you who don’t recognize the term, coordinate measuring machines (CMMs) are mechanical 3-D measureing systems designed to track a mobile measuring probe to determine the XYZ coordinates of points on a work surface. CMMs are comprised of four main components: the machine body including the measurement bed, the measuring probe, the control or computing system, and the measuring software. Machines are available in a wide range of sizes and designs and with a variety of probe technologies.
Many manufacturers are adopting CMM technology due to the prevalence of CAD in product development. CAD software facilitates the design process and provides manufacturers with virtual 3-D design models. Manufacturers use CAD software to create designs and engineering specifications for new products as well as for quantifying and modifying designs and specifications of existing products. The use of CAD can shorten the time between design revisions; current manufacturing practices must accommodate more frequent product introductions and modifications while satisfying stringent quality and safety standards. Assembly fixtures and measurement tools must be linked to the most up-to-date CAD design to allow production to keep up with the rapid pace of design changes.
The first CMM appeared in the early 1960s and was a device with a simple digital read-out that displayed the XYZ position of the probe. Traditional types of CMMs include bridge, cantilever, and gantry.
Fixed CMMs provide very high levels of precision and offer a link to the CAD model. However, because CMMs are large, they require the object being measured to be brought to the CMM (typically in a temperature-controlled room) and the object must fit within the CMM’s measurement volume. As manufactured subassemblies increase in size and become integrated into even larger assemblies, they become less portable, thus diminishing the utility of a conventional CMM. If you have a fixed CMM, you must also continue to use hand measuring tools, or expensive customized test fixtures, to measure large or unconventionally shaped objects. The fixed CMM is also complex to operate, therefore limiting the number of potential operators and users.
Many advances in CMM technology have occurred throughout the decades, including the development of portable CMMs. They provide most of the benefits of traditional CMMs but with added flexibility. The key trade-off between portable CMMs and fixed CMMs is accuracy. A high-end, fixed CMM, in a climate controlled environment, is perhaps the most accurate 3-D measurement tool for large parts. However, the accuracy of portable CMMs improves by leaps and bounds each year, with some portables giving fixed CMMs a run for their money in some cases. This makes portable CMMs an economically sound choice in many applications. In addition, portable CMMs, unlike fixed CMMs, are lightweight and can therefore be used anywhere measurement is needed (the machine goes to the part). A controlled environment is not required and operation is very simple. They provide highly accurate results and are robust enough to work in a wide range of environments. Portable CMMs are also typically much less expensive than a traditional CMM.
There are several types of portable CMMs, but the two main types, the two technologies you will most likely see on the shop floor, are articulated arms and laser trackers. You should choose the one that best fits your particular application. In many cases, manufacturers find a need within their processes for both types.
Articulated arms. An articulated arm determines and records the location of a probe in 3-D space and reports the results through software. To calculate the position of the probe tip, the rotational angle of each joint and the length of each segment in the arm must be known. Radial reach of a fully-extended extended arm typically ranges from 2 feet to 6 feet (4-foot to 12-foot diameter or working volume).
The angle of each rotating joint within the arm is determined using optical rotary encoders. These encoders count rotations incrementally via detection of accurately spaced lines on a glass grating disc. The software converts the counts into angle changes. Arms typically have six or seven axes of rotation, which means the instrument moves throughout a wide range of orientations. Laser trackers. The operation of a laser tracker is easy to understand: It measures two angles and a distance. The tracker sends a laser beam to a retroreflective target held against the object to be measured. Light reflected off the target retraces its path, re-entering the tracker at the same position it left. Retroreflective targets vary, but the most popular is the spherically-mounted retroreflector (SMR). As light re-enters the tracker, it goes to a distance meter that measures the distance from the tracker to the SMR. The distance meter may be one of two types—interferometer or absolute distance meter.
Typical applications for a laser tracker are:
Despite the wide range of options in quality measurement tools, portable CMMs continue to grow in popularity. Companies are experiencing the accuracy results they need, while gaining the flexibility to use the unit wherever and whenever it is most convenient. The savings that result from using portable CMMs include reduced scrap, shorter measurement times, and improved product quality. These savings have allowed companies to see a complete return on their portable CMM investment—in many cases within 12 months. Robert Sanville is the director of total quality for FARO Technologies Inc. Throughout his career, Sanville has held various management positions in engineering, operations, product management, and quality. He holds a bachelor of science in physics from Rensselaer Polytechnic Institute and a master of business administration from Southern New Hampshire University, as well as several patents.
Metrology Advances in Measurement Tools: Portable CMMs
Today's portable 3-D measurement tools are fast, accurate, and affordable.
Hand measurement tools
Optical comparators and machine vision systems
Coordinate measuring machines
The use of CAD
Traditional (fixed) CMMs
Portable CMMs
Types of portable CMMs
Typical applications for an articulated arm are:
Laser trackers offer extremely high accuracy levels and much larger measurement ranges (hundreds of feet in diameter). They collect coordinate data at very high speeds and require just one operator.
Summary
About The Author
Robert Sanville
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