July 11, 2002
Coordinate measuring machines have progressed since their inception in the 1970s.
Coordinate measuring machines have progressed since their inception in the 1970s. Original machines were benchtop-mounted, cumbersome to use, and limited to five axes. In addition, some complex tube applications required two software programs for a complete set of measurements. These days, low-density materials and advances in software allow portability, the ability to measure up to six axes, and the ability to measure sophisticated tubes with one program. Futhermore, CMMs can be integrated into bending operations; after measuring a bent tube on a CMM, the program sends corrected bending data to the tube bending machine.
Noncontact probes measure tube with a light beam (infrared or laser). Taking a valid measurement requires breaking the beam twice — once when moving the probe toward the item, and again when retracting the probe.
The use of portable articulated-arm coordinate measuring machines (CMMs) is growing in tube and pipe fabrication shops throughout the world. Mainly used for automotive and aerospace applications, CMMs nevertheless are useful for any fabrication that requires detailed inspection.
Compared to their modern counterparts, early CMMs were large and cumbersome. Although they were adequate for performing fairly accurate tube measurements, early models were bench-mounted, the measuring arms were heavy, and the technology was limited to five axes.
Benchtop models still exist. However, portable models now are available that allow measurements to be taken on the shop floor.
Furthermore, the weight of the measuring arms has been decreased by the use of carbon graphite tubes. Measurement capacity has improved too.
Transportation conglomerate Alstom (www.alstom.com), manufacturer of France’s TGV (Train à Grande Vitesse, or high-speed train), uses a six-axis arm on a 7-meter (23-foot) rail to measure hydraulic tubes. The rail increases the CMM's capability by providing a seventh axis. Aerospatiale, an aircraft manufacturer affiliated with the European Aeronautic Defence and Space Company (EADS, www.eads.net), uses similar machines for measuring aerospace components.
One option for obtaining a tube measurement is the T1 MP T2 (tangent-midpoint-tangent) method.
Measurement Concept. Since the inception of CMMs, the measurement concept hasn't changed. The first probes, developed in the 1970s, were V-shaped contact probes. The operator took one measurement on each straight line. With a known tube radius, the software could determine the axis of all straight lines on the tube.
While this method was fast, its accuracy was limited by the thickness of the probe.
Noncontact Probes. V-shaped contact probes have been replaced to great extent by noncontact infrared or laser beam probes. These Y-shaped probes measure twice — first when the beam is broken as the probe is advanced toward the part, then again when the beam is broken as the probe is pulled away from the part (see Figure 1). Measuring twice doubles the accuracy of the measurement.
Noncontact probes also eliminate errors associated with deflecting or moving the object. This is especially important for measuring flexible or thin-wall tube that could be moved or shifted easily by a contact probe.
A tube can be defined as successive lines that are the axes of each straight line. Based on this definition, three methods are used to determine the size and shape of the tube: XYZ, T1 MP T2, and LRA.
Control reports, when generated in a common spreadsheet format, can be customized.
The XYZ method provides each intersection point between each axis. The T1 MP T2 (tangent-midpoint-tangent) method gives the tangent points and midpoints of each axis. The LRA (length, rotation, angle) method gives the length, rotation, and angle between each axis.LRAs generate the same type of values that are used by tube bending machines. For this reason, this type of controller can interface with a bending machine.
After measurement, the operator can send corrected LRAs (CLRAs) to the bender and production does not need to stop. In fact, some software programs can communicate with several benders at one time.
In a typical manufacturing process, a table-mounted articulated arm inspects tubes from various benders. With all the active benders displayed on a main control screen, the software allows the operator to select a bender and measure a tube from that bender. The program then automatically sends corrections to the bender.
Reporting the results is just as important as taking the measurements. Two common report types are a 3-D view and a control report.
The 3-D view can be shown with deviations labeled (see Figure 2). Display options include rendering or wire frame, with nominal and measured tubes displayed on the screen at the same time in different colors. This type of view helps to show where errors have occurred.
Control reports consist of numerical information generated in spreadsheet format (see Figure 3). These reports typically can be customized to suit the needs of the fabricator or the fabricator's customers.
Some tubes have other geometric features, such as lengths and angles, which must be checked. The traditional method was to measure the tube using standard tube measurement software and then export the results to a geometry-measuring program. Time-consuming and cumbersome, this process required users to learn two programs and use the results from one as input for the other.
Conceptualizing a tube as any other geometric shape allows this program to display a tube with features such as cylinders.
One option for obtaining a tube measurement is the T1 MP T2 (tangent-midpoint-tangent) method. Advancements now enable a single program to perform all these measurements (see Figure 4). Because the program considers the tube to be a geometric entity, such as a sphere, plane, or line, it can measure the tube and create an image just as it would for any other shape. This simplifies checking angles or measuring distances between features such as flanges or brackets.
Jean-Charles Granger is vice president of Axila Inc., 24407 Halsted Road, Farmington Hills, MI 48335, phone 248-426-0919, fax 248-426-0940, e-mail firstname.lastname@example.org, Web site www.axila.com. Axila designs and manufactures coordinate measuring machines and software for measuring and inspecting solids and tubes.
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