June 29, 2009
Measuring the bends and straight sections of a bent tube can be tricky and time-consuming, especially if the tube has a large number of bends in several directions. Photogrammetry, also known as optical measurement, uses a booth equipped with several digital cameras to make a digital image of the part, allowing fast, easy measurements.
You have been bending tube and pipe for years, and while you're proficient at bending, you realize that you're spending too much time on quality control. Some of the parts are easy to handle and have just one or two bends, so measuring the straight sections and verifying the bend angles isn't too much trouble. On the other hand, you have some parts with more than a dozen bends that are long, cumbersome, and consist of small-diameter tubing that flexes easily, so the straight sections aren't too straight unless supported. Some of your finished products, such as fuel lines and brake lines, have a multitude of bends in different directions, flared ends, and threaded fasteners, creating a significant measurement challenge.
Instead of taking manual measurements, wouldn't it be faster and easier to make virtual measurements? In other words, could you make a digital 3-D image of the finished piece and compare it to a digital 3-D reference? Optical measurement systems that use photogrammetry technology can do this for you.
Photogrammetry traditionally is accomplished by taking a series of digital images of an object from various views. The images are combined and compared to a reference that provides scale to create a digital model. This technology has been automated using a booth equipped with several digital cameras, a computer, and the necessary software. The cameras are spread out across the top of the booth, and each is aimed at a slightly different angle to cover the entire measuring area. Such a system requires about 40 milliseconds to capture data from thousands of points along the straight sections and around the bends.
The software combines the images from all of the cameras to create a single 3-D model of the measured part, and compares that image with a reference model of the part. Based on this comparison, automated software provides a pass or fail indication which, as the term implies, lets you know if the part meets your specifications (see Figure 1). The analysis goes one step further and displays variance data on all sections of the tube, including the bend areas. If the tube fails the inspection, this data enables you to make immediate changes to the bending process.
In other words, photogrammetry offers more than just quality control; it enhances your shop's process control. Because the information is digital, it can be used immediately to generate a report or to make process control decisions.
After collecting images of the tube, the software uses the outside surface of the tube or pipe to determine its diameter. It then determines the location of the centerline and uses the rest of the data to construct a digital 3-D model, including bend points and the location of most external fittings. It compares this to your design data and provides the specific differences between the print and the part and an indication of whether the part passes or fails the inspection.
Most systems are set up with an interface to several CNC benders. This provides immediate corrections for length, rotation, and bend angle for each machine, even if they are producing different tubes. If the system is not automatically connected, you can still use the data, but it will be a manual operation—the bender operator makes adjustments to the bender program.
The possibilities for bent tube applications are limitless, which leads to quite a few questions about photogrammetric systems. Among the most common are:
If you need data on attachment points or flanges, you need to use adapters that are automatically recognized by the system.
It is also possible to place a greater weighting on the fit in certain parts of the tube. Constrained fits can be used to simulate the way the tube is used in its application.
It's important to choose the right reference point for meeting your customer's specifications. A linear dimension that varies 4 mm end-to-end might vary 2 mm when measured from the midpoint to the end. This amount could mean the difference between passing or failing.
If one of the cameras is out of position, it will be rejected. This type of system has sufficient redundancy so your work can continue. An annual check by the system manufacturer is a good idea. A traceable photogrammetry system is used to recheck the positions of all of the calibration targets.
It should be installed near the benders for convenience.
Because of their speed and ease of use, optical measuring systems are suitable for use on the production floor, directly integrated into the bending operation.
As more CNC benders are used in production, it makes sense to use digital reference data for measurement. This provides measurement information where it can have the biggest impact on the overall efficiency of the operation. It can reduce scrap, make adjustments to compensate for variances in the raw material's characteristics, and get your production operation online much faster after part changeovers.
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