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Coordinate Measuring Machines:
Making Great Strides as a Key Support for Automobile Manufacturing
The body is the part of each automobile that receives the greatest emphasis from manufacturers, in terms of both interior and exterior elements. This is because consumers look to the vehicle's body not only for aesthetic appeal but also for crash safety, fuel economy, comfort, low-noise operation, a sense of security, and many other factors, all of which affect the body's overall completeness and quality. Since auto body manufacturing necessarily involves large workpieces and plastic working of thin plate materials, maintaining strict error tolerances for component dimensions to ensure quality is more difficult than when manufacturing smaller components such as engine parts.
MITUTOYO already has a track record in sales of coordinate measuring machines (CMMs) designed for automotive body and mold measurements. (See Figure 1.)
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Figure 1. CMMs for Auto Body Manufacturing
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Due to differences in gravity effects and CMM structure, accuracy increases from the vertical type to the lateral type and multi-axis type in that order, although all three types use the same skills for manufacturing. Consequently, the vertical type is mainly used for molds, inspection jigs, etc., while the horizontal type is often used for molded components. The multi-axis type is typically used for parts (such as reinforcement parts and parts used in collision tests) that do not require such a high level of accuracy, or in cases where the installation site must be moveable.
Recently, a type of non-contact sensor called a line laser has been attached to an end of the coordinate measuring machine, and non-contact sensors are used alternately with contact sensors. Sales of these units has been growing as they are increasingly applied to measure molds, panels, and other body components.
Meanwhile, camera type (vision type) measuring systems have also been coming into wider use, often as add-ons to coordinate measuring machines, where they are considered useful for the following reasons.
(1)
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Measurement results from non-contact sensors are always calibrated and can be compared with results from contact-type sensors.
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(2)
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Measurements can be performed automatically, thereby enabling unmanned operation (at nighttime, etc.).
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(3)
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The sensor angles that are set for each workpiece can be set repeatedly and consistently simply by re-running the program. (The same laser angles are used for each workpiece. This helps to maintain precision during repeated operations.)
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(4)
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Non-contact sensors can be swapped with contact-type sensors to enable measurement of workpieces or sites that are difficult for non-contact sensors (such as mirror surfaces, transparent surfaces, or deep recesses).
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(5)
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The offline programming system enables offline teaching and preliminary interference checks. (See Figure 2.)
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Figure 2. Off-Line Programming System
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(6)
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There is no need to attach a target for positioning workpieces before measuring, nor is there any need to place a target bar near the workpiece.
When measuring geometric elements, contact-type sensors are still widely used due to their high precision and the fact that operators are accustomed to them, but as new types of non-contact sensors are developed they are gradually being applied to a broader range of measurement applications. (See Figure 3.)
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Figure 3. Non-Contact Sensors for Attachment to CMMs
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In the future, we can expect that a sizeable percentage (depending on the precision range) of the measurements performed up until now by coordinate measuring machines equipped with contact-type sensors will instead be handled by coordinate measuring machines combined with non-contact sensors.
Non-contact sensors have a revolutionary ability to obtain measurements from literally millions of measurement points, which is very useful when evaluating multiple curved surfaces, when measuring the thickness of materials, or for reverse engineering. (See Figure 4.) Already, non-contact sensors are widely used to evaluate overall shapes, such as when maintaining the shape of molded surfaces after insertions or when evaluating the shapes of resin parts, stamped parts, or castings (including comparisons with 3D-CAD data). They are also widely used for partial assembly of auto body parts.
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Figure 4. Overall Shape Evaluation Using Non-Contact Sensors
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When non-contact sensors are used, large numbers of measurement points can be measured quickly, and visually impressive CAD comparisons can be displayed. Naturally, products that combine the precision of a coordinate measuring machine with non-contact sensors and attractive measurement point processing software are being well received by users. In some cases, non-contact sensors are simply installed on their own stand, while in many other cases they are attached to industrial robots. Today, some coordinate measuring machines equipped with contact-type sensors (and clearly meeting all relevant standards) may come close to the precision of coordinate measuring machines equipped with non-contact sensors, but they are no longer likely to exceed that level of precision. The post-processing task of connecting measurement sites for each shot is performed using either a stand-based system or robot-based system. However, due to noise-related factors such as measurement points that are reflected irregularly from the workpiece surface (due to different positions and different measurement point thicknesses even when the workpiece surface has zero thickness), refraction that occurs in the ambient air, or lens aberrations, measurement point connections do occur after numerous shots, causing a cumulative error to occur. (It is not possible to ascertain how the stand-based systems compare with robot-based systems in terms of the number of cumulative errors that occur.)
Given the fact that the more complicated a workpiece's shape is, the more times the non-contact sensors must be realigned, it would be very convenient to have a coordinate measuring machine system that is programmed to automatically adjust them with a high degree of precision.
At present, there is no standard universally accepted by both manufacturers and users concerning how to indicate the overall precision of non-contact sensors and their corresponding machinery (coordinate measuring machines, robots, etc.), so each manufacturer has its own precision display method. Currently, MITUTOYO ensures traceability for equipment with non-contact sensors by comparing its measurement results with results from coordinate measuring machine/contact-type sensor combinations for which official standards exist.
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Published in "Navigation book for the latest-type automobile manufacturing (Japanese only)" by NESW DIGEST PUBLISHING CO LTD. October, 2004.
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