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In modern metrology, it is important to know not only the true position of a workpiece feature, but also size, runout, flatness, and other dimensional form characteristics of the entire part that can indicate how well it will perform as a component in a larger assembly. Cylindrical mating parts are a good example. The more precisely the dimensions of these parts can be controlled the better they fit together, and, consequently, the more reliably they will perform.
A precise definition of workpiece dimensions is also critical for process control applications. The more data available, the better the process can be adjusted to keep manufactured parts within tolerance.
How much data is enough to accurately measure the dimensions of workpiece features? Evaluation programs conducted by CMM manufacturers and supported by workplace experience show that the more measurements performed on machined parts, the more accurately that part can be described dimensionally. For example, on machined features with diameters of 30 to 70 mm, at least 8 to 16 probe points are required to assure stable results in describing diameter and position. For contoured shapes like sheet metal assemblies for auto bodies, thousands of data points are necessary to describe their forms accurately enough for the information to be useful in process control.
Large amounts of dimensional information must be collected quickly in order to keep pace with machine tool throughput and to provide timely feedback for process control. High speed data gathering necessary to define workpiece form requires a coordinate measuring machine (CMM) capable of scanning. Until recently those machines and the software to run them were expensive and not readily available. They also required extensive programming expertise to achieve the best results from the inspection routine. However, CMM and software manufacturers have refined and simplified scanning technology so that today the cost of scanning machines has dropped to the point where small to medium sized shops can take advantage of this technology.
The new Chameleon from Brown & Sharpe, North Kingstown, RI, is representative of this class of dimensional measuring equipment. It combines single-point measurement and multi-point scanning technology in a product its developers describe as a "fast, accurate, affordable coordinate measuring machine." It can be equipped with traditional touch trigger probes, laser triangulation scanning probes, or tactile/contact scanning probes. According to the builder, an articulated PH10MQ probe holder provides virtually unlimited access to workpiece features, effectively increasing Chameleon's 750 mm by 650 mm by 500 mm measuring range by more than 20 percent.
Chameleon is designed for users who need to perform dimensional measurements on prismatic parts using single point techniques, and who also need a machine that can gather large amounts of data quickly for accurate form definition.
The advantage of multi-mode capability is that it allows the user to not only optimize process control capability by gathering large amounts of dimensional information by scanning and applying that information to machine tool setups, but also to establish a gaging strategy that provides the optimum level of accuracy for the throughput requirements of the production operation. For example, Chameleon can be used in the scanning mode to check the dimensional accuracy of the first piece produced by a machine tool. Based on that data, tool adjustments can be made. For the part being machined, however, location may be the critical dimension. By switching to the point-to-point mode, fewer measurement points can be collected, at a higher throughput rate than the scanning mode, to determine location, and other dimensions, accurately enough for quality control purposes. If features begin to drift out of tolerance, the user can switch to the scanning mode to gather more data for readjusting the process.
Chameleon uses Dimensional Analysis Technology for Optimizing Systems (DATOS) refined by Leitz-Brown & Sharpe in ultra-high accuracy precision measuring machines. For example, the new CMM's Sharpe 32Z controller incorporates advanced DATOS algorithms to support consistent high-accuracy data evaluation at very high speed analog open and closed loop scanning, with data transfer of up to 120 points/second.
This capability, combined with an on-board temperature compensation system and all-aluminum machine construction that reduces the effects of structural deflection and vibrations, gives Chameleon a V2 accuracy of 4.2 micrometer at a scanning speed of 20 mm/second.
This CMM is equipped with Leitz-Brown & Sharpe's QUINDOS measurement software. An icon-based interface allows operators of any skill level to use the machine. A system of "feature-based measurement" reduces inspection time by allowing point-and-click measurement of selected workpiece features. It uses filtering and analysis routines designed specifically for scanning applications to help assure accurate measurement results. QUINDOS offers numerous measurement application modules, including those for gears, sheet metal, cams and cam shafts, and threads, as well as a variety of graphic output options.
Chameleon can be integrated into existing CAD systems. QUINDOS can run concurrently with DMIS (3.0), the widely-used dimensional data exchange standard, so that users can take advantage of this data analysis software and be compatible with computer-aided design (CAD) systems for reverse engineering and archiving.