shaping the future of metrology - Brown & Sharpe

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Product Profile The trend in quality control over the past few years has been to integrate inspection into the production operation. The benefit is clear. Dimensional data can be used to prevent machine tools from producing out-of-tolerance parts, and the closer the inspection operation is to production, the more efficient it is for machine operators to control the variables of the manufacturing process. Inspection close to production also brings a number of benefits to the inspection process itself, including reduced time and cost to move the parts, the real time feedback of dimensional data for inline process control, and a common environment for tooling and inspection where part temperature stabilization is often unnecessary. Putting coordinate measuring machines on the shop floor, however, has always been a challenge for CMM manufacturers.The technological problem is to guarantee a high-accuracy performance. The most dramatic problem is temperature, specifically temperature shifts compared to the calibration temperature, and temperature gradients in time and space. It is not unusual to find extreme temperature conditions on the shop floor.These conditions can be caused by such common occurrences as turning down the heat over the weekend, sunroofs, overheating due to the proximity of machine tools to inspection equipment, and any number of other conditions that may be unique to a particular shop. Both the coordinate measuring machine and the workpiece are affected by 36 mfg. Shaping the Future of Metrology There’s progress in the control of temperature effects on measurement: A.C.T.I.V. Technology On The Shop Floor by Marco Pelissero SCIROCCO ACTIV is a new coordinate measuring machine and the first to incorporate advanced Adaptive Compensation of Temperature Induced Variations thermal compensation technology. these thermal conditions, and overcoming them is the focus of continuing research into CMM behavior. Brown & Sharpe has been studying this problem for several years, through kinematic and dynamic machine simulation with finite element analysis, tests in conditioned rooms, and collaboration with customers.The result of two years of research is the introduction of A.C.T.I.V. technology (Adaptive Compensation of Temperature Induced Variations), that is being applied to the shop floor CMM line. To better understand the A.C.T.I.V. technology, it is important to understand how a typical shop floor environment affects the structure of a CMM. The Thermal Problem Shop floor thermal phenomena is composed of a linear problem, thermal expansion, and a nonlinear problem, thermal distortion. Thermal expansion does not generate distortion problems.The measuring machine’s beams and scales expand and contract with temperature changes.This generates a scaling error, but that error can be easily corrected by applying compensating factors to each of the system’s three axes. Thermal distortion is caused by temperature gradients in space. For example a temperature gradient of 1°C/m causes bending on a 2m long granite table of 10 µrad (Figure 1). This is a typical bending error on a granite CMM table. It is not difficult to see that a similar distortion increases the error formula of the machine by about 5µm per meter of the measured part in a 1 meter high CMM. Experience shows that the gradient inside the granite can be 2 to 3 times higher, due to the different thermal exchange between the top and bottom of the table. As a consequence, the error increases proportionally (Figure 2). A.C.T.I.V. Technology The A.C.T.I.V. technology compensates both types of errors with a mix of hardware and software solutions that include a complete web of up to 32 thermal sensors placed in critical points of the machine that feed a complex algorithm with a huge amount of data in real time. The idea is that the sensors read the temperature on the structure of the ma- The A.C.T.I.V. technology compensates errors with a mix of hardware and software solutions. chine and the algorithm can extrapolate expansion and distortion values from this data.Through these values, the software compensates each measured point X, Y, and Z, so that the influence of the temperature is almost cancelled over a wide range. The Results The machine becomes insensitive to temperature variations, spatial temperature gradients, and temporal temperature gradients.The SCIROCCO ACTIV ® accuracy specifications, for instance, E=U3=3.5+5L/100 are guaranteed in a temperature range of 18 to 28°C (64 to 82°F) with a temporal gradient of 10°C/day (18°F/day) and 1°C/m (1.8°F/m). (E is the new ISO volumetric accuracy standard and is equivalent to VDI/VDE U3.) The same technology has been applied with extremely good results to other shop floor CMMs, TYPHOON SF and BRAVO NT. The experimental data (Figure 2) fully confirm these theories.These results mean that on the shop floor, dimensional inspection can be performed with an accuracy comparable to that of a lab. o Marco Pelissero, a Brown & Sharpe DEA Product Manager, is a former researcher in physics at the Polytecnico of Turin, Italy. Circle 712 on the READER SERVICE CARD Figure 1. Typical bending error on a granite CMM table. Figure 2. Thermal compensation test on a bridge machine, Y axis. (measuring of Zerodur gauge with L=700 mm) mfg. The Brown & Sharpe Publication of Precision Manufacturing 37
Multifunction CMM systems speed up the design of free surface car bodies The design and production of car bodies currently rely on 3D surface models of the parts.These models are developed either mathematically on CAD workstations, physically in a stylist’s studio, or in a technician’s lab- oratory. Expensive iterations on models are always needed to refine part surfaces and transform the stylist’s concept into an efficiently producible product. 38 mfg. Shaping the Future of Metrology Left: Laser scanning probes can be fitted to CMMs, giving them the ability to digitize complex shapes for fast and accurate model making. Above: The light milling function available on a Brown & Sharpe DEA gantry CMM transforms it into a machine tool for constructing surfaces on soft materials. That is the case of styling driven designs, where the part shape is determined by a stylist working in clay or plastics; of experiment driven designs, where a prototype is modified by hand by engineers that tune its functional efficiency; and of manufacturing-driven part redesign, where the final shape of the component is determined by changes made to its tooling with the objective of improving part quality or production efficiency. In all cases, a closer interaction between stylist and model maker can help save weeks of development time. Gantry CMMs, equipped with suitable software and accessories, are excellent tools to support style model design activities and production engineering operations, as well as to improve die and mold design and manufacturing for large contoured parts, such as car bodies and subassemblies.The huge size of their measuring envelopes, plus their open construction, allows easy handling of very large workpieces. In addition, owing to the rigidity of their structure, these machines can also hold large head and probe assemblies, as well as powerful 3 and 5-axis milling heads, without compromising their high measuring accuracy. Scanning Software Adds Versatility by Marco Manganelli Gantry machines can be equipped with easy-to-use continuous path scanning software that supports tactile and laser scanning probes for fast and accurate model digitizing, and for producing part programs to control NC milling machines.The system can efficiently operate With their ability to quickly capture dimensional data from 3D surfaces, gantry CMMs like this Brown & Sharpe DEA DELTA give automobile manufacturers the flexibility to make rapid design changes. unattended. Automatic scanning routines react to surface changes, repositioning the machine or head to keep the probe in contact with the workpiece, even with complex contoured parts. Computer compensation of the machine geometry and probe deflection guarantees maximum scanning accuracy. The surface data points generated by the system can be directly imported into a CAD workstation to create a mathematical representation of the part geometry. This technology is extremely useful when an old part is available, but no CAD data is available, or when manufactured parts must be created from physical prototypes. The surface data points may also be postprocessed by the CMM computer to produce a cutter path for all machining passes, from rough cutting to finishing, for efficient model copying. Easy-to-use reverse engineering functions simplify the process of modifying the style model after engineering or manufacturing changes on the final part.These functions allow the generation of CAD/CAM data directly from the part scanning files. In a typical reverse engineering application, a model of a part may have been initially machined from a CAD file and later modified by hand to meet functional or manufacturing requirements. By scanning the modified areas of the model, the operator can create useful data to embody these changes in the original part database. Direct model copying functions for die and mold making are supported by advanced software features that allow process optimization, saving time and money.These functions include cutter radius compensation, automatic model alignment, and male/female conversion. A powerful light milling function transforms the gantry CMM into an NC milling machine to build complex sur- Gantry CMMs…are excellent tools to support style model design activities… faces on soft materials, such as clay, polystyrene, epoxy, and aluminum.This advanced technology, combined with the scanning capabilities, provides a global solution to styling model design and development, as well as die and mold making.The light milling function proves its validity in the creation of 3D physical models and prototypes from CAD data files to support the stylist’s work and to validate die addendums and modifications prior to cutting metal, thus eliminating the need for expensive iterations on prototypes. Reducing Design Time at Peugeot The Advanced Styling Department of PSA-Peugeot Citröen, a major European car manufacturer, has approached the problem of reducing the art-to-part cycle mfg. The Brown & Sharpe Publication of Precision Manufacturing 39
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