Modern CMM Design Concepts Assignment

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Modern CMMDesign ConceptsBuilding a better CMM —real-world performancefor real-world solutions.by Eric Bennett andWim WeekersCoordinate Measuring Machines (CMMs) are used in practicallyevery industry that requires precise dimensional inspection ofmanufactured parts. In today’s competitive environment,manufacturers demand CMMs that are accurate, reliable, fast,economical, and provide maximum flexibility with respect tooperating environment. In order to meet these often conflictingrequirements and provide maximum value in the products deliveredto their customers, CMM manufacturers must make informed designdecisions, intelligent material choices, and employ novel techniques.The end result should be an affordable machine that is highlyaccurate, measures parts quickly, and is relatively insensitive to itsenvironment. The key to achieving this goal is the carefulmanagement of the machine’s intrinsic error.A Historical PerspectiveHistorically, improvements in CMM measurement accuracy werealmost entirely driven by the mechanical accuracy of the CMM’shardware and the ability to maintain the thermal stability of theoperating environment. Thus, in the earliest days of the industry, inorder to achieve the ever increasing measurement accuracy requiredto support shrinking part tolerances, CMM components needed to be
Modern CMM Design Conceptsmanufactured ever more accurately: machine frames were madestiffer, guide ways straighter, drives smoother, scales more accurate,and so on. At the same time, the operating environment needed to becontrolled more and more tightly to reduce the effects of thermallyinduced measurement errors. The impact of this design direction wasincreasing expense to the customer, due to the ever increasingamounts of value added to the physical components. Clearly, this pathcould not continue indefinitely both from the perspective of expense,and from diminishing returns of tighter and tighter specifications formechanical and thermal accuracy — it is impossible to build astructure without any error at all; some intrinsic structural error willalways remain. What was needed was some sort of paradigmchanging advance in order to satisfy the increasing demand foraccurate, reliable, fast, and economical CMMs, while also providingflexibility with respect to operating environment. The introduction andcontinual refinement of software based measurement errorcompensation techniques over the past few decades have allowedCMM manufacturers to break this trend and effectively meet theircustomer’s requirements. That said, software based measurementerror compensation is not a cure-all for design problems, nor does itallow CMM manufacturers to apply sloppy design principles, makeinappropriate material choices, or skimp on build quality.Error Compensation: Building on a Solid FoundationAt its most basic level, a CMM provides a coordinate system whichdefines the location of data points in space. This coordinate systemis physically realized in the mechanical structure of the CMM usinglinear scales. Some type of probing system is used in conjunctionwith the linear scales to identify the location of measurement pointson the part being measured. In the case of a theoretical CMM with amechanically perfect” structure, the X, Y, and Z scale readings wouldperfectly correspond to the actual position of the probe tip on thepart. In reality, since the “mechanically perfect” CMM does not exist,many error sources contribute to a small difference between thescale readings and the true probe position. This is defined as themeasurement error.The concept of how to compensate for the intrinsic error present inany CMM structure was solved with software error compensation,which is based on the idea that if we can understand andmathematically characterize a CMM’s predictable measurement errorsources, the CMM controller software can automatically correct themeasurements. In this context, software error compensation is simplya method for correcting CMM scale readings for systematic errors inthe probe tip position as reported by the CMM scales. This featurewas first introduced on CMMs a few decades ago. Over time, thesecompensation techniques have grown increasingly moresophisticated, encompassing both static geometric errors and allsorts of dynamic and thermally induced geometry errors. Staticgeometric errors are those errors caused by microscopicimperfections in the shapes of the guide ways and scale systemsthat lead to errors in the measured location of the probe tip when themachine is not moving. Thermal errors are changes in the machine’sgeometry caused by changes in temperature. In its most basic form,thermal error correction includes simple linear correction of thescales due to thermal expansion and contraction to more elaboratemethods for compensating for thermally induced nonlinear changesin the structure.2
Since it is only feasible to correct for known, well characterizedsystematic errors, an important prerequisite for the successfulapplication of software error compensation is a “well behaved”,repeatable CMM. That is, a CMM based on robust design principlesand built with quality components by a staff of highly skilledtechnicians who are trained to assemble a machine with the highestpossible mechanical build quality.Stability in MotionThe main CMM structural design parameters include the weight ofthe moving mass and the static, dynamic, and thermal properties ofthe physical structure. The use of lightweight components in themoving part of the structure reduces the forces necessary toaccelerate them during machine motion, resulting in the ability to useless powerful and cooler running motors while at the same timecausing less frame distortion due to inertia. The design challenge isto find a good compromise between stiffness and weight. Thus,material selection is extremely critical, especially since it affects theother requirements of thermal stability and dynamic properties.Aluminum is especially attractive in this respect. Although it has aspecific weight that is similar to that of granite, modern extrusionprocesses allow for the manufacturing of large structural aluminumelements with the material located specifically where it has thegreatest impact on stiffness. Relatively thin wall structures with thematerial farther away from the neutral axis of bending will result inthe most structural stiffness with the least amount of weight.It is important that the build up of thermal gradients within thestructure are minimized in order to minimize frame distortion.Gradients can build up when a component slowly responds totemperature changes. A good example is a large slab of graniteexposed to a temperature change. Because of the granite’s lowthermal conductivity and large mass, heat will only slowly movethrough the material. This results in a non-uniform temperaturedistribution within the granite. If this distribution is asymmetric (topand bottom with respect to the center), this will result in differences inexpansion or contraction of the top and bottom surfaces andconsequently bend the granite. (Yes, believe it or not, granite doesbend!) On the other hand, aluminum has a high coefficient of thermalconductivity; heat enters an aluminum structure and quickly transfersthrough the material, avoiding the build up of thermal gradients andthe resulting bending.From Theory to PracticeHow do these design concepts play out in a real CMM design? TheBrown & Sharpe Global CMM, a moving bridge, air bearing CMMmanufactured by Hexagon Metrology, Inc., is a well known example ofthe application of modern CMM design principles. For Global, the maindesign drivers were accuracy, throughput, and environmental flexibility.Granite is used for the fixed base of the CMM that the moving bridgetravels on. Even with its poor thermal properties, granite is still anexcellent choice for a high quality flat work surface with extremedurability. Improvements in granite machining processes permit themanufacture of a one-piece granite base with integrated guide waysfor the air bearings. In addition, the Global granite base is heavy and,in combination with passive elastomeric isolation pads, this serves toisolate the machine from environmental vibrations which have anegative effect on accuracy and repeatability. From a thermal point ofview, the choice of granite is less favorable because the large granitebase is prone to thermal bending resulting from temperaturechanges within the installation environment. However, this thermalAluminum’s high strength to weight ratio makes itan ideal material for building extremely rigid yetlightweight structures. Shown is a Global X-BeamcomponentIts design is optimized to maximizestiffness and minimize weight. This results in aCMM that is accurate, fast, and not subject togeometry distorting inertia effects.Granite’s low coefficient of thermal conductivity results in slow heatconduction which can result in large thermal gradients within a thickslab of granite. As shown in the illustration (exaggerated for clarity),this causes the granite to bend as the opposing surfaces expand orcontract differently. Aluminum on the other hand quickly conductsheat due to its much higher coefficient of thermal conductivity, thusavoiding thermally induced geometric distortion.Granite bottom: hotGranite top: coldModern CMM Design Concepts3
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