Coordinate Measuring Machine Structure Design Evolution
The Coordinate Measuring Machine first appeared at the international machine tool show in Paris in 1959, exhibited by the British company Ferranti, who had also launched the first commercially available general purpose computer some years earlier.
Over the years various configurations of CMM have been developed; the most popular one being the bridge design that today accounts for over 90% of the CMM market. The bridge structure of the machine rests on top of a granite surface plate and usually the X, Y, Z structure glides on friction free air bearings.
The CMM mechanical structure is a carrier for the measuring sensor with the function of reporting the location of the measured points on the part under inspection. The CMM coordinate system is produced using linear scales attached to the CMM structure. A perfect CMM structure, with no intrinsic errors, would result in the exact location of the probe points being reported. The perfect mechanical structure does not exist and there are many sources of error contributing to the small deviation between the scale readings and the actual sensor position. This is identified as the CMM Measuring Error. Not only is the precision of the CMM structure critical but also the actual mounting of the measuring scales to the structure since it’s the scales that encode the actual sensor position. As an example a CMM with a separate and independent X, Y Z encoding system would result in a sensor position being recorded independently of structure errors.
The bridge structure of the CMM was manufactured in the early years from granite, cast iron or steel.
Granite – No Longer the perfect CMM Material
Granite has long been used in the metrology world in the form of surface plates, straight edges and squares. Granite has a very low absorption rate of thermal changes and a low coefficient of expansion. It is also heavy and unpredictable as a material, due to its natural occurrence, and its low modulus of elasticity can cause fractures if a CMM crash occurs. Simple parts like rectangular beams can be manufactured with relative ease from granite while complex geometry designs are much more problematic. The use of granite is therefore a design constraint when developing a modern coordinate measuring machine design.
Cast Iron – Not practical other than for tables
Again cast iron has been traditionally used for surface plates in the metrology world due to its relatively low coefficient of expansion. Since any parts are produced using the casting process each CMM model and size would require different size parts and thus each requires a separate casting. The multiple machine sizes offered by the CMM industry has rendered cast-iron impracticable other than for large table on extra-large CMM units.
Steel – used for larger CMM structures
While not the perfect metrology material steel allows mass production techniques to be introduced to the CMM manufacturing process through the use of machined weldments; steel allowed the CMM industry to move away from its hand craftsmanship production techniques. All of the major CMM suppliers manufactured steel CMM structures in the 1980’s as production volumes gained pace. The granite CMM in the 1980’s and 90’s was relegated to being produced by only the very small localised CMM manufacturers who had not the investment or production volumes to move to steel structures. Almost all of these are no longer in business. Steel fabrication are still in use today on large Gantry style CMM’s.
The early Coordinate Measuring Machines were hand crafted with mechanical precision. Many CMM builders offered two accuracy grades; the enhanced accuracy grade being achieved with more selective assembly and overall care in the assembly process. Many of these machines lost their precision over time as wear occurred in the system drives and other critical components.
The market demanded more accuracy from the CMM over the years since manufacturing tolerances have been reduced in order to satisfy improvements in product reliability and improved performance together with new design concepts. Improved accuracy, improved speed, lower costs and the ability to relax the installation environment were the demands heard by the industry coming from CMM customers.
The use of intrinsic error identification and their collection with subsequent error mapping using mathematical algorithms resident on the CMM computer were initially developed and utilised to enhance the accuracy of the mechanically precise CMM. Error mapping of a CMM involves the mathematical definition of the constant or predictable errors and their correction using software algorithms such that a perfect CMM structure is simulated and the adjusted X, Y, Z position of the sensor points reported.
The New Generation of CMM Design – Use Advanced Alloy Technology
The introduction of CAD and FEA allowed designers to start understanding the behavior of CMM structures. Computerized modeling allows the CMM designers to optimize their design to remove unnecessary mass and strengthen parts in critical areas to minimize deformation etc. The ability to strengthen a part in a localized area while reducing overall mass led to the consideration of emerging new materials such as alloys in the CMM design.
These alloys provide a lighter, stiffer structure while at the same time improving reaction to, and dispersion of, temperature changes and actually result in less deformation of the CMM structure than that experienced by the slowly reacting structures manufactured from granite or cast-iron.
The construction of the CMM structure from new high technology alloys materials is proven to provide a lighter, stiffer structure while at the same time improving reaction to, and dispersion of, temperature changes, when the CMM is installed in a less than perfect environment, and actually results in less deformation of the CMM structure than that experienced by the slowly reacting granite and cast-iron structures. Integrated with the alloy structures are new generation measuring scales, such as the Renishaw ‘Fastrack’ system, that locates the scale in a substrate mounted to the CMM structure and are free and independent of any CMM structural movements increasing dramatically CMM accuracy even when installed in non-perfect environments.
An aluminium alloy CMM structure is lighter and has an improved reaction to thermal changes resulting in less linear and torsional structural deformation and integrated with the free-floating measuring scales with thermal compensation provides the ideal platform for the next generation coordinate measuring machine.
Perfect CMM Design – High Dynamics with Excellent Thermal Response
The principle CMM design focus for a modern CMM include the moving mass, dynamics and thermal properties of the CMM frame. Using alloy components for the frame reduces the power required to accelerate the frame when in motion resulting in the use less of smaller less powerful motors creating less heat and also causing less structural inertia distortions.
Although aluminium alloys have the same specific weight as granite it’s not used as a solid block like granite and in addition production processes like extrusion allow for the manufacture of CMM structural elements with thicker material and ribbing located by design to maximise element stiffness.
The All Granite CMM was design 40+ years ago and is now Technically Obsolete
Granite structures are also less predictable due to the inconsistent material properties of the natural granite product and have their scales rigidly mounted so any granite movement directly affect the scale measuring performance creating inaccuracies. Since the accuracy of the CMM is derived from the scales and not the CMM structure the arguments for the stability of granite are false. They are valid for static granite blocks used as granite plates and straight edges where no measuring scales are involved. In addition granite has a low rate of thermal diffusion meaning that it dissipates any thermal change very slowly and therefore the inaccuracies in the structure remain longer.
The use of a thick granite plate in a CMM design appears to provide the image of great stability. In practise however the use of a thick granite plate has negative implications when the plate is subjected to thermal changes. Because of granite’s low thermal conductivity it processes thermal change very slowly resulting in a non-uniform temperature distribution in the plate causing plate bending to occur. Although this bending is only measured in microns since the CMM bridge is riding on the granite plate its sufficient to cause significant measuring errors. For this reason the granite plate of all modern CMM designs have been optimised for thickness to reduce this bending effect and minimise its impact on CMM accuracy while ensuring its thick enough to support specified component weights without distortion.
In addition many older CMM design offer an inconsistent use of materials. For example most granite CMM have the bridge beam support legs produced from steel or aluminium and the X/Z carriage supporting the Z column is manufactured from an aluminium casting for convenience; so much for their granite arguments when they have polluted their own material science arguments!. In addition aluminum air bearings are also used throughout a granite CMM. Look under their covers and see for yourself….
Modern CMM designs use a single material type throughout the entire CMM frame.
Today the largest global CMM manufacturers from USA, Japan, Germany and Italy all incorporate in their modern CMM designs a moving structure produced from aluminum alloy. All have researched the subject independent of each other and reached the identical conclusion. “Granite is an out-dated material for the moving structure of a CMM”.
Non linear motion of a Cartesian CMM such as experienced during the scanning process induces accelerations and deceleration forces that can twist and deflect the CMM machine structure; these dynamic deflections can result in measurement errors that increase with measurement speed. In addition the scanning probe deflection force can induce a potential deflection of the CMM Z column. While the modern design alloy CMM is designed for maximum stiffness the upgrade of the Z column to silicon carbide results in an improved scanning performance for the CMM.
Silicon Carbide is the new engineered CMM Material
The new 5 Axis scanning Technology from Renishaw eliminate these errors since the scanning motion is all contained within the probe head.
The COORD3 Bridge, Gantry and Horizontal Arm ranges of coordinate measuring machines embrace the modern design principles for coordinate measuring machines incorporating all of the above points.