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MICRO METROLOGY | FEATURE


Key among such technologies are co-ordinate measuring machines (CMMs). As a cost-effective and efficient measuring technology, CMMs with an accuracy of 1 micron have been used in traditional manufacturing extremely successfully. However, their usefulness is often restricted to the analysis of external geometries, and as parts and features become smaller, their usefulness is restricted by probe size, aspect ratio, probing force and the total mass of moving parts.


CMMs designed specifically for micro applications have recently been developed that aim to deliver the measurement accuracy demanded by the micro parts sector, with reduced measurement volume, high resolution machine scale and very low probing forces. The accuracy of such micro CMMs is limited by the accuracy of the probing systems, and their usefulness is restricted by the aspect ratio of the probing system.


These draw backs are as they are due mainly to the fact that micro CMMs have been developed as scaled down versions of macro CMMs. Today, increased accuracy through further reduction in scale is confronting issues because of the laws of physics at the micro level, where — for example — surface forces dominate over gravity.


CT scanning involves taking thousands of 2D X-Ray images. The object is rotated 360 degrees with images being acquired at set increments. Once imaging is complete, algorithms are used to reconstruct the 2D images in a 3D data cloud, which can be sliced and internal geometries analysed.


In order to overcome some of the issues confronted when using CMMs at the micro level, a few companies are turning to the use of X-rays and CT inspection technologies. Generally speaking, using these ‘medical’ procedures on manufactured parts is disproportionately expensive, but the data produced is good, and crucially provides measurement of external and internal data.


CT scanning involves taking thousands of 2D X-Ray images. The object is rotated 360 degrees with images being acquired at set increments. Once imaging is complete, algorithms are used to reconstruct the 2D images in a 3D data cloud, which can be sliced and internal geometries analysed.


CT scanning, however, does itself have limitations. While part complexity is not an issue, material density combined with path length (the distance through which the X-rays must pass) is important when CT scanning. Also relevant is the physical amount of material that is in a part, resolution decreasing with overall part size as well as material density. Accuracy in such instances may be maintained, but resolution can suffer badly.


One other commonly-used alternative measuring technology is optical comparators, relatively low cost and time-worn measurement tools that display a magnified image of a part onto a glass screen using illumination sources, lenses and mirrors. The image produced can be compared with 2D CAD design drawing, and variations in length and width, as well as the detection of burrs, scratches, indentations or undesirable chamfers can be


26 | commercial micro manufacturing international Vol 7 No.3


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