Принципы нанометрологии (1027506), страница 58
Текст из файла (страница 58)
Measure a point on a face orthogonal to the side face (x zero)Other alignments are possible, for example, best-fit alignments andreference point alignments are used for freeform shapes.9.1.4 CMMs and CADModern CMM software allows programming direct from a CAD model.Furthermore, once data are collected the actual points can be compared tothe nominal points and pictorial representations of the errors created.Point clouds can also be best-fitted to the CAD model for alignmentpurposes.267268C H A P T ER 9 : Coordinate metrology9.1.5 Prismatic against freeformArtefacts measured on CMMs fall into two categories:-purely prismatic components, examples of which include engineblocks, brake components, bearings, etc.-freeform components, examples of which include car doors, bodypanels, mobile phone covers, IT peripherals, etc.Prismatic components can be broken down into easily defined elements,for example, planes, circles, cylinders, cones and spheres.
A measurementwill consist of breaking down the component into these geometries and thenlooking at their inter-relationships, for example, the distance between twoholes or the diameter of a pitch circle.Freeform components cannot be broken down as with prismaticcomponents. Generally the surface is contacted at a large number of pointsand a surface approximated to the data.If a CAD model exists then the cloud of data can be compared directlyagainst the CAD model.
Having a CAD model is an advantage for freeformsurfaces, as the nominal local slope at the contact point is known in advance.The local slope is needed to correctly correct for the probe tip radius ina direction normal to the surface. For reverse engineering applications, thelocal slope needs to be approximated from measurement points adjacent tothe target point.Many real-world components are a mixture of freeform surfaces andgeometric features; for example, a mobile phone cover may have locationpins that need to be measured.9.1.6 Other types of CMMOther types of coordinate measuring systems include articulated-arm CMMsand laser trackers ([2] discusses both types of CMM).
The devices have theadvantage that they are generally portable and are better suited to measuringlarger items, for example, aerospace components.9.2 Sources of error on CMMsWhilst CMM manufacturers aim to build CMMs with small geometricerrors, no CMM is constructed perfectly.
A typical CMM has twenty-onesources of geometric error. Each axis has a linear error, three rotation errorsand two straightness errors (six per axis gives eighteen). The final three errorsTraceability, calibration and performance verification of CMMsare the orthogonality errors between any two pairs of axes. These errors arealso described briefly in section 7.3.4 for scanning probe microscopes.Traditionally these errors were minimized during manufacture of theCMM.
However, with the advent of modern computers CMMs can be errormapped (volumetric error compensation) with corrections to geometricerrors made in software [1,6–8].CMM geometric errors are measured in one of the four followingmanners:-using instruments such as straight edges, autocollimators and levels;-using a laser interferometer system and associated optics;-using a calibrated-hole plate [9];-using a tracking laser interferometer [10].9.3 Traceability, calibration and performance verificationof CMMsCalibration and performance verification are two issues that are oftenconfused when talking about CMMs [2].
To clarify, CMM calibration is themeasurement of the twenty-one degrees of freedom of a CMM to enablemechanical correction or error mapping of a CMM. Performance verificationis a series of tests that allows the manufacturer of the CMM to demonstratethat an individual machine meets the manufacturer’s specification. Notethat calibration can be part of the performance verification.The ISO 10360 series of specification standards defines the procedure forperformance verification of CMMs. The series is broken down into six parts,which are briefly described.Part 1: Vocabulary.
Part 1 [3] describes the terminology used to describeCMMs. It is important when describing CMMs to adhere to thisterminology.Part 2: CMMs used for measuring size. Part 2 [11] describes howa CMM should be specified and the necessary steps to show thata machine meets specification.The tests detailed in part 2 involve:-measuring a good-quality sphere at a number of positions andexamining the variation in indicated radius;269270C H A P T ER 9 : Coordinate metrology-measuring a series of lengths in a number of directions in the machinevolume and comparing the machine indication against the known sizeof the artefact.In addition part 2 describes how stable artefacts can be used for interimmonitoring of the CMM.Part 3: CMMs with the axis of a rotary table as the fourth axis. Part 3 [12]describes the extra steps necessary to performance-verify a CMM whichhas a rotary axis as the fourth axis.Part 4: CMMs used in scanning measuring mode.
Part 4 [13] containsthe tests necessary to demonstrate that the scanning capability of a CMMmeets specification.Part 5: CMMs using multiple-stylus probing systems. Part 5 [14] extendsthe probe test covered in part 2 to cover multiple-stylus probing systems.Part 6: Estimation of errors in computing Gaussian associated features.Part 6 [5] is concerned with assessing the correctness of the parameters ofcomputed associated features as measured by a CMM or other coordinatemeasuring system.9.3.1 Traceability of CMMsTraceability of CMMs is difficult to demonstrate. One of the problems isassociating a measurement uncertainty with a result straight off the CMM.The formulation of a classical uncertainty budget is impracticable for themajority of the measurement tasks for CMMs due to the complexity of themeasuring process.It used to be the case that the only way to demonstrate traceability was tocarry out ISO 10360-type tests on the machine.
However, if a CMM isperformance-verified this does not automatically mean that measurementscarried out with this CMM are calibrated and/or traceable. A performanceverification only demonstrates that the machine meets its specification formeasuring simple lengths, i.e.
it is not task-specific.This task-specific nature of a CMM can be illustrated with a simpleexample. Suppose a CMM measures a circle in order to determine itsdiameter. To do this the CMM measures points on that circle. The pointscan be measured equally spaced along the circumference, but may have to befrom a small section only, for example because there is no material presentat the rest of the circle. This is illustrated in Figure 9.3, which shows theeffect on the diameter and the centre location if measurements with thesame uncertainty are taken in a different manner. This means that even ifTraceability, calibration and performance verification of CMMsFIGURE 9.3 Illustration of the effect of different measurement strategies on thediameter and location of a circle.
The measurement points are indicated in red; thecalculated circles from the three sets are in black and the centres are indicated in blue.the uncertainty for a single coordinate is known, this does not simplycorrespond to an uncertainty of a feature that is calculated from multiplepoints.A better method is described in ISO/TS 15530 part 3 [15]. This specification standard makes use of calibrated artefacts to essentially use the CMMas a comparator. The uncertainty evaluation is based on a sequence ofmeasurements on a calibrated object or objects, performed in the same wayand under the same conditions as the actual measurements.
The differencesbetween the results obtained from the measurement of the objects and theknown calibration values of these calibrated objects are used to estimate theuncertainty of the measurements. However, this method requires independently calibrated artefacts for all its measurements, which is quite contradictory to the universal nature of a CMM.Alternative methods that are consistent with the GUM (see section 2.8.3)can be used to determine the task specific uncertainty of coordinatemeasurements.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
