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Eng. A: Structural Materials: Properties,Microstructure and Processing 430 79–89[59] Jordan S E, Brown C A 2006 Comparing texture characterization parameters on their ability to differentiate ground polyethylene ski bases Wear 261398–409CHAPTER 9Coordinate metrology9.1 Introduction to CMMsThis section gives an overview of coordinate metrology as an introduction tothe sections on miniature coordinate measuring machines (CMMs). Anunderstanding of the operation of normal industrial CMMs will help in theunderstanding of the principles of miniature CMMs.A CMM is a measuring system with the means to move a probing systemand capability to determine spatial coordinates on the surface of the partbeing measured.
A photograph of a typical CMM is shown in Figure 9.1.CMMs come in a number of configurations (see Figure 9.2) and a range ofsizes, from those able to measure something the size of a bus to the miniatureversions described in section 9.4. However, the majority of CMMs fall inthe range 0.5 m to 2 m. CMMs generally incorporate three linear axes anduse Cartesian coordinates, but CMMs are available with four axes where thefourth axis is generally a rotary axis. The first CMMs became available in thelate 1950s and early 1960s (see [1] for a thorough description of CMMs andsome history, and [2] for an overview of their use).CMMs measure either by single point probing, where data from singlepoints on the surface are collected, or by scanning, where data are collectedcontinuously as the stylus tip is dragged across the surface.
The stylus tip incontact with the surface is usually a synthetic ruby ball, although othergeometries are possible, for example cylindrical stylus tips.The data collected by the CMM are essentially ball centre data. The stylusin contact with the surface, therefore, needs to be qualified to determine theeffective stylus radius and the position of the centre of the tip relative to somereference point. Stylus qualification is carried out by measuring a knownartefact, usually a high-quality ceramic sphere.The data collected from the part being measured need to be aligned witheither the component drawing or a computer-aided design (CAD) model.Fundamental Principles of Engineering NanometrologyCopyright Ó 2010 by Elsevier Inc.
All rights reserved.CONTENTSIntroduction to CMMsSources of error onCMMsTraceability,calibration andperformanceverification of CMMsMiniature CMMsMiniature CMM probesCalibration ofminiature CMMsReferences263264C H A P T ER 9 : Coordinate metrologyFIGURE 9.1 A typical moving bridge CMM.This alignment is usually carried out with reference to defined datumfeatures on the drawing. However, for freeform artefacts (see section 9.1.5)a best-fit alignment may be more appropriate.
Once data are collected theyare analysed by a software package. This involves fitting substitute elements(circles, planes, etc.) to the collected data. The software can then be used tocalculate intersection points, distances between features, locations offeatures in the workpiece coordinate frame, distances between features andform errors such as roundness, cylindricity, etc.The international specification standard for CMMs is ISO 10360. CMMtypes are described in ISO 10360 part 1 [3] and include:-Fixed table cantilever CMMs (Figure 9.2a)-Moving bridge CMMs (Figure 9.2b)-Gantry CMMs (Figure 9.2c)-L-shaped bridge CMM (Figure 9.2d)-Fixed bridge CMMs (Figure 9.2e)Introduction to CMMsFIGURE 9.2 CMM configurations.265266C H A P T ER 9 : Coordinate metrology-Moving table cantilever CMMs (Figure 9.2f)-Column CMMs (Figure 9.2g)-Moving ram horizontal-arm CMM (Figure 9.2h)-Fixed table horizontal-arm CMM (Figure 9.2i and j)-Moving table horizontal-arm CMM (Figure 9.2k)Moving and fixed bridge type CMMs are the most common design.A further type of CMM also encountered is the vision system.
A visionsystem CMM is essentially a microscope mounted on one of the CMMarrangements described above. It is often referred to as being 2.5D as therange and access in the vertical, z axis is inferior to that in the x and y axes(height is measured by focusing the microscope on the relevant surfaces).9.1.1 CMM probing systemsThe probing system attached to a CMM [4] can be one of the following threetypes:-an analogue or scanning probe;-a touch trigger probe;-a probe that employs optical technology.An analogue probe is capable of working either in a mode where it collectspoints from a number of surface contacts or by scanning the componentsurface. It is a measuring probe and data are collected from the CMM scalesand the probe as it scans along the surface.A touch trigger probe works by recording the machine coordinates whenthe stylus tip contacts the surface.
It is essentially on or off.Various optical probes can be attached to CMMs, often working ona range of principles, for example, triangulation (see section 6.7.2.1). Opticalprobes have the advantage of being able to collect data significantly fasterthan an analogue contacting probe. However, they are generally less accurate.9.1.2 CMM softwareAn important part of a CMM is its software. The software needs to carry outthe following tasks:-collect data from the CMM (scales, probe, temperature sensors);-fit substitute elements to the data;Introduction to CMMs-create alignments relating to the part in question;-report the data;-compare against CAD data where necessary.CMM software needs to be tested and this is covered in ISO 10360 part 6[5]. Use is made of reference data sets and reference software to check theability of the software to calculate the parameters of basic geometricelements.9.1.3 CMM alignmentTo measure a component on a CMM, its alignment relative to the coordinatesystem of the machine needs to be described.
This alignment is usually madeusing datum features on the part in question.The alignment needs to control the following:-the part spatial rotation (two degrees of freedom);-the part planar rotation (one degree of freedom);-the part origin (three degrees of freedom).As an example, for a rectangular block the alignment process wouldtypically be:1. Measure a plane on the top surface (defines rotation axis and z zero)2. Measure a line on the side face (defines planar rotation about z axisand y zero)3. 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.
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