Cmm

for

23.4 COORDINATE MEASURING MACHINES

Coordinate metrology

coordinate measuring machine

y, z

Sec. 23.4 Coordinate Measuring Machines 721

Figure 23.3 Coordinate measuring machine

Historical Note 23.2

In the mld-1950s. applications of numerical control (NC) technology were growing (Hisrori-cal Note 6.1t A part that took hours to produce by conventional machining methods could bemachined in minutes on an I'iC machine. The problem was that it still required hours to inspectthe part by traditional measuring techniques. Among those who recogni7ed this problem wasHarty Ogden, chief engineer at Ferranti, Ltd.. a company producing NC machines in ScotlandTo address the problem, Ogden developed an inspection machine in 1956,which is consideredto be the first coordinate measuring machine (CMM).lt consisted of a freely moving measur-ing probe with electronic numerical display to indicate the location of the probe in x-y coor-dinates.1t had and movements offilO mm (24io) ~nd .181mm (15 in), T"spectively 1t "seda tapered probe tip and provided a measuring accuracy of 0.025 mm (0.001 in). The machinewas called the Ferranti Inspection Machine

Among the attendees at the Intcrnationai Machine Tool Show in Paris in 1959 wasGeorge Knopf, General Manager of the Industrial Controls Division of Bendix Corp. in theUnited States. While touring the show, Knopf visited the Ferranti exhibit and noted with greatinterest the two-axis CMM among the Ferranti products on display. Recognizing the potentialof the machine, Knopf flew from the show to the Ferranti plant in Edinburgh, Scotland, wherehe started negotiations thatled to an exclusive contract for Bendix to sell Ferranti CMMs inNorth America, Ferranti machines were exhrbited by Bendix at the National MachineTholShow in Chicago in 1960

The first Ferranti CMM sold in the United States was to Western Electric Companyplant in Winston-Salem, North Carolina. The machine was used to replace manual inspectionlechniqucs.Accuraterecord,were kept on relative inspection ti mes, manual techniques ver-sus the CMM. Inspection times were reduced from 20 minutes to 1 minute. The merit of theCMM was demonstrare<i, th", market for the CMM was establiShed

In 1961.responsibility for marketing Ferranti CMMs was assigned to Sheffield Corp .. adivision of Bendix that had hcen acquired in 1956. Between 1961 and 1964, more than 250

'~ 'Y'I.m

Chap. 23 I Inspection Technologies

Cordaxadopted. An agreement with Ferranti was reached for Sheffield to produce CMMs in the Unit-ed States. New Cordax models were introduced. CMM sales were growing, and other compa-

1965three-axis CMM, The first touch-trigger probe was developed in England in 1972. Computersoftware was developed for CMMs to perform probe offset compensation and to calculategeometric features. Improvements in CMM technology continue today.

To accomplish measurements in 3-D, a basic CMM is composed of the followingcomponents:

• probe head and probe to contact the workpart surfaces• mechanical structure that provides motion of the probe in three Cartesian axes and

displacement transducers to measure the coordinate values of each axis

In addition, many CMMs have the following components:

• drive system and control unit to move eaeh of the three axes• digital computer system with application software

In this section, we discuss (1) the construction features of a CMM;(2) operation and pro-gramming of the machine; (3) the kinds of application software that enable it to measuremore than just x-y-z coordinates; (4) applications and benefits of the CMM over manualinspection; (5) flexible inspection systems, an enhancement of the CMM; and (6) use ofC01J:lactinspection probes on machine tools.

23.4.1 CMM Construction

In the construction of a CMM, the probe is fastened to a mechanical structure that allowsmovement of the probe relative to the part. The part is usually located on a worktable thatis connected to the structure. Let us examine the two basic components of the CMM: (1) itsprobe and (2jltS mechanical structure,

Probe. The contact probe is a key component of a CMM.lt indicates when contacthas been made with the part surface during measurement. The tip of the probe is usuallya ruby ball. Ruby is a formot corundum (aluminum oxide), whose desirable properties inthis application include high hardness for wear resistance and low density for minimuminertia. Probes can have either a single tip, as in Figure 23.4(a), or multiple tips as in Fig-ure23.4(b).

Most probes today are touch-trigger which actuate when the probe makescontact with the part surface. Commercially available touch-trigger probes utilize any orvar.tous triggering mechanisms, including the following:

• The trigger is based on a highly sensitive electrical contact switch that emits a signalwhen the tip of the probe is deflected from its neutral position.

• The trigger actuates when electrical contact is established between the probe andthe (metallic) part surface.

Sec. 23.4 I Coordinate Measuring Machines

Contact probe configurations: (a) single tip and (b) mul-tiple tips .

• The trigger uses a piezoelectric sensor that generates a signal based on tension orcompression loading of the probe.

Immediately after contact is made between the probe and the surface of the object,the coordinate positions of the probe are accurately measured by displacement transduc-ers associated with each of the three linear axes and recorded by the CMM controller.Common displacement transducers used on CMMs include optical scales, rotary encoders,and magnetic scales [2]. Compensation is made for the radius of the probe tip,as indicat-ed in our Example 23.1, and any limited overtravel of the probe quill due to momentum isneglected. After the probe has been separated from the contact surface, it returns 10 itsneutral position.

The part dimension in Figure 23.5 is to be measured. The dimension is alignedwith the x-axis, so it can be measured using only x-coordinate locations. Whenthe probe is moved toward the part from the left, contact made at 68.93 isrecorded (mm). When the probe is moved toward the opposite side of the part

Figure 23.5 Setup for CMM measurement. in Example 2:.U

Tip (ruby ball)..--- -c.,

Chap. 23 I Inspection Technologies

from the right.contact made at x '" 137.44 is recorded. The probe tip diameteris 3.00 mm. What is the dimension L?

Solution: Given that the probe tip diameter D, 3.00 mm, the radius R, 1.50 mmEach of the recorded x values must be corrected for this radius.

XL'" 68.93 1.50 70.43mm

137.44 - 1.50 135.94 mm

135.94 - 70.43 65.51 mm

Mechanical Structure There are various physical configurations for achieving themotion of the probe.each with its relative advantages and disadvantages. Nearly all CMMshave a mechanical configuration that fit~ into one of the following six types. illustrated inFigure 23.6:

23.6 ,SiXtypes of construction: (a) cantilever, (b) moving bridge,(c) fixed bridge, (d) horizontal arm (moving ram type), (e) gantry, andcolumn.

(a) Cantilever. In the cantilever configuration, illustrated in Figure 23.6(a), the probe isattached to a vertical quill that moves in the z-Hxis direction relative to a horizontalarm that overhangs a fixed wnrktable.The quill can also be moved along the lengthof the arm to achieve y-axis motion, and the arm can be moved relative to the work-table to achieve » -axis motion. The advantages of this construction are: (1) convenientaccess to the worktable, (2) high throughput-the rate at which parts can be mount-ed and measured on the CMM, (3) capacity to measure large workparts (on largeCMM,). and (4) relatvely small floor space requirements. Its disadvantage is lowerrigidit) than most other CMM constructions.

(b) Moving bridge. In the moving bridge design. Figure 23.6(b). the probe monnted ona bridge structure that is moved relative to a stationary table on which is positionedthe part to he measured. This provides a more rigid structure than the cantilever de-sign. and its advocates claim that this makes the moving bridge CMM more accurate.However. one of the problems encountered with the moving bridge design is yawing{also known as walking), in which the two legs of the bridge move at slightly differ-em speeds, resulting in twisting of the bridge. This phenomenon degrades the accu-racy of the measurements. Yawing is reduced on moving bridge CMMs when dualdrives and position feedback controls are installed for both legs. The moving bridgedesign is the most widely used industry. It is well suited to the size range of partscommonly encountered in production machine shops.

(c) Fixed bridge. In this configuration, Figure 23.6(c), the bridge is attached to the (MMbed. and the worktable is moved in the .r-dircctlon beneath the bridge. This con-struction eliminates the possibility of yawing, hence increasing rigidity and accuracy.However, throughput is adversely affected because of the additional mass involvedto move the heavy worktable with part mounted on it.

(d) Horizontal arm. The horizontal arm configuration consists of a cantilevered hori-zontal ann mounted to a vertical column. The arm moves vertically and in and outto achieve y-axis and a-axis motions. To achieve .r-axis motion. either the column ismoved horizontally past the worktable (called the moving ram design), or the work-table is moved past the column (called the moving table design). The moving ram de-sign is illustrated in Figure 23,6(d). The cantilever design of the horizontal armconfiguration makes it less rigid and therefore less accurate than other CMM struc-tures. On the positive side, it allows good accessibility to the work area. Large hori-zontal arm machines are suited to the measurement of automobile bodies, and someCMM5 are equipped with dual arms 50 that independent measurements can be takenon both sides of the car body at the same time

(e) Gantry. This construction, illustrated in Figure 23.6(e), is generally intended for in-specting large objects. The probe quill (z-axis) moves relative to the horizontal armextending between the two rails of the gantry. The workspace in a large gantry typeCMM can be as great as 25 m (82 ft) in the x-direction by 11m (26 ft) in the y-diree-tion by 6 m (20 ft) in the a-direction

(f) Column. This configuration, in Figure 23.6(f), is similar to the construction of a ma-chine tool. The x- and movements are achieved by moving the worktable, whilethe probe quill is moved vertically along a rigid colnmn to achieve z· axismotion

In all of these constructions. special design features arc used to build high accuracy and pre-cision into the frame. These features include precision rolling-contact bearings and hy-drostatic air-bearings. installation mountings to isolate the CMM and reduce vibrations in

726 Chap. 23 / Inspection Technologies

the Iacrory from being transmitted through the floor, and various schemes to counterbal-ance the overhanging arm in the case of the cantilever construction [4], [17].

CMM Operation and Programming

from manual operation to direct computer control (DeC). Computer-controlled CMMsoperate much like CNC machine tools, and these machines must be programmed. In thissection. we consider: (1) types of CMM controls and (2) programming ot computer-controllcdCM\tIs.

CMM Controls. The methods of operating and controlling a CMM can be classi-fied into four main categories: (1) manual drive, (2) manual drive with computer-assisteddata processing, (3) motor drive with computer-assisted data processing, and (4) DCC withcomputet-assisted data processing.

In a manual drive CMM, the human operator physically moves the probe along themachine's axes to make contact with the part and record the measurements. The three or-thogonal slides are designed to be nearly frictionless to permit the probe to be free float-ing in the y-, and a-directions. The measurements are provided by a digital readout,which the operator can record either manually or with paper printout.Anycalculations onthe data (e.g., calculating the center and diameter of a hole) must be made by the operator.

A CMM with manual drive and computer-assisted data processing provides somedata processing and computational capability for performing the calculations required toevaluate a given part feature. The types of data processing and computations range fromsimple conversions between U.S. customary units and metric to more complicated geom-etry calculations, such as determining the angle between two planes. The probe is still freefloating to permit the operator to bring it into contact with the desired part surfaces.

A mosor-dnven CMM with computer-assisted data processing uses electric motorsto drive the probe along the machine axes under operator control.A joystick or similar de-vice is used as the means of controlling the motion. Features such as low-power steppingmotors and friction clutches are utilized to reduce the effects of collisions between theprobe and the part. The motor drive can be disengaged to permit the operator to physicallymove the probe as in the manual control method. Motor-driven CMMs are generallyequipped with data processing to accomplish the geometric computations required in fea-ture assessrnent.

A CMM with direct computer control (DCC) operates like a CNC machine tool. Itis motorized. and the movements of the coordinate axes are controlled by a dedicated com-puter under program control. The computer also performs the various data processing andcalculation functions and compiles a record of the measurements made during inspection.As with a CNC machine tool, the DCC CMM requires pari programming.

DCC Programming. There are two principle methods of programming a DCCmeasuring machine: (1) manualleadrhrough and (2) off-line programming. In the manu·al /eadthrough method, the operator leads the CMM probe through the various motionsrequired in the inspection sequence. indicating the points and surfaces that are to be mea-sured and recording these into the control memory, This is similar to the robot program-ming technique of the same name (Section 7.6.1). During regular operation, the CMMcontroller plays back the program to execute the inspection procedure.

Off-line programming is accomplished in the manner of computer-assisted NC partprogrnmming. The program is prepared off-line based on the pan drawing and then down-loaded to the CMM controller for execution. The programming statements for a comput-er-controlled include motion commands, measurement commands, and reportformatting commands.The motion commands arc used to direct the probe to a desired in-spection location. in the same way that a cutting tool is directed in a machining operationFhe measurement statements are used to control the measuring and inspection function>of the machine, calling the various data processing and calculation routines into play. Fi-nally, the formatting statements perrnnthe specification of the output reports to documentthe inspection.

An enhancement of off-line programming is CAD progrumming [2], in which themeasurement cvrle is generatcd from (Computer-Aided Design, Chapter 24) geometric data representing the part rather than from a hard copy part drawing. Off-line programming on a CAD system is facilitated by the Dimensional Mea~'uring Interface

DMTS is a protocol that perrnirs rwo-way communication between CADsystems and CMMs. Use of the DMIS protocol has thefollowing advantages [21: (1) It al-lows any CAD system to communicate with any CMM; (2) it reduces software developmentcosts for CMM and companies because only one translator is required to cornmuni-care with the ])MIS; (3) users have greater choice in selecting among CMM suppliers-and(4) user training requirements are reduced.

23.4.3 Other CMM Software

software is the set of programs and procedures (with supporting documentation) usedto operate the CMM and its associated equipment. In addition to part programming soft-ware used for programming DCC machines, discussed above, other software is also re-quired 10 achieve full funcnonality of a CMM. Indeed, it is software that has enabled theCMM to become the workhorse inspection machine that it is. Additional software can bedivided into the following categories (21: core software other than programming,(2) post-inspection software, and (3) reverse engineering and application-specific software.

consists of theminimum basic programs required for the CMM to function, excluding part programmingsoftware. which applies only to DCC machines. This software is generally applied either be-fore or during the inspection procedure. Core programs normally include the following:

• Probe calibration. This function is required to define the parameters of the probe(such as tip radius, tip positions for a multi-tip probe, and elastic bending coefficientsof the probe) so that coordinate measurements can be automatically compensated forthe probe dimensions when the tip contacts the part surface, avoiding the necessityto perform probe tip calculations as in Example 23.1. Calibration is usually accom-plished causing the probe to contact a cube or sphere of known dimensions.

• Part coordinate system definition. This software permits measurements of the partto be made without requiring a time-consuming part alignment procedure on theCMM worktable. Instead of physically aligning the part to the CMM axes, the mea-surement axes are mathematically aligned relative to the part.

Chap. 23 / Inspection Technologies

• Geometric feature construction. This software addresses the problems associated

rnent.These features include tlatness.squareness, determining the center of a hole orthe axis of a cylinder, and so on. The software integrates the multiple measurementsso that a given geometric feature can be evaluated. Table 23.4 lists a number of the

CMM software. Examples 23.2 and 23.3 illustrate the application of two of the fea-ture evaluation techniques. For increased statistical reliability, is common to mea-sure more than the theoretically minimum number of points needed to assess thefeature and (0 use curve-fitting algorithms (such as least squares) in calculating thebest estimate of the geometric feature's parameters. A review of CMM form-fittingalgorithms is presented in Lin et al.

• Tolerance lUIaly~· is.This software allows measurements taken on the part to be com-pared to the dimensions and tolerances specified on the engineering drawing.

TABLE23.4 Geometric Features Requiring Multiple Point Measurements to Evaluate-c-Subroutlnes forEvaluating These Features Are Commonly Available Among CMM Software

A dimension of a part can be determined by taking the difference between the two surfacesdefining the dimension. The two surfaces can be defined by a point location on eech surface. In twoaxes the distance between two point locations and is given by

l =± V(x, - +'(Y2- (23.3)

In three axes the distance between two point locations [x, and is given by

l "" V{~,- (23.4)

See Example 23.1.

By measuring three points around the surface of a circular hole, the"best-fit" center coordinates (8, b) of the hole and its radius Rcan be computed. Thediameter twice the radius. In the x-ypiane, the coordinate values of the three point locations areused in the following equation for a circle to set up three equations with three unknowns:

(x - + (y -

where x-coorctnare of the hole center, y-coordinate of the hole circle, and radius of thehole circle. Solving the three equations yields the values of a, b, and R. 0 2R. Example 23.2.

This is similar to the preceding problem except that the calculation dealswith an outside surface rather than an internal (hotel eurtece.

By measuring four points on the surface of a sphere, the best-fit centercoordinates (a. c) and the radius (diameter 2R) can be calculated. The coordinate valuesoftha four point locations are used in the following equation for a sphere to set up four equationswith four unknowns:

where a x:coordinate of the sphere. v-coordinate of the sphere, a-coordinate of the sphere.and R radIus of the sphere. Solvlnq the four equations yields the values of a. b. and R.

plane. Based on a minimum of two contact points on the line, the best-fit lineis determiner!. For example, the line might be the edge of a straight surface. The coordinate valuesof the two point locations a(1I used in the following equation for a line to set up two equations withtwo unknowns:

(continued on next page)

Sec. 23.4 Coordinate Measuring Machines 729

TABLE 23.4 Continued

x+Ay+B~O (23.7)

where A is a para~eter Indicating the slope of the lin.e in th.e y-axis direction and B is a co.nstant.indicating the x-axrs Intercept. Solving the two equations yields the values of Aand B, which definesthe line. This form of equation can be converted Into the more familiar conventional equation of astraightlinp.,whichis

y ~ mx b

where slope m '-liA and v-intercept -BfA.

Angle between two lines. Based on the conventional form equations o~the t.wo lines. that is, Eq~23,8), the angle between the two lines relative to the positive x-axrs is given by:

Angle between line 1 andline2 -

where" tan where m, slope of line 1; and 13 tan-'(m,), where m, slope of line 2.

(23.8)

Definition of a plane. Based on 11minimum of three contact points on a plane surface, the best-fit planeis determined. The coordinate values of the three point locations are used in the followingequation for a plane to set up three equations with three unknowns:

x ~ Ay 8z +where A and Bare parerre-ers indicating the slopes of the plane in the y- and z-axis directions, andCis a constant indicating the x-axis intercept, Solving the three equations yields the values of AB,andC,whichdefinestheplane.

Ratness. 8y measuring more than three contact points on a supposedly plane surface, the deviation ofthe surface from a perfect plane can be determined

Angle between two planes. The angle between two planes can be found by defining each of twoplanes using the plane definition method above and calculating the angle between them,

Parallelism between two planes. This is an extension of the previous function. If the angle betweentwo planes 'IS zero, then the planes are parallel. The degree to which the planes deviate fromparallelism can be determined.

Angle 8nd point of intersection between two lines. Given two lines known to intersect (e.g., two edgesof a part that meet in a corner), the point of intersection and the angle between the lines can bedeterminer! based on two points measured for each I'Ine (a total offour points)

EXAMPLE 23.2 Computing a Linear Dimension

The coordinates at the two ends of a certain length dimension of a machined

component have been measured by a CMM. The coordinates of the first end are

(23.47,48.11,0.25), and the coordinates of the opposite end are (73.52.21.70.

60.38). where the units are millimeters. The given coordinates have been cor-

rected for probe radius. Determine the length dimension that would be com-

puted by the CMM software,

Using Eq. (23.4) in Table 23.4, we have

V(23.4i - 73.52:12 + (48.11 - 2l.70? + (0.25 - 6031\)2

(26.41)" +

"" Y250S.0025' + 697.4881+-3615.6169 V6818.1075 "" 82.57mm

Chap. 23 I Inspection Technologies

EXAMPLE 23.3 Determining the Center and Diameter of a Drilled Hole

Three point locations on the surface of a drilled hole have been measured by aCMM the x-y axes. The three coordinates are: (34.41,21.07), (55.19, 30.50),and (50.10, 13.18) mm. The given coordinates have been corrected for proberadius. Determine: (a) coordinates ofthc hole center and (b) hole diameter, asthey would he computed by the CMM software

Solution: To determine the coordinates of the hole center, we must establish three equa-lions patterned after Eq. (23.5) in Table 23.4:

+ R (i)

(55.19 - + (30.50 - R

(50.10 - (13.18 - R

Expanding each of the equations. we have:

1184.0481 - 68.820 + + 443.9449 - 42.14b + b R' (i)

3045.9361 - + + 930.25 - (ii)

2510.01 - 100.2n + 173.7124 - 2fi.36b b' R'

Setting Eq. (i) Eq.

1184.0481 - + a' + 443.9449 - +3045.9361 - 110.38a a' + 930.25 - 61b b (iv)

1627.993 - 3976.1861 - 110.38a -

- 2348.1931 0

2348.1931

[24.5065 -

Now setting Bq. (ii) Eq. (iii):

(iv)

3045.9361 - 11O.38a a2 930.25 61b ~ 62

2510.01 - l00.2a + 173.7124 - + (v)

3976.1861 - 2683.7224 ~

1292.4637 - 0

1292.4637 -

126.9611 - (v)

Substituting Eq. (iv) for

126.9611 - 3.4027(124.5065 -

126.9611 - 423.6645 +6.4983a - 296.7034 45.6586 ~ 45.66

The value of can now be substituted into Eq. (iv):

Sec. 23.4 Coordinate Measuring Machines

h 124.5065 - 2.2036(45.65l'!6)

Now using the values of and b in Eq. (i) to find R (Eqs, and (iii) could alsohe used). we have

(34.41 - 45.(586)' (21.07 - 23.8932f

(-11.2486)" + (-2.8232)' 126.531 7.970 134.501

D 23.20mlll

Post-Inspection Software. Post-inspection software is composed of the set of pro-grams that are applied after the inspection procedure. Such software often adds significantutility and value to the inspection function.Among the programs included in this group arethe following

• Statistital rmalysis. This software is used to carry out any of various statistical analy-ses on the data collected by the CMM. For example. part dimension data can be usedto assess process capability (Section 21.1.2) of the associated manufacturing processor fur suuisucat process control (Sections 21.2 and 2J .sj.two alternative approach-es have been adopted by CMM makers in this area. The first approach is to providesoftware that creates a database of the measurements taken and facilitates exportingof the database to other software packages. What makes this feasible is that the datacollected by a CMM are already coded in digital fonn. This approach permits theuser to select among many statistical analysis packages that are commercially avail-able. The second approach is to include a statistical analysis program among the soft-ware supplied by the CMM builder. This approach is generally quicker and easier, butthe range of analyses available is not as great .

• Graphiral data representation. The purpose of this software i~to display the data col-lectcd during the CMM procedure in a graphical or pictorial way, thus permittingeasier visualization of form errors and other data the user.

Reverse Engineering and Application-Specific Software. Reverse engineeringsoftware is designed to take an existing physical part and construct a computer model ofthe part geometry based on a large number of measurements of its surface by a CMM.This is currently a developing area in CMM and CAD software. The simplest approach isto use the CMM in the manual mode of operation. in which the operator moves the probeby hand and scans the physical part to create a digitized three-dimensional (3-D) surfacemodel. Manual digitization can be quite lime-consuming for complex pan geometries.More autorna.ed methods are being developed, in which the CMM explores the part sur-faces with little or no human intervention to construct the model. The challenge hereis to minimize the exploration time of the CMM, yet capture the details of a complex sur-face contour and avoid collisions that woulrl damage the probe. In this context. it shouldbe mentioned that significant potential exists for using noncontacting probes (such aslasers) in reverse engineering applications.

Application-specific softwarean?/or products and whose applications arc generally limited to specific industries. Sever-al Important examples are

732 Chap. 23 I Inspection Technologies

• Gear checking. These programs are used on a CMM to measure the geometric fea-tures of a gear, such as tooth profile, tooth thickness, pitch, and helix angle.

• Thread checking, These arc used for inspection of cylindrical and conical threads.This specialized software is used to evaluate the accuracy of physical

cams relative to design specifications.• Automobile body checking. This software is designed for CMMs used to measure

sheet metal panels, subassemblies, and complete car bodies in the automotive indus-trv. Unique measurement issues arise in this application that distinguish it from themeasurement of machined parts, These issues include: (1) large sheet metal panels lackrigidity, (2) compound curved surfaces are common, (3) surface definition cannot bedetermined without a great number of measured points.

Abo included in the category of application-specific software are programs to operate ac-cessory equipment associated with the CMM. Examples ofaecessory equipment requiringit~ own application software include: probe changers. rotary worktables used on the CMM,and automatic part loading and unloading devices.

23.4.4 CMM Applications and Benefits

Many of the applications of CMMs have been suggested by our previous discussion of CMMsoftware. The most common applications are off-line inspection and on-line/post-processinspection (Section 22.4.1). Machined components are frequently inspected using CMMs.One common application is to check the first part machined on a numerically controlled ma-chine tool.lf the first part passes inspection, then the remaining parts produced in the batchare assumed to be identical to the first. Gears and automobile bodies are two examples pre-viously mentioned in the context of application-specific software (Section 23.4.3).

Inspection of parts and assemblies on a CMM is generally accomplished using sam-pling techniques. CMMs are sometimes used for 100% inspection if the inspection cycle iscompatible with the production cycle (it often takes less time to produce a part than it doesto inspect it) and the CMM can be dedicated to the process. Whether used for 100% in-spection or sampling inspection, the CMM measurements are frequently used for statisti-cal process control.

Other CMM applications include audit inspection and calibration of gages and fix-tures. Audit inspection refers to the inspection of incoming parts from a vendor to ensurethat the vendor's quality control systems are reliable. This is usually done on a samplingbasis. In effect, this application is the same as post-process inspection. Gage andflxturt cal-ibration involves the measurement of various gages, fixtures, and other inspection and pro-duction tooling to validate their continued use.

One of the factors that makes a CMM so useful is its accuracy and repeatability.ical values of these measures are given in Table 23.5 for a moving bridge CMM. It can beseen that these performance measures degrade as the size of the machine increases.

Coordinate measuring machines arc most appropriate for applications possessingthe following characteristics (summarized in the checklist of Table 23.6 for potential usersto evaluate their inspection operations in terms of CMM suitability):

1 Many inspectors petforming repetitive 1R4JJual inspection operartons. If the inspec-tion function represents a significant labor cost to the plant, then automating the in-spection procedures will reduce labor cost and increase throughput.

Sec. 23.4 Coordinate Measuring Machines 733

TABLE 23.5 Typical Accuracy and Repeatability Measures for Two Different Sizes of

CMM; Data Apply to a Moving Bridge CMM

Measuring range 650mmI25.6in) 900 mm (35.4 in)

600mm (23.6 inl 1200 mm (47.2 inl

500mm 119.7 in) 850 mm (33.5 in)

Accuracy. x 0.004mm(0.00016in) 0,006 mm {o.00024 in)

0.004mm W.00016 in) 0,007 mm (0.00027 in)

0.0035 (0.00014 in) 0,0065 mm 10.00026 in)

Repeatability 0.0035mmlO.00014in) 0.004mm(0,00016in)

Reso'ution 0.0005 mm (0.00002 in) 0.0005 mm(0.00002 in)

TABLE 23.6 Checklist 10 Determine Suitability of CMMs for Potential Applications-TheMore Check Marks in the YES Column. the MorR likRly tTechnology Is Appropriate

(Few or No (Many

1 Many inspectors performing repelitive manualinspection operations,

2. Post-process inspection--------------~-

3. Measurenent of geometric features requiringmultiple contact points,

----------- -------------Multiple inspection are required if parts

are manually inspected-----------

Complex;:>3rtgeometry.

6. High variety of parts to beinspected

7. Repeat orders.

Total check marks in each column.

2. Post-process inspection. CMMs are applicable only to inspection operations per-formed after the manufacturing process.

3. Measurement of geometric features requiring multiple contact points. These kindsof features arc identified in Table 23.4, and available CMM software facilitates eval-uation of these features.

4. Multiple inspection setups art: required parts are manually inspected.spections are generally performed on surface plates using gage blocks, height gages,and similar devices, and a different setup is often required for each measurement.

Chap. 23 / Inspection Technologies

The same group of measurements on the part can usuaily be accomplished in ODesetup on a

5. Complex part geometry. If many measurements are to be made on a complex part,and many contact locations arc required, then the cycle time ofa CMM will besignificantly less than the corresponding time for a manual procedure.

is a programmable machine, ca-pable of dealing with high parts variety.

7. Repeat orders. Using a CMM, once the part program has been prepared for thefirst part.subsequent parts from repeat orders can be inspected using the same program.

When applied in the appropriate parts quantity-parts variety range, the advantagesof using CMMs over manual inspection methods are [17]:

• Reduced inspection cycle time. Because of the automated techniques included in theoperation of a CMM, inspection procedures are speeded and labor productivity isimproved. A DeC CMM is capable of accomplishing many of the measurement taskslisted in Table 23.4 in one-tenth the time or less, compared with manual techniques.Reduced inspection cycle time translates into higher throughput.

• Flexibility. A CMM is a general-purpose machine that can be used to inspect a va-riety of different part configurations with minimal changeover time. In the case of theDeC machine, where progranuning is performed off-line, changeover time on theCMM involves only the physical setup.

• Reduced operator errors. Automating the inspection procedure has the obvious ef-fect of reducing human errors in measurements and setups.

• Great" inherent tU:CUI'tU:y and precision. A CMM is inherently more accurate and pre-cise than the manual surface plate methods that are traditionally used for inspection.

• Avaidance of multiple setups. Traditional inspection techniques often require mul-tiple setups to measure multiple part features and dimensions. In general, all mea-surements can be made in a single setup on a eMM, thereby increasing throughputand measurement accuracy.

The technologyofCMMshas spawned other contact inspection methods. We discuss twoof these extensions in the following Sections.flexible inspection systems and inspectionprobes.

23.4.5 Flexible Inspection Systems

A flexible inspection system (FIS) takes the versatility of the CMM one step further. In con-cept, the FIS is related to a eMM in the way a flexible manufacturing system (FMS) is re-lated to a machining center. Aflexible inspection s,stem is defined as a highly automatedinspection workcell consisting of one or more CMMs and other types of inspection equip-ment plus the parts handling systems needed to move parts into, within, and out of the cell.Robots might be used to accomplish some of the parts-handling tasks in the system. Aswith the FMS, aU of the components of the FIS are computer controlled.

All example of an FIS at Boeing Aerospace Company is reported in Schaffer [19J. Asillustrated in the layout in Figure 23.7, the system consists of two DCC CMMs, a roboticinspection station, an automated storage system, and a storage-and-retrieval cart that in-terconnects the various components of the cell. A staging area for loading and unloadingpallets into and out of the cell is located immediately outside the The CMMs in the cell

Sec. 23.4 Coordinate Measuring Machines 735

Figure 23.7 Layout plan of flexible inspection system (FlS).

perform dimensional inspection based on programs prepared off-line. The robotic stationis equipped with an ultrasonic inspection probe to check skin thickness of hollow wing sec-tions for Boeing's aerospace products.

23.4.6 Inspection Probes on Machine Tools

In recent years there has been a significant growth in the usc of tactile probes as on-lineinspection systems in machine tool applications. These probes are mounted m toolholders..inserted into the machine tool spindle, stored in the tool drum. and handled by the auto-matic tool changer in the same way that cutting tools are handled. When mounted in thespindle, the machine tool is controlled very much like a CMM. Sensors in the probe de-termine when contact has been made with the part surface. Signals from the sensor aretransmitted by anv of several means (e.g .. direct electrical connection, induction-coil, in-frared data transmission) to the controller that performs the required data processing tointerpret and utilize the signal.

Touch-sensitive probes are sometimes referred to as in-process inspection devices,but by our definitions they are on-Iine/post-process devices (Section 22.4.1) because theyarc employed immediately following the rnaehming operation rather than during cutting.However, these probes are sometimes used between machining steps in the same setup; forexample, to establish a datum reference either before or after initial machining so that sub-sequent cuts can be accomplished with greater accuracy. Some of the other calculation fea-tures or machine-mounted inspection probes are similar to the capabilities of CMMs withcomputer-assisted data processing. The features include: determining the centerline of acylindrical part or a hole and determining the coordinates of an inside or outside corner.

One of the controversial aspects of machine-mounted inspection probes is that thesame machine tool making the part is also perfonning the inspection. The argument againstthis is that cenain errors inherent in the cutting operation will also be manifested in the rnea.suring operation. For example, if there is misalignment between the machine tool axes,thus producing out-of-square parts, this condition will not be identified by the machine-mounted probe because the movement of the probe is affected by the same axis misalign-ment. To generalize, errors that are common to both the productiuu prUCI;:SSand themeasurement procedure will go undetected by a machine-mounted inspection probe. Theseerrors include [2]: machine tool geometry errors (such as the axis misalignment problem

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736 Chap. 23 Inspection Technologies

identified above), thermal distortions in the machine tool axes, and errors in any thermalcorrection procedures applied to the machine tool. Errors that are not common to both sys-tems should be detectable by the measurement probe. These measurable errors includetool and/or toolholder deflection, workpart deflection, tool offset errors, and effects of tool

off-line inspection operations.

23.5 SURFACE MEASURfMENT

The measurement and inspection technologic, discussed in Sections 23.3 and 23.4 are con-cerned with evaluating dimensions and related characteristics of a part or product. An-other measurable attribute of a part or product is its surface. The measurement of surfacesis usually accomplished by instruments that use a contacting stylus. Hence, surface metrol-ogy is most appropriately included within the scope of contact inspection technologies.

23.5.1 Stylus Instruments

Stylus-type instruments are commercially available to measure surface roughness. In theseelectronic devices, a cone-shaped diamond stylus with point radius of about 0.005 mm(0.0002 in) and 90~ lip angle is traversed across the test surface at a constant slow speed.The operation is depicted in Figure 23.8. As the stylus head moves horizontally, it alsomoves vertically to follow the surface deviations. The vertical movements are converted intoan electronic signal that represents the topography of the surface along the path taken bythe stylus. This can be displayed as either: (1) a profile of the surface or (2) an averageroughness value.

use a separate flat plane as the nominal reference against whichdeviations are measured. The output is a pIal of the surface contour along the line tra-versed by the stylus. This type of system can identify roughness, waviness, and other mea-sures of the test surface. By traversing successive lines parallel and closely spaced witheach other, a "topographical map" of the surface can be created.

reduce the vertical deviations to a single value of surface rough-ness. As illustrated in Figure 23.9, surface roughness is defined as the average of the verti-cal deviations from the nominal surface over a specified surface length. An arithmeticaverage (AA) is generally used, based on the absolute values of the deviations. In equa-tion form,

R

where arithmetic mean value of roughness (m, in); vertical deviation from thenominal surface converted to absolute value (In, in);and sampling distance, called thealtoff length,over which the surface deviations are averaged. The distance L", in Figure 23.9is the total measurement distance that is traced by the stylus. A stylus-type averaging de-vice performs Eq. 11) electronically. To establish the nominal reference plane, the de-

'Portions of this section are based on Groover [lOJ,Seclion 5.2 and 41.4 .1.