correlation of machine tool and cmm c.sims,' … of machine tool and cmm ... there are...

Correlation of machine tool and CMM

accuracy and precision

C.Sims,'" AD.Hope,'" G.T.Smith/" M.Guir

"' Systems Engineering Faculty, Southampton Institute, U.K.

Abstract

This paper seeks to explore the practical possibilities of developing a mechanicalartifact that can be used to verify the accuracy and precision of a CNC machinetool. It is acknowledged that regular interim checks of machine tools ensure ahigh quality assurance within the manufacturing loop. The development of thisartifact is biased towards volumetric and geometric calibration using stereometryas the mathematical foundation, and can be used on both vertical and horizontalmachine tools.

Introduction

It was in the early part of the 20th century that the work in establishingacceptance standards for machine tools first began. Schlesinger [1] whocommenced this work published a comprehensive set of acceptance testspecifications in 1927. It was this work that became the foundation of futureadvancement throughout the industrial world. These acceptance criteria wereoriginally for manually operated machines where the individual skills of theoperator were paramount in setting up a machine for production purposes. Thearrival of the NC machine tool meant that new functional tests had to beestablished, and this entailed upgrading the statutory acceptance tests withmodern technology. The NC machine tool was inherently more accurate than itspredecessor and was able to perform more complex machining operations.The process for testing machine tool accuracy has quickly developed over thelast two decades, especially in conjunction with 'state of the art' technology, thebest known in use being the laser interferometer and the ballbar.Calibration of a machine tool is aimed at measuring all the attributes that canaffect the machine tool's overall accuracy [2] and in turn enables the machineuser to quantify the overall accuracy and capability of the machine tool.There are three scenarios whereby calibration is carried out. They are:-

Transactions on Engineering Sciences vol 23, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533

390 Laser Metrology and Machine Performance

4 measuring the various elements of error individually usinginstrumentation in conjunction with artifacts to obtain a geometricevaluation within the working volume of the machine

+ machining tests which can be either a recognised test, or a bespoketestpiece which is solely attributed to a customer's design

+ a combination of both

The purpose of this paper is to present the current research work beingcarried out in this area at Southampton Institute.One of the objectives of this project is to try and determine the machinetool's optimum performance, by tracing and eliminating if possible thecauses of any uncertainties. Following on, the final objective is to confirmthis performance output by designing and machining an artifact that can becalibrated on a CMM, and ultimately decide whether a CMM is required inthe manufacturing process. All this work has principally been involved withcalibrating a machine tool in an environment similar to that expected in amedium size manufacturing plant. Ambient temperature, humidity and airpressure are relatively uncontrolled when machining is taking place, andlikewise when calibration data is being obtained from the machine tool orCMM.In view of these possible sources of error and in addition to the internalspecific errors of the machines themselves, it was considered prudent tocalibrate the machine tool under different environmental conditions such asambient temperature variation.

Machining tests

Historically the traditional method of establishing the accuracy capabilities ofa machine tool was to machine a desired test piece, whether it was a standardpart or a customer chosen design, inspect it for its dimensional accuracy anddetermine the overall accuracy of the machine from the derived data. Inaddition surface finish quality could be judged.Knapp [3] states that there are several advantages in using test-pieces asinterim checks. They are as follows:-$ the machining and programming correspond to the everyday use of the

machine+ the operator of the machine tool carries out the test, and subsequently is

allied to that test$ there are no requirements for additional equipment for the machine tool4 the calibration of the test piece is carried out on external equipment i.e. a

CMM and therefore the interruption in the production line is minimisedHowever, there are limitations with this route, and it is probable that the


Laser Metrology and Machine Performance 391

machine tool accuracy may be concealed by other error sources. These canbe identified through inspection accuracy, process errors, the inability touncover individual sources of error and the accuracy data obtained cannotnecessarily be attributed to other part geometries.Therefore to this extent it is not recommended to solely rely on a machiningtest to determine overall accuracy, especially if contouring accuracy iscritical.The accuracy of the test-piece can be affected in other ways which are notdirectly attributable to the machine tool. Some of these external variables arecaused by material properties, temperature changes in the workplace, coolantflow and tool wear. In view of this ASME BS.54-1992 [4] recommends thatmachine positioning accuracies are tested by the Standard's procedures e.g.laser interferometer. Test work pieces have the advantage that they may beused to correlate thermal growth, repeatability, productivity and machinestability.When the test piece has been machined it is likely that three processes will beintroduced to measure its accuracy:-f CMM inspection4 roundness measuring machine4 surface finish measuring machineBy its very nature this is a time consuming operation, and is susceptible toadditional measurement errors introduced through the measuring apparatusand operator technique.Machining tests are divided into either standard tests which have beenderived from industry and institutions, or a 'bespoke' test which is usually areplica of the production line workpiece. Typically standard test pieces areNAS 979 [5], BAS [6] and a Hemisphere [7].

Methodology

The following calibration methods were used to try and obtain the machinetool's characteristics during environmental changes, and supplementingthese criteria with varying positional work spaces on the machine. Thisinvolves calibrating the machine tool under no-load conditions as well asinspecting the machine's cutting accuracy using CMMs and roundnessmeasuring machines.All the calibration tests were carried out on a Sabre 500 Vertical MachiningCentre with FANUC controller in the AMT workshops at the SouthamptonInstitute. The machine tool is situated in the workshop with no airtemperature control, other than general heating facilities. A temperaturemonitoring system with five probes and a chart recorder was used to track the



various temperature deviations around the machine tool [8]. The methods ofcalibration undertaken were as follows:-+ Renishaw ballbar4 Renishaw laser interferometer4 machining test pieces with analysis performed on an Eastman CMM and

Talyrond measuring machine

Renishaw ballbar

The machine tool was warmed up one hour before calibration tests began.The temperature monitoring probes were positioned on the machine, andexcept for the ambient temperature probe, the sensors were enclosed with aninsulation cover to minimise outside influences. The complete operation wasperformed over four hours.The machine tool's table travel dictated the size of ballbar and extension armthat could be used which was 150mm overall length. Contouring tests wereperformed in the 'XY' plane, with the first test being at 65mm above theworktable. Further contouring tests were performed at pre-set heights fromthe worktable beginning at 100mm through to 250mm in 50mm increments.Surface plates and cubes were used to achieve the desired height formounting the ballbar's magnetic base.Temperature monitoring was operational throughout the calibration processwith continual data logging. A print out of the temperature progress wastaken continually so as each ballbar test could be allied to the prevailingthermal conditions.The ballbar was calibrated at the commencement of tests and handling of thebar was limited in order to minimise and thermal errors caused by theseactions.The ballbar was programmed to perform a bi-directional trace at twofeedrates of 250mm/min and 750mm/minAs mentioned in the introduction the artefacts for checking interim machinetool's performance are biased to the 'XY' plane, and the philosophy behindthis research is to examine a larger volume relative to the vertical axis. Inother words if the potential to use the machine tool at higher distances abovethe worktable is available, then it follows that accuracy and precision at thosepositions is a natural progression for calibration.

Ballbar results

It was found that over the four hours the ballbar tests took place there was a



3 deg C rise in the ambient temperature. This increase in temperature wasreflected by the other probes. The traces obtained at the four horizontal planeheights showed that they all followed a common pattern. A trend analysiswhich superimposes each trace on each other verified this observation (seefigures land 2). The circularity values are similar but they represent thesummarisation of all the errors. The scale mismatch which indicates theerror in the linear movements of the axes X and Y does not deviatesignificantly at

Circularity ranges between13.5pm and 19.8pm

: Z-O H"*^diwFigure 1 : Ballbar trace at 250 mm\min at four preset heights

the different heights. This is graphically represented by the elliptical shapeof the trace. Also showing on the profile are axis spikes, these are caused bya form of hysteresis at the reversal points. One of the sources of hysteresis isfrom backlash on the drive and position feedback system. The diagnosticschart pointed to backlash in both 'X' and ' Y' axes. These did not alter to anyextent with either feedrate or height positioning from the worktable surface.

— Circularity ranges between- 16.8pm and 18.3pm~~ Settle: Z.O I'm/d i«

Figure 2: Ballbar trace at 750mm\min at four preset heights



Laser interferometer test results

A complete calibration test was performed on the Sabre 500, which consistedof positioning, angular (pitch & yaw), straightness and squareness in all threeaxes. The data recorded from these tests were used as a foundation forcomparison against all other methods of testing. For practical purposes andbecause of time constraints one axis was chosen to observe any irregularitiesand trends. The 'Y' axis was used for all tests with continual temperaturemonitoring. The laser interferometer was set up together with theenvironmental station with full signal strength obtained over the full run ofthe machine. The temperature monitoring commenced twenty-four hoursbefore calibration took place. The data recorder indicated a common trendon all the probes as the thermal conditions altered.The machine was started up and allowed to warm up for one hour bytraversing the 'Y' axis continually. Following on the 'Y' axis displacementwas calibrated every thirty minutes giving a total of nine sets of five runsbefore the process was terminated. The feedrate was programmed to250mm/min, and as time progressed the ambient temperature rose withcorresponding rises from the other sensors. Over the period of the machineoperating the ambient temperature rose by 3 °C, and the overall accuracyincreased by 5pm as seen in figure 3. The trend analysis is shown in figure 4.

21.95 222 22.42 22.68 22.85 2Z91 23.08 23.09Temperature deg C

-+-Accuracy

Figure 3: Accuracy variation against temperature over 5 hours



TREND ANALYSIS Linear thermal

Target (mill 1

Figure 4: Trend analysis with temperature variation over 5 hours

Positioning test with enclosure

The previous test was repeated with an 'enclosure' over the machine tominimise air turbulence. The temperature monitoring system indicated asteady rise over the six hour period of the ambient temperature, but thesensors within the enclosure displayed a shallower gradient. During thistesting process a set of five runs were performed every 30 minutes, giving atotal of nine individual sets before closing down the machine.

22.01

Figure 5: Accuracy variation against temperature within enclosure



The main observation from the data was that the overall accuracy remainedapproximately constant throughout ( see figure 5). The 'trend analysis'demonstrates this graphically (see figure 6).

TREND ANALYSIS Linear encl.temp.mem

Cm!11Inctr

Figure 6: Trend analysis with enclosure over 5 hours

150 200Hetgtt millimetres

~»-Accuracy

34

32

30 O

•f* i24 H

22

Figure 7: Accuracy variation with height and temperature change over atunescale of 5 hours


Laser Metrology and Machine Performance

Positioning tests at preset heights

397

This set of tests were performed without an enclosure but again withtemperature monitoring. While the tests were in progress, a steeptemperature gradient was taking place caused by a 6 deg C ambient rise.This again was reflected in the other sensors. The temperature indicated infigure 7 is the temperature variation within the machine's workspace. Thetime period was five hours. Four preset heights were chosen to perform thelaser displacement tests starting at 100mm from the worktable rising in50mm increments until a maximum height of 250mm was obtained. Insimilar mode five runs were performed at each height, and the accuracy wasshown to increase in value as the height increased. It is probable that thesevalues were obtained in part to the wide temperature span that was takingplace in the workshop. A trend analysis is shown in figure 8.

TREND ANALYSIS Linear var.ht.

Figure 8: Trend analysis at preset heights over a 5 hour timescale

Cutting tests

A series of tests involved the light machining of a set of 300mm diameteraluminium disks at the preset heights used for calibration [9]. A simplerobust jig was constructed which enabled each disk to be held firmly readyfor cutting. Each disk had been 'roughcut' to allow for the end mill cutter tobe lightly applied for machining by circular interpolation. Two feedrates of250mm/min and 750mm/min were used for both clockwise and counter-clockwise directions. The top of the cutter was used in order to reduce any



side forces.The disks were inspected for roundness on a CMM and a roundnessmeasuring machine. It was found that the surface finish of the disk'scircumference contributed to the mediocre circularity error obtained on bothinspection machines. However the roundness comparison between the twomodes of inspection showed a similar trend. The data obtained gave a broadnumerical span, this can be attributed to spurious points on the disk causedby the 'finish' of the cut, the height off the worktable and rotational direction.It was also observed that the machining performed in a clockwise directionneeded very little or no filtering, yet the majority of counter clockwisemotion plots had to be filtered. Even so, the 'spikes' at the point of transitionshowed up clearly. These 'reversal spikes' can contribute to the roundnesserror, and it can be seen from the plots that there are some large spikes. Thespikes shown between the point of axis change-over can be associated withthe finish of the machining process, and possibly with the type of stylus usedon the roundness testing machine. The stylus used was a 'chisel' type, and itis suspected that some scoring may have taken place on the aluminiumsurface. Similar patterns are found from probing eleven points around thedisk on the CMM. Another factor which could contribute to the roundnessfigures is temperature variation which is graphically illustrated from theperiod when machining the disks. As the temperature in the workshopincreases, there is a similar but erratic increase within the working volume ofthe machine tool. Over the period of two hours machining, although notcontinuous, the minimum and maximum temperature above the worktablewas 18°Cand23°C

Concluding remarks and future work

Research at this stage of the experimental work has indicated that machinetools outside a relatively controlled environment are heavily subjected totemperature changes [10]. This was not a new concept but a parameter thathas to be included if the detection and minimising of errors are to beobtained. In this work the temperature monitoring system graphicallyillustrated the wide temperature spectrum that occurs in the machine shop,and in particular the laser calibration data when compared with the samemachine within an enclosure. Although the tests have been initially based onone axis i.e. 'Y', it is probable that similar variations would be found in the'X'and'Z'axes as well.The calibration tests were also repeated in the linear positioning mode at fourdifferent heights above the worktable. The results in this work indicated agrowth in positioning error as the height increased above the worktable. This



positioning error has partly been attributed to a squareness error in the Y-Zplane and a sharp temperature increase within the confines of the machine.Similarly the ballbar was used to calibrate the machine in the X-Y plane atthe same four pre-selected heights. Two feedrates were used namely;250mm/min and 750 mm/min and it was observed that the higher feedrateinduced an increased roundness error. The trend patterns between the fourgraphical plots were superimposed upon each other when overlapped, and thetrend betweenThe height versus roundness and height versus squareness were again veryuniform in their appearance.The machining of sixteen aluminium disks of 300mm diameter gave theopportunity to compare the plot traces between the ballbar, the Talyrond andthe CMM. Again , errors became apparent when measuring the disks mainlybecause of the finish of the 'cut'. Likewise, only eleven points were probedon the disk by the CMM and this could have given rise to some inaccuraciesbecause of the surface finish. It has been suggested that thirty-three points bechecked on a disk so as to obtain a better trace. Inspection showed that byignoring a few spurious points there is a similarity in the trace between thethree calibration techniques. Due to the unloaded condition of the machinewhen a ballbar is being used the plot gives a very accurate condition of themachine. However it has been shown that other calibration methods mightimport other criteria i.e. cutting forces, loads. All the tests have beenundertaken within similar temperature ranges which gives a certain amountof credence when comparisons are made between the differing methods ofcalibration.The main thrust in the final part of the experimental work is to design andmanufacture a prototype artifact. Its purpose is that the artifact can be simplymachined to confirm the machine tool's accuracy and precision via theinterpolation of volumetric calibration and ultimately exclude the CMM fromthe machining process.The proposed artifact after machining will be allied solely to that machine,and when the artifact is dismantled will in effect lose all calibrationtraceability. As the artifact proportions will be constant for each timecalibration confirmation is required, a standard equation can be used tocompare the volume of the artifact against the CMM's volumetricinterpolation. Similarly, a roundness measuring machine may be used toconfirm the circularity of the artifact as a third choice of calibration. Anotheradvantage of this preliminary design is that the artifact can be used to verifyany other machine tool as well. A first draft for the artifact design is shown infigures 9 and 10. It is proposed that the diameter of the disks will be thesame as the diameter of the ballbar's circular travel. A straight comparison



between the two circular interpolations can then be obtained. Once themachine artifact has been manufactured there are various experimentalpossibilities that can be tried. For example, machining the disks on themachine tool after different warm-up times over a period of twenty fourhours and evaluating the dimensional repeatability, thermal changes andpositional deviation.

References

1. Schlesinger G. Testing Machine Tools 8th Edition, Revised byKoenigsberger F. and Burdekin M. Pergamon Press, 1978.

2. Wills-Moren W.J. 'An Introduction to Machine Calibration' from a shortcourse 'Precision Engineering The State-of-the-Art'. College ofManufacturing, Cranfield Institute of Technology.

3. W.Knapp.'Interim Checks for Machine Tools', Laser Metrology andMachine Performance IILLamdamap '97. pp!61-168.

4. ASME B5.54-1992. Methods for Performance Evaluation of ComputerNumerically Controlled Machining Centres.

5. National Aerospace Standard (NAS) 979, National Standards AssociationInc., 1321 Fourteenth Street,N.W., Washington,DC 20005. 1969.

6. Week M. Handbook of Machine Tools. Vol 4. Metrological Analysis andPerformance Tests. 1984.

7. Gull M.'Contouring Accuracy Assessment of CNC Machine Tools-auser's view'. Laser Metrology and Machine Performance HI. Lamdamap'97. pp401-412

8. Rudder F.F. 'Characterisation of the Vertical Machining Centre', ProgressReport of the 'Quality in Automation' Project for FY90. Edited byM.A.Donmez, U.S.Dept. of Commerce, National Institute of Standards andTechnology, March 1991, pp5-23.

9.Smith G.T.,Hope A.D., and Painter P.'Performance Evaluation of aMachining Centre using Laser Interferometer and Artefact_basedTechniques', Proceedings of F AIM, CRC Press Inc, pp962-974, 1992.

10. McClure R. 'The Significance of Thermal Effects in Manufacturing andMetrology', CIRP Annals , Vol.15, 1967.



BaJlbar set to; 150mm

(Aluminium)

Precision mandrel(Hardened & groundSteel) - with protectedcentres

Disk 2 (holes)

Disk 1 (holes)

Disk 3 (holes)

1mm stock - to bemachined off by circularinterploation

Figure 9. Machining & turning centre artifact-for volumetric calibration



Solid content (I* C)

Figure 10. Stereometry of A; conic frustum,cylinder & rectangle


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