performance assessment of machine tools and coordinate measuring machines some recent ... ·...

Performance assessment of machine tools and coordinate measuring machines - some recent developments at UMIST

M . Burdekin Department of Mechanical, Aerospace and Manl<acturing Engineering, UMIST, Manchester, UK

Abstract

The paper presents an overview of a number of research projects undertaken at UMIST and relating to the calibration and verification of Machine Tools and CMMs. Applications of a laser interferometer under static and dynamic conditions are featured together with ball-link bars and other artefacts.

1 Introduction

The Department of Mechanical Engineering at UMIST has been involved with the teaching and research in engineering metrology and machine tool performance for nearly 120 years. The concept of a Metrology Club was created in the 1940s by the late Professor H. Wright-Baker, who, with regular subscriptions from local industry, refurbished a metrology laboratory which included equipment such as a Talysurf 1 (circa 1942) and a Talyrond 1 (circa 1951).

Developments in machine performance and accuracy, commenced with the appointment, in 1960, of Professor Franz Koenigsberger to the first UK Chair in Machine Tool Engineering. Being an ex-research student, as well as the Son-in- Law of the famous G. Schlesinger, the classic book on "Testing Machme Tools1'

Transactions on Engineering Sciences vol 34, © 2001 WIT Press, www.witpress.com, ISSN 1743-3533

was used for teaching the fundamentals of machine tool alignments to engineering students. The 8" and final edition of this book [l] was updated in the 1970s along with a complementary book [ 21 on Machine Tool Structures..

A significant impetus in developments on Performance Assessment of Machine tools was provided by a project funded by the then Ministry of Technology on the Specification and Tests of Metal Cutting Machine Tools. This covered aspects of accuracy, thermal errors, vibrations, noise, reliability and wear. This state of the art review was disseminated into the public domain by a Conference [3] held at UMIST in 1970. Much of this is still relevant today after more than 30 years. A sequel to thls work was produced in the USA in 1980 [4] with some contributions from UMIST.

Research in performance assessment has continued steadily over the past 30 years, with 25 or so students from the department graduating in this area with a Ph.D; many of whom are now in senior positions around the world. The emphasis of much of this research has been on the development of practical and cost effective solutions. This research was initially facilitated by the acquisition of a HP laser interferometer in 1972, together with a Ferranti CMM, and various types of NC machines and computers. This research theme continued over the years with more recent developments being reviewed in the following sections.

2 Dynamic Metrology

2.1 CMM Machines

There are practical measurements where the data has to be sampled whilst the machine is in motion, otherwise the results do not represent the true operating conditions of the equipment. Calibration of Coordinate Measuring Machines is one such application where the machine axes are generally not stationary during the measurement of a component. Therefore, if a laser interferometer is used to determine the parametric errors of each axis, this should be applied whilst the axis is in motion ie not stationary. Linear displacement measurements are the most critical type of the so called "on the fly" calibration as this requires simultaneous sampling of both the laser position and the CMM scale position. The original development of this "on-the fly" type of calibration of CMMs [5] was applied to a Ferranti manual CMM whose PDP8 mini computer running machine code, sampled sequentially the CMM display and the BCD output of the HP5526 laser system. For the wavelength output, 2000 samplesls could be achieved enabling the resulting calibration data to be interpreted in terms of the usual progressive, periodic and hysteresis components as well as the random errors. With the replacement IEEE interfacing on the 5528 system, the sampling


rate was only 401s. However, with the recent PC based HP5529 laser system, it has now been possible to expand this sampling rate to tens of kHz.

To make thls calibration system portable, HP9825 and 9826 computers, operating under HPL Basic computer language and a BCD interface, were adopted. As these computer systems were too slow for sequential reading of the laser and CMM displays, a hardware freeze and trigger command had to be generated by building an external interface known as CALPAC. [6],[7],[8]. With these freeze and trigger commands now being virtually instantaneous, realistic on-the fly measurements could be achieved for linear positioning as well as angular, straightness and squareness. Examples of data acquisition for in-plane and volumetric components on cantilever and moving bridge type CMMs are shown in [6],[7], [8].

Although this system was initially developed more than 20 years ago, it was discontinued because the replacement HP5528 laser system was incompatible with the former development of on-the fly sampling. Recent work using the 10887 PC calibrator board, which accepts the laser input, trigger pulses as well as A quad B waveforms from scales, it is now practical to undertake on-the fly measurements at 30k samples per sec. A typical set-up and results for an axis of a moving bridge CNC version of a CMM is shown in Figla and lb.

Figure la: Set-up for on the fly measurement on CMM


Figure lb: Results from a CMM axis measurement

2.2 Kinematic Accuracy of lathes

When lathes and thread grinding machines are operating in the screw-cutting mode, the relationship between the spindle rotation and linear motion of the tool is critical for generating thread forms with accurate pitches, free from periodic errors.

To undertake such an assessment with an Agilent laser interferometer, the axial displacement of the carriage is measured by mounting the linear interferometer onto the moving carriage with the retroreflector located on the axis of the rotating spindle. With some interferometer systems, alternative optical configurations may have to be used as the interferometer cannot be located on the moving element. During the simulated screwcutting mode, the laser interferometer must be sampled on-the-fly at known angular positions of the rotating spindle. To achieve this, accurate trigger pulses must be generated from the rotating spindle. Unfortunately the built-in encoder on the machine cannot be used for this purpose as this forms part of the system being investigate. An auxiliary-mounted encoder was used by [9] as an angular pulse generator. However, with this there is always uncertainty as eccentricity in the mounting can result in periodically spaced pulses being generated. To overcome the potential problem of triggering the laser from an encoder, a rotating polygon was used as the basis of a pulse generator [10]. This had the advantage of being


easily mounted on the machine and also ~nsensitive to eccentricity in mounting. Recent development of this system has incorporated a laser diode pulse generator and the 5529 laser. The outline of the system is shown in Fig. 2.

Polygon Cube Corner L~near Interferometer

Laser Head ,'

l Laser Diode

System

Pulse l Generator

Figure 2: Set-up for screw cutting calibration on lathes

2.3 Spindle Rotation Accuracy

Techniques for spindle rotation accuracy are well established usmg non- contacting displacement transducers against a rotating master ball. For assessments where the tool position is fixed and the workpiece rotates. as in the case of lathes and internal grinding operations, only one displacement transducer is necessary in order to determine the radial error motion.

The use of a laser interferometer was investigated using a focused beam onto the surface of the rotating sphere. Using a lens of l00 mm focal length. displacements up to 100 pm could be detected around the focal point. The basic principle is shown in Fig 3 with a typical result for a toolroom lathe shown in Fig 4. Carbide balls of grade 5 were used having a roundness value in the order of 0.1 pm. For high precision spindles, it is possible to use the ball error map to


reduce uncertainties. The system can also be used to investigate radial thermal drift.

Linear Interferometer

Masterball

Lens

Beam Incldent

Lens.= Beam Reflected

Figure 3: Linear interferometer operating against rotating sphere

. I Error nation

tlachine : OSB-19101

Tesr Data : 8/4/1999

No. of Rev. : 5

Spindle Speed : 235.99 rpm

Bal l Dianoter : 12.000 mm

Error Band (pm) : 4.26

Eccen t r i c i t y (pm) : X = 3.2318 Y = 1.4810

Rcss (Fl+F3) 10 Escape a Plot

PT-3 F3 t o Plot

<E%> EXIT F2 PRINT

Figure 4: Example of radial error motion on lathe spindle


3 Ball bars

Contouring assessment of NC machine tools using the NAS 979 cutting test piece was investigated [3 ],[l l ] together with master part trace tests. Kinematic ball-link bars were developed in the 1980s [l31 which comprised two carbide reference spheres mounted on the table and spindle. A kinematic link supporting two LVDTs, was kinematically located between the two spheres. Each LVDT contacted directly onto the respective ball and operating differential such that the output represented distance between the balls as the machine performed a circular contouring motion. The absolute length of the link-bar was set against a calibrated setting fixture, giving traceable measurements. Potentially the system was very accurate as the measurements were taken directly from the ball surface that could be error mapped when uncertainties better than the ball sphericity of 0.1 pm was required.

Figure 5: An early kinematic link on a diamond turning machine


Figure 6: Digital kinematic ball-link on CNC machining centre

Various designs were evaluated using non-contact probes and with invar and carbon fibre links for testing nano-accuracy turning centres etc. For most industrial rnachnes subrnicron accuracy is adequate and current link designs are based upon a single LVDT with digital output having a resolution of 0.1 pm over a range of 2 mm. This link is held between the balls by a spring force withn the LVDT. The commercial version of this system is known as "Contisure" [l21 and marketed by CD Measurements.. Fig. 5 shows an early design of a link in use on a diamond turning lathe. Fig 6. shows a 100 mm digital version.

One of the additional features of the instrumented circle test is the possibility of diagnosing the geometric errors. In addition to the error band and least squares circle as shown in Fig 7 error features such as axis reversal, can be clearly seen in the X axis. Other features such as squareness, servo mismatch, straightness etc can be evaluated by decomposing the circle data [14]. Further examples and case studies are reported in [l 5][16][17]


CONTISURE : LEAST SQUARES CENTRE PLOT FL02A. XY 1

Identity : FL02-XY , +y Anticlock Lsq. Radius

Machine : Flow Europe

Serial Ho.: 111-105 Test Date : Feedrate : 1000 mm/min

No. Scans : 1 Anticlockwise Qngles. Stt : 22 End : 382

Results at 20 deg. C

Eccentricity :-

X = 139.8 Y = -36.5um

Nom. Rad. : 100.0000 mm

L. Sqr. Rad. : 99.9638 mm

Error Band : 216.9 urn

Min. Rad. : 99.8192 mm (-144.6 um)

Max. Rad. : 100.0361 mm ( t72.3 um) Microns x 10 -15105 0 5 +

Figure 7; Example of circle plot from water jet cutter

3.1 Direct volumetric calibration

Research has recently been completed on the direct measurement of volumetric accuracy using various calibrated lengths of kinematic links. ie. the basic 100 mm link shown in Fig. 6 plus carbon fibre extensions. Using four reference balls, positioned on the table of the machme, the xyz coordinates of the spindle are determined by trilateration using calibrated lmk lengths. This is a relatively quick and low cost procedure and has the added advantage of providing diagnostics such as scale error; squareness., roll, pitch, yaw. The influence of thermal errors over the working volume due to spindle rotation and axis motions can also be investigated using this technique.

3.2 Servo feed-drive characteristics

The feed rates used on some machines, including laser and water-jet cutters, can have a significant influence on the component accuracy depending upon the


characteristics of the servo feed drive. Machmes are expected to cut small circles and squares at high feed rates and consequently using the standard 100 mm kinematic link, realistic conditions such as high angular frequencies cannot be achieved.

To overcome the limitations of using a standard kinematic link length for what is essentially a closed-loop frequency response test on the axes, a system using two orthogonally mounted linear transducers has been devised. This system enables the actual X-y displacement of the machine to be measured whilst it is performing either circle or square motions .The stystem shown in Fig. 8 incorporated used two Heidenhain digital linear transducers, orthogonally mounted and acting against a precision cube, located in the machine spindle. To measure the yz and xz planes, the digital linear transducers were orientated in the corresponding axes using a common mounting fixture. If three transducers are available, all three planes can be measured at one setting. The system was used for both small circle and square paths and detailed results reported in [18].

To avoid using hardware which is incompatible with the standard kinematic link, for small circle and square testing, a system incorporating three reference spheres and two standard kinematic links was developed and is shown in Fig. 9. The performance is similar to the cube system but of lower cost and with the added advantage that it utilises the standard hnematic link. Because both X & y coordinates are sampled with time, the changes in instantaneous feed velocity can be assessed during the motion. Both circle and square tests have been developed together with Windows based software.

Figure 8: Small circle test using precision cube


Figure 9: Small circle testing using two standard kinematic links

3.3 Robot performance

Because the robots accuracy is relatively large compared with machine tools, performance tests were undertaken using a kinematic link having a range of 10 mm. These relatively large errors usually result from an incorrect geometric robot model used within the controller. Using the results from the circle test this model can be modified until the errors are minimised. Reversal errors in the joints are highlighted in a similar manner to a two orthogonal axis test but in the case of the robot these reversal points do not take place at the 90 degree points.

4 Coordinate measuring machines

Developments in accuracy assessment of CMMs have continued since the 1970s. The research has centred around measurement of parametric errors using computer aided on-the-fly laser techniques, combined with volumenic error mapping [5][6][7][8]. Compensation of volumetric errors using a hybrid hardware and software system was also developed to improve accuracy [19].

An interesting approach using the CMM base as a metrology reference was also considered [20]. With granite bed machines the base area supporting the component does not have to be very flat for functional purposes but nevertheless this is usually in the order of grade 0 or 1. Calibrating the flamess of this base using an electronic level with a rectangular grid [21], provides an in-situ fundamental metrology reference which can be used to establish the geometric errors. For example, the calibrated plane can be used as a reference artefact in


terms of its flatness, and probed by the machine. The locus of the probe [22] obtained from the difference between the calibrated flatness and the probing data represents a probing reference plane of the machine. This data can also be interpreted in terms of the effective parametric components such as straightness in x and y and the roll motion if the axis. Other effective parametric components such as squareness, pitch and roll about a vertical axis can also be measured by placing suitable artefacts on the calibrated grid points of the base reference plane.

4.1 Virtual CMMs

The development of virtual CMMs is an interesting concept [ 231. It is bases upon mathematical models which represent a range of practical geometries and configurations and enables the errors to be determined anywhere in the working volume. This is a very practical mathematical tool which can be applied to different perfect artefacts such as cylinders, spheres etc to produce results representing actual measurements on these objects. Current research in this area has developed interactive volumetric analysis software based upon either actual machine calibration data or synthetic parametric data. The virtual CMM approach can also be used to assess whether a particular CMM is capable of measuring any component to the specified tolerance. This is of course fundamental to any measurement process but cannot be specified at present for CMMs.

4.2 Calibration & verification

The emphasis with current research is on the development of rapid calibration and verification techniques using calibrated mechanical artefacts together with computer aids. The use of hole plates [24], ball bars etc. have been investigated and shown to be remarkably efficient in providing detailed calibration results which can be interpreted in terms of error maps or diagnostics. For example, on our LK G80 with a PH9 and CMES software, the volumetric error map together with the 21 parametric error components can be obtained in less than 2 hours. Ball-bar variable space structures (Tetrasure) [l21 has been applied for both calibration and verification purposes. The variable space structure enables measurements to be taken at reference points in the working volume and, using regression type techniques, diagnostic information such as the corresponding parametric errors can be obtained.


Laser llt.trolog and llrrchrne Performance 103

5 Conclusions

The paper attempts to give an overview of recent developments in machine tool and CMM metrology. Space limitations restrict the details provided but references quoted can be consulted when necessary. The presentation however will cover additional details.

References

Schlesinger. G. Testing Machine Tools gth Edition edited by Koenigsberg F., & Burdekin.M Pergamon Press. Koenigsberger F., & T1usty.J. Machine Tool Structures Vol 1 Pergamon Press.

Tlusty, J., Cowley.A., Koenigsberger. F. & Burdekin.M. Specifications and tests of metal cutting machine tools.Vol.land 2. Proceedings Conference held at UMIST, Feb.1970. Based upon the Final report of a Ministry of Technology Funded Project. Technology of Machine Tools. Proceedings of the Machine Tool Task Force Conference, Chlcago, October, 1980. Vol 5 Machine Tool Accuracy.. Lawrence Livennore National Laboratory University of California. Burdekin. M.& Voutsadopolous. C. Efficient calibration of co-ordinate measuring machines. NELEX 78 International Metrology Conference, November, 1978. Burdekin. M. & Voutsadopoulos. C. Computer aided calibration of the geometric errors of multi-axis coordinate measuring machines. Proc. 1.Mech.Eng. 198 1, Vol. 195 i\'o:20.pp 23 1-239 Burdekin. M & Voutsadopou1os.C. Computer aided volumetric calibration of CMM's. Paper 6.2 NELEX 82 Int. Metrology Conf. September1982 Burdekin.M., Di Giacomo. B. & Z. Xijing - Calibration software & application to coordinate measuring machines. NPL Conf. on 'Software for CMM's September 1985. ISBN 0946754039 Vanherck P,.Bagiasna.k. & Peters. J. Continuous measurement of linear motion errors in single tool cutting. Proc of the 19" MTDR Conf. 1978 pp 375-38 1

[l01 Butterworth. A. & Burdelun.M., A computer aided system for kinematic error calibration on lathes and thread grinding machines. Proc. 26" MTDR Conf., Manchester 1986 pp 87-91

[l l ] Burdekin. M. - Test part machining for accuracy assessment - Machine tool task force conference, Chicago, October, 1980.pp 9.10-1 to 9.10-8


[l21 "Contisure"; CD Measurements Ltd. Hadfield, Glossop SKI3 2ER [l31 Burdekin. M & Park J., - Contisure - A computer aided system for

assessing the contouring accuracy of N.C. machine tools. Proc. 27th Int. MATADOR Conference 1988. pp197-203

[l41 Kwon H D & Burdekin M,, - Measurement and diagnostics of machine tool errors during circular contouring motions, Proc. Instn. Mech. Engs., Vo1.212, Part B, pp.343-356, 1998.

[l51 Burdekin M & Jywe. W - Optimising the contouring accuracy of CNC machines using the Contisure System. Proc. 29th Int. MATADOR Conference, 1992 pp.371-378. Macmillan ISBN 0333 553 17-9.

[l61 M Burdekin & Jywe W - Application of Contisure for the verification of the contouring performance of precision m achines. Proc. 6th Int. Precision Engineering Seminar, Braunschweig 199 1. Pub. Springer-Verlag. Progress In Precision Engineering ISBN 3-540-53986-7

[l71 Kwon H D & Burdekin M - Adjustment of CNC machine tool controller setting values by an experimental method, Int. J, of Machine Tools & Manuf., Vol.3 8, pp. 104.5- 1065, 1998.

[ l 81 Kwon H D & Burdekin M - Development and application of a system for evaluating the feed drive errors on computer numerically controlled machined tools. Prec. Eng., Vo1.19. pp. 133-140, 1996.

[l91 Burdekin M & Voutsadopoulos C - An error compensation system for coordinate measuring machines Paper 6.6. NELEX 82 Int. Metrology Conference, September 1982.

[20] Burdekin M & Pahk H J - Computer aided volumetric error calibration of coordinate measuring machines using the base as a metrological reference. Proc. 28th Int. MATADOR Conference 1990. pp 469-476

[21] Burdekin M & Pahk H J - The application of a microcomputer to the on- line calibration of the flatness of engineering surfaces. Proc. 1.Mech.Eng. Vol203, PART B Journal of Engineering Manufacture 1989.pp127-137

[22] Pahk H J& Burdekin M - Evaluation of the effective parametric errors in coordinate measuring machines using the locus of stylus on the horizontal plane. Proc. 1.Mech.E. Part B. Vol. 205, pp 123 - l38> l99 1.

(231 Pahk H J Burdekin M, & Peggs G N, Development of virtual coordinate measuring machines incorporating probe errors, Proc.I.Mech.E., Vo1.212 PART B, pp.533-548,1999.

[24] Lee E S, Wi H G & Burdekin M:- 3-Dimensional Error Calibration of CMMs Using a Hole-Plate Artefact. Journal of the Korean Society of Precision Engineers, Vol. 13, No.4, pp.67-74, 1996.


performance assessment of machine tools and coordinate measuring machines some recent ... ·...

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