a.j. white, s.r. postlethwaite, d.g. ford · 2014. 5. 17. · a.j. white, s.r. postlethwaite, d.g....

An identification and study of mechanisms

causing thermal errors in CNC machine tools

A.J. White, S.R. Postlethwaite, D.G. Ford

Precision Engineering Unit, School of Engineering, University ofHuddersfield,Queensgate, Huddersfield, HD1 3DH, United KingdomEmail: [email protected]

Abstract

This paper describes tests that have been performed on 10 different CNCmachine tools of widely differing configurations. These tests aim to identifythermal errors and methods by which these errors may be eliminated fromcomponents produced by these machines. The paper consists of three sectionswhich describe the general testing methodology used, the data recorded andanalysed from the tests, and a list of actions which may be performed in order toreduce the thermal errors in the machines tested. The general testingmethodology has evolved from both the BSI 3800 part 3 standard and the need toidentify the factors affecting thermal errors. Thermal imaging, non-contactprobes, level sensing equipment, material constructed from invar and appropriateexercise of machine functions have all been used to identify thermal errors. Thedata recorded has identified thermal errors that can be associated with particularmechanism types. Mechanisms investigated include ball-screws, hydro-screws,linear scales, hydrostatic bearings, structures, and cooling systems withinstructures. The list of actions required to reduce the thermal errors has includeddesign changes, improved chiller settings, application of linear scales and revisedprobing systems. It has become clear from this study that a fundamentalunderstanding of the mechanical arrangement of a particular machine is essentialto reducing thermal errors in an economic and effective manner.

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

102 Laser Metrology and Machine Performance

Introduction

It is generally accepted that geometric, load and thermal errors on CNC machinetools all contribute to errors on a finished part. Postlethwaite* has produced acompensation system that can reduce geometric errors by up to an order ofmagnitude. However, Bryan stated that 70% of the errors on a finished part aredue to thermal effects. The complex nature of thermal errors makes correction forthese errors troublesome in that they are unpredictable and are difficult to correctin normal machining environments. Previous studies have succeeded in reducingthermal errors significantly in one-off applications using neural networks andmulti-regression models (Chen et al/, Yang et al/,Tseng , Wang et al.%However, these methods of error correction suffer from the drawback ofrequiring a lot of testing time on the machine to produce an accurate model. Alsoit has been found in this study that thermal errors in a machine change over time,reducing the accuracy of a model optimised when the machine is new. Inaddition, these models have been shown to be susceptible to changing machineusage conditions such as dry or wet cutting and differing machine duty cycles(Chen?).

The sources of thermal errors can come from the environment, heating andcooling sources within the machine, work-piece cutting/design, and themanufacturing processes. Consequently no single approach to reducing thermalerrors exists. However, CNC machine tools are built with a limited range ofinternal mechanisms designed to move the tool relative to the work-piece (or viceversa), and to hold the tool against the cutting forces produced. The aim of thisstudy has been to test and analyse the thermal performance of the internalmechanisms of many different machine configurations. The machines testedrange from 3 and 4 axis grinders, multiple head milling machines, verticalmachining centres with and without moving columns, portal type millingmachines, and horizontal type milling machines. The thermal errors experiencedat the part on each machine have been broken down into their constituentcomponents caused by the particular construction of the machine. This study hasidentified many different causes of thermal errors on many different machineconfigurations, which allows a generalised testing and correction strategy to beproposed for all machine tool types.

1 Testing methodology

1.1 General

Testing of the machines to identify thermal errors was based on BS3800 part 3(updated version shortly to be ratified as ISO230-3). The test specification forboth the spindle running test and the axis expansion test is not designed toprovide sufficient information for thermal error compensation, but to providemachine tool users with a quantitative evaluation of machining performanceunder specific machining conditions. The BS3800 part 3 tests have beenmodified whenever a significant increase in accuracy will result, or when data


Laser Metrology and Machine Performance 103

that represents an actual machining situation is required. Wherever possible thethermal errors have been measured between the tool and a work-piece. This givesa true measure of the actual thermal error occurring on the work-piece. However,this total thermal error consists of many error components that are caused byvarious parts of the machine structure. In order to perform thermal errorcompensation, it is necessary to separate the thermal errors from one-another andcorrect for them individually, or to 'short-circuit' the thermal errors bycompensating for them directly at, or near, the tool.The requirements for a full understanding of all the thermal errors that exist in aparticular machine requires the following process to be performed:1. Evaluate the mechanical construction of the machine, noting the fixture

points of the axes relative to one-another and the construction of thestructural members.

2. Identify the thermal errors that are likely to exist and the machine factorsthat effect the size and direction of these errors. From this, design tests thatcan be performed to isolate these errors from one-another and from non-thermal errors. In some cases this may be difficult, and certain thermalerrors may have to remain a sum of two or more thermal errors. In this case,try to estimate the factors affecting the thermal errors and the conditionsunder which the error is at its maximum size.

3. Ensure the machine has been statically powered for several hours with theemergency stop released. This ensures that heat generated by axes understatic conditions can be identified. Perform an initial survey of the machineusing thermal imaging to identify unexpected areas of heat build-up. Noteany thermal effects of activating the emergency stop system such as shutdown of chillers/pumps etc.

4. Exercise all the machine axes and powered devices on the machine such ashydrostatic and coolant pumps in both realistic and worst-case ways,recording the effect of the thermal error at the work-piece, or at a datum thatcan be related to the work-piece. It is important that changes in factorsaffecting thermal errors are kept to minimum during the tests.

5. Order the thermal errors according to size, rate of change, and dependencyaccording to their effect on a representative work-piece. Apply the mostcost-effective solution(s) based upon the overall production strategy toreduce or eliminate the thermal errors.

1.2 Equipment used to perform thermal testing

1.2.1 Inductive proximity non-contact displacement transducersThese inductive probes are unaffected by coolant and other non-metallicsubstances between them and the target. They are calibrated over a range of up to0.5mm, with a resolution of 1pm. Whilst they are calibrated for a steel target,they can be re-calibrated for use with other metallic materials such as aluminium.



1.2.2 Dual Talyvel electronic levelThis differential electronic level allows the tilt of one part of the machine toolrelative to another to be measured with a resolution of 0.1 arc seconds. It isnormal to place the reference level on the machine table (representing the work-piece) and the other level on the part of the machine that is moving. Digital datain serial format can be logged using a personal computer.

1.2.3 Agema Thermovision 900 thermal imaging systemThis infra-red thermal imaging system is cooled by liquid nitrogen, has a spectralresponse of 8-12 m, an accuracy of ±1°C and basic repeatability of ±0.5°C.Images have a resolution of 272x136 pixels, and can be captured using either a20° or 40° angle lens. During machine testing it has been common to averageover either 16 or 32 frames to reduce noise and to automatically record imagesevery 30 seconds or 1 minute during heating and cooling tests. The sequences ofimage files produced are loaded into, and interpreted by a piece of softwarewritten in MATLAB. This software allows the images to be manipulated,unwanted features to be cleaned off them, accurate temperature measurements tobe made, and analyses of the change in thermal gradients to be made. Analysesallow temperature and distortion models to be produced that can then be used asa basis for thermal error compensation. Care has been taken to ensure that thetemperatures recorded by the thermal imaging system are correct. This isachieved by heating a part, imaging this, recording the temperature using athermocouple, and then setting the correct emissivity. Alternatively, it has beenfound that adhesive tape can be placed over machine parts of low emissivity toset that emissivity to a known value of 0.96.

1.2.4 Thermocouple measurementsTemperatures are recorded using K type disc thermocouples that are connectedinto a PROSIG data logging system. This system has a basic resolution of 0.1 °C.Absolute accuracy is not high, but it is more important that changes intemperature are recorded correctly during the tests. This system can record up to16 channels on almost any time-base. Thermocouples are attached to the surfaceof the machine tool using tape, often with thermal compound to ensure a lowthermal resistivity, and sometimes with an insulator to reduce the effect offluctuating air or coolant temperatures. Temperatures recorded using boththermocouples and the thermal imaging system are compared wherever possibleto ensure they are similar.

1.2.5 Laser interferometerA Renishaw laser interferometer is used to measure static thermal movements,and thermal effects on moving axes. Data is recorded onto a file that is thenanalysed using MATLAB. The automatic material temperature compensation inthis equipment is disabled during these tests.

1.2.6 Other equipmentA bar made from Invar is often used to move a thermal datum from one part ofthe machine to another. This allows a non-contact displacement transducer to



measure thermal expansion over long distances (such as machine beds). Dial testindicators are often used as a visible indication of movement. An aluminiummirror is used to allow the thermal imaging camera to view obscured parts. Adigital camera is often used to record details of machine construction such as piperoutings and motor mountings.

1.3 Measurement Methodology

1.3.1 Measuring spindle growthSpindle growth is measured using a 5 non-contact displacement probe spindleerror analyser that is also used to measure x and y movement and tilt. The erroranalyser is securely attached to the machine table or other part of the machinestructure that allows the movement to be related to an error on the component.During the heating tests a problem was discovered with the invar mandrel usedfor testing spindle growth. The tool holder expanded diametrically relative to themandrel, resulting in the mandrel being pulled up into the tool holder taper, andnegative growth being recorded by the non-contact probe at the bottom of themandrel. This was rectified by using an all-steel mandrel, measuring thetemperature gradient of the mandrel from thermal images, and then correcting forthe growth seen. It should be noted that in actual machining the temperature ofthe tool could be anything from the natural temperature caused by conductionand convection, to a temperature near that of a strong jet of coolant. It should alsobe noted that a spindle running test often causes the structure holdim the spindlebearings to expand, and that the true spindle expansion is that measured betweenthe support structure and the end of the spindle at the nominal tool length.

1.3.2 Measuring ball-screw expansionAs stated above, the BS3800 part 3 ball-screw heating test was used as a basis fortests performed in this study. However in one of the specialised crank grinders itwas necessary to identify the effect of thermal growth of the hydro-screw underrealistic worst-case conditions. For this reason a test regime was designed thatcycled the grind head in and out a prescribed number of times at realistic speedsand distances. Total thermal drift over a batch of components could be estimatedfrom these tests. The ball-screw heating test on a new pre-tensioned ball-screwfitted to a vertical machining centre caused the ball-screw to go into tension andlock up. Reducing the traverse rate of the axis rectified this.

1.3.3 Measuring machine distortionThe laser interferometer, non-contact displacement transducers and Talyvel havebeen used to measure distortion of the machine structure. The Talyvel hasrecorded bending of the column of a vertical machining centre, and the laser hasrecorded yaw of a head caused by unequal cooling of the structure. Each of thesethermal errors caused a linear movement of the tool relative to the work-pieceand also an angular error that cannot be corrected on 3 axis machines.



2 Data recorded and analyses

2.1 Ball-screw and hydro-screw expansion

Tests showed some very serious thermal errors when axes with rotary encodersand ball-screws were exercised according to BS3800 part 3. Figure 1 shows anexpansion error of over SOOjim on a pre-tensioned ball-screw with 800mm travel,and an increase in reversal error from 4 m to 90jim. It is anticipated that thisdramatic increase in error is due to the extreme tightness of the ball-nut on theball-screw which generates a lot of heat on this new mechanism. A hydro-screwon a grinder was tested using a simulated crank grinding cycle to give a realisticthermal error. This showed a maximum expansion of 30 m over a length of200mm during repeated fast (8400mm/min) traverses and quick directionreversals. An increase of less than 1°C was recorded in the temperature of thehydro-nut. A maximum axis offset of approximately 15jim was recorded. Thisoriginated from the conventional thrust bearing near the axis drive that showed atemperature rise to about 7°C above ambient in a less aggressive test.

Ball-screw thermal errors have been found to consist of two components.Position independent errors are caused by ball-screw expansion between thethrust bearing and ball-nut position closest to the thrust bearing. Positiondependent errors change as the axis moves and are caused by ball-screwexpansion between the axis position closest to the thrust bearing and the currentposition. It should be noted that on pre-tensioned ball-screws the zero offset pointis nominally at the centre point between the two thrust bearings. This may not beat the centre of the axis travel, and also, as the ball-screw heats up this zero offsetmay 'wander' depending upon the relative stiffness of the thrust bearings at eachend or uneven temperature distributions along the length of the ball-screw. Inaddition very little ball-screw expansion is required before pre-tension iscompletely lost.

> o s it lo n (mm)

Figure 1. Ball-screw expansion and reversal error test.



2.2 Thermal errors affecting a linear scale

A grinder has been tested with a linear scale on the axis that sets the position ofthe grind wheel relative to the radius of the part. The pre-load and lubrication ofthis axis was by means of a hydrostatic bearing system with two inlet portspositioned near the front and the rear of each guide-way. The linear scale waspositioned in close proximity to the guide-way and pinned in the middle.Thermal images showed a temperature rise along the guide-way, centred at thetwo hydrostatic oil input ports (figure 2). A temperature rise of 7°C was recordedon the guide-way and this caused both offset and scale errors in the linear scalereadings. The offset was caused by heat flowing into the bed and increasing thedistance between the component centre-line and the axis minimum position. Thescale error was caused by expansion of the scale itself and resulted in an error of15 urn over 200mm, or lum in 7mm of part diameter from the probe datumposition. A part whose nominal diameter is furthest away from the datum radiuswould suffer the greatest error as a result of this scale error.

Figure 2. Detail of temperature rise along the guide-way of a grinder.

2.3 Spindle growth

Two basic types of spindle have been tested, conventional angular contact ballbearings, and hydrostatic bearings. It was found that the most important aspect ofthe bearing was the position of the thermal datum. A hydrostatic bearing had thethermal datum position at the opposite end of the spindle to the tool. Thus theentire length of the spindle was responsible for the spindle growth. This resultedin 160|um of spindle growth at the tool. All conventional spindle bearings testedhad the thermal datum positioned near the tool.

2.4 Thermal expansion and distortion of machine structures

All of the machine tools tested have exhibited expansion of some part of thestructure that sets the positions and angle of the axes relative to one-another.Most have also exhibited some form of distortion due to unequal heat



distributions within those structures. Thermal imaging makes measurement oftemperature gradients simple and quick. A machining centre exhibited bending ofthe head due to uneven heat generation in the upper and lower spindle bearings.Figure 3 shows movement of the tool tip resulting from uneven temperaturegradients during a simple heating and cooling test. On this 3 axis machine, it isnot possible to compensate for the effect of tilt, and so deep drilling or facemilling would produce errors. Another identical (but older) machine was tested.This machine showed the same patterns of heat build-up, but less total heatresulting in significantly different expansion and distortion patterns whencompared with the first machine. Figure 4 shows thermal images of the twomachines, with the newer machine on the left. Image differences other thanthermal gradients are caused by the application of tape to equalise emissivity.

Spindle Movement10

8

-m 6<DCD

IT *mcz

Q

0

-2

Axis

-5 -3 -2 -1Movement (metres) xicr

Figure 3. Tool movement due to head distortion

[ 86. 68) = 32.01I totherm interval 0.3 *€ | ltotheirointefvalO.3 *C

Figure 4. Thermal images of two 'identical' heads.



3 Actions to reduce thermal errors

3.1 Ball-screw expansion errors

It has become clear during the series of tests that ball-screw expansion cancontribute more to total thermal error in a machine than any other error source(thermal, load or geometric) unless a major mechanical problem is present. Thetests have shown the ball-screw thermal error can far outstrip spindle growth (bya factor of 2) even when the thrust bearing for a particular spindle arrangement ispositioned away from the measured end. At present there are a limited number ofstrategies that can be used to eliminate ball-screw expansion. The most obviousof these is fitting a linear scale. It should be noted however that the linear scaleon one of the grinding machines was heated by a hydrostatic guide-way andexhibited significant thermal errors. Fitting linear scales to existing machineswhilst still achieving good protection from coolant/impact, and without sufferingfrom Abb6 errors is often difficult. Ball-screw cooling is employed on somemachines. This either takes the form of cooled liquid being circulated through ahollow ball-screw, or the entire ball-screw being immersed in cooling fluid. Bothof these solutions require much engineering change to the machine, and thesecond one cannot be applied to vertical ball-screws without significant sealingproblems. Probing just before critical cuts are made is often an option, althoughthis can increase the machine cycle time significantly. Also the act of toolchanging and probing can generate ball-screw heat.

3.2 Moving the thermal datum

One of the grinding machines had the spindle thermal datum positioned at theend of the spindle away from the grind wheel. It was suggested that this shouldbe moved as near to the tool as possible. Future designs will have the thermaldatum positioned near the grind wheel. A machining centre had the Z axis ball-screw thrust bearing positioned at the top of the column, and the bottom of theball-screw unsupported. Thus growth of the column affected the total axisposition. Re-positioning the thrust bearing at the base of the column wouldeliminate the column from the total axis error. It should be noted here that on thismachine the column thermal error and the ball-screw thermal error were inopposite directions for heating. Even if the thermal characteristics of the twoparts were identical they would not cancel each other out as one error (the ball-screw) consists of a position independent thermal movement (PITM) and positiondependent thermal movement (PDTM), whilst the other error consists of a PITMonly. A combination of the two errors can only increase the total Z axis positionuncertainty resulting from thermal errors.



3.3 Intermittent probing

One of the grinding machines exhibited a gradual increase in part size up tolOOum over the course of the day. Testing revealed that this change was causedby a total of 5 thermal errors in various parts of the machine. Significantreduction of the thermal errors would have required much mechanical re-design.Compensation for these errors by modelling the thermal errors and moving themachine axes would have been difficult to achieve with the desired accuracy. Itwas decided that the best solution would be to measure the total thermal error atthe start of each component and compensate accordingly. The probing takesplace during the automatic unloading of the machined part and loading of a newpart, and so no increase in cycle time results. This system is in the process ofbeing designed and the level of error reduction achieved will be identifiedshortly.

3.4 Cooling the machine structure

Two of the machines had chilled fluid flowing through parts of the machine tool,one through the head, and the other through the spindle drive motor. Initial testsusing the thermal imaging camera showed both structures were being over-cooled by up to 8°C. On one structure this resulted in Z axis movement. On theother structure this resulted in a yaw of the head which was reversed by heatingfrom the spindle bearings during running (recorded using the laserinterferometer). The PID control of the chiller on the first machine was tuned andthe temperature sensor was moved, resulting in a chilled fluid temperaturefluctuation of just ±0.5°C either side of 20°C. The second machine required anupgraded cooling system to achieve better temperature control.

3.5 Errors in a probing system

On one of the grinding machines, thermal imaging and the use of non-contactdisplacement transducers revealed errors in the probing system. Four errors wereidentified. These are listed below, and shown in figure 5.A Hot exhaust from the spindle motor causing the probe arm to expand.B Expansion of the structure between the probe arm end-stop and the grind

point.C Movement of the probe datum relative to the component centre-line.D Heating of the linear scale measuring the position of the grind axis relative to

the component caused inaccurate measurement of axis position.Recommendations have been made to the manufacturer to divert the exhaustedair from the spindle motor out of the machine enclosure. Flooding the outside ofthe spindle box with coolant will reduce expansion of the structure between theprobe arm end-stop and the grind point. Movement of the probe datum wascaused by hot hydrostatic oil heating the structure to which the datum wasattached and causing this to move away from the component centre-line. Analternative mounting point for the datum was suggested. Hot hydrostatic oilwithin the guide-way nearby was heating the linear scale. Better control of



hydrostatic oil temperature, or applying a layer of insulation between the linearscale and the guide-way was suggested to reduce this problem.

Probe Datum

Work-head

Figure 5. Probing errors on a grinder.

Conclusions

This study of a wide variety of machine tool configurations with many differenttypes of internal mechanisms and machining functions has identified thefollowing with regard to thermal errors:1. Mechanism types and their configuration within a machine tool indicate a

predisposition to thermal errors of particular types. Axes using a ball-screwwith rotary encoder as the sole positioning device generate larger thermalerrors than any other single error source.

2. Thermal imaging is a very powerful tool for identifying very low levels ofunexpected heat build up within machine structures, and finding the sourceof this heat. Thermal imaging can also show the temperature gradients on thesurface of machine tool structures and on rotating parts.

3. Machine wear and other variable factors such as coolant flow directionreduces the accuracy of modelling techniques that use a limited number oftemperature sensors to predict thermal errors.

4. Thermal errors on a component can be the result of many factors includingmachine shop temperature, component design, coolant flow, probing errorsand frequency, and machine internal thermal errors. The relative sizes ofthese errors are dependent upon the component design and many aspects ofthe manufacturing process.

5. The cost-effective reduction of thermal errors on a component requires thewhole manufacturing cycle of the component to be taken into account.Actions to reduce the thermal errors may include modification of themachine shop environment, modification of the machine, improving theprobing strategy or compensation using the machine axes.



References

1. Postlethwaite S R., Electronic based accuracy enhancement ofCNCmachine tools, PhD thesis, Huddersfield Polytechnic, 1992

2. Bryan J., International status of thermal error research, Annals of the CIRP,Vol. 39, No. 2, P645-537, 1990

3. Chen J S., Chiou G., Quick testing and modeling of thermally induced errorsof CNC machine tools, InternationalJournal of Machine ToolsManufacture, Vol. 35, No. 7, pp. 1063-1074, 1995

4. Yang M, Lee J., Measurement and prediction of thermal errors of a CNCmachining center using two spherical balls, Journal of Materials ProcessingTechnology, Vol. 75, pp. 180-189, 1998

5. Tseng P-C., A real-time thermal inaccuracy compensation method on amachining centre, InternationalJournal of Advanced ManufacturingTechnology, Vol. 13, pp. 182-190, 1997

6. Wang Y., Zhang G., Moon K S., Sutherland J W., Compensation for thethermal error of a multi-axis machining center, Journal of MaterialsProcessing Technology, Vol. 75, pp. 45-53, 1998

7. Chen J S., A study of thermally induced machine tool errors in real cuttingconditions, InternationalJournal of Machine Tools Manufacture, Vol. 36,No. 12, pp. 1401-1411, 1996


a.j. white, s.r. postlethwaite, d.g. ford · 2014. 5. 17. · a.j. white, s.r. postlethwaite, d.g....

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