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DOI 10.1007/s00170-003-1866-9ORIGINAL ARTICLEInt J Adv Manuf Technol (2005) 25: 19H. Huang S. Kanno X.D. Liu Z.M. GongHighly integrated and automated high-speedgrinding systemfor printer headsconstructed bycombination materialsReceived: 21 January 2003 / Accepted: 11 July 2003 / Published online: 2 June 2004 Springer-Verlag London Limited 2004Abstract The fabrication of heavy-duty printer heads involvesa great deal of grinding work. Previously in the printer manu-facturing industry, four grinding procedures were manually con-ducted in four grinding machines, respectively. The productivityof the whole grinding process was low due to the long loadingtime.Also, the machine floor spaceoccupation was large becauseof the four separate grinding machines. The manual operationalsocausedinconsistentquality.Thispaperreportsthesystemandprocess development of a highly integrated and automated high-speed grinding system for printer heads. The developed system,which is believed to be the first of its kind, not only producesprinter heads of consistently good quality, but also significantlyreduces the cycle time andmachine floor space occupation.Keywords High-speed grinding Printer head Processoptimisation Robot System development1 IntroductionThe fabrication of heavy-duty printer heads involves in a greatdeal of machining work. Grinding is the final process for machin-ing the printer heads. In the printer manufacturing industry fourgrindingproceduresareneededandareperformedinfourseparategrinding machines. Rough grinding is first required to machinethe topsurface ofa printer head. Thena deepslot isground onthebacksideoftheprinterhead.Astheworkpieceisheavilydistortedafter rough and slot grinding, the reference datum on the backside has to be ground prior to the fine grinding of the top surface.The productivity of the whole grinding process is low due to thelong loading time and the machine space occupation is large dueH. Huang (u)School of Mechanical Engineering,The University of Western Australia,WA6009, AustraliaE-mail: .auS. Kanno X.D. Liu Z.M. GongSingapore Institute of Manufacturing Technology,71 Nanyang Drive, 638075 Singapore, Singaporeto the engagement of the four grinding machines. Another majorproblem isinconsistent quality inthe production, which iscausedby inconsist manual loading. The assurance of grinding quality issomewhat dependent onskilledworkers.To reduce the machining costs, machine floor space occupa-tion and dependence on skilled workers, the industry has beenseeking an automation solution. This paper reports the systemand process development of a highly integrated and automatedhigh-speed grinding system for printer heads.2 Characteristicsof workpiece and machiningsystem requirementsA printer head is illustrated in Fig. 1. The workpiece has a lengthof about 360 mm, a width of 55 mm and a height of 25 mm.The frame of the workpiece is made of aluminium alloy witha large slot in the middle of the back surface (see the inset fig-ure in Fig. 1). This long, hollow structure makes the workpieceextremely weak and greatly increases the machining difficultiesbecause it is easy to be deformed and distorted during grinding.On the top surface of the workpiece, there are a number ofsmall slots, which are filled with different materials from theframe. Asshowninthe insertfigure atthetopleftcornerinFig. 1,thetopsurfaceconsistsoffourdifferentmaterials,i.e.,aluminiumalloy, epoxy aluminium oxide, magnet material and inconel. Theproperties ofthesematerialsare listedinTable 1. Allofthesema-terials, with very different material properties, have to be groundsimultaneously in the rough and fine grinding procedures. Thegrinding wheel loading is significant. As a result, the wheel hasto be selected in a compromised way to accommodate the fourdifferent materials 1,2. Besides wheel selection, conventionalgrindingmethodologieshavedifficultiesinhandlingthecombina-tion materials.High-speed grinding needs tobe employed.The dimensional accuracy of the machined workpiece ishigh. The overall flatness requirement on the top surface (of36055 mm) is less than 15 m. No specific surface roughnessis required, but chatter marks should not be seen on the groundsurface. The automatic grinding system should:2Fig.1. Illustration of a printer headMaterialsChemical compositionHardnessGrinding area percentage(kg/mm2)on top surfaceSi 0.6%0.8%, Fe 0.2%0.7%Al 6061-T6Cu 0.3%0.4%, Cr 0.2%0.35%11025.88Mg 1%1.2%, Al BalanceMagnetcobalt+iron4208.96epoxy resin +90%Epoxyof spherical alumina96057.99(nominal 98% Al2O3)Inconel 7185407.17Table1. Properties of the combina-tion materials Complete all the four grinding procedures which are carriedout in four separate machines in the manual production line,i.e., rough, slot, datum and fine grinding Automatically load/unload workpieces during the four grind-ing procedures Meet quality and productivity requirements.3 System developmentThe system, illustrated in Fig. 2, consists of a grinding machine,a workpiece holding fixture, a Scala robot for loading/unloadingworkpieces and safety devices. The holding fixture fixed on themachine working table has positioning, clamping and cleaningdevices. The systems working sequence is briefly described asfollows:1. After completing a grinding procedure, the side door of thegrinder opens and the grinding wheel moves to its homeposition.2. The holding fixture releases the workpiece. At the same time,the machine controller sends a loading/unloading signal tothe robot.3. The robot then enters the grinder from the side door, gripsthe workpiece, turns it around 180 degrees and puts it backinto the holding fixture. The holding fixture then positionsand clamps the workpiece.4. The robot returns to its home position. The robot con-troller sends a job completion signal to the machine con-troller. The side door closes and the next grinding procedurestarts.Fig.2. Front view of the robot system with grinding machine3Fig.3. The grinding system: the ELB grinder and the robot loading system3.1 Selection of grinding machineAn Elb micro-cut, high-speed grinding machine, shown in Fig. 3,was selected because Elb-Schliff has long been recognised asthe worlds leading manufacturer of surface grinding machines,both in conventional and high-speed designs 3. Of significanceare its 3-axis moving spindle design, which greatly reduces themachine floor occupation. The conventional machine design hasa moving work table along the x-axis, and the spindle only movesalong the other two axes. The enclosure is much larger to coverthe extremes of travel of the moving table. It is also noted thatthe Elb machine spindle system is very rigid and is able to carrythree grinding wheels of 300 mm in diameter running at speedsof up to 8000 rpm with high rotational accuracy.3.2 Combination of grinding wheelsAs mentioned earlier, the four grinding procedures have to beconducted in one grinding machine for the automated system,Fig.5. Illustration of the workpiece holdingfixtureFig.4. The assembly configuration of grinding wheels on the spindleso all the grinding wheels must be assembled into one spindle.This requires a very rigid spindle system, particularly for runningat high speeds. Also because the spindle moving distances arelimited, the wheels must be designed to meet stringent space re-quirements. The assembly of the three wheels is shown in Fig. 4.Wheel No. 1 is used for rough grinding, wheel No. 2 is for datumand fine grinding and wheel No. 3 for slot grinding. The wheelselection and specifications will be described in the process de-velopment section.3.3 Workpiece holding fixtureThe fixture, shown in Fig. 5, is made of stainless steel to resistrust and coolant corrosion. It is positioned on the machine tablevia two positioning keys to ensure the machining straightness.The fixture has three working stations for the four grinding pro-cedures. The functions of the working stations are described asfollows (also refer to the inset figure in Fig. 1): Station 1 is for grinding the slot. The datum plate of the sta-tion has a flatness of less than 8 m. Station 2 is for rough and fine grinding of the top surface. Itsdatum plate has a flatness of less than 5m, which is essen-tial to meet the workpiece flatness requirements.4 Station 3 is for grinding the workpiece datum plane. Three-pin positioning method and floating-point support are usedto meet the flatness requirements for the workpiece datumplane.The workpiece holding fixture provides: Appropriate holding of the workpiece to ensure that aftergrinding the workpiece meets the flatness requirements Automatic clamping and releasing of the workpiece withoutdistortion Automatic cleaning of the grinding swarfsThe special features of the holding fixture are described asfollows.Adjustable force and self-locking clamping: The clamp-ing system for each working station has two air cylinders, whichare pneumatically controlled through the robot controller. Theclamping force is adjustable through the air pressure valves. Themaximum design clamping force for each cylinder is 5kg withthe standard air pressure of 7bar. For the fine grinding at Sta-tion 2 and the datum grinding at Station 3, the clamping forcemust be adjusted to be as small as possible to reduce distortionscaused by clamping. To maintain solid holding during grinding,the clamping systems at Station 2 and 3 are self-locked after theclamping.Fig.6. Distribution of positioning and supporting pins for datum grinding.The three solid circles represent fixed pins and the rest of the circles standfor floating pinsFig.7. The Scala robot with the de-veloped gripperThree-pin positioning and floating pin support at Sta-tion 3: The workpiece is heavily distorted after the rough andslot grinding. The workpiece would be wobbled during grindingif a plane were used to position and support it, making it impos-sible to achieve the required flatness accuracy for the workpiecedatum plane. In this system, a three-pin positioning and float-ing pin support method was developed for the datum grinding atStation 3. The location of these pins with respect to the work-piece is illustrated in Fig. 6. The workpiece is first positionedon the three fixed pins. This makes sure that the grinding ref-erence datum is supported by a plane determined by the threepins. An air cylinder is then used to press the workpiece to main-tain its good contact with the fixed pins. The four floating pinssuspended on springs are also pressed down to contact the work-piece. After clamping the workpiece, a hydraulic locking systemis then activated to hold the four floating points, which providea solid support for the workpiece together with the three fixedpins. Each floating pin can withstand 15kg with a positioningaccuracy of 5m.Positioning and cleaning: Side and end pneumatic pushersat each station are used to position the workpiece before clamp-ing. Pressurised air is used to blow coolant, dirt and swarfs awayfrom the datum plates and pins during loading and unloading.3.4 Loading/unloading systemRobot selection: The selection of a robot for the loading/unload-ing of workpieces was a difficult task because of the limitationson positioning accuracy, machine space and costs. In this sys-tem, a Yamaha Scala robot (YK440A) was selected after com-prehensive studies 4. The robot, shown in Fig. 7, has fouraxes: x, y, z and R. It has a positioning accuracy of 20 m andcan hold a weight of 20kg. The robot controller has a digi-tal I/O interface. This provides a platform for users to developthe communications between the grinding and loading/unloadingsystems.5Robot gripper design: As shown in Fig. 1, the printer headis quite long and difficult to handle, which presented a greatchallenge to the gripper design due to the space and weightlimitations. The final robot gripper design is shown in Fig. 8.The gripper consists of a rotary driving hand (right) and a slavehand (left). The robot gripper can thus firmly hold the work-piece and canrotate freelywithout distorting ortwistingit. Basedon the weight consideration, the robot gripper is mainly madeof aluminium alloy with special treatment for wear resistanceand corrosion. The driving motors and cylinders are all standardparts, thus easy to maintain and replace. The gripper also has anair cylinder pusher in the middle, which is used for pressing theworkpiece before clamping. This is essential for the floating pinsupport.3.5 System integration & communication between machine &robot controllersThe robot controller has a digital I/O interface. This providesthe users great convenience because there is no need for an ex-tra controller for the workpiece holding fixture. In the automaticsystem, the loading/unloading mechanisms and the workpieceFig.8. Robot gripper designAction descriptionAction conductor1Loading w/p and clampingOperator, press button2Grinding cycleMachine controller3Wheel back to home positionMachine controller4Machine side door openMachine controller5Robot advancing to working zoneRobot controller6Robot approaching to w/pRobot controller7Unclamping w/pPneumatic clamper18Robot picking up w/pPneumatic clamper29Lifting and rotating (180) w/pRobot controller10Reloading w/p and clampingRobot controller11Robot back to home positionRobot controller12Sending w/p ready signal to machine controllerRobot controllerTable2. System processing actionsand action conductorsholding fixture are both controlled by the robot controller, andthe grinding system is controlled by the machine CNC controller.The machine and the robot controllers are basically two indepen-dent systems. Only when the grinding operation is stopped, theyare allowed to communicate. This arrangement makes the oper-ation safe and the maintenance simple. Communication betweenthe robot controller and the CNC controller are completed via thedigital I/O interface. The robot and the workpiece holding fix-ture can start their operations only after receiving the grindingcompletion signal from the machine controller and making surethat the grinders side door is open. When all these operationsare completed, the robot controller sends the workpiece readysignal to the machine controller. System processing proceduresare listed in Table 2, which describes the actions during a typicalgrinding process.3.6 Safety issuesFor safety purposes, preventive safety measures have been takento protect the operator, equipment and workpiece from damageby any robot and fixture errors. Laser sensors are used to exam-ine that the side door is open before starting the robot, the robot6gripper is open before loading and unloading, and the workpieceis in the right position before grinding. There is also an interlockbetween the robot and the grinder. When one party is in opera-tion, the other must be in a stand-by mode. The robot system isenclosed by a plastic fence and is separated from the operator.There are also three emergency stop switches for the operator tostop the system.4 Process development4.1 Selection of grinding wheelsAs mentioned earlier, the rough and fine grinding of the work-piece top surface has to deal with four types of materials withdifferent mechanical properties. The selection of the grindingwheels is critical, particularly for rough grinding, which in-volves a large amount of material removal. Conventional abra-sive wheels are suitable for grinding aluminium and magnet,but the much harder inconel and alumina will wear them outFig.9. Comparison of grinding forces generated by wheels using differentbond types. Wheel speed = 85m/s, feed rate = 1200 mm/minFig.10. a Porous structure of Gressowheel, and b loaded wheel topogra-phy after grindingat a much quicker rate. This necessitates a frequent truing anddressing 2. Superabrasive wheels can handle all four materialsand have a much longer life than conventional wheels. The grind-ing experimental studies showed that diamond abrasive has ad-vantages over CBN in terms of wheel life. This could be becauseof the existence of epoxy alumina. However, another severeproblem for grinding printer heads using superabrasive wheels isthe wheel loading and chip adhesion at high removal rates due tothe existence of aluminium alloy, a very soft material. Therefore,the selection of the bond material is important. Figure 9 showsthe grinding force results under the same conditions, but withdifferent wheel bonds. Electroplated (EP), vitrified and Gressowheels were used to grind printer heads. The grinding force ismuch smaller for the Gresso wheel than those for the other twowheels due to less loading, particularly at the highest removalrate. The bond of Gresso wheels is made from synthetic resin,consisting of phenolic and polyimide resins. The wheel thus hasa porous structure (Fig. 10a) and performs as a wheel with a bondbetween vitrified and normal resin. This structure is particularlyimportant for the wheels self dressing when the wheel loadingbecomes significant (see Fig. 10b). The Gresso wheel was thusselected for rough grinding and the experimental results indi-cated that it coped well with the combination of materials.A resin bond CBN wheel has been used to grind the deep slotat the back of the workpiece because in slot grinding the wheelmainly deals with epoxy, inconel and magnet material. A muchsmaller material removal is needed with the datum grinding andthe fine grinding of the top surface than the rough grinding. Sincethe vitrified diamond wheel performs for the combination ma-terials when grinding at small removal rates (see Fig. 9), it hasbeen selected for the datum and fine grinding. Another reason touse the vitrified wheel is that it produces a smaller flatness errorthan the Gresso or resin bond wheel. The wheel specificationsand their functions are summarised in Table 3.4.2 Selection of grinding parametersHigh-speed grinding technology 5 has been employed in thissystem because it has apparent advantages over the conventionalspeed grinding for the combination of materials. Comprehensive7Wheel specificationGrinding procedureMaterials to be groundWinter Gresso diamondRough grindingAluminium alloy, epoxyD252SP2002C50alumina, inconel, magnetWinter resin CBNSlot grindingMagnet, epoxy, inconelB126KSS007C100Winter vitrified diamondDatum grindingAluminium alloyD46V+2038C75Fine grindingAluminium alloy, epoxyalumina, inconel, magnetTable3. Wheel specifications and applicationsManual operationAutomated systemGrindingMRRTimeGrindingMRRTimeconditions(mm3/mm.s)(min.)conditions(mm3/mm.s)(min.)Rough63 m/s85 m/sgrinding1143 mm/min2.4281200 mm/min450.127 mm0.2 mmSlot83 m/s100 m/sgrinding889 mm/min3.7671000 mm/min54 mm0.25 mmDatumTraverse100 m/sgrindinggrinding 2.51.51500 mm/min1.2530.05 mmFine83 m/s100 m/sgrinding1500 mm/min1.2541500 mm/min1.2540.05 mm0.05 mmTotal62.5loading timeTotal time26.520.8Table4. Comparison of grinding conditions andcycle timesgrinding experiments were carried out to find out the appropri-ate grinding parameters. It was found that the grinding speed isthe critical factor influencing the whole process. Figure 11 showsthe effect of specific material removal rate (SMRR) on grind-Fig.11. Relationship between specific force and specific material removalrate for rough grinding. Coolant was supplied using the twin nozzles de-scribed in Fig. 15ing force for the rough grinding, which is the bottleneck in thewhole grinding process. It is clearly seen that for all the nor-mal and tangential speeds applied, the grinding forces increasewith increasing SMRR, as expected. It is also seen that thereis a significant increase in grinding force when SMRR reachesa certain level for the lower speeds of 35 and 63 m/s. The signifi-cant force increase is often associated with severe wheel loadingand a grinding burn event. The higher the speed applied, thehigher the value of SMRR can be achieved without deteriora-tion of grinding quality. When the wheel speed is increased to85 m/s, the maximum value of SMRR without burn marks ismore than two times higher than that at 35 m/s. This suggeststhat increasing the grinding wheel speed enables a higher pro-ductivity. Similar grinding experiments were completed to selectthe grinding parameters for the slot, datum and fine grinding. Theoptimised grinding conditions are listed in Table 4.4.3 Coolant supplyThe coolant supply in the grinding of printer heads plays animportant role because when the grinding is conducted at high-speeds, the coolant flow must be sufficiently strong to breakthe air barrier formed around the wheel periphery. More impor-tantly, due to severe wheel loading, the coolant has to play theother role, i.e., clearing grinding swarfs from the wheels 6.Therefore, a significantly high coolant pressure is needed for the8Fig.12. Effect of coolant pressure on grinding force and surface qual-ity in rough grinding when using grinding speed of 85 m/s, feed rate of1200 mm/min and depth of cut of 0.2 mm. Single nozzle (i.e., only Nozzle 1in Fig. 15) was usedgrinding system. Figure 12 shows the effect of coolant pressureon grinding force and grinding quality during the rough grindingprocess. It is seen from Fig. 12 that the coolant pressure has littleeffect on the tangential grinding force, but influences the normalgrinding force. The pressure also significantly affects the groundsurface quality because the lowest pressure cant clear grindingswarfs away from the wheels properly and thus causes burn onthe workpiece surface (as shown in Fig. 13a). When the pressureis increased to 15bar, the ground surface quality (see Fig. 13b)can consistently meet the requirement.The higher coolant pressure also causes a larger grindingforce 7. As seen in Fig. 12, the normal grinding force inducedby the coolant can reach more than half of the total normal grind-ing force. The increased force then results in a larger flatnesserror. This becomes very critical in the datum and fine grindingprocedures. Figure 14 shows the effect of the coolant pressure onthe final surface flatness for the single nozzle used. It can be seenthat the increase in coolant pressure led to a larger flatness error.However, the employment of a greater coolant pressure of 15baris needed for a required surface quality. To solve the problem,Fig.13a,b. Surfaces ground usingcoolant pressures of a 7bar andb 15 bar. Grinding speed of 85 m/s,feed rate of 1200 mm/min and depthof cut of 0.2 mm were employed.Single nozzle was usedFig.14. Effect of coolant pressure on flatness errors using single and twinnozzlesa separate nozzle with a high coolant pressure of 15 bar (Noz-zle 2 illustrated in Fig. 15) was adopted for clearing swarfs awayfrom the wheel specifically. This nozzle is mounted on the tophalf of the wheel, which can prevent the coolant flow from di-rectly hitting the workpiece. Nozzle 1 in Fig. 15 is for coolingwith a much smaller coolant pressure of 5bar. With the twinnoz-Fig.15. Illustration of nozzle and prop arrangement in the grinding system9Fig.16. Burn partially occurred on the ground surface due to the shortage ofcoolant supply at the beginning of grindingTable5. Comparison of surface roughness (Rain m) after fine grindingAluminiumEpoxySoft magnetInconelManual operation0.620.770.520.33Automated system0.550.720.480.35zles installed for the rough, datum and fine grinding wheels, thefinal flatness error is greatly reduced, as shown in Fig. 14.A coolant prop was also installed in front of the workpiece toguide the coolant into the grinding zone at the beginning stageof grinding, as illustrated in Fig. 15. Without the coolant prop,the front end surface of the workpiece was easily burnt, as shownin Fig. 16, since the coolant was blocked by
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