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Int J Adv Manuf Technol (2002) 20:639648 Ownership and Copyright 2002 Springer-Verlag London Limited An Advanced Ultraprecision Face Grinding Machine J. Corbett 1 , P. Morantz 1 , D. J. Stephenson 1 and R. F. Read 2 1 School of Industrial 2 Cranfield Precision, Division of Landis Lund, Cranfield University, Cranfield, Bedford, UK Cranfield Precision, Division of Landis Lund, has recently developed an ultraprecision face grinding machine which incor- porates several automatic supervision features. The company supplied the machine to Cranfield Universitys Precision Engin- eering Group in order that the group can undertake research, particularly in the area of damage-free grinding with high surface and subsurface integrity. The paper discusses the design of the machine, initial machining trials and potential research projects. Such projects will benefit from the avail- ability of such an advanced machine system which incorporates many state-of-the-art features for the automatic supervision and control of the machining process. Keywords: Automatic supervision; Grinding; Machine tool design; Precision machining 1. Introduction Cranfield Precision, which is a UNOVA Company, specialises in the design and manufacture of machines for cost-effective production of components in advanced materials including ceramics, glasses, intermetallics and hard alloy steels. The School of Industrial and Manufacturing Science (SIMS), Cran- field University, places great importance on developing close collaborative links with industry and is currently undertaking a range of ultraprecision and high-speed machining research projects including superabrasive machining, ductile machining of brittle materials and precision machining for the automotive industry. The complementary research interests of the two organisations have resulted in Cranfield Precision developing and supplying an advanced ultraprecision face grinding machine to the Precision Engineering Research Group within SIMS. This will enable the group to undertake a wide range of research programmes, particularly in the area of damage-free grinding with high surface and sub surface integrity. Correspondence and offprint requests to: Prof. J. Corbett, School of Industrial and Manufacturing Science, Cranfield University, Bedford MK43 0AL, UK. E-mail: j.corbettL50560cranfield.ac.uk. Materials processing with nanometric resolution and control is viewed as a mid- to long-term solution to the cost and time problems that plague the manufacturing of electro-optics and other precision components. For example, ductile grinding of brittle materials can provide surfaces, as ground, to nanometre smoothness and figure accuracy at higher production rates than usually encountered 1. More significantly, a ductile ground surface experiences little or no subsurface damage, thereby eliminating the need for the subsequent polishing step associa- ted with conventional grinding. The performance of many “microfeatured” products (e.g. semiconductor, optical communi- cations systems, computer peripherals, etc.), as well as larger components for aerospace and automotive applications, depends increasingly on higher geometric accuracies and micro- and nanostructured surfaces. Recently, the automotive industry has indicated a future requirement for the surfaces of certain key transmission components to be of “optical” quality, with targets of 10 nm Ra surface finish to be economically produced on hardened steel by direct machining, without polishing. The conditions under which damage-free surfaces can be produced on glasses and ceramics, and “optical” surfaces can be produced on hardened steel, are exacting, requiring (a) the use of a class of machine tool not normally found in even the best production facilities, e.g. high accuracy, smoothness of motion, loop stiffness 2, (b) the incorporation of ancillary features specially developed to suit the particular application (e.g. grinding wheel truing and conditioning), and (c) the use of the correct grinding technology for the application (many variables wheel type, coolant, speeds, feeds, etc). All the conditions must be satisfied and the wafer face grinding machine has been developed to meet them. 2. Objectives In order to meet the demands of surface integrity and pro- ductivity mentioned above, for a wide range of components, the principal objectives include the development of: 1. A machining process capability for the manufacture of sizeable components with high levels of surface/subsurface integrity. 640 J. Corbett et al. 2. Optimised “ductile mode” machining processes for brittle materials (glasses and ceramics). 3. A single process, with only one set-up, to replace the typical three-stage lapping, etching and polishing process, resulting in much higher productivity. 3. The Process A prime requirement of the process is that it should be capable of machining extremely flat surfaces on workpieces up to 350 mm diameter. Further, the surfaces should be smooth (50 nm Ra) and have minimum subsurface damage. Ideally the surface should be close to the quality obtained by polishing. In order to meet these demanding requirements rotation grinding is utilised. A feature of rotation grinding is that unlike conven- tional surface grinding, it has a constant contact length and constant cutting force. Figure 1 illustrates the grinding prin- ciple. Both the cup grinding wheel and workpiece rotate and the axial feed of the grinding wheel removes stock from the surface of the workpiece until it reaches its final thickness/geometry. 4. The Machine The demanding requirements of the process and component quality necessitate a machine of extremely high loop stiffness. The design targets for the face grinding machine (Fig. 2) are: 1. Loop stiffness better than 200 N H9262m H110021 with good dynamic damping, required to achieve submicron subsurface damage. 2. Control of pitch (wheel to component surface) to better than 0.333 arc seconds, required to achieve a total thickness variation (TTV) tolerance of 0.5 H9262m. 3. Control of cut-depth to better than 0.1 H9262m, required to achieve submicron subsurface damage. 4. Axial error motions of spindles better than 0.1 H9262m, required to achieve submicron subsurface damage. 5. Measurement and feedback of component thickness to 0.5 H9262m, required to achieve micron thickness tolerance. Fig. 1. Face grinding operation. The geometry of the ground flat surface is determined by the relative position of the rotary axes of the grinding wheel and workpiece. Figure 3 indicates the relative machine motions and axes. There are 11 axes, plus three automatic robot loading motions (not shown), all driven under servo control. These are: S1 Grinding spindle C Workhead spindle Z Infeed X Crossfeed S2 Truing spindle W Dressing axis A Tilt pitch B Tilt yaw S3 Chuck wash brush P Probe thickness Wash arm As described below, the flatness accuracy can be achieved by the superimposed rotations of the rotary axes coupled with an appropriate spindle alignment strategy. Further, this prototype research machine benefits from the incorporation of the following state-of-the-art features for the automatic supervision and control of the machining process. 4.1 Adjustment of the Workpiece and Grinding Wheel Planarity The relative alignment of the two rotary spindles S1 and C (Fig. 3) is simplified because the geometry of the ground surface can be described by geometrical equations. The grind- ing process requires a specific angle between the plane of the grinding wheel and the plane of the workpiece to be maintained as the Z-axis infeed is applied. This angle is typically much less than a degree, so that the workpiece and wheel are nearly parallel. This angle is monitored by three gauging LVDT sensors which measure the displacement between the grinding spindle housing, and a precision-machined surface on the work- spindle housing. The gauging sensors are placed around the grinding spindle housing, roughly equidistant from the centre of the wheel spindle axis in the plane of the wheel, at a known separation. The information from these sensors is fed back into the control system to amend the control for the A- (pitch), B- (yaw) and Z- (infeed) axes. This is a unique feature of the machine, to maintain workpiece flatness because, as the workpiece subsurface damage reduces and the surface finish improves the grinding forces increase significantly. This has the effect of distorting the grinding spindle to workhead align- ment, which then produces non-flat surfaces. On conventional machines this alignment is adjusted by mechanical trial-and- error adjustment, and relies on the force and deflection always being uniform. However, on this machine if the process con- ditions are changed, the alignment is automatically compensated for. This can then be optimised to suit the material and wheel conditions by changes in the software of the control system. A functional block diagram for the servo control of the Z-axis is illustrated in Fig. 4. Ultraprecision Face Grinding Machine 641 Fig. 2. Face grinding machine. Fig. 3. Axes nomenclature. 4.2 Grinding Wheels The roughing and finishing wheels are concentrically mounted on one spindle via a patented system which incorporates an advance/retract mechanism for the roughing wheel, as shown in Fig. 5. In order to maximise component throughput, a coarse- grained wheel is first used to obtain a high material removal rate. The fine-grained finishing wheel is then used to obtain the finished size and surface integrity. 4.3 Detecting Grinding Wheel Contact Acoustic emission (AE) sensors are used to establish the initial touch between the grinding wheel and component. Because of the importance of establishing first touch to very fine limits, when finish grinding, ring sensors are used on the workhead and grinding spindles. These are extremely sensitive and are located at the front of the spindles, close to the signal source. An on-machine grinding wheel truing spindle is also fitted with AE sensors which enables “touch dressing” of the grind- ing wheel. 4.4 Automatic Measurement of Grinding Forces Grinding forces are measured via sensors placed within the force loop away from external forces, such as lead screw nuts and their associated friction. Measurement of the grinding forces gives a good indication of grinding wheel wear. 4.5 Measurement of Grinding Wheel Wear and Component Thickness Grinding wheel wear is monitored together with component thickness. A specially designed anvil and LVDT probe assembly are used to measure component thickness. This is done by initially datuming the anvil and probe on to the porous ceramic vacuum chuck face to which the component is fixed. When measuring the component thickness, the anvil, which is on the same slideway as the probe, contacts the chuck datum and the LVDT probe makes contact with the face of the component, thus giving a measurement of thickness. Grinding wheel wear can be found by reading directly the position of the Z-axis, and relating this to the chuck face datum position. Thermal growth is measured by pairs of eddy current probes mounted on the workhead and grinding spindles. Any growth is automatically compensated by adjusting the relative positions of the two spindles. 642 J. Corbett et al. Fig. 4. Z-axis functional block diagram. Fig. 5. Single axis dual wheel system. 4.6 Ancillary Features The machine also has facilities for on machine component and chuck washing and also a robotic loading and unloading capa- bility to load and unload automatically components onto and from the workhead spindle. 5. Machine Commissioning Machine services consist of an air supply, grinding coolant supply and motor coolant supply, as well as three-phase electri- cal power. Air is provided by a high performance supply and conditioning system, which delivers clean dry air at 13 bar at over 5000 l min H110021 . The air consumption within the face grinder machine is around 2000 l min H110021 in the air bearings, the remain- der being for the various air purge and cleansing systems. The air is filtered to 0.1 H9262m and dried to a pressure dewpoint of H1100240C as required for the operation of ultraprecision air bear- ings. Coolant supply is by recirculating water. This is pumped at 4.5 bar at a flow rate of up to 100 l min H110021 . Coolant is distributed to various coolant nozzles, under individual control, as required by the machine process. Used coolant is returned to the collection tank, and then fed to the main coolant tank by a scavenge pump. Some water-borne debris (workpiece and grinding wheel residues) settles out here; the remainder is removed by filtration on the machine in various stages down to 0.01 H9262m. Services provision requires a multiplicity of controlled pro- cess fluid distribution points, together with appropriate safety interlock and performance monitoring systems. 5.1 The Control System The control system is in two parts, based on an industry standard Isagraph PLC, and a Cranfield Precision CNC system. Machine I/O is on a distributed Interbus S system and servo control is implemented by a Sercos fibre-optic ring. The PLC program required only minor modification during commissioning, most effort being concentrated in further devel- opment of the CNC program, particularly in grinding touch sensing, and in the truing and grinding operations. 5.2 Machine Preparation In preparation for grinding operations, an assessment of several areas was critical: 1. Machine alignments. 2. Balance of spindles. 3. Condition of wheels. 4. Application of coolant. 5. Control of machine motions. Ultraprecision Face Grinding Machine 643 Fig. 6. Grinding spindle horizontal amplitude response to out-of-balance forces. These are the major determinants of grinding surface quality, and were tackled in order. 5.3 Machine Alignments The machine tool builders had set most machine alignments accurately; metrology checks confirmed these to be in order. However, the critical alignment datum (alignment between workspindle axis and grinding spindle axis) had been lost, since the grinding spindle had to be removed, prior to relocating the machine in the Precision Engineering Laboratory. This alignment datum had to be re-established by using a miniature eddy current probe (with a measurement range of around 6 H9262m) mounted on the grinding spindle faceplate. A special purpose alignment jig was mounted on the workspindle faceplate. Measuring the variation of distance of the probe from features on the alignment jig, as the two spindles were independently rotated, allowed the angular alignment of the two spindle axes to be determined, using a multiparameter least squares fit. Fig. 7. Grinding spindle horizontal phase response to out-of-balance forces. 5.4 Wheel Balance The machine is configured to enable automatic balancing on the grinding spindle. This is included on the machine to accommodate the automatic selection of grinding wheels. The grinding spindle carries two concentric segmented cup wheels, rough and fine grit; the rough wheel is of slightly larger diameter. The roughing wheel can be selected automatically by sliding it parallel to the spindle axis, under air piston control, to engage in one of two face tooth couplings, so that it either projects or is just below the face of the fine wheel. These two configurations, with rough or fine wheel selected, have marginally different out-of-balance moments, and the automatic balancing is included to compensate for this. Figures 6 and 7 show the amplitude and phase response for a balance (displacement) sensor in a horizontal orientation, located at the grinding spindle nose, over a range of revolutions per minute (rpm) shown along the x-axis of the figure. The y- axis represents a nominal peak-to-peak displacement at that rotation rate of the grinding spindle. These data were obtained subsequent to fine (single plane) balancing of the grinding 644 J. Corbett et al. Fig. 8. Grinding spindle vertical amplitude response to out-of-balance forces. spindle. A strong resonant response can be seen at around 1200 rpm (or 20 Hz). Figure 8 shows the amplitude response for the balance (displacement) sensor in a vertical orientation. The resonant behaviour is entirely absent. Subsequent investi- gation has revealed that the source of the resonance is com- pliance in the “B” grinding wheel tilt axis (a vertical axis), as shown in Fig. 3, which is why this is only apparent to a horizontally oriented sensor. The truing wheel balance is also critical. Figure 9 shows the horizontal response of the truing spindle, subsequent to fine balancing. Again a small resonance is apparent in the horizontal direction, at around 4000 rpm (67 Hz). The truing spindle is mounted on the X-axis carriage, and this horizontal resonance is in the direction of the X-axis motion. Once more this is due to the compliance of the motion system in its drive direction. This has a lower impact on grinding performance than grinding spindle balance, although truing the spindle out- Fig. 9. Truing spindle horizontal amplitude response to out-of-balance forces. of-balance motion will impart a small-scale cyclic topography on to grinding wheels, which in turn affects grinding quality. 5.5 Wheel Condition On this machine, wheel form is imparted through a truing operation, and wheel condition is maintained through sub- sequent dressing, in between relatively infrequent truing oper- ations. The truing (forming) operation specified in the machine design involves a plunge “grind” similar to the wafer grinding operation, although in this case the cup grinding wheel and parallel truing wheel make contact at their periphery. The grinding wheel form was found to be in error by 0.2 (12 minutes of arc). The truing operation was amended to rectify this. Truing is now accomplished by a plunge and a subsequent traverse of the X-axis. Correctly applied, this can produce the Ultraprecision Face Grinding Machine 645 correct (planar) wheel form, since the grinding spindle axis had previously been set accurately to be perpendicular to the X-axis motion. 5.6 Coolant Application Considerable effort was concentrated in the alignment of the coolant nozzles, in order to deliver sufficient coolant into the grinding interface. This is particularly important here where the grinding arc of contact is so long, at approximately 200 mm for a 200 mm wafer. 5.7 Motion Control New grinding routines and complex motion profiles were developed for the grinding. A full wafer grinding cycle consists of an initial rough grind, followed by a finish grind. In each grinding phase, the work and grinding spindles are set to rotate, coolant is then applied, and the grinding spindle is fed into the work rapidly until the acoustic emission sensor detects a touch. Following rapid deceleration, the spindle is fed in further, in three stages, at successively lower feedrates, and for successively smaller feed depths. Finally, after a dwell (spark out) time, the grinding spindle is retracted. This in-feed motion sequence is complicated by the simul- taneous slaved motion of the A and B tilt axes. Completely coplanar plunge grinding is likely to result in non-planar grinding results. In order to achieve planarity in the finished surface, a slight angle must be maintained between spindle axes at first contact, and this angle is gradually reduced to zero (nominally) at the conclusion of grinding. A further modification to the infeed motion is imposed by the results of the three-point gauging, which monitors, in-process, the deflec- tion of the machine due to the high static grinding forces. The measurements of the gauges modify the demanded angle between spindle axes throughout the grinding process. 6. Initial Grinding Trials Initial grinding trials were conducted on monocrystalline sili- con, using 200 mm wafers. Initial grinding trials were conduc- ted, using parameters selected