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Figure 255. Some typical micro-tooling: drills and boring tools. Machining and Monitoring Strategies otherwise lead to premature tool breakage. While an- other note of concern when micro-drilling is its pene- tration rate. If too high a drill feedrate is programmed, then the micro-drill will immediately fracture. Some micro-drilling manufacturing companies either rec- ommend a single-ute assymetric drill geometry – al- lowing high chip loads, coupled with an ecient chip evacuation process, conversely, another approach is to increase the number of drill utes, but this may cause chip evacuation problems with ‘sticky’ workpiece materials. At present, micro-drills can normally drill holes with L/D ratios of 5:1, but it is anticipated that these L/D ratios will soon be up to 10:1. In any micro- drilling operation, the rst few revolutions of the drill are crucial (Figs. 49 and 50), as the drill’s point expe- riences eccentric forces as it enters the cut, with any workpiece irregularities causing the drill to ‘walk’ – re- sulting in its bending, breakage, or at the very least some ‘helical wandering’ (i.e axially – see Fig. 70) as the drill penetrates into the part. In order to mini- mise the eccentric forces as a micro-drill enters a hole, many micro-drilling manufacturers recommend that a pilot hole (Fig. 50b) of between 1-to-2 drill diameters deep is produced, utilising a short and rigid pilot drill. A pilot drill’s point angle (i.e. see Fig. 46 – top) should have an included angle that is either identical, or greater than that micro-drill producing the nal hole. If smaller included angles were selected, as the drill en- ters the pilot drilled hole, this causes the micro-drill’s cutting edge to chip. is tool wear-eect is because, as the micro-drill’s more shallow point angle initially contacts the previous pilot-drilled hole – with its more acute angled geometry, as the micro-drill enters work- piece, this contact will take place at the outer edges of the lips before the drill point touches the hole’s surface. In lieu of a pilot-drilled hole, then begin the feedrate at somewhat less than the nishing feed, or perhaps, utilise a ‘pecking-drilling action’ – drill to a predeter- mined depth, partially withdraw the drill, then drill deeper into the workpiece, once more partially with- draw the drill, then repeat this sequence. ‘Pecking’ has the further benet of avoiding dwell at the bottom of a ‘blind hole’ , this being an important surface integrity feature with work-hardening materials. Hole tolerances that have been satisfactorily micro- drilled in a range of workpiece materials are of the order: ± 5 µm, with tolerance-in-roundness (TIR) of <2.5 µm. By utilising coolant delivery at high pressure, either through-the-drill, with the ‘larger drill sizes’; or alternatively ood coolant for minute micro-drills; usually allows a 30% increase in cutting speeds coupled to extended drill life. Although care should be made when utilising through-coolant drills 65 , as their small coolant hole diameters will simply clog unless the cool- ant has been passed through some form of micro-ltra- tion unit, to remove ‘nes’ and other types of potential clogging debris. Micro-Mills and Milling Over the last few years, with the advances in cutting tool materials in combination with that of cutting tool technology, has led to signicantly smaller milling cutter diameters with more complex geometries be- ing produced (Fig. 255-bottom le). In fact, several important technologies have developed during the last decade to assist the cutting tool manufacturers to cater for the micro-machining industries. Probably the most important of these new technology applications is the design and development of very high accuracy six-axis CNC tool grinding machines, having temperature con- trol and coolant condition monitoring – these being key elements in the cutter-grinding process. Grinding tolerances held by these machines on say, an φ0.12 mm ball-ended end mill, must be within <2.5 µm (TIR). Complementary to these multi-axes CNC grinders, has been the improvements in diamond grinding wheel technology, in conjunction with appropriate grinding wheel metrological inspection techniques, that have contributed to the signicant advancement in micro-tool manufacturing quality and productivity. Whilst continuing our discussion on the cutting tool material front, micro-milling cutters are now being produced from extra-ne grain cemented carbides, al- lowing sharp cutting edges in conjunction with good milled surface nishes. For example, one Japanese mi- cro-tooling manufacturer oers a standard range of ‘micro-mills’ , from: φ5 µm to φ55 µm; in incremental 65 ‘Ethanol’ , is an alternative applied lubrication strategy, to that of either the usual water-based ood coolant method, or by through-the-tool delivery. Here, ethanol is a form of alcohol, occurring naturally in the sugar fermentation process, its benets are that it has less-than-water viscosity, enabling it to penetrate into the tool/chip interface in a superior fashion to that of other coolants. Ethanol is usually delivered to the cut- ting zone in the form of a spray-mist. While another bonus of the application of ethanol is its low evaporation point, giv- ing it eciency in cooling and as a lubricating agent for tool spindle speeds (i.e. of up to and including) >60,000 rev min –1 . Moreover, ethanol simply evaporates and this eect negates any disposal costs, while it provides a slight ‘chilling-eect’ on the part, minimising thermal growth problems on miniature- sized components. Chapter sizes of 25 µm. is range includes four-ute square- ended cutters (i.e. similar to the one depicted in Fig. 255 – bottom le), together with ball-nose end mills, plus some customised micro-mill tooling. Complementary to these micro-milling cutters, are their respective toolholders 66 , which must hold and securely contain the tool’s shank in an accurate and precise manner. Any form of tool runout when held in the toolholder must be kept to absolute minimum. e toolholder’s importance in the micro-milling op- eration is oen overlooked, at the user’s peril! By way of illustrating this fact, if one has a two-ute φ0.5 mm end mill then the chip-load will be <0.010 mm tooth –1 . So, if the micro-mill has a runout of 5 µm, the cutter is only utilising one of its utes – by a factor of 100%. is micro-mill’s runout condition, leads to cutter in- stability coupled with a poor milled surface situation, with the potential for either reduced tool life, or break- age. Micro-milling tool deection and its subsequent breakage when utilised for micro-machining opera- tions, are principally caused by three main factors, these are: 1. Micro-machining creates a substantial increase in the specic energy, as the chip thickness decreases – meaning that here, as the chip gets thinner with smaller D OC ’s, the micro-mill is subject to greater resistance, when compared to that of ‘macro-mill- ing’. Moreover, it is almost as if the workpiece mate- rial becomes harder during micro-machining. is resistance force to machining here, is strong enough to exceed the bending strength of tool – even prior to any wear occurring, leading to tool breakage. A method of minimising tool breakage and prevent- ing its occurrence, is to ensure that the chip thick- ness is smaller than the radius of the tool’s edge, 2. During micro-milling, a sharp rise in the cutting forces and stresses resulting from chip-clogging may cause tool breakage – when say, utilising a two- ute cutter, each cutting edge removes chips from the machining vicinity by only a half rotation of the tool. Likewise, if chip-clogging occurs – within a few micro-tool revolutions, the cutting forces and 66 ‘Micro-milling/-drilling toolholders’ , are typically manufac- tured with taper-/face-tments of the : ISO 15 to 30, or HSK- E25 to 32 types. A typical range of micro-toolholders, might cover tool shank sizes in a range from: φ0.5 mm to φ2 mm, in 0.01 mm increments. With ‘matching micro-tooling’ , to typi- cal tolerances of: <+0/-4 µm, with these micro-tools typically being, for example, rated @ 40,000 rev min –1 , having a runout of <3 µm @ 4xD. bending stresses increase beyond the limit of the tool’s bending strength, thus causing it to break. A possible solution to this problem, is to utilise alter- native tooling materials such as micro-grained M-2 HSS, as they are more exible and as such, they can tolerate any likelihood of chip-clogging in a more compliant manner than their cemented carbide counterparts, 3. While micro-machining very ductile workpieces, micro-mills can lose cutting eciency as a result of BUE – this results in increased lateral pressure (i.e. as feeding occurs) on the micro-mill, causing it to slightly deect. is increasing tool deection due to the presence of BUE, increases the stress gener- ated with every cutter rotation, quickly causing the micro-tool to break. is well-known BUE phe- nomena is termed: ‘extensive stress-related break- age’ , which could possibly be minimised by adopt- ing a somewhat more ecient and benecial cutting uid lubrication strategy. NB Due to these (above) micro-tooling related phenomena, many of the latest micro-milling ma- chines are equipped with sensors to dynamically- measure and monitor the cutting forces acting on the micro-mills. Micro-Boring Tools and Internal Machining Operations e production problems of drilling small holes in workpieces is a big challenge, but this is nothing com- pared to that of the technological complexity of bor- ing very minute holes and other internal features in components. Some tooling manufacturers oer insert- style tools that are specically-designed to bore-out small hole diameters, even down to just ≈φ0.3 mm. A typical micro-boring tool is illustrated in Fig. 255c (i.e. ≈φ0.3 mm), where the main features of the tool- ing are explained. In the enlarged diagram of a mi- cro-boring insert geometry schematically illustrated in Fig. 255 – middle right (i.e. for a ≥ φ0.7 mm boring insert). is particular micro-boring insert geometry, is designed for both boring and proling operations into holes of ≥φ0.7 mm, although the clearance (‘t max ’) will only cope with shallow prole-depth features, be- fore fouling on the tool’s shank. Micro-tool stiness is quite high and occurs due to the enlarged shank di- ameter, allowing reasonably large L/D ratios of >10:1 to be bored – which is remarkable, considering the minute size of these insert’s. Other micro-machin- Machining and Monitoring Strategies ing 67 operations that can be undertaken include (Fig. 255 – middle-right): grooving; threading; face-groov- ing; back-boring (i.e. not depicted). Some important micro-machining factors need to be addressed, prior to boring-out previously drilled holes with these micro-boring tools (Fig. 255c), such as: • Setting the micro-boring tool at the hole’s centre height (Fig. 255d – top) – this is the most impor- tant preliminary step when about to commence a micro-boring operation. A micro-boring tool that is incorrectly set below the hole’s centre-height (Fig. 255d – bottom) – adversely aects its performance in several ways. It could foul on the curvature at the bottom of the pre-drilled hole, through a reduced edge clearance angle (primary relief). Moreover, the ‘tool fouling’ causes the insert to rub against the hole impeding the cutting action, which in turn, creates vibration causing the insert’s tip to be ‘driven-down’ still further below centre. As a conse- quence, the tip is forced deeper into the workpiece material – due to the radial sweep of the bore. us, as the top rake angle is increased – relative to that of the workpiece, the clearance angle is reduced. is geometric change in the micro-boring tool’s geom- etry, causes the insert to ‘snatch, or grab’ material rather than cut it, which then increases vibration, so tool breakage will shortly ensue, NB Due to the minute dimensions of these micro- tools 68 , it is very dicult to set the tool exactly on 67 ‘Micro-machining’ , has been dened according to a dierent approach, namely, concerning the actual workpiece’s volume, as follows: ‘It is the [workpiece] size in which the work envelope is smaller than 490 cm 3 ’. (Source: Destefani, 2005) 68 ‘Meso-scale machining’*, this term has been coined and has been dened as: ‘Millimetre-sized parts, with micron-sized fea- tures’. For example, minute component features that can have micron-sized tolerances (<1 µm). Recently in the USA, these ‘meso-machining technologies’ have included both micro- milling and -turning, with in the former case, utilising micro- end mills of ≈φ20 µm, while in the latter case, using micro- turning tools of 10 µm in width. Hence, these technologies can machine part features in the 25 µm range.(Source: Kennedy, 2006) *e term: ‘meso-’ meaning: ‘middle’ , or ‘intermediate’. (Source: Concise Oxford Dictionary) NB In this case, the so-called ‘meso-machining operations’ , refer to machining in between the:nano- and micro-machin- ing ranges. centre-height – the ‘ideal positioning’. erefore, the tool can be set marginally higher than ‘true’ cen- tre-height, which increases the angular clearance – relative to the hole, thereby allowing a freer cutting condition. Further, if any potential vibrations occur the micro-tool is both deected downward toward the centre and (radially) slightly out-of-cut, some- what reducing this ‘grabbing tendency’. • Choosing the right speed and feed – when the bore is <φ6 mm, then ‘standard’ speeds and feeds cannot be used. For example, when boring say, a φ1 mm hole, one would expect to utilise a cutting speed of perhaps ≈140 m min –1 , which equates to a spindle speed of ≈44,600 rev min –1 – which is totally un- realistic for most types of turning machine tool. If any vibration occurred, then the micro-boring tool would be immediately destroyed. So pragmatically, if we limit the spindle speed to 6,000 rev min –1 , which would signicantly drop the cutting speed to ≈19 m min –1 , we would also need to complemen- tarily reduce the micro-boring insert’s pressure by reducing the D OC to <0.1 mm which – to minimise tool deection and potential breakage. Feedrates for ‘macro-boring operations’ , are normally dic - tated by the ‘close-relationship’ between its tool nose radius and the bored-hole surface texture re- quirements. Conversely, for micro-boring these are not the controlling factors anymore. Here, minimi- sation of cutting forces is vitally important by se- lecting a feedrate that should not exceed 0.125 mm rev –1 – which automatically overcomes any micro- bored surface texture issues, • Ensuring adequate chip evacuation – is a real dif- culty with such small bored holes, as little in the way of unlled volumetric capacity exists with the micro-boring tool situated inside the hole. So, with the micro-boring tool inside the hole – accounting for 60% of the available volumetric space, how can the chips escape? By utilising a ‘through-insert- coolant’ micro-boring insert (Fig 255c and d) with the coolant under pressure, it can reach the cutting edge. is coolant aids in both forcing and ush- ing chips out of the bore’s mouth, which minimises any of these work-hardened chips, creating a chip- packing tendency in the bore, with the potential of causing tool breakage, • Providing adequate tool stability and location – these are important factors for micro-boring tooling. Micro-boring inserts and their respective toolhold- ers are designed for quick, simple and repeatable Chapter setups, with some tooling manufacturers design- ing the tooling assembly to avoid inserts twisting in-cut. ese design innovations range from: in- sert-clamping ats; inserts having angle-ground back-ends; to that of ‘teardrop-shaped inserts’ (i.e see Fig. 255d) – this latter type rmly ‘wedging’ the insert as it attempts twist – under the torque and bending moments while boring. If these micro-tooling factors are adhered to, then the problems that are likely to be encountered when mi- cro-machining are signicantly reduced, which means that any form of micro-machining activities can be achieved – with due diligence. So that a considerable amount of micro-machining can be undertaken, ma- chine tool companies have been developing a range of specialised machines to cater for this market. Hence, with the expansion of micro-machining activities this being a somewhat ‘growth industry’ 69 , let us briey consider these specialised micro-machine tools and the technical challenges they had to overcome in order to cope with such minute cutters and invariably minis- cule workpiece volumetric dimensions. .. Micro-Machine Tools CNC machine tools designed to machine parts, or moulds that have small dimensional size, typically with a linear dimension of ≤ 10 mm, or having detailed part features of ≤ 0.1 mm, require some signicant ac- curate and precision enhancements, if they are to cope with the micro-machining demands of late. A typical micro-machining machine tool will be mentioned, ‘high-lighting’ some of its important design features, so that one can gain an insight into the careful at- tention to detailing necessary for minute component manufacturing. One such machine tool produced by the Japanese company Makino (not shown), has a ‘footprint’ of ≈ 1.8 m × 2.4 m, with a signicant weight of ≈ 5 tonnes, with a worktable size of ≈ 300mm × 200 mm, having three axis travel of: 200 mm in X-axis; 150 mm in Y- 69 As a ‘total aside and not related to the main topic’ and, for you ‘acionados’ of English language – isn’t this above statement (i.e. in italics), the basis for a: ‘double’ – oxymoron? ** Oxymoron: expression with contradictory words – e.g. ‘Wise fool’ or, ‘Legal murder’. axis; and 150 mm in Z-axis. Obviously for such minute micropart features to be machined, the positional ac- curacy and repeatability of the slideways are of crucial importance, as such, the machine’s positional accuracy is ± 0.3 µm, with a repeatability of ± 0.2 µm. is ma- chine features unique workholding equipment, such as a direct-chucking spindle, which has been designed to eliminate the toolholder-induced variables, en- abling miniature components to be produced, such as: medical instruments; semi-conductor devices; optical products; etc. A major factor with any micro-machining machine tool like the one mentioned above, is its machining environment and more specically, its ‘thermal condi- tions’ 70 . As the minute part’s temperature changes along with that of the machine’s spindle – during machining, any dimensional modications on say, a ‘macro-scaled part’ could normally be considered as negligible, but on micro-sized workpieces, these linear variations be- come signicant dimensional issues. In order to vir- tually eliminate spindle growth the machine tool has to be designed for a stable environment, when one is attempting to hold ‘micrometre-accuracies’. Machine tool features necessary in reducing this spindle/ma- chine expansion, include, an automatic spindle lu- bricant temperature controller – to reduce spindle growth, coupled with the machine’s granite base – as granite has only 10 to 20% of the thermal conductivity 70 ‘ermal conditions and eects’ , in particular will aect either the machine tool’s linear expansion/contraction – depending upon whether there is a temperature rise, or fall, respectively. Simplistically and in this instance, ignoring any uncertainty factors, then , ‘Coecient of thermal expansion’ which is nor- mally denoted by the symbol ‘α’ , can be dened as: ‘A measure of the change in length of a material subjected to a change in temperature’. us: α = Change in length L (�T) = strain (�T) = є (�T) Where: L o = original length (mm), ∆T = change in temperature (°C). Or, this thermal expansion equation can be alternatively written, as follows: ∆L = (L) (Cα) (∆T) Where: ∆L = change in length – by thermal expansion (mm), L = original length (mm), Cα = coecient of thermal expansion, ∆T = change in temperature (°C). NB e uncertainty contributions here, may be combined, in the following manner: u d = √(u 0 ) 2 + (u c ) 2 Where: u d = ‘design- stage’ uncertainty, u 0 = interpolation uncertainty, u c = instru- ment/equipment uncertainty. (Source: Figliola and Beasley et al., 2000) Machining and Monitoring Strategies of an equivalent cast iron structure, thus structurally- minimising the eects of ambient temperature changes. Moreover, for any form of extremely critical and preci- sion miniature part manufacture, the entire machine tool can be situated in a ‘thermal chamber’ , this in ef- fect acts as a ‘controlled-temperature environment’. Micro-tools of for example, φ50 µm – as has already be mentioned, are almost impossible to see, let alone attempting to set them to length. In order to facilitate this minute tool setting operation – to sub-µm accu- racy, the machine tool company developed a hybrid automatic tool-length measuring system. is system of tool length measurement is achieved by combining a static low-pressure contact sensor in conjunction with non-contacting sensing 71 , this being performed while the tool is rotating at speed. Together, these two sensing techniques permit sub-µm tool positioning accuracy, during machining operations. Obtaining the most advantageous micro-machine tool for a particular type of minute ‘workpiece group’ , should be taken by considering the part’s features – in- cluding its geometric conguration, together with the level of accuracy and precision required and whether these parts are produced as ‘one-os’ (i.e. as custom- ised-specials), or in various batch sizes. .. Nano-Machining and Machine Tools When attempting to machine components to toler- ances of nanometric dimensions, the actual problems considerably exacerbate, even when compared to that of machining in the sub-micrometre range. Usually, conventional cutting edges that have been honed, can- not hope to cope with miniscule D OC conditions, as the edge is just simply not sharp enough and will tend 71 ‘Non-contacting tool sensing’ , is ahieved as follows: while the spindle is rotating the tool’s tip position is measured via a non-contact electro-magnetic sensor. is tool measurement takes into account the thermal displacement of the tip, caused by its rotation. us, the system’s contolller merges these two measurements. NB e tool’s length measurement operation occurs by mea- suring the spindle growth and waiting until it stabilises – within specic user-dened limits, such as for example, within2 µm. Once the spindle is ‘stable’ , it can machine the workpiece at the desired level of accuracy and precision. to ‘plough’ , instead of cut. In order to machine such components, oen on either very ductile workpiece materials, or glasses, monolithic diamond tooling is invariably utilised with the tool orientated so avoiding its natural fracture planes. It is normal practice when machining components to a few billionth parts of a metre (i.e. 1 nm = 10 –9 m), that a wide range of ‘pro- cess, environmental and machine tool inuences’ are acknowledged and then subsequently minimised. In fact, the Japanese Professor Nakazawa (1994) consid- ered the machine tool’s inuences when machining at high precision and, stated they could be broken down into the three following requirements, mentioning that the: 1. Machine tool’s built-in reference must not vary, 2. Machine tool’s must follow this kinematic reference at its highest precision, 3. Machine’s movement must be accurately trans - ferred to the workpiece. Moreover, Nakazawa presented a useful table of the ma- jor factors that disturb the relative tool-to-workpiece po- sitions in ‘forced machining’ , as presented in Table 17. In fact these ‘requirements’ are usually adequate for the level of machining into the sub-micrometre range, but once one proceeds to ‘ultra-high precision machining’ operations within the nano-range, then many other factors contribute to the overall success of the cutting process (Fig. 256). As previously men- Table 17. Factors that disturb relative tool-to-workpiece cut- ting positions Factors: Internal/ External: Conditions: Heat source Internal Machining energy at machining point motor, ball leadscrew, bea- ring, guide, hydraulics External Convection (air conditioning) Radiation (lighting, body heat) Vibrations Internal Vibrations created by the ma- chining mechanism motor, ball leadscrew, joints, bearings, guides External Other machines, moving vehicles, people Dust Internal Matter from the workpiece, or tool External Dust in surrounding air, particles in cuting uid [Source: Nakazawa, 1994] . Chapter Figure 256. The error sources from the processes; environment and machine tool; on workpiece accuracy. [Source: Breuck- mann & Langenbeck, 1989] . Machining and Monitoring Strategies tioned, ultra-precision machining does not usually en- tail machining very diminutive components, but it is normally concerned with holding exceptionally tight tolerances on macro-sized parts (Fig. 257b). In fact, a diamond turning machine tool that can manufacture components to nano-tolerances is depicted in Fig. 257a. is ultra-precision CNC lathe is considered to be the most accurate and precise machine of its type currently available. erefore, it is worth discussing the machine and its environment, as its installation and operation encapsulates all of the error sources shown in Fig. 256. Nano-Machine Tool and its Facilities In Fig. 257a, is shown a very expensive and highly sophisticated machine tool, it was delivered to the Atomic Weapons Establishment (Aldermaston, UK) from the manufacturers Craneld Precision Engi- neering (UK). Its intended purpose was to machine Perspex optics and large thin-walled aspheric shells having form errors of <5 µm, together with wafer-thin laser targets having micro-machined features, but held to tolerances of ± 10 nm, by single-crystal (monolithic) diamond tools – these diamond cutting edges being orientated along their correct crystallographic plane, allowing turned surface texture values (Ra) of <1 nm to be achieved. If an ultra-precision machine tool is required to work at nanometric resolution, then if it needs to be located within a manufacturing plant, the machine tool must have a special-purpose facility designed, constructed and built to exceptionally-stringent and specied requirements. Prior to discussing this facility, it is worth describing some features of this a machine tool, so that one can comprehend the signi- cant technical problems that had to be overcome, in order for it to achieve a ‘true’ nano-machining capabil- ity. e machine tool was constructed on a polymer concrete base, that consisted of 8 tonnes of synthetic granite, giving the desirable properties of: excellent thermal stability; high stiness; plus good vibration damping characteristics. Strangely for a diamond turning machine, the headstock was equipped with a hybrid hydraulic/air spindle rather than an air-bear- ing design, because this spindle’s specially-designed construction enabled an increased load-carrying ca- pacity (57 kg), coupled to superior stiness. Spindle speed range was from: 200 to 5,000 rev min –1 . ree- axes were tted, two linear axes – running on fully- constrained hydrostatic dovetail bearings with linear motor drives and one rotational axis. ese axes kine- matics were: X-axis (520 mm); Z-axis (220 mm); B- axis (360°) – as illustrated in Fig. 256-middle le. A la- ser-positioning system was tted to the axes, having a resolution of 1.25 nano-metres, incorporating a wave- length tracker, to compensate for any environmental changes to the air: temperature; pressure; humidity; hydrocarbon content; etc. e CNC controller utilised a high resolution (1 nm) fast feed-forward operation, thereby reducing servo-following errors, combined with real-time axis compensation together with a two-dimensional error compensation capability – for straightness and orthogonality. Tool-setting errors on the monolithic diamond tooling, were minimised by a unique probe and optical setting technique (Fig. 256 – middle-right), reducing form errors, when utilising the rotational B-axis for spherical/aspherical turned component geometry 72 . e nanocentre machine tool, was housed in a tem- perature-controlled environmental enclosure, consist- ing of the temperature being held at: 20°C ± 0.5°C, en - closed within 100 mm thick high density polyurethane foam panelling – providing high thermal resistance, with an air-tight seal. e positive-pressure air-supply unit was situated outside – in an adjoining plant room, ducted into the enclosure by dra-free ducts, with ten platinum resistance temperature probes (i.e. resolu- tion 0.001°C) strategically situated within the volume space, continually monitoring the enclosure’s temper- ature. Lighting from the uorescent lights had their chokes removed from the enclosure, thereby reducing the localised air temperature ‘stratication eects’ by 72 ‘Form errors’ , the principal causes of form error when spheri- cal/aspherical diamond turning components, can be sum- marised in the following manner: Conical error – due to spindle misalignment, Chevron and Ogive errors* – due to rst-order tool wear and centring errors,Waviness error – resulting from sub- strate vibration and tool prole error, Astigmatism – created by the component xturing arrange- ments and material stiness. *Chevron error occurs when turning a convex spherical formed component – due to the tool ‘over-shooting’ , while, an Ogive error results from the tool stopping short – this latter error creating an ‘ogival arch eect’ , with both these prole errors being due to an incorrect tool centre height setup.(Sources: Myler and Page, 1988; Wheeler, 2001) – – – Chapter Figure 257. Nanometric machining to ultra-precise dimensional and surface texture characteristics. [Source: Lamb & Gull, 1999] . Machining and Monitoring Strategies over 1°C. e hydraulic and electronic cabinets were temperature – controlled to ± 1°C. e ‘refractive index of the air’ had to be corrected – based upon a modi- cation to the Edlen (1966) equation, as the laser path positional monitoring system would otherwise be af- fected, with a correction factor being entered into the CNC controller. e ‘T-shaped base’ of the Nanocentre was sup- ported on three pneumatic mounts that were ‘tuned’ to eliminate oor-borne vibrations of ≥ 2.5 Hz. Two types of vibrational sources occur, namely forced- and self- excited, with the forced vibrations originating from external sources – through the foundations, while the self-excited vibrations normally being the result of in- ternal sources. A ‘oor vibration audit’ on the vibra- tional inuences was conducted, to establish whether the overall enclosure was suciently vibration absor- bent. e oor vibration spectra gave typical vibra- tional readings of 1 nm (rms) at frequencies of 25 Hz, during the tests, with the external air compressors emitting a oor borne 25 Hz frequency component, which had to be subsequently nullied. Further testing procedures were undertaken, including ‘modal analy- sis’ and ‘thermal imaging’ of the machine’s structure, together with a full calibration of the machine tool’s kinematics. Once all of these tests and various others had been completed and compensated for, then a machining testing program could then be undertaken. A typi- cal test piece is illustrated in Fig. 257b, where an al- uminium 6061-T6 part was heat treated and then stabilised, of φ250 mm copper-plated (200 µm depth coating) part. ese testpieces were faced-o with a monolithic diamond tool – taking very shallow D OC ’s of just a few micrometres, producing for example, a face-turned surface texture averaging ≈ 2.8 nm Ra. Later, proling tests were also conducted, prior to - nal operational acceptance by AWE, from the machine tool builder. Prior to completing this summation of the just some of the rigorous testing procedures carried out to ensure that the Nanocentre machine tool could oper- ate within the nanometric range of ultra-high machin- ing operation, it is worth making an unusual point concerning human intervention at this exacting-level of machining. It was found that when several person- nel were within the machine tool enclosure while machining took place, then the thermal output from these people, inuenced the part’s dimensional size – without actually contacting the machine, by simply acting as a heat-emitting source 73 . Moreover, it was also found that when diamond-turning 74 by facing-o a very ductile testpiece similar to that depicted in Fig. 257b, when these people were in conversation during the nanometric cutting of the part, their ‘voice signa- tures’ – in the form of air-borne vibrations were ‘ma- chined’ into the surface – in a similar manner to that of an acrylic recording of a record in the past! erefore, in order to ensure that both the human thermal eects and the vibrational perturbations (i.e. by air-borne vi- brations – talking), the personnel had to be removed while any ultra-precision machining operations were in progress. Ultra-precision machining at these nano-metric levels of operation, severely stretches today’s levels of technological challenges for: machine tools, me- trology, plant and equipment, as we approach that of atomic-levels of precisional uncertainties 75 . It is not just the case of purchasing an extremely accurate and 73 ‘Human body – as a heat source’. e average body – in a ‘sedentary state’ , will emit ≈100 W of heat. So, here in this case, when there are two people present in the machine tool enclosure, they will radiate ≈200 W of heat – inuencing the machine’s and hence, the workpiece’s thermal expansion – when machining at nanometric levels of accuracy and preci- sion. (Internet source: Burruss, R.A.P., Virtual People, 2005) 74 ‘Monolithic diamond’ , has some of the following characteris- tics: hardness of ≈8,000 Hv; Density (ρ) of 3,515 kg m –3 ; com- pressive strength of 7,000 MPa; and a Young’s modulus (E) of 930 GPa.(Source: Cardarelli et al., 2000) 75 ‘Atomic radius’ , for example, for some typical elements, ranges from that of: carbon, having an atomic radius of ≈0.071 nm (i.e. its atomic ≈φ0.142 nm)*, iron’s atomic radius is ≈0.124 nm (i.e. ≈φ0.248 nm, or ≈¼ nm)*, Aluminium’s atomic radius is ≈0.143 nm (i.e ≈φ0.286 nm, or >¼ nm), Cesium’s atomic radius is ≈0.265 nm (i.e ≈φ0.530 nm, or >½ nm).(Source: Callister, Jr. et al., 2003)*Slight digression here, may help explain why these atomic radii are important, when certain elements are alloyed together, such as iron and carbon, these being the main con- stituents of plain carbon steel.When an allotropic change oc- curs to the iron’s atomic lattice structure (i.e. BCC→FCC @ ≈910°C), then the carbon being somewhat smaller, can t into these (now) larger interstitial sites – voids – within the FCC iron lattice – distorting the adjacent iron atoms. Upon rapid cooling (e.g. by water quenching), some of the carbon is ‘trapped’ and severely distorts the structure as it attempts to transform back to the original BCC form. Hence, this dis- torted structure of iron-carbon – termed martensite, is both a very hard, but brittle structure, requiring tempering: if it is to act as a hardened and tempered workpiece material. is is the basis (i.e. somewhat simplied), behind this iron-carbon heat-treatment process. Chapter . [Source: Breuck- mann & Langenbeck, 1989] . Machining and Monitoring Strategies tioned, ultra-precision machining does not usually en- tail machining very diminutive components, but it is. dimension of ≤ 10 mm, or having detailed part features of ≤ 0.1 mm, require some signicant ac- curate and precision enhancements, if they are to cope with the micro -machining demands of late uncertainty, u c = instru- ment/equipment uncertainty. (Source: Figliola and Beasley et al., 2000) Machining and Monitoring Strategies of an equivalent cast iron structure, thus structurally-
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