Cutting Tool Technology Episode 8 docx

8 Cutting Fluids ‘Everything ows and nothing abides.’  (540 – 480 BC) [An early Metaphysician from Ephesus (Asia Minor), in: On Nature] 8.1 Historical Development of Cutting Fluids General speaking, metalworking mass production techniques can be traced back to the 16 th century, but it was really not until the late 18 th century that engi- neers in the industrialised countries paid close atten - tion to increasing production, due to the vast rise in their populations and signicant industrial growth. In Europe at that time, two countries where important areas of applied machining and uid research were being pursued was: in France, where the machining of metals was being investigated and developed into a science – specically in terms of the eects of tool feeding and lubrication and its aect on surface nish; also in the mid-19 th century in England, where the ef- fects of water as a coolant to enhance tool performance was also studied. us, as these research activities pro - gressed, complementary advances were taking place into the study of tool materials their heat treatment and in particular, tool hardening techniques. As has happened on many occasions in the past, considerable advances took place as a result of the enormous de - mands for armament manufacturers and their produc - tion needs during times of war, signicantly adding advancements and renements to the: machine tools; tooling; as well as for lubricants. As these research programmes developed, it soon became clear that for cutting uids, while water may have had the optimum specic heat capacity of all available uids, it brought real problems due to corrosion of the machined com - ponents and to the exposed surfaces of machine tools. Frequently, such related losses far outweighed the ben - ets of increased production throughput and the im - provement in tool life that it imparted to the overall manufacturing process. A simple solution was at hand in the form of corrosion inhibition, via the use of: animal oils; fatty acids; soda; which when combined with water to form a ‘soap’ , oered an improvement in product protection against rusting, while eectively retaining the overall cooling properties of water. In particular in the North of England during the mid-19 th century, notably around Manchester and the Hudderseld areas – where the world’s major cotton and woollen industries were now in full production. ese areas, had for centuries used the benets of ‘so-water’ , which unfortunately had a tendency for the coolant solutions to generate large quantities of foam, hence the term ‘suds’ which is sometimes used to the present day, although this name is hardly rele - vant to modern-day cutting uids. Almost by accident and as an incidental benet of these ‘soap solutions’ , they were found to impart improved lubricating prop - erties between the tool and the component, through a ‘machining mechanics’ and ‘chemical relationship’ that was at the time, not fully comprehended. In the meantime, mineral oil: which had advanced from being simply thought of and used as just an ali - phatic additive to vegetable oil, to that of becoming recognised as a useful lubricant, which was at the time currently and widely available. is mineral oil was de - monstrably shown at the time, to oer improvements to both the machined surface nish quality and en - hancing the tool’s lubrication. At the beginning of the 20 th century, experimental studies into topics such as the initial studies into: boundary lubrication; lubricity; plus its relative viscosity; for the newly-developing en - gines in the automotive industries were being rapidly developed. Moreover, when general manufacturing industries started the mass production of consumer goods, the resultant quality was of prime importance and basic water lubrication was now no longer su - cient. By the mid-20 th century, the preliminary forays into the basic development of today’s modern-day cut - ting uids occurred. At this juncture, it soon became evident that it was essential to combine the properties of several dissimilar uids to produce an early, but ‘workable’ form of cutting uid, these ‘ingredients’ were: • Oil – to act as a lubricant between the chip, tool and machined workpiece, • Water – for cooling, to extract the heat from the cutting process, • Detergent – to break down the ‘surface tension’ 1 be- tween the oil and water, 1 ‘Surface tension’ , this is oen generally dened as the: ‘Inter- facial tension between two phases, one being a liquid, while the other is a gas’. More specically, surface tension is a physical force in the surface of the liquid that arises as a result of the liquid’s atoms pulling their neighbours in all directions. While atoms deep in the liquid have no net force applied to them, conversely, surface atoms have no neighbours above them, as a result they experience a net inward force from the bulk of atoms below them. Hence, this net inward force is known as its surface tension, with the greater the radius of curvature, the higher the surface tension (i.e see Fig. 208a). Hence, a ‘droplet of water’ sitting on a at surface – termed a ‘spherical cap’ has  Chapter  • Sulphur – to act as an ‘extreme pressure’ (EP) ad- ditive to reduce frictional eects at the various cut - ting interfaces. NB Sulphur was soon to prove unpopular as a sat- isfactory EP additive, as it had a tendency to stain, or erode certain decorative machined nishes for specic metals and alloys. 8.2 Primary Functions of a Cutting Fluid In the previous section, it was recognised that two of the primary functions of a cutting uid was to cool and lubricate both the workpiece and cutting tool’s edge. In addition, one could add the improvement of machined surface quality and an increase in tool life (i.e. see Fig. 193b). Further, it has been shown that a reduction in spindle power is an added bonus to many machining processes, which oers considerable savings when this reduction in electrical demand is accrued per annum. If a problem occurs where work-hardened swarf dis - posal from the cutting vicinity presents an obstacle to ecient cutting, then ushing this zone with ood coolant, may eliminate this diculty. Eective chip re- moval by the application of ood coolant (Fig. 194a: showing a twist drilling operation, 194b: milling with the periphery using a ‘porcupine cutter’), can mini - mise an otherwise serious problem on machining cen - tres where large volumes of densely-packed swarf can impede the cutting process. Even on continuous cut - ting operations such as when undertaking external/in - a high contact angle (i.e. the angle of tangency that the spheri- cal cap, or a bubble makes with the surface). When this angle is considered on a ‘wetability scale’ it has a high contact angle and as such, is not considered as ‘wet’!, due to its high curva- ture (Fig. 208a), as indeed does a typical lubrication oil. Con- versely, a liquid detergent does not have particularly a high contact angle and as such, will chemically react with both the oil and water and breaks-down this surface tension between the concentrations of an unmixed oil and water. is loss of surface tension between these two ‘products’ thereby produces a basic mixture, or suspension and it then becomes somewhat ‘milky’ in appearance, thus it is oen termed a (basic) ‘emul- sion’. ternal turning processes, an insert’s chip-breaker will break the swarf into convenient shapes and sizes, but these chips may still necessitate ushing-away – being deposited into a swarf conveyor and then onward into an adjacent skip. Swarf removal has become of sig - nicant importance as material removal rates have in - creased with latest tooling advances and high-produc - tion machine tools, where they may be continually fed wrought material allowing them to operate untended for 24 hours per day. Possibly the most stringent test for any cutting uid is in deep-hole drilling applica - tions (Fig. 58), where coolant is delivered under high- pressure through suitable coolant holes and is forced up to the cutting edge to not only cool the drill, but provide lubrication and ush any swarf back and away from the cutting vicinity. In fact, with extremely high- pressure coolant delivery systems having pressures >300 MPa, such as when using a through-the-nose indexable insert short-hole drill as illustrated in Fig. 195, the advantages are: increased speeds; penetration rates; more holes per insert edges are achievable – see the following section for more details on this high- pressure coolant delivery topic, with particular refer - ence to turning operations. 8.3 High-Pressure Jet- Assisted Coolant Delivery Probably the most important criteria in many metal cutting operations is an acceptable chip control, with respect to its: chip form; chip-ow; plus its chip-break - ing ability. It has been mentioned earlier in Chapter 2: Section 2.5, that good chip control will have an aect on: tool life, machined surface texture; cutting forces; reliability; etc. Productivity is strongly inuenced by poor chip control, as the machine tool must be fre - quently stopped to manually remove the vast quanti - ties of swarf present in the working area. is problem becomes especially acute when turning smaller inter - nal diameters on products, since limited space soon becomes lled and compacted with work-hardened chips, that can damage the recently machined surfaces. Reasonable chip control can oen be achieved by in - dexable inserts with an appropriate cutting geometry that is having chip-formers, these being developed to meet the requirements for specic machining opera - tions. Cutting Fluids  Figure 193. Heat dissipation during machining can be lessened by utilising appropriate cutting uids.  Chapter  Figure 194. ‘Standard-pressure’ (i.e. <40 bar) coolant supply for drilling and milling operations. Cutting Fluids  Trends of late, have been toward either ‘dry-’ , or ‘near-dry’ machining strategies – more will be men- tioned on these machining applications later in the chapter, however many modern materials cannot be machined dry, even with the latest coated cutting in - serts, because of the high temperatures generated in the cutting vicinity. Typically, alloys such as: austenitic stainless steel; high-temperature alloys; titanium, etc.; demand the application of appropriate cutting uids. In industrial machining applications, the availability of ‘high-pressure cooling’ (HPC) of cutting tools has proven to be very eective when machining the met - als just mentioned, while at the same time increasing production throughput. By utilising a high-pressure uid jet, it is possible to signicantly decrease cut - ting zone temperatures, while extending tool life – in certain instances by >200%, operating with lower cut - ting forces because of the improved frictional condi - tions between tool/chip interface, with an attendant reduction in machining-induced vibration levels. All of these advantages will improve the machined surface texture and oer better and more consistent dimen - sional accuracy, by a reduction in component process variability. Figure 195. High-pressure coolant supply for high penetration rate drilling. [Courtesy of Sumitomo Electric Hardmetal Ltd.].  Chapter  When utilising high-pressure jet-assisted machining at cutting uid pressures of >110 MPa with a velocity of >122 ms –1 , some precautions need to be considered, prior to applying this cutting uid strategy. Caution should be made, when using certain types of cutting tool geometries and grades, as they may not have been designed for this increased level of coolant delivery, which could if inappropriately applied, actually lower productivity. e above mentioned eects do not only depend on eective heat dissipation, but require the contact length between the chip and the rake face to be reduced. Since the application of a coolant by a high- pressure jet, partially penetrates between the tool/chip interface, via a ‘hydro-wedge’ 2 which here is created and, then provides hydrodynamic lubrication at this position in the ‘friction zone’. Hence, the shorter the contact length the lower the friction, causing a larger shear angle, which in turn lowers the chip compres - sion factor (Fig. 196). is ‘hydraulic wedge’ – as a re- sult of HPC, inuences the chip formation in several ways, it aects both the ‘up- and side-curl’ , thus break - ing them into manageable pieces as well as vectoring the chips. By aiming the HPC cutting uid jet to either the main, or secondary cutting edges this will inu - ence and aect chip-curling behaviour. is chip-curl - ing action in turn, aects the resultant tool life, as it is thought that a reason for this dierence in respective tool life is due to the temperature distribution on the rake’s face – as a result of the vectoring angle of the jet- assisted coolant application. 8.4 Types of Cutting Fluid Introduction Modern cutting uids can be sub-divided into two major classications: ‘Oil-’ , or ‘Aqueous-based’ , with further sub-division into ‘Semi-synthetic’ , or ‘Synthetic’ uids (i.e. see Fig. 197, for a ‘family-tree’ and break- 2 ‘Hydrodynamic wedge’ , as its name implies, cannot actually penetrate into the chip/tool interface, as the separation pres- sures here – at the interface – are simply far too high. How- ever, this hydrodynamic wedge acts as a sort of ‘lever’ (Fig. 196) on the emerging formation of the curling chip, changing its contact length, which in turn, modies the shearing zone and as a result, inuences the chip compression factor. down of these cutting uid groupings). In most tech- nological countries, relevant Standards for both the chemical and technical requirements are published concerning their: storage, usage and disposal, along with their pertinent operator health needs. e ‘Aque - ous-based’ cutting uids can be divided into either: ‘emulsiable’; or ‘water-soluble’ types (Fig. 197). e former ‘Oil-based’ cutting uids are supplied as ready- to-use products, while the ‘Aqueous-based’ products are normally oered in the form of a concentrate, which must be admixed with water to the desired con - centration, prior to use. Once these latter products have been mixed with water, the ‘emulsiable’ versions form an ‘emulsion’ 3 , whereas the ‘soluble’ type forms a ‘solution’ 4 . In both cases, the resulting cutting uid is termed ‘water-mixed’. The ‘Ideal’ Cutting Fluid Having accepted the fact that a cutting uid is a re- quirement for the machining of many of today’s en - gineering materials, be they either metallic, non-me - tallic in composition and are necessary for various production processes. en one must ensure that the selected ‘uid’ achieves its intended purpose, more - over, that it does not create additional problems. ese conditions imply that there are many and varied spe - cic characteristics that an ‘ideal’ cutting uid should possess, such as: • Optimum cooling and lubrication – clearly, the ‘ideal’ cutting uid would have the most favourable cooling and lubricating properties, to ensure para - mount cutting performance as measured by: pro - duction rate; tool life; surface texture, 3 ‘Emulsions’ , are a disperse system (consisting of several phases), which arises through mixing of two liquids which are not soluble in each other. Hence, one liquid forms the inner, or disperse phase, distributed in droplet form in the carrier liquid (the outer, or continuous phase). NB e emulsiable metalworking uids are what is known as ‘oil-in-water’ emulsions, that is the oil forms the inner phase, conversely, its counterpart is formed by emulsifying metal- working uids, which are ‘water-in-oil’ emulsions. (Source: Cincinnati Milacron/Cimcool, 1991) 4 ‘Solutions’ , are a metalworking uid solution, these are water- soluble uids mixed with water. (Source: Cincinnati Milacron/ Cimcool, 1991) Cutting Fluids  Figure 196. The eect of rake angle on chip thickness – with and without coolant supply .  Chapter  Figure 197. The main types of cutting uids for machining operations. Cutting Fluids  • Acceptability to the operator (i.e. when machine tool is manned) – as all operators have some de - gree of exposure/contact with the cutting uid. Op - erators will consider the lubricant’s overall perfor - mance, but even when the uid is ‘perfect’ in every other respect, complaints are likely if, for example, the smell was unpleasant. e following features are likely to be of particular interest to the operator: – Smell – ideally, the cutting uid should have no perceivable odour, but if present, it certainly should not be objectionable, – Colour and clarity – most operators prefer prod- ucts which are perceived to be ‘clean and fresh’ throughout their life and, some operators prefer dye-coloured translucent products for this rea - son, – Misting – high-speed cutting operations tend to generate a mist. Occasionally these mists may be associated with operator health problems: dry-throats; stinging eyes; etc.; leading to com - plaints. Although misting is largely dependent on the: machine tool; its operation; atmospheric ventilation; etc.; dierent uids have diverse misting characteristics and, ‘ideally’ the uid should be non-misting – more will be said on the operator’s health issues later in the chapter, – Irritation to the skin and eyes – these operator issues have been associated with physical con - tact from cutting uids, such as: skin and eye irritation; itching; rashes, swellings; stinging; etc. Once again, uid formulations that are ‘kind and gentle’ are preferred. As mentioned above, more will be mentioned on these health issues shortly, • Long ‘sump-life’- with all machining uids having a nite life, at some point, the machine’s cutting- uid system must be completely emptied, cleaned, ushed through and relled with new uid. ere are numerous reasons why the cutting uid might be regarded as ‘dead’ – these points will be raised when investigating the ‘problems’ with cutting uids later in the chapter. e uid’s life is an im - portant economic consideration in terms of: uid usage; labour costs; down-time; etc. Some leading- manufacturer’s cutting uid formulations are capa - ble of achieving signicantly better overall perfor - mance and have an extended ‘sump-life’ over their cheaper contemporaries. e increased ‘sump-life’ will enable better use of a company’s maintenance department’s manpower resources, thereby en - abling it to be more ecient in their anticipated ‘planned maintenance scheduling’ 5 over prescribed shut-down periods, or ‘maintenance windows’ , • Corrosion protection – all cutting uids are for- mulated to provide corrosion protection to the machine tool and the workpiece during and, for a short time aer the cutting operation. In the main, these uids should preserve their corrosive-protec - tion properties throughout their useful life, to avoid the potentially expensive problems of rusting of the machine and machined components alike, the lat - ter being rejected by the customer. More informa - tion will be given on corrosion protection later in the chapter, • Low foaming properties – on some of today’s ma- chine tools, they incorporate uid systems that agitate the cutting uid to such an extent that foam spills out of the coolant tank and onto the oor. e ‘ideal’ uid will withstand: swarf-washing jets; high-pressure uid delivery; centrifuges; etc.; even when prepared with the soest quality water sup - ply. e subject of foaming will be addressed spe - cically later in the chapter, • Machine tool compatibility – no self-respecting engineer wants to see their newly purchased, or well-maintained older machine being attacked by its cutting uid. e optimum uid should create no detrimental eects on the machine tool’s: paint nishes; seal materials; screens and guarding; etc., • Workpiece compatibility – means that the widest possible range of workpiece materials should be machined with a particular, but versatile grade of 5 ‘Planned maintenance scheduling’ , many companies adopt either a ‘Total Productive Maintenance’ (TPM), or ‘Reliabil- ity-centred Maintenance’ (RCM) philosophical and practical approach to their overall maintenance organisational needs. NB TPM is dened as: ‘A system of maintenance which cov- ered the entire life of every piece of equipment in every division including planning, manufacturing and maintenance. (Source: Japan Institute of Plant Maintenance – JIPM, 1971). RCM is dened as: ‘A process used to determine the maintenance re- quirements of any physical asset* in its operating context’. (Source: Moubray, 1996) Oen, companies run both a TPM and RCM strategy together, to achieve an overall high level of maintenance planning discrimination, coupled to plant secu- rity – in association with individual asset reliability. * A physical asset is any piece of operating plant, or equip- ment that requires a maintenance function to be undertaken upon it at some prescribed time period, or requiring various modications to it for its operating context.  Chapter  [...]... of the components? Are the machine tools supplied with cutting fluid individually, or delivered from a centralised system? Are particular cutting fluids recommended by the manufacturer of these machine tools? 4 08 Chapter 8 • Protection of people and the environment – to mally be made regarding the type of cutting fluid to what extent are the personnel exposed to cutting fluids: before; during; and... are the norm The production criteria for the choice of cutting fluid includes the: type of machining process; cutting conditions – workpiece material, cutting data, tool material, etc., together with the machine tool type and its configuration 18 Thus, on the basis of these production-based decision criteria, an initial choice will nor- 18 ‘Machine tool configuration’ , take for example, the case of an... even at some point later, support from the cutting tool manufacturer in the form of: systematic sampling procedures; laboratory testing and technical advice could prove very informative – particularly for applications where heavy cutting fluid consumption is anticipated 8. 8 Care, Handling, Control and Usage – of Cutting Fluids So that the properties of a cutting fluid can be maintained giving it a... leaks from the machine tool and contaminates the cutting fluid Most modern machines are equipped with ‘total-loss’  slideway lubricating systems which can contaminate the cutting fluid with up to a litre of oil per day – on a large machine tool The ‘ideal’ cutting fluid would be capable of tolerating this contamination without any detrimental effects on its operating performance Some cutting fluids are... ‘emulsifiable’ cutting fluids form an emulsion, conversely, the ‘soluble’ variety forms a solution In both of these cases, the resultant cutting fluid product is termed: ‘watermixed’ In the following section, the various types of cutting fluids currently available will be briefly mentioned 392 Chapter 8 8.4.1 Mineral Oil, Synthetic, or Semi-Synthetic Lubricant? Mineral Oil In order to manufacture cutting. .. fractions), by distillation Cutting Fluids Figure 1 98.   The basic structure of an oil-based cutting fluid and an ‘oil-in-water’ emulsifying molecule [Courtesy of Cimcool] 393 394 Chapter 8 Figure 199.  The principle of polar and passivating corrosion protection and the minimum requirements for water quality [Courtesy of Cimcool] Cutting Fluids the oil, keeps the machine tool cleaner and will delay... facilities are necessary, this assistance is often provided by the cutting fluid manufacturer’s laboratory service department 8. 8.2 Monitoring, Maintenance and Testing of Cutting Fluid – in Use While in use the cutting fluid is subject to various influences that may affect its properties Such influential factors include: leakages from the machine tool s lubrication and hydraulic systems (i.e ‘tramp-oil’);... analysis equipment, plays a vital role in any new cutting fluid development process An important criterion for the quality of the final cutting fluid formulation is its stability By comparison, synthetic cutting fluids produce fewer problems than semi-synthetic and emulsion cutting fluids, in their development In the case of the semi-synthetic and emulsion cutting fluids, not only must cooling water and... any lubricant manufacturer 8. 4.2 Aqueous-Based Cutting Fluids A large proportion of cutting fluids used for machining operations are still of the aqueous-based types (Fig 197), as they combine the excellent heat-absorbing capacity of water, with the lubricating power of chemical substances Such cutting fluids offer excellent cooling, lubricating and wetting properties Machine tools require protection... for the effectiveness of a water-mixed cutting fluid, it is essential to know the quality of the water source available and to take account of this factor when selecting a concentrate Cutting fluid manufacturers undertake water analysis, as do local water companies In Fig 199b, the minimum requirements for water quality for aqueous-based cutting fluids is shown 8. 5 Cutting Fluid Classification – According . 8 Cutting Fluids ‘Everything ows and nothing abides.’  (540 – 480 BC) [An early Metaphysician from Ephesus (Asia Minor), in: On Nature] 8. 1 Historical Development of Cutting. alloys. 8. 2 Primary Functions of a Cutting Fluid In the previous section, it was recognised that two of the primary functions of a cutting uid was to cool and lubricate both the workpiece and cutting. requirements for water quality for aqueous-based cutting uids is shown. 8. 5 Cutting Fluid Classification – According to Composition Generally speaking, cutting uids are purchased under the following