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Figure 202. A cutting uid emulsion’s diametral size (0.2 to 1.5 µm) in comparison with micro-organisms and ‘tramp oil’, together with the ‘pH scale’. [Courtesy of Kuwait Petroleum International Lubricants] . Cutting Fluids Figure 203. Bacterial contamination: aqueous cutting uid. Chapter Figure 204. Computer-Aided Design (CAD), utilised to select a corrosion inhibitor for an aqueous-based cutting uid. [Courtesy of Cimcool] . Cutting Fluids gies 14 . As an example of this phenomenon, anionic emulsiers normally have corrosion inhibiting charac - teristics, but these properties are usually so slight that any side-eects are usually disregarded. However, by using CAD, it is possible to nd emulsiers – normally several are needed, whose side-eects add up syner - getically. When the correct emulsiers are selected and in the right proportions, not only is the desired emulsifying action obtained, but at least some of the required cor - rosion protection also occurs. In Fig. 204, an example of the ‘construction’ of a corrosion inhibitor system using a variety of inhibitors – either singly, or in com - bination – can be comprehended. Here, the ‘zero-line’ on the vertical axis of the graph represents: ‘no eect’ , while values greater, or less than zero represent a: posi - tive; or negative eects; respectively. Such CAD for chemical compounds makes it pos - sible to develop ‘atomised’ cutting uids far faster than by previous techniques and oers the prospect of discovering entirely new cutting uid combinations. Computer analysis, oers a way to develop, analyse and test new cutting uids, enabling very rapid modi - cations to be incorporated in order to meet new tech - nical and commercial requirements. Further, these CAD-based techniques guarantee a chemically-stable product with the optimum properties, reducing the risk of selecting the wrong type of cutting uid both by the manufacturer and user. CAD product develop - ment still necessitates practical product testing, during its development phase utilising standardised proce - dures of: ‘calibration and laboratory test methods’ – to model the computerised-uid data in a real-time cut - ting environment. .. Cutting Fluid – Quality Control For practical reasons industrial cutting uid manufac- turers have to use mass produced raw materials and chemicals, which may be less pure than those used in their formulations in the laboratory (Fig. 205). Not only are there variations in quality, owing to variance in the production process, but dierences can also oc - 14 ‘Synergy’ , refers to the outcome when substances are com- bined and produce ‘side-eects’ , which add to, or even amplify each other, giving rise to a much stronger resultant eect. cur depending on the raw material source and the sea- sons of the year. In order to ensure constant quality of the nished product despite these variations and the factors which determine the quality of the raw materials, they must be checked prior to entering the cutting uid production processing stage. e labo - ratory-based technique of computer-aided statistical process control that ensures: ‘preventative quality con - trol’ , will enable the researcher to set the upper and lower quality levels for a particular raw material – un - der test. us, on the basis of these user-dened sta - tistically-acceptable levels, the correlation between the analysis and the practical results can be determined. Raw materials analysis using computer-aided design in conjunction with sophisticated analysis equipment, plays a vital role in any new cutting uid development process. An important criterion for the quality of the - nal cutting uid formulation is its stability. By com - parison, synthetic cutting uids produce fewer prob - lems than semi-synthetic and emulsion cutting uids, in their development. In the case of the semi-syn - thetic and emulsion cutting uids, not only must cool - ing water and lubricating oil be brought together – two naturally incompatible substances, they must also ‘persuaded’ to remain mixed together under widely varying and extreme cutting and environmental con - ditions. When dierent degrees of water hardness, varying mix ratios and a diverse range of impurities occur, they will strongly inuence the overall water-oil system. e conventional way of stabilising such a sys - tem is to add plenty of emulsiers. is action can lead to excessive foaming, especially if the water is so, which in itself necessitates adding anti-foam agents. Anti–foam agents are an expensive alternative – par - ticularly in a large central-based ‘Niagara’ reservoir- type system feeding several machine tools in say, an FMC/S (Fig. 203a), which here, by ‘anti-foaming dop - ing’ in any event, will only work for a limited period of time. More important for cutting uid stability, is the size and distribution of the oil droplets in the water phase (i.e. see Fig 206-inset photomicrograph). It is the even distribution of the many oil droplets which ensures that the oil-water system is stable. e growth of micro-organisms (Fig. 203b) aects the droplet size and as a result, as these droplets spherically-increase in size, the number and distribution of droplets decreases. us, an oil-water system with many evenly-spaced and small droplets, will be more stable than systems Chapter Figure 205. Laboratory-based testing procedures on cutting uid coolant products. Cutting Fluids where there are bigger, but fewer droplets present. Both the size and distribution of these oil droplets has an important inuence on the emulsion’s consequent foaming behaviour, which in turn, is strongly aected by the water hardness and any turbulence produced by the machine tool, or from a centralised coolant supply system (Fig. 203a). Product Testing ere are a number of possible tests for checking the quality of a cutting uid and those most commonly utilised are ‘stability tests’. Such tests measure not only the physical stability, but also tests for: bacteriologi- cal stability – this latter term is sometimes known as Figure 206. The method used to check a soluble cutting uid’s dilution, utilising a Refractometer. [Courtesy of Rocol Ltd.]. Chapter a biostability test; biostatic properties; or resistance to bacteria growth 15 . Foaming behaviour: a slight ten- dency to foam is important for some types of machin - ing operations, particularly when deep-hole drilling operations are undertaken and more specically, in grinding operations. ere are certain cleaning sys - tems available, such as ‘full-jacket cyclones’ and ‘hydro- cyclones’ , which promote foam formation. However, there are no standard techniques for the measure- ment of foam formation and collapse, although the laboratory circulating pump method, is a reasonable approximation of practical conditions. In this method, the cutting uid is forced through a spray-head so that the resultant spray falls onto the surface of the liquid and the time taken for foam to form and then collapse, is a measured – giving an indication of foaming be - haviour. Adhesion tests are oen undertaken, with low adhesion representing a tendency for the cutting uid product to build-up layers of deposit. is adhesion test is usually used for synthetic products and again, there is no recognised standard test method; although one technique used to soak a pile of washers in the cutting uid for a certain time, then drying them out and subsequently testing for adhesion, is sometimes employed. Compatibility tests for cutting uids, with particular reference to paints and elastomers by visual inspection of painted sheet metal is oen car - ried out 16 . Acid/alkaline tests are oen undertaken on the cutting uid, as this aects both the machine tool and health of the setter/operator 17 . Once a cutting uid manufacturer has produced a new formulated product, which has been subject to a stringent laboratory testing programme, followed by exhaustive practical trials and consequent analyses, it is then made available, initially to their ‘prime cus - tomers’ – for an ‘alpha-trial’ testing programme aer which, the cutting uid is oered on the ‘open-market’ 15 ‘Bacteriological test methods’ , typical of this type of testing regime are the German Standards, denoted by DIN51367 and DIN513368, but similar test Standards are listed in most of the world’s technological countries. 16 ‘Compatibility tests’ , specially-prepared painted sheet metal is fully- immersed in the cutting uid for a certain time, then visually inspected for paint de-lamination, etc., according to the German Standard: DIN53521. 17 ‘Acid/alkaline tests’ , are important cutting uid tests and they are normally measured under laboratory conditions with an electrochemical pH meter, in accordance to say, the German Standard DIN51369. for customers world-wide. erefore, it is vital that the correct cutting uid is used in the machine tools, as it will have serious consequences to the: machine tool’s subsequent maintenance programme; likely produc - tion output; machined workpiece quality and reliabil - ity; together with the various health issues relating to that of the setter/operator. e following section has been included to help with the important decisions re - lating to the choice of cutting uid selection. 8.7 Selecting the Correct Cutting Fluid When choosing a cutting uid many factors have to be considered, with the relative importance varying with each individual circumstance. For the latter reason, it is not possible to oer general rules for the selection of a cutting uid. Inevitably, a compromise is neces - sary, although a comprehension of the factors involved makes it possible to achieve the best choice, under given circumstances. So, when choosing a cutting uid it should always be borne in mind that the machining process plays an important role in: productivity and eciency; operator health; safety; plus the quality of work created. .. Factors Affecting Choice Probably the main factors that must be considered when selecting a specic cutting uid are not always apparently obvious, but some questions should be raised, which might include: • Business philosophy – what are the relative con- cerns and weightings given to goals such as: e - ciency; quality-conciousness; market and economic position; of the company? • Production programme – what is the scale of pro- duction, is it a: single item (i.e. one-o); a batch; or mass production? Moreover, what machining pro - cesses are involved in the part’s production? • Hardware – what production plant and equipment is there available for the machining of the compo - nents? Are the machine tools supplied with cutting uid individually, or delivered from a centralised system? Are particular cutting uids recommended by the manufacturer of these machine tools? Cutting Fluids • Protection of people and the environment – to what extent are the personnel exposed to cutting uids: before; during; and aer use? Are there local constraints on uid disposal? e above criteria concerning cutting uid selection, can be sub-divided into two distinct groupings – com - mercial and production, as follows. Commercial Criteria e commercial criteria determine the ‘weight’ to be given to various production decisions. For instance, if the time factor is more important than the cost factor, then higher cutting speeds will be used and so the de - mand placed on the cutting uid will be greater. If, dif - ferent materials and types of machining processes are involved in the production process, then a ‘universal’ cutting uid might be a better choice than a number of dierent uid products, even if the latter uid compo - sitions individually produced a better performance. Production Criteria In either one-o, or batch production, individual machines tools are likely to have their own separate cutting uid supply, however in a mass production environment, uid centralised systems are the norm. e production criteria for the choice of cutting uid includes the: type of machining process; cutting con - ditions – workpiece material, cutting data, tool mate - rial, etc., together with the machine tool type and its conguration 18 . us, on the basis of these produc- tion-based decision criteria, an initial choice will nor - 18 ‘Machine tool conguration’ , take for example, the case of an ‘orthogonal’ * machining centre, it relates to whether the machine has a horizontal, or vertical spindle orientation, with one, or multiple spindles present, having three, four, or mul- tiple linear and rotary axis control and of the conventional, or high-speed machining (HSM) variety. Most machine tools today fall into the ‘orthogonal’ machining category, but some ‘non-orthogonal’ machining centres exists, which oer con- tinuous kinematics that have multiple-axis control – for even simple straight-line motion, oen available with omni-direc- tional spindle orientations – for ease of cutter access to say, a complex, or sculptured machine part geometry. *Orthogonality of axes means that each axis is positioned at 90° with respect to each other, such as on a three-axis verti- cal machining centre (i.e. X-, Y- and Z-axes, with the Z-axis normally incorporating the machine’s spindle). mally be made regarding the type of cutting uid to be used – whether it is an aqueous-, or oil-based uid type that is required. .. Selection Procedure When selecting a cutting uid, it is important to take the uid manufacturer’s instructions into consider - ation. If their recommendations are ignored, it may render any previous guarantees invalid. Many of these cutting uid manufacturers specify that certain products be utilised, normally only applying them to either special machining circumstances, or where dif - cult cutting conditions are likely to be encountered. Equally, other cutting uid producers only specify the general type of instructions, such as: what type of cut - ting uid to employ, for example, aqueous, or not, to be used, on the contrary, some uid manufacturers do not specify anything! Usually it is possible to rely on the manufacturer’s specication tables which indicate their most suitable product for a particular range of machining operations. However, before consulting a cutting uid manufacturer’s set of tables, the follow - ing factors must be either known, or established. ese crucial decisions include: • Type of machining operation – care must be taken to ensure that correct and ecient planning of the machining strategy for the successful completion of the part is known and, the optimum machining techniques to achieve this objective are conrmed, • Water characteristics – hard/so, chloride, sul- phate, bicarbonate content has been both tested and accurately established, • Type of workpiece material to be machined – with due regard to machining of: work-hardening mater- ials, or ‘sticky’ materials – aluminium and copper alloys, etc., • Type of machine tool ltration system – having ei- ther: no ltration; sump only; paper lter; centri - fuge; etc. NB In the case of the centrifuge, semi-synthetic products are not recommended, while washable l- ters should only be utilised with synthetic, or semi- synthetic products. As a general ‘rule of thumb’ , emulsions with EP addi - tives should be employed for heavy cutting work, whilst synthetic products are normally best when cutting at high speeds. By way of an illustration, if one consid - Chapter ers multiple machining operations undertaken on machining centres, the cutting uid should be chosen for the range between the highest and lowest cutting speeds. Once a particular uid product has been se - lected, it is still necessary to carry out a practical test as only aer such testing, will it conrm if the correct choice was made. At this stage in the selection proce - dure and 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 uid consumption is anticipated. 8.8 Care, Handling, Control and Usage – of Cutting Fluids So that the properties of a cutting uid can be main- tained giving it a long and reliable service life, correct: storage and handling; usage and mixing with labelling – having instructions for use; and care; are essential requirements. An indispensable part of any overall cutting uid control, is suitable handling and storage at the user’s premises, as it provides a continuous replenishment and service facility for the monitoring of uids at, or near to the machine shop. All containers of cutting oil should always be clearly marked with their end usage for use within the factory. When storing these cutting oils, the grade most frequently utilised are best kept on simple robust stillages, or for larger quantities, in covered tanks, with an adequate supply of: taps; hand- pumps; funnels; measures; and drip-trays; to ensure proper handling and to avoid intermixing of grades. is potential uid contamination is one of the most important aspects when dealing with soluble oils, in order to obviate any emulsication troubles and pos - sible coolant separation issues. In large manufacturing companies, where bulk delivery of cutting uids are being made-up prior to delivery, it is important that all storage tanks for the reception are properly identied, with appropri - ate markings immediately changed once a new grade is adopted. Unfortunately, one oil looks much the same as any other and if not properly identied (i.e. marked-up), as to the grade identication and its end usage, this could lead to some costly machining mis - takes (e.g. part-scrappage problems) within the pro - duction machining facility. e reception of drums of cutting uid and oils, requires certain safeguards, to protect the mutual interests of both the supplier and user alike. For instance, the supplier should be noti - ed at once if there is obvious drum damage, or the odd drum is leaking. Full drums of uid should never be just dropped-o the delivery vehicle from its plat - form, as a rim, or seam may be damaged and leakage of the contents could potentially be a problem, apart from the real risk of personal injury of a heavy drum’s unchecked motion! For example, a typical full drum weighs about 200 kg and if dropped from the vehicle’s loading platform – normally a height of just over a metre, the impact force will be considerably greater than its ‘dead weight’ – due to gravitational inuence, thus the uncontrolled careering drum is a major ac - cident waiting to happen! So, when unloading full drums from a road vehicle, always use: a drum skid 19 ; hoist, or fork-li truck. When the handling of cutting and lubricating oils has developed into a lthy job, this is reected in the storage facility, which in some companies is the most neglected part of the factory. Under such conditions, it is oen dicult, if not impossible to avoid both wast - age and contamination of lubricants, thereby leading to issuing the wrong oil for the present machine tool’s cutting requirements. Equally, a clean well-organised oil storage facility is an invaluable asset to any engi - neering plant and no eort should be spared to create and maintain these optimum conditions, this being es - sential to a trouble-free operation. If soluble cutting uid – ‘coolant’ , is mixed in bulk, the storage tank should be clearly marked with the: product’s name; reference code; and concentration. is simple but vital tank identication avoids mis - takes in issuing the wrong coolant, or dilution concen - tration to a prescribed machine tool, while providing a reminder of the required contents when mixing a fresh uid batch. Soluble oil concentrate must not be exposed to cold climatic conditions and allowed to freeze, since this ‘cryogenic eect’ might adversely aect the 19 ‘Oil-drum skids’ , when robustly designed/produced, allow the controlled sliding to ground-level of full drums o a vehicle’s loading platform, without potential health risks to the compa- ny’s work-force. Cutting Fluids concentrate’s stability – when subsequently used. Further, bulk drum storage outside should if possible, be under protective cover, but if this is not the case, then drums should be placed onto their sides and not stored upright (i.e. ‘bung-side up!’). If they were to be stored upright, then any standing rain water can collect on the drum’s top and owing to daily tem - perature uctuations, any water can be sucked into the barrel – even through the unopened top. ese small amounts of water may destabilise the concentrate, leading in the worst scenario to complete separation of the concentrate into distinct layers. In this state, the concentrate becomes completely unusable, as uid in such a condition, will be unlikely to adequately remix. Consequently, it is advisable that some form of space heating for undercover uid storage is desir - able, to minimise wide temperature uctuations, this being particularly relevant during winter months, as the oil viscosity appreciably changes (i.e. thickens), with more sluggish ow-rates causing delays in both its delivery and usage. When not in use, all lubricant containers should be kept closed, thus avoiding entry of abrasive foreign matter. So, whenever possible, indoor storage of cutting uids and lubricants is to be recommended. As all oil storage facilities have a real potential for a catastrophic and explosive re hazard. Sucient re extinguishers – of the correct type and size, should be strategically placed at easily-accessible points around the oil storage facility and positioned at ground level with unobstructed access to them. Any oily paper waste and sawdust 20 present, requires prompt disposal to minimise re risk. Any oil storage facility is strictly a non-smoking zone, for obvious reasons. .. Product Mixing – Preparation of a Aqueous-Based Cutting Fluids As mentioned, cutting uids are usually supplied to the customer in the form of concentrates and the ease 20 ‘Sawdust’ soaked in oil is a likely re source, so rather than use this to soak up oil-spillages, use specially-produced gran- ules that are non-ammable and oil absorbent. NB ese oil absorbent granules are usually readily available and can be purchased from most leading-lubricant supply companies. with which they can be mixed together with water var- ies, depending upon the amount of oil they contain. Concentrates with high oil concentration may require vigorous stirring in order to form an emulsion. While other products containing little oil are oen supplied as ‘preformed emulsions’ , in which the concentrate has previously been mixed by the cutting uid manufac - turer with water to form a stable emulsion. ese lat - ter ‘preformed concentrates’ , only require them to be stirred into water at the correct dilution ratio, in order to prepare them for use. Correct preparation of the emulsion is essential if the cutting uid is to provide its optimum perfor - mance. By preference, the water supply should not be excessively hard (i.e. <300 ppm – total hardness) and, it should be of ‘drinking quality’ – thereby not infected with any bacteria. Hand Mixing With the production need for relatively small volumes of aqueous-based coolants, the emulsion can be pre - pared as follows: 1. Choose a suitable-sized volume mixing vessel – not the machine sump, nor a galvanised container, 2. Carefully ll with the measured amount of water, 3. Slowly add a measured amount of concentrate while continuously stirring – with a top-to-bottom stir - ring action – this being the most ecient action, until the emulsion is fully-formed, 4. Steadily add the newly-mixed emulsion to the ma - chine tool’s sump. Automatic Mixing When larger volumes of emulsion cutting uid are required, then it is advisable to use a purpose-built and designed uid mixer (Fig. 208). ese proprietary mixing devices incorporate the following features: • ey can directly-connect to a standard water tap, • ey can be screwed directly into a large uid con- centrate drum, • ere is adjustment for the automatic dispensing of the correct emulsion dilution ratio of: ‘oil-to-wa - ter’ , • e uid mixer incorporates a non-return valve, to ensure that the emulsion cannot leak-back into the drum’s contents. Chapter . methods’ , typical of this type of testing regime are the German Standards, denoted by DIN5 136 7 and DIN5 133 68, but similar test Standards are listed in most of the world’s technological countries. 16 . systematic sampling procedures; laboratory testing and technical advice could prove very informative – particularly for applications where heavy cutting uid consumption is anticipated. 8.8 Care, Handling, Control and Usage – of Cutting Fluids So that the properties of a cutting uid can be main- tained giving it a long and reliable service life, correct: storage and handling; usage and mixing with labelling – . be given to various production decisions. For instance, if the time factor is more important than the cost factor, then higher cutting speeds will be used and so the de - mand placed on the cutting uid will be greater. If, dif - ferent materials and types of machining processes are involved in the production process, then a ‘universal’ cutting uid might be a better choice than a number of dierent uid products, even if the latter uid compo - sitions individually produced a better performance. Production
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