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Cutting fluids — pet or pest?
Cutting fluids- pet or pest?. A review of staining and corrosion tendencies and effects on machine tool paints and seals D. A. Hope* There is no truly universal cutting fluid: a variety of different types are used to cater for the range of machining operations demanded by modern production techniques. This review outlines the types of fluids and additives that are currently used to formulate cutting fluids and then discusses at greater length the potentially harmful side effects that can ensue if the wrong fluid is selected, or the correct fluid is abused, for a particular combination of workpiece and cutting operation. The aim of the article is to indicate to the cutting fluid user the kind of information and assistance he should seek from his supplier, not only in solving existing problems but also in anticipating potential difficulties in the future.
There are few human activities that do not depend on the use of articles that at some stage in their manufacture have required the application of metal cutting techniques. Modern high volume production is only possible by the application of cutting fluids during the cutting operation. The vast range of different workpiece materials and machining operations demanded by the design engineer has produced the stimulus for the development of a multiplicity of cutting fluids to cater for the almost infinite number of combinations involved in practice. There are many aspects to be considered before choosing which fluid to use for a particular cutting operation: the major consideration is adequate performance capability. All fluids currently marketed have their limitations and many will have adverse side effects if used on metals or for cutting operations for which they were not designed. It is of vital importance that the user is aware of what type of cutting fluid he is using and what the possible side effects are so that he can ensure adequate safeguards are taken before expensive problems occur. Some of the commonest problems encountered are those involving staining and corrosion of workpieces and machine tools, usually due to the misuse of the fluid but occasionally to the use of an incorrect fluid. In the UK alone there are about 150 oil companies; many hundreds of individual cutting fluids of a vast range of different chemical compositions are available in the market place. No one, least of all the average user, can be expected to have detailed knowledge of all of them. Each supplier will, of course, have this data on his own products and it is essential that the user can obtain such knowledge as he requires to avoid problems in practice. It is hoped that the following discussion will indicate the type of information the user should demand from his supplier.
Types of cutting fluid There are two main types of cutting fluid: neat oils, which are used as supplied, and water.mix fluids that have to be mixed with water before use. The amount of water-mix * BP Oil Ltd, Victoria Street, London, UK
fluid in the fluid as used can vary between wide limits from as little as 1% or less for grinding to 25% or more for some arduous duties, although the concentration in the majority of applications will be about 4-5%. Neat oils come in a wide variety of compositions ranging from straight mineral oils containing no additives to blends of additives containing no oil. The additives themselves cover a wide range of different types although the most commonly encountered are fatty materials, both ester and free acid, chlorinated paraffins, sulphurised oils and fats and free sulphur. 1 Other less common materials include organic phosphorus compounds, of which zinc dialkyl dithiophosphates and aryl and alkyl phosphates are the main types. These additives are added to increase the cutting performance for specific operating conditions but all can cause staining or corrosion if used with the wrong workpiece material. Water-mix fluids are produced in three main types: emulsifiable oils, chemical or pure synthetic fluids, and the recent so-called semi-synthetics. These are further subdivided into two main categories: ep and non-ep, the former being compounded with additives to improve cutting performance which, as with neat oils, can have adverse staining and corrosive properties if used with the wrong materials.
Neat oils and additives Straight oils are the simplest type of cutting fluid since they contain no additives and problems are rarely encountered in their use. Addition of fatty materials is the first step in improving the cutting performance and again these 'compounded' oils present few problems. The use of galvanised tanks, pipes and fittings should be avoided, however, because fatty acids can react with the zinc to produce insoluble zinc soaps that can promote filter blockage, foaming and generally dirty machines and working conditions. This ban on galvanised materials in the cutting fluid system is applicable to all cutting fluids, and indeed to all lubricants, since most will have a potentially
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aggressive reaction to zinc. These fatty acids are only weakly acidic so that they attack only the very reactive metals such as zinc and magnesium. 2 They are almost completely inert with ferrous metals and indeed fatty acids are commonly incorporated in temporary corrosion preventives for ferrous parts. Another aspect to be considered when using fatty oil based fluids is their relatively low resistance to oxidation and the unfortunate fact that copper is a highly active catalyst for this reaction. This means that when machining copper and copper-based alloys where very fine swarf is produced, the freshly exposed metal surface can catalytically oxidise the fat to produce organic acids which in some circumstances may react with freshly exposed copper surfaces to produce copper soaps, most of which are green in colour. Copper deactivators are generally ineffective in combating this degradation because of the high rate of depletion on the fresh surface generated in the cutting operation. Addition of anti-oxidants is an expensive remedy which does not always overcome the problem. Even so, it is fortunately evident that few such problems occur in practice where relatively large chips of swarf are generated: the main troubles occur in rolling where micron size particles are produced. Chlorine
When fatty additives fail to give the desired performance the next class of chemical compounds to be added is that based on chlorine. The poisonous gas cannot be used for obvious reasons and currently it is included in lubricant formulations as a chlorinated paraffin. These compounds have high stability at low temperatures, are readily soluble in mineral oils, and are non-irritating to the skin, yet they have the ability to react with metals at high temperatures to produce a solid metal chloride film that is easily sheared: a good solid lubricant can be produced in situ as and when required. This metal chloride is firmly bonded to the metal surface in discrete patches corresponding to the distribution of asperities on the original metal surface and remains on the workpiece after the cutting operation. In the majority of applications this is of no significance and no problems arise. However, with some alloys where the end use of the component involves exposure to high temperature it may eventually be of dire consequence. For example, trace amounts of chloride can cause intercrystalline corrosion in most stainless steels, nimonics, and titanium, leading to stress corrosion cracking at elevated temperatures. 3,4 Magnesium is a highly reactive metal and it is probably true to say that, outside the highly specialised aerospace industry, there is little call for machining this metal. Nevertheless it is worthy of note that chlorinated oils should not be used for these applications since chlorine stains are rapidly produced on this metal, even in low severity operations. In cutting operations lubricated by a chlorinated oil, most of the chlorine released from the additive immediately reacts with the metal being cut and the tool, but a small proportion is liberated as hydrogen chloride which forms the highly corrosive hydrochloric acid in the presence of minute traces of moisture. This hydrochloric acid will rapidly attack most metals, leaving the affected surfaces highly susceptible to further staining, corrosion, and rust.
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Because hydrochloric acid is so reactive and is normally produced in very small quantities, it is a relatively simple exercise for the cutting fluid supplier to incorporate a chemical inhibitor to neutralize the acid as soon as it is formed and before it can react with the metal workpiece or machine tool. This can, however, only be expected to cover normal use and if conditions are unusually severe it may be necessary to re-inhibit from time to time until the conditions can be altered. This may be accomplished by several means: increasing the flow of cutting oil into the cutting zone, increasing the capacity of the oil system, installing oil coolers, etc. Again, fortunately, it is a relatively simple procedure to monitor a fluid in use by determination of strong acid number s which will give an immediate indication if re-inhibition is required.
Sulphur Sulphur is by far the most widely used additive to improve the performance of neat cutting oils either in its 'active' or
Tube mill problem solved In arduous cutting or metal forming operations using chlorinated oils, the formation of hydrochloric acid can cause problems. Incorporation of chemical inhibitors in the formulation may ease the problem, but in the long term it is probably better to change operating conditions, for example by increasing the coolant flow rate, increasing the capacity of the oil system, or installing oil co~lers. This approach is illustrated by the solution to problems experienced at a tube reducing mill. The mill, producing nimonic alloy tubes, was designed on the basis of similar mills reducing mild steel tubes which used a soluble oil coolant. To achieve satisfactory production rates and surface finish on the nimonic alloy tubes, it was necessary to use a high viscosity, heavily chlorinated neat oil. The low power pumps and small bore pipework that were satisfactory for a soluble oil were woefully inadequate for the neat oil; the flow rate was too low in the work area and much fuming was evident. The main oil tank installed in a cellar permitted little cooling and bulk oil temperature stabilised between 80°C and 90°C. Operators complained of the acrid and irritating fumes, micrometers left in the vicinity of the machine rapidly corroded, and even steel trusses in the roof area began to rust. No inhibitor system could be expected to overcome the major design fault and immediate steps were taken to redesign the oil system. Higher capacity, high pressure pumps were installed, 1 in pipework was replaced by 3 in pipes, the tank capacity was substantially increased and a cooler installed. These measures were completely successful in overcoming the problems and although the metal working operation involved is particularly arduous, simple regular monitoring of the oil s is now sufficient to achieve a highly satisfactory operation, with a bulk fluid temperature of no more than 30°C.
'inactive' form: the staining and corrosive effects of such additives are well known. It is sufficient, therefore, to summarise these briefly since most users are already aware of the potential problems. The staining and corrosion of copper and its alloys by the formation of black copper sulphide is the most frequent problem, although many of the 'inactive' type sulphurised oils can be used on yellow metals with little practical difficulty. It should be recognised, however, that all extreme pressure additives are designed to react chemically with metals at the elevated temperatures generated by rubbing asperities on the metal surfaces and to do this the additives must have some degree of thermal instability. As in the case of chlorinated additives, the 'inactive' type sulphurised additive will release free, active sulphur under in-use conditions so that a new, unused 'inactive' type sulphurised cutting oil will tend to become more 'active' in use. The rate of generation of 'active' sulphur is directly related to the severity of the cutting operation and hence the temperatures involved. For this reason, many but not all of these oils contain irthibitors to counteract an increasing tendency to stain yellow metals as the original additive is degraded in use. The user would be well advised, therefore, to check that any 'inactive' type oil being used for machining yellow metals is inhibited. Sulphur has a similar effect to chlorine on stainless steels, nimonics, and titanium and therefore its use should be avoided when the machined components are to be exposed to high temperatures in the end use. The 'active' type sulphurised oils are, of course, not intended for use with yellow metals and, although some may be quite satisfactory for such use by the inclusion of effective inhibitors, the general rule must be: do not so use them without first consulting the supplier. Not so well known is the effect of 'active' sulphur on magnesium and aluminium 2 since both metals are susceptible to rapid staining and corrosive attack. It is most unlikely that this would present any difficulty other than in a small jobbing shop where one oil may be preferred whatever workpiece material is involved. Many machine tool manufacturers still fit their machines as standard practice with cutting oil pipes, valves and sprays o made from yellow metals or aluminium alloys so the user should employ his purchasing power with discretion to ensure that any machine he purchases is satisfactory, as supplied, for any cutting oil he might wish to use. The use of copper heat exchanger coils should also be avoided. The majority of additives containing phosphorus are not particularly effective in improving the performance of cutting fluids and hence their use is not widespread. They give little practical difficulty in use since, although their degradation products are acidic they do not generally give rise to staining or corrosion. Indeed the reverse is often the case since phosphating is a recognised form of surface treatment.
De-greasing One of the functions of a cutting oil is to provide protection against corrosion of machined components. This must be remembered when, for instance, a component is washed in a solvent to remove the oil before checking its dimensional accuracy. Immediately after such a check, the component should be protected again, either by re-immersing in the
cutting oil or by application of a temporary corrosion protective. Trichlorethylene is one of the most widely used degreasing solvents since it is, without doubt, one of the most effective fluids available. It presents several harmful side effects however. The toxic effects of its fumes and the ready generation of phosgene at high temperatures, by a lighted cigarette for instance, are frequently overlooked because of its nonflammability. When used in a vapour degreasing bath, the constant boiling and condensing of the fluid in the presence of water and iron particles steadily generates hydrochloric acid which is highly corrosive and will initiate rapid staining and corrosion of any components passed through the tank. Development of a green colouration around the condenser coils or tank bottom is an indication that acidity has developed in the solvent. Control of this situation is simple: addition of sodium carbonate (washing soda) to the tank with a regular check to ensure maintenance of alkalinity is sufficient to prevent formation of free hydrochloric acid. Another problem with trichlorethylene is that vapours in the atmosphere will readily condense on any oily film it encounters and, being completely miscible with oil will pass through the oil film to a metal surface below. Once there, it can react with traces of moisture to initiate corrosion underneath the supposedly protective oil layer. Ultraviolet light will generate the acid even in the absence of moisture and machine shops with north-facing windows or skylights are particularly prone to this problem. There is no known protection against this attack. It is strongly recommended that such degreasing baths are at least provided with good fume extraction equipment and preferably are sited in a separate degreasing area fully partitioned off from the main shops and storage areas.
Water-mix fluids At in-use concentrations these fluids consist largely of water ancLtherefore rust is a constant enemy. Suppliers of such fluids are well aware of this potential problem and devote much effort to ensuring that corrosion preventing properties are adequate for the machining applications and dilutions for which they are intended. 6 All this effort is frequently negated by users who mishandle and abuse fluids. Cast iron is particularly susceptible to rusting, 7 probably due to its porous surface which in effect presents a series of capillaries to the fluid which preferentially absorb the surface active components and thereby reduce the concentration of the base fluid in the emulsion. It is therefore good practice to employ a higher strength fluid when machining cast iron. A water-mix fluid is usually supplied as a concentrate that has to be mixed with water by the user. It is essential that the mixing is carried out correctly so that the intended composition is produced 7-9 at the correct concentration. Not the least important factor is the use of good quality water. Mixing the fluid correctly is only the first step however, since the concentration usually alters in use. All fluids lose water by evaporation, soluble oils lose oil by preferential drag out on components and swarf, etc. Fluid concentration should always be closely monitored and top-up
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strength controlled to maintain the desired mixture strength. Soluble oils form emulsions of finely dispersed oil droplets in water when mixed. This is achieved by incorporating emulsifiers which can be considered as forming an envelope of molecules around the oil droplets: the more emulsifier present, the greater the surface area of oil they can cover and hence the smaller the oil droplets. If tramp oil enters the emulsion it has the effect of reducing the emulsifier concentration and the oil droplets increase in size. When they reach a critical size they can no longer remain suspended and rise to the surface to form a cream and eventually free oil. This results in a bulk fluid with a lower than required oil content and corrosion is the probable result. Problems with water-mix fluids
If the emulsion becomes contaminated with viable microorganisms which feed on the emulsifiers, oil droplet size, 1° will increase, reducing the degree of protection against corrosion. Coupled with this will be contamination of the fluid with the waste products generated by the microorganism's metabolic processes, which are themselves largely composed of corrosive acids. 11 One of the most troublesome species of bacteria multiplies in the absence of air to generate hydrogen sulphide which is extremely corrosive as well as having a foul odour. Thus, all soluble oil emulsions and particularly those in large centralised systems should be regularly monitored for the presence of micro-organisms and populations controlled by an appropriate method. TM 13 A frequent complaint from users of water-mix fluids is the staining and corrosion, sometimes severe pitting, of cast iron beds and slideways of machines where swarf has been allowed to accumulate and remain undisturbed for several hours or longer. It cannot be over emphasised that this is a common inbuilt inevitable fact of life where water-mix fluids are used at critically low concentrations. If it is not possible, for whatever reasons, to ensure that swarf is removed at frequent intervals, then a much higher concentration fluid than is really required for the machining operation must be used. All water-mix cutting fluids are alkaline since this is necessary to achieve good ferrous corrosion inhibition. It also reduces the viability of any of the common microorganisms that gain entry to the fluid: for these the higher the pH the better. Unfortunately there are several factors which effectively limit the degree of alkalinity that can be tolerated, one of the main ones being potential irritancy to human skin: a sensitive person may well suffer irritation at a pH above 10. Also at around this level of alkalinity there is a strong possibility of aluminium being chemically attacked and corroded. Zinc is more reactive than aluminium and when machining this metal it is even more important to ensure that the pH of the fluid is low enough to avoid problems. It is a good general rule to avoid using water-mix fluids with zinc if at all possible.
produced can readily react with moisture to generate highly poisonous phosphine gas, but the normal ventilation in most machine shops is adequate to disperse the extremely small quantities generally encountered. 14 Petroleum sulphonates are the most widely used emulsifiers for soluble oil cutting fluids and they also have good corrosion inhibiting properties for ferrous materials. They do, however, have a major disadvantage in that they promote staining and corrosion of copper. This can be almost entirely prevented by removing the cutting fluid from the component immediately after machining since the stains only become evident after standing in contact with the emulsion for some time. It is possible to incorporate inhibitors of the metal de-activating type, but their effect is not permanent because of their depletion on freshly generated metal surfaces of swarf and workpiece. In addition to the staining effect on copper, the fine swarf particles can also react with the emulsifier to produce insoluble green copper soaps which can block filters, form sludges in tanks and pipes etc. This also depletes emulsifier in the fluid and thus causes ferrous corrosion. Therefore before machining copper with a soluble oil emulsion, the user should seek guidance from his fluid supplier. Many soluble oils also contain extreme pressure additives to make them suitable for more arduous machining operations and these additives are essentially of the same types as used in neat oils, that is fats, sulphur and chlorine. They have exactly the same effect as for the neat oils, which must obviously be taken into account. Because soluble oils contain emulsifiers they are surface active and therefore are strongly attracted to metal surfaces. When splashes of emulsion settle on machine surfaces, factory walls and roofs etc, the water will evaporate to leave a sticky, oily film that readily attracts metal fines, dirt and dust. Therefore if regular cleaning is not carried out, the factory rapidly becomes a dirty and unpleasant place in which to work. Soluble oils are far worse than neat oils or synthetics in this respect.
S~/nthetics Let us now look more closely at the synthetic water-mix fluids and consider what disadvantages they present in relation to the soluble oils. That they have many advantages as cutting fluids is not in dispute Is but they certainly do present specific problems on today's machine tools. By definition they contain no mineral oil and therefore do not provide any machine lubrication. Machine tool designers have for years relied on conventional soluble oils to provide lubrication: many of them have not yet realised that for satisfactory operation with synthetics, automatic lubrication of slideways and screws is essential.
Magnesium is so reactive that it will ignite even under water at temperatures commonly encountered at a cutting tool tip. Therefore this metal should never be machined with a water-mix fluid.
Many of the first generation synthetics allowed the build up of solid deposits when splashed fluid lost water by evaporation. Is Most fluids on the market today have overcome this problem but some of the older type are still available. Even though deposits are not now solid, nevertheless they do still tend to be highly alkaline and will therefore corrode aluminium and zinc, the former frequently used for 'one off' splash guards etc.
SG iron is essentially a cast iron containing small amounts of magnesium and phosphorus. Because of its free cutting qualities, a water-mix fluid is commonly used. The swarf
One of the major constituents of most of the synthetic fluids on the market is triethanolamine and this compound reacts with the copper in yellow metals to form cupramines
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(prussian blue). Yellow metal components will rapidly acquire dark blue or black stains if they are left in contact with fluid after machining. A side effect is that the fluid will acquire a blue colouration but this is harmless as long as the colour does not become too intense. An inhibitor can be included in the formulation to cater for the odd yellow metal job in an otherwise essentially ferrous machining shop, but most synthetic fluids are not really suitable for continuous operations on yellow metals. Most of the synthetic fluids are unsuitable for machining aluminium because of their corrosive action mentioned above, although a few are now available that have been designed specifically for use with this metal. The fluid suppliers will be only too pleased to give advice on this aspect. Several advantages of synthetic fluids are due to one common factor: the absence of oil and emulsifiers. They are largely unaffected by tramp oil, are clean to work with, and do not contribute to dirty factory conditions.
Semi-synthetics Semi-synthetics are a relatively new development and unfortunately the term covers a wide variety of basically different types. For instance some suppliers use this term to apply to simple micro-emulsions, that is soluble oils containing a high concentration of emulsifier such that the size of the suspended oil droplets is well below 1 micron. Others are basically true synthetics containing a small amount of emulsified oil to improve machine tool lubrication etc. Hence there is no general advice that can be given in relation to these fluids and the user must refer to his supplier to ascertain what type is involved and the side effects that may be encountered.
Water-mix fluids affect paintwork A final point for discussion is the effect on the paintwork of machines by water-mix fluids in general. For many years, up to about 1960, the machine manufacturers had little to worry about: cutting fluids were neat oils and conventional soluble oils, some containing mild ep additives. With these fluids the relatively cheap, conventional oilresistant paint was adequate. We are now in the era of high performance ep soluble oils and synthetic fluids, many of which attack the old type of cheap paints. Despite a wealth of data now available on paints that are completely resistant to the modern fluids, few of the machine manufacturers are willing to use them because of the increased costs involved, unless specifically requested to by their customers. The users remedy is therefore in his own hands.
Seals Modern machine tools require a variety of different lubricants for their efficient operation, of which the cutting fluid is only one: it is necessary to prevent their intercontamination. Hence efficient seals are required. There have been many attempts in the past to formulate 'multipurpose' oils that will adequately serve as machine lubricants and cutting fluid but they have had little success mainly because they can offer only a compromise in performance parameters and they tend to be more expensive than any one of the single purpose oils they were intended to replace. Most machine tools are provided with seals that are satisfactory for use with all types of cutting fluid and mineral
oil based lubricants since any of the common oil resistant sealing materials will be suitable with water-mix cutting fluids. Corrosion of shafts at seal-lip contact is sometimes encountered: the cutting fluid is usually blamed. There are other factors involved however, such as the choice of seal and shaft materials, deposition of colonies of microorganisms, differential aeration gradients etc.16 These should be thoroughly investigated rather than simply changing the grades of oil used which would, in all probability, not overcome the trouble. The major problem is to prevent the contamination of machine lubricants with water-mix coolants: the effect of such contamination on internal corrosion and machine maintenance costs is too well known to require elaboration here. It is nevertheless a fact that the cutting operation on most multi-spindle automatics could be adequately served by using a water-mix fluid, but the current inadequate sealing demands the continued use of neat oils. Most sealing compounds are flexible elastomers which are far softer than the materials being machined and therefore prone to physical damage by dirt, debris and swarf in the fluids they are designed to seal. It is therefore important to keep the fluids and machines as clean as possible to prevent such damage.
References I 2 3 4 5 6
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8
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10 11 12 13 14 15 16
MachineTools, Metals and Cutting Fluids. BP Trading Ltd (1972) 91-105 DrakeF. B., James K. W, An Approach to Cutting Fluids. Cutting Tools. Jan 1969. 29-31 SinclalreM., Phillips E., The Causes and Prevention of Corrosion of Stainless Steel Metal Progress U.S.A. Vo180, 3, (1961) 92.96 VaughnR. L. Cutting Fluid Additives Tame Titanium. Metalworking Production (15 Feb 1967) 70.72 I.P. Standards for Petroleum and its Products. The Institute of Petroleum (1976) 182.1-182.2 MortonI. S. DevelopmentTesting of Metalworking Lubricants Part Three: Tests on Aqueous Fluids. Industrial Lubrication and Tribology (Nov(Dec 1972) 267-270 NieholsonR. A. Trouble Shooting in the Field of Cutting Oils and Coolants. Industrial Lubrication and Tribology (May(June 1975) 89-94 Ellis E. G. Fundamentals of Lubrication Part 20 - Process Lubricants, Cutting Oils. Industrial Lubrication and Tribology (Dec 1967) 467-471 CourseyW. M. The Application, Control and Disposal of Cutting Fluids. Lubrication Engineering (May 1969) 200-204 Hill E. C. Microbial Pests in Industry. New Scientist (25 Sept 1969) 21-24 BennettO. E. The Biologyof Metalworking Fluids. Lubrication Engineering Vo128, 6 (1972) 237-247 Hill E. C. Biocides for Petroleum Products. IP Journal Vo158, 563 (1972) 248-253 WheelerH. O., Bennet O. E. Bacterial lnhibitors for Cutting Oil. Applied Microbiology Vol 4, 3 (I 956) 122-126 BowkerJ, IL The Liberation of Phosphine in the Machining of Spheroidal-Graphite Iron. Trans Assn o f lnd Med Officers Vol 8 (Ju11958) 50-53 Morton1. S. Water Base Cutting Fluids Still a Question Mark. Industrial Lubrication and Tribology (Feb 1971) 57-62 DegaR. L. Corrosion at the Seal. Mechanical Engineering (Nov 1966) 48-52
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There are few human activities that do not depend on the use of articles that at some stage in their manufacture have required the application of metal cutting techniques. Modern high volume production is only possible by the application of cutting fluids during the cutting operation. The vast range of different workpiece materials and machining operations demanded by the design engineer has produced the stimulus for the development of a multiplicity of cutting fluids to cater for the almost infinite number of combinations involved in practice. There are many aspects to be considered before choosing which fluid to use for a particular cutting operation: the major consideration is adequate performance capability. All fluids currently marketed have their limitations and many will have adverse side effects if used on metals or for cutting operations for which they were not designed. It is of vital importance that the user is aware of what type of cutting fluid he is using and what the possible side effects are so that he can ensure adequate safeguards are taken before expensive problems occur. Some of the commonest problems encountered are those involving staining and corrosion of workpieces and machine tools, usually due to the misuse of the fluid but occasionally to the use of an incorrect fluid. In the UK alone there are about 150 oil companies; many hundreds of individual cutting fluids of a vast range of different chemical compositions are available in the market place. No one, least of all the average user, can be expected to have detailed knowledge of all of them. Each supplier will, of course, have this data on his own products and it is essential that the user can obtain such knowledge as he requires to avoid problems in practice. It is hoped that the following discussion will indicate the type of information the user should demand from his supplier.
Types of cutting fluid There are two main types of cutting fluid: neat oils, which are used as supplied, and water.mix fluids that have to be mixed with water before use. The amount of water-mix * BP Oil Ltd, Victoria Street, London, UK
fluid in the fluid as used can vary between wide limits from as little as 1% or less for grinding to 25% or more for some arduous duties, although the concentration in the majority of applications will be about 4-5%. Neat oils come in a wide variety of compositions ranging from straight mineral oils containing no additives to blends of additives containing no oil. The additives themselves cover a wide range of different types although the most commonly encountered are fatty materials, both ester and free acid, chlorinated paraffins, sulphurised oils and fats and free sulphur. 1 Other less common materials include organic phosphorus compounds, of which zinc dialkyl dithiophosphates and aryl and alkyl phosphates are the main types. These additives are added to increase the cutting performance for specific operating conditions but all can cause staining or corrosion if used with the wrong workpiece material. Water-mix fluids are produced in three main types: emulsifiable oils, chemical or pure synthetic fluids, and the recent so-called semi-synthetics. These are further subdivided into two main categories: ep and non-ep, the former being compounded with additives to improve cutting performance which, as with neat oils, can have adverse staining and corrosive properties if used with the wrong materials.
Neat oils and additives Straight oils are the simplest type of cutting fluid since they contain no additives and problems are rarely encountered in their use. Addition of fatty materials is the first step in improving the cutting performance and again these 'compounded' oils present few problems. The use of galvanised tanks, pipes and fittings should be avoided, however, because fatty acids can react with the zinc to produce insoluble zinc soaps that can promote filter blockage, foaming and generally dirty machines and working conditions. This ban on galvanised materials in the cutting fluid system is applicable to all cutting fluids, and indeed to all lubricants, since most will have a potentially
TRIBOLOGY international February 1977
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aggressive reaction to zinc. These fatty acids are only weakly acidic so that they attack only the very reactive metals such as zinc and magnesium. 2 They are almost completely inert with ferrous metals and indeed fatty acids are commonly incorporated in temporary corrosion preventives for ferrous parts. Another aspect to be considered when using fatty oil based fluids is their relatively low resistance to oxidation and the unfortunate fact that copper is a highly active catalyst for this reaction. This means that when machining copper and copper-based alloys where very fine swarf is produced, the freshly exposed metal surface can catalytically oxidise the fat to produce organic acids which in some circumstances may react with freshly exposed copper surfaces to produce copper soaps, most of which are green in colour. Copper deactivators are generally ineffective in combating this degradation because of the high rate of depletion on the fresh surface generated in the cutting operation. Addition of anti-oxidants is an expensive remedy which does not always overcome the problem. Even so, it is fortunately evident that few such problems occur in practice where relatively large chips of swarf are generated: the main troubles occur in rolling where micron size particles are produced. Chlorine
When fatty additives fail to give the desired performance the next class of chemical compounds to be added is that based on chlorine. The poisonous gas cannot be used for obvious reasons and currently it is included in lubricant formulations as a chlorinated paraffin. These compounds have high stability at low temperatures, are readily soluble in mineral oils, and are non-irritating to the skin, yet they have the ability to react with metals at high temperatures to produce a solid metal chloride film that is easily sheared: a good solid lubricant can be produced in situ as and when required. This metal chloride is firmly bonded to the metal surface in discrete patches corresponding to the distribution of asperities on the original metal surface and remains on the workpiece after the cutting operation. In the majority of applications this is of no significance and no problems arise. However, with some alloys where the end use of the component involves exposure to high temperature it may eventually be of dire consequence. For example, trace amounts of chloride can cause intercrystalline corrosion in most stainless steels, nimonics, and titanium, leading to stress corrosion cracking at elevated temperatures. 3,4 Magnesium is a highly reactive metal and it is probably true to say that, outside the highly specialised aerospace industry, there is little call for machining this metal. Nevertheless it is worthy of note that chlorinated oils should not be used for these applications since chlorine stains are rapidly produced on this metal, even in low severity operations. In cutting operations lubricated by a chlorinated oil, most of the chlorine released from the additive immediately reacts with the metal being cut and the tool, but a small proportion is liberated as hydrogen chloride which forms the highly corrosive hydrochloric acid in the presence of minute traces of moisture. This hydrochloric acid will rapidly attack most metals, leaving the affected surfaces highly susceptible to further staining, corrosion, and rust.
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Because hydrochloric acid is so reactive and is normally produced in very small quantities, it is a relatively simple exercise for the cutting fluid supplier to incorporate a chemical inhibitor to neutralize the acid as soon as it is formed and before it can react with the metal workpiece or machine tool. This can, however, only be expected to cover normal use and if conditions are unusually severe it may be necessary to re-inhibit from time to time until the conditions can be altered. This may be accomplished by several means: increasing the flow of cutting oil into the cutting zone, increasing the capacity of the oil system, installing oil coolers, etc. Again, fortunately, it is a relatively simple procedure to monitor a fluid in use by determination of strong acid number s which will give an immediate indication if re-inhibition is required.
Sulphur Sulphur is by far the most widely used additive to improve the performance of neat cutting oils either in its 'active' or
Tube mill problem solved In arduous cutting or metal forming operations using chlorinated oils, the formation of hydrochloric acid can cause problems. Incorporation of chemical inhibitors in the formulation may ease the problem, but in the long term it is probably better to change operating conditions, for example by increasing the coolant flow rate, increasing the capacity of the oil system, or installing oil co~lers. This approach is illustrated by the solution to problems experienced at a tube reducing mill. The mill, producing nimonic alloy tubes, was designed on the basis of similar mills reducing mild steel tubes which used a soluble oil coolant. To achieve satisfactory production rates and surface finish on the nimonic alloy tubes, it was necessary to use a high viscosity, heavily chlorinated neat oil. The low power pumps and small bore pipework that were satisfactory for a soluble oil were woefully inadequate for the neat oil; the flow rate was too low in the work area and much fuming was evident. The main oil tank installed in a cellar permitted little cooling and bulk oil temperature stabilised between 80°C and 90°C. Operators complained of the acrid and irritating fumes, micrometers left in the vicinity of the machine rapidly corroded, and even steel trusses in the roof area began to rust. No inhibitor system could be expected to overcome the major design fault and immediate steps were taken to redesign the oil system. Higher capacity, high pressure pumps were installed, 1 in pipework was replaced by 3 in pipes, the tank capacity was substantially increased and a cooler installed. These measures were completely successful in overcoming the problems and although the metal working operation involved is particularly arduous, simple regular monitoring of the oil s is now sufficient to achieve a highly satisfactory operation, with a bulk fluid temperature of no more than 30°C.
'inactive' form: the staining and corrosive effects of such additives are well known. It is sufficient, therefore, to summarise these briefly since most users are already aware of the potential problems. The staining and corrosion of copper and its alloys by the formation of black copper sulphide is the most frequent problem, although many of the 'inactive' type sulphurised oils can be used on yellow metals with little practical difficulty. It should be recognised, however, that all extreme pressure additives are designed to react chemically with metals at the elevated temperatures generated by rubbing asperities on the metal surfaces and to do this the additives must have some degree of thermal instability. As in the case of chlorinated additives, the 'inactive' type sulphurised additive will release free, active sulphur under in-use conditions so that a new, unused 'inactive' type sulphurised cutting oil will tend to become more 'active' in use. The rate of generation of 'active' sulphur is directly related to the severity of the cutting operation and hence the temperatures involved. For this reason, many but not all of these oils contain irthibitors to counteract an increasing tendency to stain yellow metals as the original additive is degraded in use. The user would be well advised, therefore, to check that any 'inactive' type oil being used for machining yellow metals is inhibited. Sulphur has a similar effect to chlorine on stainless steels, nimonics, and titanium and therefore its use should be avoided when the machined components are to be exposed to high temperatures in the end use. The 'active' type sulphurised oils are, of course, not intended for use with yellow metals and, although some may be quite satisfactory for such use by the inclusion of effective inhibitors, the general rule must be: do not so use them without first consulting the supplier. Not so well known is the effect of 'active' sulphur on magnesium and aluminium 2 since both metals are susceptible to rapid staining and corrosive attack. It is most unlikely that this would present any difficulty other than in a small jobbing shop where one oil may be preferred whatever workpiece material is involved. Many machine tool manufacturers still fit their machines as standard practice with cutting oil pipes, valves and sprays o made from yellow metals or aluminium alloys so the user should employ his purchasing power with discretion to ensure that any machine he purchases is satisfactory, as supplied, for any cutting oil he might wish to use. The use of copper heat exchanger coils should also be avoided. The majority of additives containing phosphorus are not particularly effective in improving the performance of cutting fluids and hence their use is not widespread. They give little practical difficulty in use since, although their degradation products are acidic they do not generally give rise to staining or corrosion. Indeed the reverse is often the case since phosphating is a recognised form of surface treatment.
De-greasing One of the functions of a cutting oil is to provide protection against corrosion of machined components. This must be remembered when, for instance, a component is washed in a solvent to remove the oil before checking its dimensional accuracy. Immediately after such a check, the component should be protected again, either by re-immersing in the
cutting oil or by application of a temporary corrosion protective. Trichlorethylene is one of the most widely used degreasing solvents since it is, without doubt, one of the most effective fluids available. It presents several harmful side effects however. The toxic effects of its fumes and the ready generation of phosgene at high temperatures, by a lighted cigarette for instance, are frequently overlooked because of its nonflammability. When used in a vapour degreasing bath, the constant boiling and condensing of the fluid in the presence of water and iron particles steadily generates hydrochloric acid which is highly corrosive and will initiate rapid staining and corrosion of any components passed through the tank. Development of a green colouration around the condenser coils or tank bottom is an indication that acidity has developed in the solvent. Control of this situation is simple: addition of sodium carbonate (washing soda) to the tank with a regular check to ensure maintenance of alkalinity is sufficient to prevent formation of free hydrochloric acid. Another problem with trichlorethylene is that vapours in the atmosphere will readily condense on any oily film it encounters and, being completely miscible with oil will pass through the oil film to a metal surface below. Once there, it can react with traces of moisture to initiate corrosion underneath the supposedly protective oil layer. Ultraviolet light will generate the acid even in the absence of moisture and machine shops with north-facing windows or skylights are particularly prone to this problem. There is no known protection against this attack. It is strongly recommended that such degreasing baths are at least provided with good fume extraction equipment and preferably are sited in a separate degreasing area fully partitioned off from the main shops and storage areas.
Water-mix fluids At in-use concentrations these fluids consist largely of water ancLtherefore rust is a constant enemy. Suppliers of such fluids are well aware of this potential problem and devote much effort to ensuring that corrosion preventing properties are adequate for the machining applications and dilutions for which they are intended. 6 All this effort is frequently negated by users who mishandle and abuse fluids. Cast iron is particularly susceptible to rusting, 7 probably due to its porous surface which in effect presents a series of capillaries to the fluid which preferentially absorb the surface active components and thereby reduce the concentration of the base fluid in the emulsion. It is therefore good practice to employ a higher strength fluid when machining cast iron. A water-mix fluid is usually supplied as a concentrate that has to be mixed with water by the user. It is essential that the mixing is carried out correctly so that the intended composition is produced 7-9 at the correct concentration. Not the least important factor is the use of good quality water. Mixing the fluid correctly is only the first step however, since the concentration usually alters in use. All fluids lose water by evaporation, soluble oils lose oil by preferential drag out on components and swarf, etc. Fluid concentration should always be closely monitored and top-up
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strength controlled to maintain the desired mixture strength. Soluble oils form emulsions of finely dispersed oil droplets in water when mixed. This is achieved by incorporating emulsifiers which can be considered as forming an envelope of molecules around the oil droplets: the more emulsifier present, the greater the surface area of oil they can cover and hence the smaller the oil droplets. If tramp oil enters the emulsion it has the effect of reducing the emulsifier concentration and the oil droplets increase in size. When they reach a critical size they can no longer remain suspended and rise to the surface to form a cream and eventually free oil. This results in a bulk fluid with a lower than required oil content and corrosion is the probable result. Problems with water-mix fluids
If the emulsion becomes contaminated with viable microorganisms which feed on the emulsifiers, oil droplet size, 1° will increase, reducing the degree of protection against corrosion. Coupled with this will be contamination of the fluid with the waste products generated by the microorganism's metabolic processes, which are themselves largely composed of corrosive acids. 11 One of the most troublesome species of bacteria multiplies in the absence of air to generate hydrogen sulphide which is extremely corrosive as well as having a foul odour. Thus, all soluble oil emulsions and particularly those in large centralised systems should be regularly monitored for the presence of micro-organisms and populations controlled by an appropriate method. TM 13 A frequent complaint from users of water-mix fluids is the staining and corrosion, sometimes severe pitting, of cast iron beds and slideways of machines where swarf has been allowed to accumulate and remain undisturbed for several hours or longer. It cannot be over emphasised that this is a common inbuilt inevitable fact of life where water-mix fluids are used at critically low concentrations. If it is not possible, for whatever reasons, to ensure that swarf is removed at frequent intervals, then a much higher concentration fluid than is really required for the machining operation must be used. All water-mix cutting fluids are alkaline since this is necessary to achieve good ferrous corrosion inhibition. It also reduces the viability of any of the common microorganisms that gain entry to the fluid: for these the higher the pH the better. Unfortunately there are several factors which effectively limit the degree of alkalinity that can be tolerated, one of the main ones being potential irritancy to human skin: a sensitive person may well suffer irritation at a pH above 10. Also at around this level of alkalinity there is a strong possibility of aluminium being chemically attacked and corroded. Zinc is more reactive than aluminium and when machining this metal it is even more important to ensure that the pH of the fluid is low enough to avoid problems. It is a good general rule to avoid using water-mix fluids with zinc if at all possible.
produced can readily react with moisture to generate highly poisonous phosphine gas, but the normal ventilation in most machine shops is adequate to disperse the extremely small quantities generally encountered. 14 Petroleum sulphonates are the most widely used emulsifiers for soluble oil cutting fluids and they also have good corrosion inhibiting properties for ferrous materials. They do, however, have a major disadvantage in that they promote staining and corrosion of copper. This can be almost entirely prevented by removing the cutting fluid from the component immediately after machining since the stains only become evident after standing in contact with the emulsion for some time. It is possible to incorporate inhibitors of the metal de-activating type, but their effect is not permanent because of their depletion on freshly generated metal surfaces of swarf and workpiece. In addition to the staining effect on copper, the fine swarf particles can also react with the emulsifier to produce insoluble green copper soaps which can block filters, form sludges in tanks and pipes etc. This also depletes emulsifier in the fluid and thus causes ferrous corrosion. Therefore before machining copper with a soluble oil emulsion, the user should seek guidance from his fluid supplier. Many soluble oils also contain extreme pressure additives to make them suitable for more arduous machining operations and these additives are essentially of the same types as used in neat oils, that is fats, sulphur and chlorine. They have exactly the same effect as for the neat oils, which must obviously be taken into account. Because soluble oils contain emulsifiers they are surface active and therefore are strongly attracted to metal surfaces. When splashes of emulsion settle on machine surfaces, factory walls and roofs etc, the water will evaporate to leave a sticky, oily film that readily attracts metal fines, dirt and dust. Therefore if regular cleaning is not carried out, the factory rapidly becomes a dirty and unpleasant place in which to work. Soluble oils are far worse than neat oils or synthetics in this respect.
S~/nthetics Let us now look more closely at the synthetic water-mix fluids and consider what disadvantages they present in relation to the soluble oils. That they have many advantages as cutting fluids is not in dispute Is but they certainly do present specific problems on today's machine tools. By definition they contain no mineral oil and therefore do not provide any machine lubrication. Machine tool designers have for years relied on conventional soluble oils to provide lubrication: many of them have not yet realised that for satisfactory operation with synthetics, automatic lubrication of slideways and screws is essential.
Magnesium is so reactive that it will ignite even under water at temperatures commonly encountered at a cutting tool tip. Therefore this metal should never be machined with a water-mix fluid.
Many of the first generation synthetics allowed the build up of solid deposits when splashed fluid lost water by evaporation. Is Most fluids on the market today have overcome this problem but some of the older type are still available. Even though deposits are not now solid, nevertheless they do still tend to be highly alkaline and will therefore corrode aluminium and zinc, the former frequently used for 'one off' splash guards etc.
SG iron is essentially a cast iron containing small amounts of magnesium and phosphorus. Because of its free cutting qualities, a water-mix fluid is commonly used. The swarf
One of the major constituents of most of the synthetic fluids on the market is triethanolamine and this compound reacts with the copper in yellow metals to form cupramines
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TRIBOLOGY international February 1977
(prussian blue). Yellow metal components will rapidly acquire dark blue or black stains if they are left in contact with fluid after machining. A side effect is that the fluid will acquire a blue colouration but this is harmless as long as the colour does not become too intense. An inhibitor can be included in the formulation to cater for the odd yellow metal job in an otherwise essentially ferrous machining shop, but most synthetic fluids are not really suitable for continuous operations on yellow metals. Most of the synthetic fluids are unsuitable for machining aluminium because of their corrosive action mentioned above, although a few are now available that have been designed specifically for use with this metal. The fluid suppliers will be only too pleased to give advice on this aspect. Several advantages of synthetic fluids are due to one common factor: the absence of oil and emulsifiers. They are largely unaffected by tramp oil, are clean to work with, and do not contribute to dirty factory conditions.
Semi-synthetics Semi-synthetics are a relatively new development and unfortunately the term covers a wide variety of basically different types. For instance some suppliers use this term to apply to simple micro-emulsions, that is soluble oils containing a high concentration of emulsifier such that the size of the suspended oil droplets is well below 1 micron. Others are basically true synthetics containing a small amount of emulsified oil to improve machine tool lubrication etc. Hence there is no general advice that can be given in relation to these fluids and the user must refer to his supplier to ascertain what type is involved and the side effects that may be encountered.
Water-mix fluids affect paintwork A final point for discussion is the effect on the paintwork of machines by water-mix fluids in general. For many years, up to about 1960, the machine manufacturers had little to worry about: cutting fluids were neat oils and conventional soluble oils, some containing mild ep additives. With these fluids the relatively cheap, conventional oilresistant paint was adequate. We are now in the era of high performance ep soluble oils and synthetic fluids, many of which attack the old type of cheap paints. Despite a wealth of data now available on paints that are completely resistant to the modern fluids, few of the machine manufacturers are willing to use them because of the increased costs involved, unless specifically requested to by their customers. The users remedy is therefore in his own hands.
Seals Modern machine tools require a variety of different lubricants for their efficient operation, of which the cutting fluid is only one: it is necessary to prevent their intercontamination. Hence efficient seals are required. There have been many attempts in the past to formulate 'multipurpose' oils that will adequately serve as machine lubricants and cutting fluid but they have had little success mainly because they can offer only a compromise in performance parameters and they tend to be more expensive than any one of the single purpose oils they were intended to replace. Most machine tools are provided with seals that are satisfactory for use with all types of cutting fluid and mineral
oil based lubricants since any of the common oil resistant sealing materials will be suitable with water-mix cutting fluids. Corrosion of shafts at seal-lip contact is sometimes encountered: the cutting fluid is usually blamed. There are other factors involved however, such as the choice of seal and shaft materials, deposition of colonies of microorganisms, differential aeration gradients etc.16 These should be thoroughly investigated rather than simply changing the grades of oil used which would, in all probability, not overcome the trouble. The major problem is to prevent the contamination of machine lubricants with water-mix coolants: the effect of such contamination on internal corrosion and machine maintenance costs is too well known to require elaboration here. It is nevertheless a fact that the cutting operation on most multi-spindle automatics could be adequately served by using a water-mix fluid, but the current inadequate sealing demands the continued use of neat oils. Most sealing compounds are flexible elastomers which are far softer than the materials being machined and therefore prone to physical damage by dirt, debris and swarf in the fluids they are designed to seal. It is therefore important to keep the fluids and machines as clean as possible to prevent such damage.
References I 2 3 4 5 6
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10 11 12 13 14 15 16
MachineTools, Metals and Cutting Fluids. BP Trading Ltd (1972) 91-105 DrakeF. B., James K. W, An Approach to Cutting Fluids. Cutting Tools. Jan 1969. 29-31 SinclalreM., Phillips E., The Causes and Prevention of Corrosion of Stainless Steel Metal Progress U.S.A. Vo180, 3, (1961) 92.96 VaughnR. L. Cutting Fluid Additives Tame Titanium. Metalworking Production (15 Feb 1967) 70.72 I.P. Standards for Petroleum and its Products. The Institute of Petroleum (1976) 182.1-182.2 MortonI. S. DevelopmentTesting of Metalworking Lubricants Part Three: Tests on Aqueous Fluids. Industrial Lubrication and Tribology (Nov(Dec 1972) 267-270 NieholsonR. A. Trouble Shooting in the Field of Cutting Oils and Coolants. Industrial Lubrication and Tribology (May(June 1975) 89-94 Ellis E. G. Fundamentals of Lubrication Part 20 - Process Lubricants, Cutting Oils. Industrial Lubrication and Tribology (Dec 1967) 467-471 CourseyW. M. The Application, Control and Disposal of Cutting Fluids. Lubrication Engineering (May 1969) 200-204 Hill E. C. Microbial Pests in Industry. New Scientist (25 Sept 1969) 21-24 BennettO. E. The Biologyof Metalworking Fluids. Lubrication Engineering Vo128, 6 (1972) 237-247 Hill E. C. Biocides for Petroleum Products. IP Journal Vo158, 563 (1972) 248-253 WheelerH. O., Bennet O. E. Bacterial lnhibitors for Cutting Oil. Applied Microbiology Vol 4, 3 (I 956) 122-126 BowkerJ, IL The Liberation of Phosphine in the Machining of Spheroidal-Graphite Iron. Trans Assn o f lnd Med Officers Vol 8 (Ju11958) 50-53 Morton1. S. Water Base Cutting Fluids Still a Question Mark. Industrial Lubrication and Tribology (Feb 1971) 57-62 DegaR. L. Corrosion at the Seal. Mechanical Engineering (Nov 1966) 48-52
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