Further, these CAD-based techniques guarantee a chemically-stable product with the optimum properties, reducing the risk of selecting the wrong type of cutting fluid both by the manufac
Trang 1Figure 202 A cutting fluid 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]
.
Trang 2Figure 203 Bacterial contamination: aqueous cutting fluid
.
402 Chapter 8
Trang 3Figure 204 Computer-Aided Design (CAD), utilised to select a corrosion inhibitor for an aqueous-based
cutting fluid [Courtesy of Cimcool]
.
Trang 4gies. As an example of this phenomenon, anionic
emulsifiers normally have corrosion inhibiting charac-teristics, but these properties are usually so slight that
any side-effects are usually disregarded. However, by
using CAD, it is possible to find emulsifiers – normally
several are needed, whose side-effects add up
syner-getically
When the correct emulsifiers 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 effect’ ,
while values greater, or less than zero represent a: posi-tive; or negative effects; respectively.
Such CAD for chemical compounds makes it pos-sible to develop ‘atomised’ cutting fluids far faster
than by previous techniques and offers the prospect of
discovering entirely new cutting fluid combinations.
Computer analysis, offers a way to develop, analyse
and test new cutting fluids, enabling very rapid modi-
fications 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 fluid 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-fluid data in a real-time cut-ting environment.
8.6.1 Cutting Fluid – Quality Control
For practical reasons industrial cutting fluid 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 differences can also oc- ‘Synergy’ , refers to the outcome when substances are
com-bined and produce ‘side-effects’ , which add to, or even amplify
each other, giving rise to a much stronger resultant effect.
cur depending on the raw material source and the sea-sons of the year. In order to ensure constant quality
of the finished product despite these variations and the factors which determine the quality of the raw materials, they must be checked prior to entering the cutting fluid production processing stage. The 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. Thus, on the basis of these user-defined 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 fluid development process.
An important criterion for the quality of the fi-nal cutting fluid formulation is its stability. By com- parison, synthetic cutting fluids produce fewer prob-lems than semi-synthetic and emulsion cutting fluids,
in their development. In the case of the semi-syn- thetic and emulsion cutting fluids, not only must cool-ing water and lubricatthetic and emulsion cutting fluids, not only must cool-ing 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 different degrees of water hardness, varying mix ratios and a diverse range of impurities occur, they will strongly influence the overall water-oil system. The conventional way of stabilising such a sys-tem is to add plenty of emulsifiers. This action can lead
to excessive foaming, especially if the water is soft, 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 fluid 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. The growth
of micro-organisms (Fig. 203b) affects the droplet size and as a result, as these droplets spherically-increase in size, the number and distribution of droplets decreases. Thus, an oil-water system with many evenly-spaced and small droplets, will be more stable than systems
404 Chapter 8
Trang 5Figure 205 Laboratory-based testing procedures on cutting fluid coolant products
.
Trang 6where there are bigger, but fewer droplets present.
Both the size and distribution of these oil droplets has
an important influence on the emulsion’s consequent
foaming behaviour, which in turn, is strongly affected
by the water hardness and any turbulence produced by
the machine tool, or from a centralised coolant supply
system (Fig. 203a).
Product Testing
There are a number of possible tests for checking the quality of a cutting fluid 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 fluid’s dilution, utilising a Refractometer [Courtesy of Rocol Ltd.]
.
406 Chapter 8
Trang 7a biostability test; biostatic properties; or resistance to
bacteria growth. Foaming behaviour: a slight
ten-
dency to foam is important for some types of machin-ing operations, particularly when deep-hole drilldency to foam is important for some types of machin-ing
operations are undertaken and more specifically, in
grinding operations. There 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 fluid 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 often undertaken, with low
adhesion representing a tendency for the cutting fluid
product to build-up layers of deposit. This 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 fluid for a certain time, then drying them out
and subsequently testing for adhesion, is sometimes
employed. Compatibility tests for cutting fluids,
with particular reference to paints and elastomers by
visual inspection of painted sheet metal is often car-ried out. Acid/alkaline tests are often undertaken on
the cutting fluid, as this affects both the machine tool
and health of the setter/operator
Once a cutting fluid 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 after
which, the cutting fluid is offered on the ‘open-market’
‘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.
‘Compatibility tests’ , specially-prepared painted sheet metal
is fully- immersed in the cutting fluid for a certain time, then
visually inspected for paint de-lamination, etc., according to
the German Standard: DIN53521.
‘Acid/alkaline tests’ , are important cutting fluid 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. Therefore, it is vital that the
correct cutting fluid 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. The following section has been included to help with the important decisions re-lating to the choice of cutting fluid selection.
8.7 Selecting the Correct
Cutting Fluid
When choosing a cutting fluid many factors have to be considered, with the relative importance varying with each individual circumstance. For the latter reason, it
is not possible to offer general rules for the selection
of a cutting fluid. 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 fluid
it should always be borne in mind that the machining process plays an important role in: productivity and efficiency; operator health; safety; plus the quality of work created.
8.7.1 Factors Affecting Choice
Probably the main factors that must be considered when selecting a specific cutting fluid 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: effi-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-off); 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 fluid individually, or delivered from a centralised system? Are particular cutting fluids recommended
by the manufacturer of these machine tools?
Trang 8• Protection of people and the environment – to
what extent are the personnel exposed to cutting
fluids: before; during; and after use? Are there local
constraints on fluid disposal?
The above criteria concerning cutting fluid selection,
can be sub-divided into two distinct groupings – com-mercial and production, as follows
Commercial Criteria
The 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 fluid will be greater. If, dif-ferent materials and types of machining processes are
involved in the production process, then a ‘universal’
cutting fluid might be a better choice than a number of
different fluid products, even if the latter fluid compo-sitions individually produced a better performance.
Production Criteria
In either one-off, or batch production, individual
machines tools are likely to have their own separate
cutting fluid supply, however in a mass production
environment, fluid centralised systems are the norm.
The production criteria for the choice of cutting fluid
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
configuration. Thus, on the basis of these
produc-
tion-based decision criteria, an initial choice will nor- ‘Machine tool configuration’ , 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 offer
con-tinuous kinematics that have multiple-axis control – for even
simple straight-line motion, often 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 fluid to
be used – whether it is an aqueous-, or oil-based fluid type that is required.
8.7.2 Selection Procedure
When selecting a cutting fluid, it is important to take the fluid manufacturer’s instructions into consider-ation. If their recommendations are ignored, it may render any previous guarantees invalid. Many of these cutting fluid manufacturers specify that certain products be utilised, normally only applying them to either special machining circumstances, or where dif-ficult cutting conditions are likely to be encountered. Equally, other cutting fluid producers only specify the general type of instructions, such as: what type of cut-ting fluid to employ, for example, aqueous, or not, to
be used, on the contrary, some fluid manufacturers do not specify anything! Usually it is possible to rely on the manufacturer’s specification tables which indicate their most suitable product for a particular range of
machining operations. However, before consulting a
cutting fluid manufacturer’s set of tables, the follow-ing factors must be either known, or established. These
crucial decisions include:
• Type of machining operation – care must be taken
to ensure that correct and efficient planning of the machining strategy for the successful completion
of the part is known and, the optimum machining techniques to achieve this objective are confirmed,
• Water characteristics – hard/soft, 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 filtration system – having ei- ther: no filtration; sump only; paper filter; centri-fuge; etc.
NB In the case of the centrifuge, semi-synthetic
products are not
recommended, while washable fil- 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-408 Chapter 8
Trang 9ers multiple machining operations undertaken on
machining centres, the cutting fluid should be chosen
for the range between the highest and lowest cutting
speeds. Once a particular fluid product has been se-lected, it is still necessary to carry out a practical test
as only after such testing, will it confirm 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 fluid consumption is
anticipated.
8.8 Care, Handling,
Control and Usage –
of Cutting Fluids
So that the properties of a cutting fluid 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 fluid
control, is suitable handling and storage at the user’s
premises, as it provides a continuous replenishment
and service facility for the monitoring of fluids 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.
This potential fluid contamination is one of the most
important aspects when dealing with soluble oils, in
order to obviate any emulsification troubles and pos-sible coolant separation issues.
In large manufacturing companies, where bulk
delivery of cutting fluids are being made-up prior
to delivery, it is important that all storage tanks for
the reception are properly identified, 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 identified (i.e.
marked-up), as to the grade identification and its end
usage, this could lead to some costly machining mis-takes (e.g. part-scrappage problems) within the pro-duction machining facility. The reception of drums of cutting fluid and oils, requires certain safeguards, to protect the mutual interests of both the supplier and user alike. For instance, the supplier should be noti-fied at once if there is obvious drum damage, or the odd drum is leaking. Full drums of fluid should never
be just dropped-off 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 influence, 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; hoist, or fork-lift truck
When the handling of cutting and lubricating oils has developed into a filthy job, this is reflected in the storage facility, which in some companies is the most neglected part of the factory. Under such conditions, it
is often difficult, 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 effort should be spared to create and maintain these optimum conditions, this being es-sential to a trouble-free operation.
If soluble cutting fluid – ‘coolant’ , is mixed in bulk, the storage tank should be clearly marked with the: product’s name; reference code; and concentration. This simple but vital tank identification 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 fluid batch.
Soluble oil concentrate must not be exposed to cold climatic conditions and allowed to freeze, since this ‘cryogenic effect’ might adversely affect the
‘Oil-drum skids’ , when robustly designed/produced, allow the
controlled sliding to ground-level of full drums off a vehicle’s loading platform, without potential health risks to the compa-ny’s work-force.
Trang 10concentrate’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 fluctuations, any water can be sucked into the
barrel – even through the unopened top. These 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 fluid
in such a condition, will be unlikely to adequately
remix. Consequently, it is advisable that some form
of space heating for undercover fluid storage is desir-able, to minimise wide temperature fluctuations, this
being particularly relevant during winter months, as
the oil viscosity appreciably changes (i.e. thickens),
with more sluggish flow-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 fluids and lubricants is to be
recommended.
As all oil storage facilities have a real potential for
a catastrophic and explosive fire hazard. Sufficient fire
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 sawdust0 present, requires prompt disposal
to minimise fire risk. Any oil storage facility is strictly a
non-smoking zone, for obvious reasons.
8.8.1 Product Mixing – Preparation of
a Aqueous-Based Cutting Fluids
As mentioned, cutting fluids are usually supplied to
the customer in the form of concentrates and the ease
0 ‘Sawdust’ soaked in oil is a likely fire source, so rather than
use this to soak up oil-spillages, use specially-produced gran-ules that are non-flammable and oil absorbent.
NB These 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 often supplied
as ‘preformed emulsions’ , in which the concentrate has
previously been mixed by the cutting fluid manufac- turer with water to form a stable emulsion. These 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 fluid 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 fill 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 efficient 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 fluid are required, then it is advisable to use a purpose-built and designed fluid mixer (Fig. 208). These proprietary mixing devices incorporate the following features:
• They can directly-connect to a standard water tap,
• They can be screwed directly into a large fluid con-centrate drum,
• There is adjustment for the automatic dispensing
of the correct emulsion dilution ratio of: ‘oil-to-wa-ter’ ,
• The fluid mixer incorporates a non-return valve, to ensure that the emulsion cannot leak-back into the drum’s contents
410 Chapter 8