Cutting Fluid Mists Mists resulting from machining operations and their subsequent collection resulting from the application of cutting fluids, are usually given a low priority by most m
Trang 1Figure 211 Some typical, but extreme skin disorders – attributed to exposure to cutting fluids [Courtesy of Castrol Industrial]
.
Trang 2Many other skin conditions can occur and their
causes can emanate from a number of MWF sources
– going beyond the current scope and objectives of
this chapter. Although through the application of bar-rier and conditioning creams, together with clean and
suitable protective clothing, coupled to good washing
facilities, these factors will inevitably lessen the pos-sibility of allergic reactions and skin disorders
Tumours and Cancerous Effects
However, less well known than the allergic and skin
condition previously mentioned, are the other more
serious debilitating health effects on the machine tool
personnel exposed to MWF’s. Industrial experience
suggests that continuous and long exposure to certain
mineral oils can give rise to skin thickening, known as
keratosis, whereby ‘warty-elevations’ (i.e. see Fig. 211b)
can slowly develop over a period of some years. Hence,
these warts will either: remain as they are; disappear;
or in the worse case scenario, become malignant.
A considerable volume of research in both the
chemical and biological fields has been undertaken,
in particular, into the effects of mineral oils in
cut-ting fluids and their affect on worker’s health. Mineral
oils may contain carcinogens – chemical compounds
which are active in causing cancer, with currently, a
number of these compounds having been identified.
They occur in the main, as polycyclic aromatic hydro-carbons and, when present in modern refined mineral
oils exist in extremely small proportions – making their
‘positive’ chemical identification exceedingly difficult
to define. Oil refinement by acid treatment has now
been replaced by more modern refining techniques,
including solvent-refined treatment and hydrogenera-tion – greatly reducing the undesirable proportions
of aromatic compounds (i.e. these latter compounds
being potential carcinogens). Moreover, chemical
coolants were originally based on diethanolamine
and sodium nitrate, which for some time have been
suspected of forming ethanolnitrosamine – another
suspected carcinogen. In order to remove this
pos-sible carcinogen, in 1984, cutting fluid manufacturers
removed the nitrates from their formulations. Finally,
‘N-nitrosamines’ , and its chemical compounds are a signifi-cant danger to worker’s health and, the American
Environ-mental Protection Agency (EPA), stated in a report of their
findings in 1974, that: ‘As a family of carcinogens, the
nitrosa-mines have no equal.’
if one considers permissible exposure levels (PEL’s) from nitrosamine sources. Then, it has been stated that
smoking twenty (untipped) cigarettes per day will de-liver 0.8 micrograms of various nitrosamines which al-most equates to eating a kilogram of fatty bacon per day
(i.e. 6 microgrames), thus, when undertaking these seriously debilitating smoking/eating toxicity habits over a significant period of time, they would consider-ably increase the risk of cancer
Cutting Fluid Mists
Mists resulting from machining operations and their subsequent collection resulting from the application of cutting fluids, are usually given a low priority by most manufacturers when compiling a list of potential capi- tal items for the workshop. To press this point still fur-ther, many companies would much sooner purchase a new machine tool, than install a special-purpose air cleaner. In the automotive industries interest in the level of air quality has some degree of importance, while elsewhere in smaller production workshops it is somewhat of a hit-or-miss affair. Given the potential worker health risks involved today, with high-speed machining (HSM) coupled to increased tooling cut-ting data and higher-pressure coolant supplies (i.e. see Fig. 195 – top), possibly the greatest threat posed
to a worker is from atomised mists (i.e. sub-µm size) within the local atmosphere. Many companies that incorporate mist collection filtering, will only remove particles of >4 µm in size, leaving the critical sub-µm particles still present in the atmosphere.
The earliest chemical interventions to reduce mist-ing were high-molecular-weight polymer additives, that act to stabilise MWF’s and thus suppress mist for-mation. With conventional petroleum-based fluids, polyisobutylene has been the preferred anti-mist ad- ditive. While, for aqueous-based cutting fluids, poly-ethylene oxide (PEO) has been utilised. Due to the susceptibility of PEO’s to shear degradation, repeti- tive additions of the PEO polymer are needed to main-tain mist reduction. Today, a newer class of shear-stable polymers has been developed to overcome the shear degradation as indicated by PEO’s. These latest polymer products have been derived from complex: 2-acrylamido-2methlypropane sulphonic acid mono-mers, hence, providing longer-term performance in continuously recirculating aqueous-based MWF sys-tems.
So, very high concentration cutting fluid mists will over a short period of time cause: ‘smarting’ of the
Trang 3eyes; irritation of exposed skin; result in slight irrita-tion of the mouth and throat; by inhalation, will
ir-ritate the lungs; by ingestion, of the stomach – it may
promote nausea; and affect other internal organs.
If exposed to toxic mists over a long period of time,
this could cause lasting damage to both external and
internal bodily-parts, with at the extreme condition,
promoting the growth of malignant tumours. In order
to restrict misting and minimise operator health risks,
then special-purpose filtering systems have been
de-veloped, which will be briefly reviewed below
The conventional mist-collection technology,
such as: filters; rotating drums; or cyclones; will col-lect particles of >1 µm in diameter, but cannot cope
with smaller sub-µm particles. Further, it has been re-ported that fibrous filters once they are wet, lose
ef-
ficiency over time – see Fig. 212. Therefore, the opti-mum manner of removing sub-µm mists are by fitting
one of the following: High-efficiency Particulate Air
fil-ters (HEPA); Electrostatic Precipitators
(ESP’s); or Fi-bre-bed
systems. Probably the two best systems for re-moval of sub-µm mist particles are the HEPA and ESP
systems. Each one has its disadvantages, with HEPA fil-
ters being expensive and become clogged, thereby los-ing efficiency. So, when disposable filter replacements
are needed this hidden replacements cost, will result
in both costly maintenance and disposal. While, ESP’s
need frequent maintenance and cleaning, thus
rep-
resenting a continuous on-going cost burden. Mean-while, Fibre-bed systems offer high efficiency in mist
collection, but with ease of maintenance, although
they are larger requiring more electrical power to op-erate them.
Vegetable Oil-Based MWF’s
Driven by the health and safety concerns of both
workers and manufacturers alike, vegetable oil-based
MWF’s have been developed, to substitute for the same
machining operations as either the mineral-, or petro-leum-based fluids, currently undertake. It has been
reported that compared with mineral oil-based
cut-
ting fluids, the alternative vegetable-based MWF’s, en-hance cutting performance by extending tool life while
improving machined surface texture, with the
addi-tional benefit of being an environmentally-friendly
MWF. In particular, Soybean oils have shown
con-siderable promise as a practical alternative to
‘tradi-tional’ MWF’s, where they have improved component
surface texture and reduced tool chatter. One of the
principle reasons for these surface texture and
ma-chining improvements, is that the vegetable oil-based
MWF’s have enhanced lubricity, coupled with a slight
‘polar-charge’ – which acts to attract the vegetable oil molecules to the metallic surface being tenacious enough to resist any subsequent wipe-off. The oppo-site is true for a mineral-based oil, where there is no molecular charge, so offers little improvement in lu-bricity.
Mineral-based MWF’s are just straight hydrocar-bon, while their vegetable oil counterparts contain oxygen, which is tenaciously-attracted to the sterile elevated temperature of the recently-machined work-piece’s metallic surface, thus it bonds more strongly – acting as a result as a better lubricant. Yet another performance benefit of utilising vegetable-based oils over their mineral-based equivalents, is that they have
a higher ‘flash-point’ , which reduces both the ten-dency for smoke formation and fire-risk. Yet another reason for selecting a vegetable-based MWF over its mineral-based counterpart, is that it has a high natural viscosity. Hence, as the machining temperature
increases, the viscosity of the vegetable oil drops more slowly than for that of a mineral oil. Conversely, as the temperature falls, the vegetable oil remains more fluid than its counterpart mineral oil. Thus, facilitating more
efficient and quicker drainage from both the swarf and
workpiece. The high viscosity index of vegetable oil ensures that it provides more lubricity-stability, across the operating temperature range being found during a range of machining operations. High viscosity allows vegetable oils to be used as a slideway lubricant and for gear lubrication in gearboxes, acting as a so-called:
‘multi-functional fluid’ (i.e. see Section 8.9).
Along with the above stated benefits, there is also
a down-side to vegetable-based fluid applications, the
limitations are that they lack sufficient oxidative sta- ‘Flash-point’ of oils, is the instantaneous ignition of the oil at
a specific temperature, without the aid of a flame. So, in the case of a Soybean oil it has a flash-point of 232°C, while a typi-cal mineral oil has a flash-point of just 113°C.
‘Viscosity’ , can be defined* as: ‘The resistance of a fluid to shear
force.’ Therefore, the shear force per unit area is a constant
times the velocity gradient, with the constant being the coef-ficient of viscosity. SI units are: Newton-seconds per square metre (Ns m–), denoted by the Greek symbol: ‘µ’. [Source: Carvill, 1999]
*While another definition for a fluid’s viscosity is: ‘The bulk property of a fluid, semi-fluid, or semi-solid substance that causes it to resist flow.’ [Kalpakjian, 1984]
‘Viscosity index’ , can be defined as: ‘A measure of a fluid’s
change of viscosity with temperature: the higher the index, the smaller the relative change in viscosity.’ [Kalpakjian, 1984]
Trang 4bility for many machining applications. Thus, a low
oxidative stability means that the oil will oxidise quite
quickly during use, becoming thick as it polymerises
to a plastic-like consistency. Once the oil has become
too thick, or even too thin for that matter, the cutting
tool’s edge(s) will quickly wear-out. Vegetable oils be-come oxidised and as a result, will chemically change,
along with their viscosity and lubricating abilities. There is some concern among users of vegetable-based cutting fluids, that this oil reacts with the environment (i.e. oxygen and metals), thus breaking-down, which
is not the case for petroleum-based products. Both of these fluid products oxidise with heat, but vegetable oils are more susceptible to oxidation. While another
Figure 212 At the filter some droplets and volatiles are
re-moved from the atmosphere, but the remainder pass through
and are entrained Other particulates are ‘indefinitely’
re-tained, but with time reduce filter efficiency Optimum filters
re-entrainment – at a reasonable pressure-drop [Source: Raynor
P & Leith, D – Univ of North Carolina, 2003]
Trang 5of hydrolytic stability. Typically, when making an
emulsion; obviously oil and water are present; so if ox-ygen and some form of alkaline component is at hand,
it may cause certain ester linkages within the vegetable
oil to break down. These broken-down components
act in a different manner to that of the original vegeta-ble oil, thereby affecting its cutting fluid performance.
Conversely, mineral-based cutting oils are resistant to
hydrolytic reactions. Vegetable oils can support micro-
bial growth more readily than the equivalent mineral-
based cutting fluids. Although this vegetable oil’s bio-degradability is ideal for subsequent waste treatment,
conversely, when this product is ‘festering’ in a
ma-chine’s sump, it becomes both smelly and sour, via its
bactericide and fungicide reactions. Finally, for many
companies, probably the biggest limitation in changing
over to vegetable-based products in machining opera-tions is its purchase cost. For example, canola oil, can
cost up to 300% more than its equivalent petroleum-based product and to compound the financial problem
still further, costly ingredients are necessary to control
oxidation and enhance its biological stability – consid-erably adding to the finished product’s cost
8.12 Fluid Machining
Strategies: Dry;
Near-Dry; or Wet
So far, this chapter has been principally concerned
with all aspects of flood/wet coolant applications to the
overall machining process. Several other complemen-tary cutting strategies can be adopted, these include:
dry; near-dry; together with wet machining; thus, in
the following sections a discussion of these important
issues and concerns will be briefly mentioned
‘Hydrolytic stability’: ester molecules consist jointly of con-densed fatty acids and alcohols; so the vegetable oils will
naturally exist as esters – often termed ‘triglycerides’ , these
being a condensation of fatty acid, plus glycerine. Under the
right conditions, the triglyceride can split and revert back to a
fatty acid and glycerine, which acts differently from that of the
original ester. In the case of mineral-based oils, they do not
contain these ester linkages and as such, will not break down,
nor ‘hydrolise’.
8.12.1 Wet- and Dry-Machining –
the Issues and Concerns
In the past twenty-five years the cost of cutting flu-ids has risen from just 3% of the overall cost of the machining process, to that of >15% of a production shop’s cost. Cutting fluids and especially ones that are oil-based products have become something of a liabil-ity of late, this is due in the main, to many countries
‘Environmental Protection Agencies’ , strictly regulat-ing their ensuing disposal – at the end of their natural life. In many countries ‘spent’ cutting fluids have been re-classified as either ‘toxic-’ , or ‘hazardous-waste’ , moreover, if they have been found to have machined certain alloyed and exotic material workpieces, they are under even harsher disposal regulations. Due to the increasing popularity today of high-speed machin-ing (HSM) – more will be said on this subject in the following chapter – coupled to increased cutting data and the application of coolants via high-pressure sys-tems, these factors have significantly contributed to the creation of air-borne mists within the workshop environment (i.e. see Fig. 212). Such coolant mists now have even stricter permissible exposure levels (PEL’s) imposed in the working environment, to regu-late and control these air-borne particulates, thereby minimising workers health risks. Thus, the cost of: fluid maintenance; record-keeping; with strict compli-ance to current and proposed regulations, have rapidly increased the overall price of cutting fluids. In many manufacturing companies involved in a significant amount of machining operations, they are consider-ing the strategy of cutting dry, to overcome the cutting fluid-based costs and disposal concerns during and after their subsequent usage.
For many companies involved in significant work-piece machining operations, they are unsure if they could cut all their components ‘dry’. Furthermore, they are under the impression that to achieve higher cutting data and ‘hard-part’ machining, then cutting fluids are essential in achieving these objectives. Moreover, many companies also believe that the cost of chang-ing from a ‘wet-’ to ‘dry-machining’ operations would
be prohibitively high. Neither of these impressions are true. So, by machining ‘dry’ it can be considered as a standard operational procedure for most metal-cut- ting operations, including: turning, drilling and mill- ing operations (i.e see Figs. 39, 49 and 168a, respec-tively). Moreover, it is not only possible to ‘hard-part’: turn (Fig. 15) and bore (Fig. 65b); or mill (Fig. 172); etc.; but these can now be classified as highly-profit-able ‘dry-machining’ activities.
Trang 6Probably the chief obstacle to dry-machining
ac-ceptance, is that conventional wisdom dictates that
MWF’s are vital in attaining acceptable machined fin-ishes and will considerably extend the tooling’s life. In
many circumstances these are valid points, but with
some of the advanced cemented carbide grades and
high-technology coatings, such tooling can be
oper-ated at higher cutting data than was previously the
case and, cope with their elevated machining tempera-tures. In fact, if interrupted cutting occurs, the hotter
the cutting zone becomes, the more unsuitable will be
the application of a cutting fluid – as the thermal shock
becomes greater with a ‘wet-machining’ strategy
Present tool coating technologies are vital to dry-
machining applications, as they both control the tem-perature fluctuations, while restricting heat transfer
from the cutting vicinity to the insert, or tool.
Mul-tiple coatings act as a heat barrier because they
of-fer a lower thermal conductivity to that of the tool’s
substrate and the workpiece material. Thus, coated
inserts/tooling absorb less heat and as a result, can tol-
erate higher cutting temperatures, allowing more ag-gressive cutting data, whilst not debilitating the tool’s
life. Coating thickness is also important, as the thin-ner the overall coatings, the better they can withstand
temperature fluctuations, that might otherwise arise,
if thicker coatings were utilised. The main reason for
this improved thermal shock performance of thinner,
rather than thicker tool coatings, is that a thinner coat
is less likely to incur the same stresses, hence, they are
less susceptible to cracking as a result. So, by running
thin coatings in ‘dry-machining’ operations, normally
extends tool edge life by up to 40%, over thicker coat-ings.
‘Thermal shock/fatigue’ , the cyclical nature of both rapid
heating followed by immediate cooling – in for example face-milling (i.e see Fig. 213 – top), or when interrupted turning
(e.g. when eccentric turning, or OD/ID machining with either
splines and keyways present), promotes potential tool edge
fracturing – resulting from the cyclic thermal stresses and in-
creased temperature gradients, being exacerbated by the ap-plication of a cutting fluid.
‘Thin coats-v-thick coats’ , the former coating offers longer life
than the latter coating process. Today, it is normal to utilise the
coating process of: physical vapour deposition (PVD) as this
type of coating is thinner and will adhere/bond more strongly,
than the alternative chemical vapour deposition process. For
example, a TiAlN PVD coated insert/tool can have a hardness
of 3,500 Hv, withstanding cutting temperatures up to 800°C.
‘Dry-machining’ – some Factors for Consideration
• Adopting a ‘dry machining’ strategy will only make
sense, if all the cutting processes in the part’s manu-facture can be performed without coolant,
• Only by utilising specialised cutting tool
geome-tries, can ‘dry-machining’ be possible and effective,
• Tooling typically having special hard multi-layered,
or diamond-like coatings, etc., to isolate heat and create minimal thermal conduction across the tool/ chip interface,
• Employing cutting tool materials producing sharp edge geometries – to reduce heat,
• For drilling operations, utilise ‘soft-glide’ coatings – for lubrication, with the necessary and appropriate
efficient chip transportation geometries,
• Speedy and efficient removal of both chips and as-sociated steam – by suction – are important factors
in ‘dry-machining’ ,
• Utilise new machining concepts, plus the latest fully-enclosed machine tools – whenever possible,
• Employ faster, rather than slower cutting data, to al-low the majority of heat to be confined to the evacu-ated chips.
8.12.2 Near-Dry Machining
The strategy of ‘near-dry’ machining is not a new con-cept, it has been in existence for more than 50 years. However, this machining and lubricating approach
is still not a universal practice, which is surprising when one considers the real benefits that accrue from the practice over its ‘wet-machining’ counterpart. As its name implies, in ‘near-dry’ machining little lubri-cant is used – normally vegetable-based oils, meaning that both cutting fluid treatment and its disposal are eliminated. Further, instigating a ‘near-dry’ machining strategy means that there are fewer worker health risks
from resultant mists, which might otherwise create: re-NB From a metallurgical/materials science viewpoint, the:
TiAlN – PVD tool coating can attribute its superior mechani-cal/physical properties to an amorphous aluminium-oxide film that forms at the tool/chip interface, as some of the alu- minium of the coating surface oxidises at these elevated ma-chining temperatures. While, even more exotic multiple-type
diamond-like coatings
can be applied and their like, which of-fer even greater cutting performance – in certain machining circumstances, when applied to the tool’s cutting edge(s).
Trang 7cutting approach can be exploited across a wide range
of either ferrous, or non-ferrous workpiece metals.
Most machine tools are equipped with the capabi-lity of supplying flood coolant to the cutting process,
together with ‘through-coolant’ tooling systems, mean-ing that the cost to reconfigure for that of a ‘near-dry’
technology is not prohibitive. Assuming the
worse-case scenario of requiring a through-coolant
tool-ing system, then probably just over $5,000 at today’s
prices should prove sufficient capital to complete the
task. Some re-tooling to complement the ‘near-dry’
machining production techniques may be necessary,
allowing the precise application of lubricant to the cut-ting edge(s). Further, the user must consider a method
for efficient chip removal from the cutting area. Usu-ally, with external ‘near-dry’ cutting operations, the
lubricant is transported within the media of a
com-pressed air application, via the correct-sized aperture
nozzle – pointed toward the cutting zone. Control of
the volume of lubricant delivery to the tool and work-piece area is critical, with the common misconception
being that more lubricant is better! The optimum ar-rangement for ‘near-dry’ lubricant application, is when
the minimum of over-spray and resultant misting does
not occur.
With external ‘near-dry’ operations, dispensing
systems usually consists of reservoir metering pumps
and valves, being mounted on the machine tool’s exte-rior – at some convenient location. While the nozzles
are strategically-mounted so that they can easily be
directly aimed at the tool’s cutting edge(s). Normally,
the nozzles are a manufactured from either copper,
or plastic and ‘snap-together’ – being much smaller
in size than their ‘wet-machining’ counterparts. For
internal machining operations, having tooling with
‘through-the-nose’ delivery, the lubricant is mixed
with compressed air prior to delivery to the cutting
zone. The admixing of compressed air and lubricant
keeps the lubricant in suspension, with these oil par-ticles being broken-down into minute particles prior
to being fed into the compressed air jet stream – on
their way to the tooling
For ‘conventional’ flood coolant delivery the
sys-tems, the coolant channels are filled with cutting fluid,
which inevitably finds its way to the cutting zone. If
however, in a ‘near-dry’ machining configuration, a
heavy mist of lubricating oil floats through the com-pressed air, attempting to negotiate all of the twists
and turns on its way to the cutting zone, this may pres-ent a potential lubrication clogging/starvation
prob-lem. Hence, for a successful ‘near-dry’ delivery system, the lubricant channels need to be smooth and even, with direct flows from the coolant pump to the cutting zone.
A basic misapprehension by some machine tool designers and manufacturers, is that copious volumes
of flood coolant are necessary to remove large quanti-ties of swarf. In fact, just the opposite can occur, as wet chips will not only pack tightly together but have
a surface tension property to them, tending to make them adhere to machine tool surfaces (i.e. see
Foot-note 29, ‘Lang’s chip-packing ratio’ in Chapter 2). This
is not generally the case for ‘near-dry’ lubrication, as the chips here, have a thin layer of non-oxidising lu-bricant surrounding them and with their evacuation velocity – after being machined, coupled to gravita-tional effects, means that they will fall to the bottom
of the swarf tray, or into the chip conveyor. It is good working practice to use the external air-only supply’s blow-off nozzles to clear away chips form the cutting area0, however, it is not recommended to use the oil/ mist to achieve chip clearance, as it will simply blow the lubricant straight past the cutting edge(s), while probably creating an unwanted oil-misting problem.
It is possible to incorporate both ‘wet-’ and ‘near-dry’ lubrication systems onto the same machine tool. It has been reported that for external/internal work the change-over from one system say, from ‘wet-’ , to the other – ‘near-dry’ , takes about 3 minutes to complete. For ‘near-dry’ machining to be successful, it de- pends upon various factors, including: workpiece ma-terial to be machined; tool geometry and its coating(s); speeds and feeds selected; plus other important fac- tors. If applied correctly, ‘near-dry’ machining has sig-nificant direct and indirect benefits to the machining process as a whole.
Economics of: ‘Dry-’; ‘Near-Dry’;
and ‘Wet-Machining’.
For any tool and workpiece lubrication strategy to operate effectively, a range of cost factors need to be considered, regardless of the method of machining
0 ‘Chilled compressed air’ , has been successfully utilised in the
past for not only removing chips from the cutting vicinity, but on certain materials, the continuous application of chilled compressed air acts simply as a form of ‘basic lubricant’ for the cutting process in hand.
Trang 8to show the relative merits of the three machining
strategies previously discussed, namely: ‘dry-’;
‘near-dry’; or ‘wet-machining’. The cost component for each
of these lubrication strategies has been broken down
into its relevant parts, with some of them not being ap-plicable to every lubrication application. If one ignores the individual cumulative factor in the overall cost and simply looks at the ‘bottom-line’ , namely, the total relative costs for each process, then a clear message is being given here! Explicitly, that ‘wet-machining’- in certain cases, when compared to ‘dry-machining’ is
Figure 213 Indicates the comparative costs for utilising either: ‘dry-’, ‘near-dry-’ or ‘wet-machining’ strategies
.
Trang 9>330% more expensive overall, this being a good rea-son to look carefully at employing ‘dry-machining’
techniques – when applicable!
In Appendix 14, a MWF ‘trouble-shooting guide’
has been included, to help establish the relative causes
and remedies for certain fluid-related problems – as
they arise
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