This geometric change in the micro-boring tool’s geom-etry, causes the insert to ‘snatch, or grab’ material rather than cut it, which then increases vibration, so tool breakage will shor
Trang 1Figure 254 Relative dimensional sizes and scales, for:
• machining of accurate and presicion parts or
• equated to their respective measurement.
[Source: Smith, et al., 2002]
.
Trang 2industrial-versions of what constitutes a ‘micro-tool’ ,
we obtain the following dimension for ‘our tooling’:
<φ0.95 + 0.55 + 0.06/3 (mm) = ≈<φ0.52 mm, or by
sim-ply and conveniently rounding-down, let us consider
in our discussion, that any form of ‘micro-tooling’ is to
be set at, or below: φ0. mm
The geometric features of any ‘micro-tools’ cannot
simply be considered as minute version of
‘macro-tool-ing’ More specifically, we simply do not just
scale-down say, a φ12 mm drill, then manufacture its
geom-etry as a micro-tool of, for example: φ0.25 mm drill
(Fig 255a) Micro-tools have to be ‘engineered’ to
pro-vide effective chip evacuation, this is particularly
rel-evant when hole-making (Fig 255b), yet remain rigid
enough to withstand the cutting forces generated and
not fracture under these conditions In the following
sections concerning this review on micro-tooling, the
production processes of: drilling; milling; and boring
tools; will be briefly mentioned
Micro-Drills and Drilling
Probably the most significant difference when about
to utilise these minuscule tools, compared to their
macro-drilling counterparts, is that an operator
can-not even see what size they are, without suitable visual
magnification! This means that a micro-drill’s careful
handling – of these fragile tools, plus their safe
stor-age are vital Micro-drills require correct containment
and appropriate labelling, allowing them to be readily
identified Due to their minute diameters,
micro-tool-ing require rotational speeds of ≈100,000 rev min–1 in
order to obtain the correct peripheral speeds, thereby
minimising the cutting forces acting on the
micro-geometry of the cutting edges So that the minute
geometric features of the cutting edges are maintained
(Fig 255b), it is desirable to have a very fine carbide
grain structure – to strengthen the tool’s edges If just
the smallest amount of uncontrolled lateral force
oc-curs, it can cause either tool edge fracture, or instigate
complete breakage Thus, if a micro-tool’s edge is just
slightly chipped then this in itself may not adversely
‘Micro-tooling materials’ , cemented carbide’s increased
ri-gidity over other tooling materials, makes is susceptible to
fracture A good substitute micro-tool material is M-35
co-balt steel, it is a compromise between carbide drills and those
manufactured of M-2 and M-7 HSS Heat generated drilling
holes, will ‘roll’ a drill’s edge, thus it becomes: dull; ploughs;
and breaks Cobalt improves drill ‘red-hardness’.
affect cutting, but a 5 µm edge-chipping on a φ100 µm tool will radically modify the tool’s geometry and seri-ously impair its cutting performance This miniscule cutting edge modification can cause the tool to break,
or damage the part’s features – requiring very careful handling of such micro-tooling, in order to obtain the optimum cutting performance
A drill’s feature that needs to be modified from that
of its comparable ‘macro-drilling’ equivalent, is the
drill’s web ( i.e see Fig 47- bottom), this being the
cen-tral portion of the tool that extends axially along the flute – gradually thickening as the distance increases from the tool’s point So for a micro-drill, the web is proportionally thicker, because there has to be some
‘core-strength’ to a micro-tool By way of illustration,
on a micro-drill a web of say just 25 µm is simply not robust enough, as such, it would not work To reduce
a micro-drill’s stress and prevent it from binding in
the hole, a back-taper is purposely ground – playing
a major role in drilling efficiency While, another mi-cro-geometric consideration for these minute drills is its cutting edge sharpness, becoming of critical impor-tance as the relative tool diameter gets smaller – this being a limitation to effective micro-machining For example, on a macro-drill, if the cutting edge has a
25 µm cutting edge radius, it is considered somewhat sharp, but this would hardly be the case for a micro drill Moreover, on a micro-drill if the cutting edge radius is 10 µm and it is taking a 2 µm chip load, its not just considered as ‘dull’ , but it is highly-negative
raked! The PVD-coating process on micro-drills can be successfully accomplished, if the drill is manufactured with a reduced width of edge preparation, thus
main-taining its sharpness (i.e by way of illustration, Fig 18 shows typical ‘edge-preps’ for macro-cutting inserts)
In any form of micro-drilling operation, very high spindle speeds are necessary, for example, if a φ0.2 mm micro-drill is utilised, then the spindle speed should
be ≈80,000 rev min–1, in order to prevent the creation
of high drill thrust and torque forces, which might
‘Micro-drill back-taper’ , this is where a slight decrease in
the drill’s diameter is being specifically peripherally-ground, decreasing in size from the drill point up to and toward the shank
NB Back-taper on a micro-drill is generally relieved by
be-tween: 5 µm and 13 µm; because the flute’s lengths are usually
<25 mm Conversely, on a ‘macro-drill’ this back-taper lies be-tween: 13 µm and 25 µm per 25 mm of length
Trang 3Figure 255 Some typical micro-tooling: drills and boring tools
.
Trang 4otherwise lead to premature tool breakage While
an-other note of concern when micro-drilling is its
pene-tration rate If too high a drill feedrate is programmed,
then the micro-drill will immediately fracture Some
micro-drilling manufacturing companies either
rec-ommend a single-flute assymetric drill geometry –
al-lowing high chip loads, coupled with an efficient chip
evacuation process, conversely, another approach is to
increase the number of drill flutes, but this may cause
chip evacuation problems with ‘sticky’ workpiece
materials At present, micro-drills can normally drill
holes with L/D ratios of 5:1, but it is anticipated that
these L/D ratios will soon be up to 10:1 In any
micro-drilling operation, the first few revolutions of the drill
are crucial (Figs 49 and 50), as the drill’s point
expe-riences eccentric forces as it enters the cut, with any
workpiece irregularities causing the drill to ‘walk’ –
re-sulting in its bending, breakage, or at the very least
some ‘helical wandering’ (i.e axially – see Fig 70) as
the drill penetrates into the part In order to
mini-mise the eccentric forces as a micro-drill enters a hole,
many micro-drilling manufacturers recommend that a
pilot hole (Fig 50b) of between 1-to-2 drill diameters
deep is produced, utilising a short and rigid pilot drill
A pilot drill’s point angle (i.e see Fig 46 – top) should
have an included angle that is either identical, or
greater than that micro-drill producing the final hole
If smaller included angles were selected, as the drill
en-ters the pilot drilled hole, this causes the micro-drill’s
cutting edge to chip This tool wear-effect is because,
as the micro-drill’s more shallow point angle initially
contacts the previous pilot-drilled hole – with its more
acute angled geometry, as the micro-drill enters
work-piece, this contact will take place at the outer edges of
the lips before the drill point touches the hole’s surface
In lieu of a pilot-drilled hole, then begin the feedrate
at somewhat less than the finishing feed, or perhaps,
utilise a ‘pecking-drilling action’ – drill to a
predeter-mined depth, partially withdraw the drill, then drill
deeper into the workpiece, once more partially
with-draw the drill, then repeat this sequence ‘Pecking’ has
the further benefit of avoiding dwell at the bottom of a
‘blind hole’ , this being an important surface integrity
feature with work-hardening materials
Hole tolerances that have been satisfactorily
micro-drilled in a range of workpiece materials are of the
order: ± 5 µm, with tolerance-in-roundness (TIR) of
<2.5 µm By utilising coolant delivery at high pressure,
either through-the-drill, with the ‘larger drill sizes’;
or alternatively flood coolant for minute micro-drills;
usually allows a 30% increase in cutting speeds coupled
to extended drill life Although care should be made
when utilising through-coolant drills, as their small
coolant hole diameters will simply clog unless the cool-ant has been passed through some form of micro-filtra-tion unit, to remove ‘fines’ and other types of potential
clogging debris
Micro-Mills and Milling
Over the last few years, with the advances in cutting tool materials in combination with that of cutting tool technology, has led to significantly smaller milling cutter diameters with more complex geometries be-ing produced (Fig 255-bottom left) In fact, several important technologies have developed during the last decade to assist the cutting tool manufacturers to cater for the micro-machining industries Probably the most important of these new technology applications is the design and development of very high accuracy six-axis CNC tool grinding machines, having temperature con-trol and coolant condition monitoring – these being key elements in the cutter-grinding process Grinding tolerances held by these machines on say, an φ0.12 mm ball-ended end mill, must be within <2.5 µm (TIR) Complementary to these multi-axes CNC grinders, has been the improvements in diamond grinding wheel technology, in conjunction with appropriate grinding wheel metrological inspection techniques, that have contributed to the significant advancement
in micro-tool manufacturing quality and productivity Whilst continuing our discussion on the cutting tool material front, micro-milling cutters are now being produced from extra-fine grain cemented carbides, al-lowing sharp cutting edges in conjunction with good milled surface finishes For example, one Japanese mi-cro-tooling manufacturer offers a standard range of
‘micro-mills’ , from: φ5 µm to φ55 µm; in incremental
‘Ethanol’ , is an alternative applied lubrication strategy, to that
of either the usual water-based flood coolant method, or by through-the-tool delivery Here, ethanol is a form of alcohol, occurring naturally in the sugar fermentation process, its benefits are that it has less-than-water viscosity, enabling it to penetrate into the tool/chip interface in a superior fashion to that of other coolants Ethanol is usually delivered to the cut-ting zone in the form of a spray-mist While another bonus
of the application of ethanol is its low evaporation point, giv-ing it efficiency in coolgiv-ing and as a lubricatgiv-ing agent for tool spindle speeds (i.e of up to and including) >60,000 rev min–1 Moreover, ethanol simply evaporates and this effect negates any disposal costs, while it provides a slight ‘chilling-effect’ on the part, minimising thermal growth problems on miniature-sized components
Trang 5sizes of 25 µm This range includes four-flute
square-ended cutters (i.e similar to the one depicted in Fig
255 – bottom left), together with ball-nose end mills,
plus some customised micro-mill tooling
Complementary to these micro-milling cutters, are
their respective toolholders, which must hold and
securely contain the tool’s shank in an accurate and
precise manner Any form of tool runout when held
in the toolholder must be kept to absolute minimum
The toolholder’s importance in the micro-milling
op-eration is often overlooked, at the user’s peril! By way
of illustrating this fact, if one has a two-flute φ0.5 mm
end mill then the chip-load will be <0.010 mm tooth–1
So, if the micro-mill has a runout of 5 µm, the cutter
is only utilising one of its flutes – by a factor of 100%
This micro-mill’s runout condition, leads to cutter
in-stability coupled with a poor milled surface situation,
with the potential for either reduced tool life, or
break-age Micro-milling tool deflection and its subsequent
breakage when utilised for micro-machining
opera-tions, are principally caused by three main factors,
these are:
1 Micro-machining creates a substantial increase in
the specific energy, as the chip thickness decreases
– meaning that here, as the chip gets thinner with
smaller DOC’s, the micro-mill is subject to greater
resistance, when compared to that of
‘macro-mill-ing’ Moreover, it is almost as if the workpiece
mate-rial becomes harder during micro-machining This
resistance force to machining here, is strong enough
to exceed the bending strength of tool – even prior
to any wear occurring, leading to tool breakage
A method of minimising tool breakage and
prevent-ing its occurrence, is to ensure that the chip
thick-ness is smaller than the radius of the tool’s edge,
2 During micro-milling, a sharp rise in the cutting
forces and stresses resulting from chip-clogging
may cause tool breakage – when say, utilising a
two-flute cutter, each cutting edge removes chips from
the machining vicinity by only a half rotation of the
tool Likewise, if chip-clogging occurs – within a
few micro-tool revolutions, the cutting forces and
‘Micro-milling/-drilling toolholders’ , are typically
manufac-tured with taper-/face-fitments of the : ISO 15 to 30, or
HSK-E25 to 32 types A typical range of micro-toolholders, might
cover tool shank sizes in a range from: φ0.5 mm to φ2 mm, in
0.01 mm increments With ‘matching micro-tooling’ , to
typi-cal tolerances of: <+0/-4 µm, with these micro-tools typitypi-cally
being, for example, rated @ 40,000 rev min–1, having a runout
of <3 µm @ 4xD
bending stresses increase beyond the limit of the tool’s bending strength, thus causing it to break A
possible solution to this problem, is to utilise alter-native tooling materials such as micro-grained M-2 HSS, as they are more flexible and as such, they can tolerate any likelihood of chip-clogging in a more compliant manner than their cemented carbide
counterparts,
3 While micro-machining very ductile workpieces,
micro-mills can lose cutting efficiency as a result of BUE – this results in increased lateral pressure (i.e
as feeding occurs) on the micro-mill, causing it to slightly deflect This increasing tool deflection due
to the presence of BUE, increases the stress gener-ated with every cutter rotation, quickly causing the
micro-tool to break This well-known BUE phe-nomena is termed: ‘extensive stress-related break-age’ , which could possibly be minimised by adopt-ing a somewhat more efficient and beneficial cuttadopt-ing fluid lubrication strategy.
NB Due to these (above) micro-tooling related
phenomena, many of the latest micro-milling ma-chines are equipped with sensors to dynamically-measure and monitor the cutting forces acting on the micro-mills
Micro-Boring Tools and Internal Machining Operations
The production problems of drilling small holes in workpieces is a big challenge, but this is nothing com-pared to that of the technological complexity of bor-ing very minute holes and other internal features in components Some tooling manufacturers offer insert-style tools that are specifically-designed to bore-out small hole diameters, even down to just ≈φ0.3 mm
A typical micro-boring tool is illustrated in Fig 255c (i.e ≈φ0.3 mm), where the main features of the tool-ing are explained In the enlarged diagram of a mi-cro-boring insert geometry schematically illustrated
in Fig 255 – middle right (i.e for a ≥φ0.7 mm boring insert) This particular micro-boring insert geometry,
is designed for both boring and profiling operations into holes of ≥φ0.7 mm, although the clearance (‘tmax’) will only cope with shallow profile-depth features, be-fore fouling on the tool’s shank Micro-tool stiffness is quite high and occurs due to the enlarged shank di-ameter, allowing reasonably large L/D ratios of >10:1
to be bored – which is remarkable, considering the minute size of these insert’s Other
Trang 6micro-machin-ing operations that can be undertaken include (Fig
255 – middle-right): grooving; threading;
face-groov-ing; back-boring (i.e not depicted)
Some important micro-machining factors need to be
addressed, prior to boring-out previously drilled holes
with these micro-boring tools (Fig 255c), such as:
• Setting the micro-boring tool at the hole’s centre
height (Fig 255d – top) – this is the most
impor-tant preliminary step when about to commence a
micro-boring operation A micro-boring tool that
is incorrectly set below the hole’s centre-height (Fig
255d – bottom) – adversely affects its performance
in several ways It could foul on the curvature at the
bottom of the pre-drilled hole, through a reduced
edge clearance angle (primary relief) Moreover,
the ‘tool fouling’ causes the insert to rub against
the hole impeding the cutting action, which in
turn, creates vibration causing the insert’s tip to be
‘driven-down’ still further below centre As a
conse-quence, the tip is forced deeper into the workpiece
material – due to the radial sweep of the bore Thus,
as the top rake angle is increased – relative to that of
the workpiece, the clearance angle is reduced This
geometric change in the micro-boring tool’s
geom-etry, causes the insert to ‘snatch, or grab’ material
rather than cut it, which then increases vibration,
so tool breakage will shortly ensue,
NB Due to the minute dimensions of these
micro-tools, it is very difficult to set the tool exactly on
‘Micro-machining’ , has been defined according to a different
approach, namely, concerning the actual workpiece’s volume,
as follows: ‘It is the [workpiece] size in which the work envelope
is smaller than 490 cm’ (Source: Destefani, 2005)
‘Meso-scale machining’*, this term has been coined and has
been defined as: ‘Millimetre-sized parts, with micron-sized
fea-tures’ For example, minute component features that can have
micron-sized tolerances (<1 µm) Recently in the USA, these
‘meso-machining technologies’ have included both
milling and -turning, with in the former case, utilising
end mills of ≈φ20 µm, while in the latter case, using
micro-turning tools of 10 µm in width Hence, these technologies can
machine part features in the 25 µm range.(Source: Kennedy,
2006)
*The term: ‘meso-’ meaning: ‘middle’ , or ‘intermediate’
(Source: Concise Oxford Dictionary)
NB In this case, the so-called ‘meso-machining operations’ ,
refer to machining in between the:nano- and
micro-machin-ing ranges.
centre-height – the ‘ideal positioning’ Therefore, the tool can be set marginally higher than ‘true’ cen-tre-height, which increases the angular clearance – relative to the hole, thereby allowing a freer cutting condition Further, if any potential vibrations occur the micro-tool is both deflected downward toward the centre and (radially) slightly out-of-cut, some-what reducing this ‘grabbing tendency’
• Choosing the right speed and feed – when the bore
is <φ6 mm, then ‘standard’ speeds and feeds cannot
be used For example, when boring say, a φ1 mm hole, one would expect to utilise a cutting speed of perhaps ≈140 m min–1, which equates to a spindle speed of ≈44,600 rev min–1 – which is totally un-realistic for most types of turning machine tool If any vibration occurred, then the micro-boring tool would be immediately destroyed So pragmatically,
if we limit the spindle speed to 6,000 rev min–1, which would significantly drop the cutting speed to
≈19 m min–1, we would also need to complemen-tarily reduce the micro-boring insert’s pressure by reducing the DOC to <0.1 mm which – to minimise tool deflection and potential breakage Feedrates for ‘macro-boring operations’ , are normally dic-tated by the ‘close-relationship’ between its tool nose radius and the bored-hole surface texture re-quirements Conversely, for micro-boring these are
not the controlling factors anymore Here, minimi-sation of cutting forces is vitally important by se-lecting a feedrate that should not exceed 0.125 mm
rev–1 – which automatically overcomes any micro-bored surface texture issues,
• Ensuring adequate chip evacuation – is a real
dif-ficulty with such small bored holes, as little in the way of unfilled volumetric capacity exists with the micro-boring tool situated inside the hole So, with the micro-boring tool inside the hole – accounting for 60% of the available volumetric space, how can the chips escape? By utilising a ‘through-insert-coolant’ micro-boring insert (Fig 255c and d) with the coolant under pressure, it can reach the cutting edge This coolant aids in both forcing and flush-ing chips out of the bore’s mouth, which minimises any of these work-hardened chips, creating a chip-packing tendency in the bore, with the potential of causing tool breakage,
• Providing adequate tool stability and location –
these are important factors for micro-boring tooling Micro-boring inserts and their respective toolhold-ers are designed for quick, simple and repeatable
Trang 7setups, with some tooling manufacturers
design-ing the tooldesign-ing assembly to avoid inserts twistdesign-ing
cut These design innovations range from:
in-sert-clamping flats; inserts having angle-ground
back-ends; to that of ‘teardrop-shaped inserts’ (i.e
see Fig 255d) – this latter type firmly ‘wedging’ the
insert as it attempts twist – under the torque and
bending moments while boring
If these micro-tooling factors are adhered to, then the
problems that are likely to be encountered when
mi-cro-machining are significantly reduced, which means
that any form of micro-machining activities can be
achieved – with due diligence So that a considerable
amount of micro-machining can be undertaken,
ma-chine tool companies have been developing a range of
specialised machines to cater for this market Hence,
with the expansion of micro-machining activities this
being a somewhat ‘growth industry’ , let us briefly
consider these specialised micro-machine tools and
the technical challenges they had to overcome in order
to cope with such minute cutters and invariably
minis-cule workpiece volumetric dimensions
9.10.2 Micro-Machine Tools
CNC machine tools designed to machine parts, or
moulds that have small dimensional size, typically
with a linear dimension of ≤ 10 mm, or having detailed
part features of ≤ 0.1 mm, require some significant
ac-curate and precision enhancements, if they are to cope
with the micro-machining demands of late A typical
micro-machining machine tool will be mentioned,
‘high-lighting’ some of its important design features,
so that one can gain an insight into the careful
at-tention to detailing necessary for minute component
manufacturing
One such machine tool produced by the Japanese
company Makino (not shown), has a ‘footprint’ of
≈ 1.8 m × 2.4 m, with a significant weight of ≈ 5 tonnes,
with a worktable size of ≈ 300mm × 200 mm, having
three axis travel of: 200 mm in X-axis; 150 mm in
Y- As a ‘total aside and not related to the main topic’ and, for you
‘aficionados’ of English language – isn’t this above statement
(i.e in italics), the basis for a: ‘double’ – oxymoron?
** Oxymoron: expression with contradictory words – e.g
‘Wise fool’ or, ‘Legal murder’.
axis; and 150 mm in Z-axis Obviously for such minute micropart features to be machined, the positional ac-curacy and repeatability of the slideways are of crucial importance, as such, the machine’s positional accuracy
is ± 0.3 µm, with a repeatability of ± 0.2 µm This ma-chine features unique workholding equipment, such
as a direct-chucking spindle, which has been designed
to eliminate the toolholder-induced variables, en-abling miniature components to be produced, such as: medical instruments; semi-conductor devices; optical products; etc
A major factor with any micro-machining machine
tool like the one mentioned above, is its machining environment and more specifically, its ‘thermal condi-tions’0 As the minute part’s temperature changes along with that of the machine’s spindle – during machining, any dimensional modifications on say, a ‘macro-scaled part’ could normally be considered as negligible, but
on micro-sized workpieces, these linear variations be-come significant dimensional issues In order to vir-tually eliminate spindle growth the machine tool has
to be designed for a stable environment, when one is attempting to hold ‘micrometre-accuracies’ Machine tool features necessary in reducing this spindle/ma-chine expansion, include, an automatic spindle lu-bricant temperature controller – to reduce spindle growth, coupled with the machine’s granite base – as granite has only 10 to 20% of the thermal conductivity
0 ‘Thermal conditions and effects’ , in particular will affect either
the machine tool’s linear expansion/contraction – depending upon whether there is a temperature rise, or fall, respectively.
Simplistically and in this instance, ignoring any uncertainty
factors, then , ‘Coefficient of thermal expansion’ which is
nor-mally denoted by the symbol ‘α’ , can be defined as: ‘A measure
of the change in length of a material subjected to a change in temperature’ Thus:
(�T) =
strain
є (�T)
Where: Lo = original length (mm), ∆T = change in temperature
(°C) Or, this thermal expansion equation can be alternatively
written, as follows: ∆L = (L) (Cα) (∆T) Where: ∆L = change
in length – by thermal expansion (mm), L = original length (mm), Cα = coefficient of thermal expansion, ∆T = change in
temperature (°C).
NB The uncertainty contributions here, may be combined, in
the following manner: ud = √(u0) + (uc) Where: ud =
‘design-stage’ uncertainty, u0 = interpolation uncertainty, uc = instru-ment/equipment uncertainty (Source: Figliola and Beasley et al., 2000)
Trang 8of an equivalent cast iron structure, thus
minimising the effects of ambient temperature changes
Moreover, for any form of extremely critical and
preci-sion miniature part manufacture, the entire machine
tool can be situated in a ‘thermal chamber’ , this in
ef-fect acts as a ‘controlled-temperature environment’
Micro-tools of for example, φ50 µm – as has already
be mentioned, are almost impossible to see, let alone
attempting to set them to length In order to facilitate
this minute tool setting operation – to sub-µm
accu-racy, the machine tool company developed a hybrid
automatic tool-length measuring system This system
of tool length measurement is achieved by combining
a static low-pressure contact sensor in conjunction
with non-contacting sensing1, this being performed
while the tool is rotating at speed Together, these two
sensing techniques permit sub-µm tool positioning
accuracy, during machining operations
Obtaining the most advantageous micro-machine
tool for a particular type of minute ‘workpiece group’ ,
should be taken by considering the part’s features –
in-cluding its geometric configuration, together with the
level of accuracy and precision required and whether
these parts are produced as ‘one-offs’ (i.e as
custom-ised-specials), or in various batch sizes
9.10.3 Nano-Machining
and Machine Tools
When attempting to machine components to
toler-ances of nanometric dimensions, the actual problems
considerably exacerbate, even when compared to that
of machining in the sub-micrometre range Usually,
conventional cutting edges that have been honed,
can-not hope to cope with miniscule DOC conditions, as
the edge is just simply not sharp enough and will tend
‘Non-contacting tool sensing’ , is ahieved as follows: while
the spindle is rotating the tool’s tip position is measured via a
non-contact electro-magnetic sensor This tool measurement
takes into account the thermal displacement of the tip, caused
by its rotation Thus, the system’s contolller merges these two
measurements
NB The tool’s length measurement operation occurs by
mea-suring the spindle growth and waiting until it stabilises – within
specific user-defined limits, such as for example, within2 µm
Once the spindle is ‘stable’ , it can machine the workpiece at
the desired level of accuracy and precision.
to ‘plough’ , instead of cut In order to machine such components, often on either very ductile workpiece materials, or glasses, monolithic diamond tooling is invariably utilised with the tool orientated so avoiding its natural fracture planes It is normal practice when machining components to a few billionth parts of a metre (i.e 1 nm = 10– m), that a wide range of ‘pro-cess, environmental and machine tool influences’ are acknowledged and then subsequently minimised In fact, the Japanese Professor Nakazawa (1994)
consid-ered the machine tool’s influences when machining at high precision and, stated they could be broken down
into the three following requirements, mentioning that the:
1 Machine tool’s built-in reference must not vary,
2 Machine tool’s must follow this kinematic reference
at its highest precision,
3 Machine’s movement must be accurately trans-ferred to the workpiece
Moreover, Nakazawa presented a useful table of the
ma-jor factors that disturb the relative tool-to-workpiece po-sitions in ‘forced machining’ , as presented in Table 17.
In fact these ‘requirements’ are usually adequate for the level of machining into the sub-micrometre range, but once one proceeds to ‘ultra-high precision machining’ operations within the nano-range, then many other factors contribute to the overall success
of the cutting process (Fig 256) As previously
men-Table 17 Factors that disturb relative tool-to-workpiece
cut-ting positions
Factors: Internal/
External: Conditions:
Heat source Internal Machining energy at machining
point motor, ball leadscrew, bea-ring, guide, hydraulics
Radiation (lighting, body heat)
Vibrations Internal Vibrations created by the
ma-chining mechanism motor, ball leadscrew, joints, bearings, guides
people
in cuting fluid [Source: Nakazawa, 1994]
.
Trang 9Figure 256 The error sources from the processes; environment and machine tool; on workpiece accuracy [Source:
Breuck-mann & Langenbeck, 1989]
.
Trang 10tioned, ultra-precision machining does not usually
en-tail machining very diminutive components, but it is
normally concerned with holding exceptionally tight
tolerances on macro-sized parts (Fig 257b) In fact, a
diamond turning machine tool that can manufacture
components to nano-tolerances is depicted in Fig
257a This ultra-precision CNC lathe is considered to
be the most accurate and precise machine of its type
currently available Therefore, it is worth discussing
the machine and its environment, as its installation
and operation encapsulates all of the error sources
shown in Fig 256
Nano-Machine Tool and its Facilities
In Fig 257a, is shown a very expensive and highly
sophisticated machine tool, it was delivered to the
Atomic Weapons Establishment (Aldermaston, UK)
from the manufacturers Cranfield Precision
Engi-neering (UK) Its intended purpose was to machine
Perspex optics and large thin-walled aspheric shells
having form errors of <5 µm, together with wafer-thin
laser targets having micro-machined features, but held
to tolerances of ± 10 nm, by single-crystal (monolithic)
diamond tools – these diamond cutting edges being
orientated along their correct crystallographic plane,
allowing turned surface texture values (Ra) of <1 nm
to be achieved
If an ultra-precision machine tool is required to
work at nanometric resolution, then if it needs to be
located within a manufacturing plant, the machine
tool must have a special-purpose facility designed,
constructed and built to exceptionally-stringent
and specified requirements Prior to discussing this
facility, it is worth describing some features of this a
machine tool, so that one can comprehend the
signifi-cant technical problems that had to be overcome, in
order for it to achieve a ‘true’ nano-machining
capabil-ity
The machine tool was constructed on a polymer
concrete base, that consisted of 8 tonnes of synthetic
granite, giving the desirable properties of: excellent
thermal stability; high stiffness; plus good vibration
damping characteristics Strangely for a diamond
turning machine, the headstock was equipped with a
hybrid hydraulic/air spindle rather than an
air-bear-ing design, because this spindle’s specially-designed
construction enabled an increased load-carrying
ca-pacity (57 kg), coupled to superior stiffness Spindle
speed range was from: 200 to 5,000 rev min–1
Three-axes were fitted, two linear Three-axes – running on fully-constrained hydrostatic dovetail bearings with linear motor drives and one rotational axis These axes kine-matics were: X-axis (520 mm); Z-axis (220 mm); B-axis (360°) – as illustrated in Fig 256-middle left A la-ser-positioning system was fitted to the axes, having a resolution of 1.25 nano-metres, incorporating a wave-length tracker, to compensate for any environmental changes to the air: temperature; pressure; humidity; hydrocarbon content; etc The CNC controller utilised
a high resolution (1 nm) fast feed-forward operation, thereby reducing servo-following errors, combined with real-time axis compensation together with a two-dimensional error compensation capability – for straightness and orthogonality
Tool-setting errors on the monolithic diamond tooling, were minimised by a unique probe and optical setting technique (Fig 256 – middle-right), reducing form errors, when utilising the rotational B-axis for spherical/aspherical turned component geometry The nanocentre machine tool, was housed in a tem-perature-controlled environmental enclosure, consist-ing of the temperature beconsist-ing held at: 20°C ± 0.5°C, en-closed within 100 mm thick high density polyurethane foam panelling – providing high thermal resistance, with an air-tight seal The positive-pressure air-supply unit was situated outside – in an adjoining plant room, ducted into the enclosure by draft-free ducts, with ten platinum resistance temperature probes (i.e resolu-tion 0.001°C) strategically situated within the volume space, continually monitoring the enclosure’s temper-ature Lighting from the fluorescent lights had their chokes removed from the enclosure, thereby reducing
the localised air temperature ‘stratification effects’ by
‘Form errors’ , the principal causes of form error when
spheri-cal/aspherical diamond turning components, can be sum-marised in the following manner:
Conical error – due to spindle misalignment, Chevron and Ogive errors* – due to first-order tool wear
and centring errors,Waviness error – resulting from
sub-strate vibration and tool profile error,
Astigmatism – created by the component fixturing
arrange-ments and material stiffness.
*Chevron error occurs when turning a convex spherical formed component – due to the tool ‘over-shooting’ , while, an Ogive error results from the tool stopping short – this latter error
creating an ‘ogival arch effect’ , with both these profile errors being due to an incorrect tool centre height setup.(Sources: Myler and Page, 1988; Wheeler, 2001)
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