Chatter is not unduly affected by the feedrate selected, but feed does have an effect on the predictable severity of vibration during machining, NB As no cutting force exists if the vibr
Trang 1either entirely eliminating it, or at the very least,
min-imising its affect on the overall machining process
Chatter during machining can result from a range of
multifarious and often linked-factors, they include:
• Depth of cut (DOC) – can be considered as the
prin-cipal cause and, for the prospective control of
chat-ter The DOC delineates the chip width, acting as
the feed-back gain within the closed-loop cutting
process,
NB The machining processes ‘stability limit’ –
be-ing the threshold between stable cuttbe-ing and
chat-ter – can be dechat-termined from trial-and-error by
simply incrementally increasing the DOC until the
commencement of chatter, then‘backing-off’ at this
level The prediction of chatter’s onset can be found
analytically, this value being based upon thorough
knowledge of material stiffness and cutting system
dynamics
• Rotational speed – is probably the simplest
param-eter to modify, thereby altering chatter and its
as-sociated amplitude,
NB The peripheral speed of either the rotating
tool, or workpiece, affects the phase-shift between
overlapping surfaces and its associated vibration
regeneration
• Feed – for milling operations the feed per tooth
de-fines the average uncut chip thickness (t),
influenc-ing the magnitude of the cuttinfluenc-ing process Chatter
is not unduly affected by the feedrate selected, but
feed does have an effect on the predictable severity
of vibration during machining,
NB As no cutting force exists if the vibration
oc-curs in the ‘Y’ direction – resulting in loss of
con-tact between the tool and workpiece – the
maxi-mum amplitude of chatter vibration will be limited
by its feed
‘Gain’ , can be practically defined in the following way: ‘The
ratio of the magnitude of the output of a system with respect
to that of the input – the conditions of operation and
measure-ments must be specified’ (Smith, 1993, et al.).
• Cutting stiffness (Ks) – is a material property con-nected to: shear flow stress; hardness, as well as work-hardening characteristics of the workpiece, this factor often being referred to in a metaphorical sense of its material’s machinability characteristics,
NB Materials that might offer poorer comparative
machinability, for example titanium, require con-siderably higher cutting forces leading to a greater
displacement in the ‘Y’ direction and as such, offer
a less stable cutting action
• Width of chip (total) – is equivalent to the product
of the DOC multiplied by the number of cutting edges engaged in the cut Furthermore, the total cut width will influence the stability of the cutting process,
NB At a preset D OC corresponding to that of the
‘stability limit’ , increasing the number of engaged cutting edges, will result in chatter, or vice-versa
• Cutting tool geometry – influences both the
direc-tion and the magnitude of the cutting force, in particular the quantity of the force component in
the modulation direction ‘Y’ So, an increased force occurring in the ‘Y’ direction, causes amplified
dis-placement and vibration at 90° to the surface, creat-ing ideal conditions for chatter Other cuttcreat-ing insert geometrical factors that can influence the cutting stability include the following:
– Back rake angle (α) – as it is inclined to a more
positive angle, the length of the commencement
of the shearing zone decreases, this in turn,
re-duces the magnitude of the cutting force (F c) As the back rake inclination becomes larger, then this directs the cutting force in a more tangential manner, thereby reducing the force component
in the ‘Y’ direction – creating improved stability
at higher speeds,
NB An insufficient feedrate in comparison to the
insert edge radius produces a less efficient cutting action, with more tool deflection and reduced ma-chining stability
– Clearance angle – reduction (γ) – has the effect
of increasing the frictional contact at the inter-face between the tool and workpiece, possibly having a process damping effect This potential stabilising effect could be the result of energy
Trang 2dissipation – heat transformation, which could
result in decreased tool life, with the
superflu-ous effect of thermal distortion of the machined
part, or an increase in the workpiece’s
heat-af-fected zone (HAZ),
NB On a newly-fitted cutting insert, if initial wear
occurs, this can sometimes have a stabilising effect
for the onset of chatter
– Nose radius – size, insert shape – diamond
tri-angular, square, round, plan approach angle
– positive, neutral, negative – all influence the
area of the chip shape and its corresponding ‘Y’
direction The orientation of the modulation
direction ‘Y’ toward a dynamically more-rigid
direction angle, allows a decrease in vibrational
response, giving greater overall process stability
– having notably less chattering tendencies
As machining process stability is a direct result of
characteristics of dynamic force displacement between
both the workpiece and the cutting insert, all of the
various factors of a machining system: machine tool;
spindle; tooling; workpiece; workholding – in varying
degrees, can influence chatter To increase process
sta-bility of the machining system, it is necessary to
maxi-mise the dynamics, this being the overall product of its
static stiffness and damping capacity Further,
machin-ing stability can be increased by utilismachin-ing toolmachin-ing with
the greatest possible diameter with the minimum of
tool overhang By way of a caution concerning chatter
frequency, this normally occurs near the most flexible
vibrational mode of the machining system
7.3.3 Stability Lobe Diagrams
In Fig 157c, a ‘Stability lobe diagram’ (SLD) is
de-picted, which relates to the: total cut width that can be
machined, to the tooling’s rotational speed, for a
speci-fied number of cutting inserts For example referring
to the: Degarmo, et al (2003) diagram, suppose the
total width of cut was maintained below a minimum
level, then the process stability would exhibit ‘speed
If the total cut width was maintained below a minimum level,
in practical terms this would be of limited value for many
ma-chining systems.
independence’ , or an ‘unconditional stability’ Hence,
at relatively slow speeds an increased stability can
be achieved within the process damping region – as
shown The ‘conditional stability’ lobe regions of the
diagram, permit an increased total cut width (i.e the
DOC x number of cutting edges, these being engaged
in the cut) at dynamically preferred speeds, at which
the phase-shift ‘ε’ between overlapping, or consecutive
cutting paths approaches zero In Fig 157c, stability
lobe number ‘N’ refers to the complete vibration cycles
existing between overlapping surfaces Moreover, the higher speeds correspond to lower lobe numbers, pro-viding the utmost potential increase in the total cut width and material removal rate – this being due to the greater lobe height and width If the total cut width exceeds the stability threshold – even assuming that the cutting process is operating at the desired speed, chatter will occur So, the larger the total cut width above the ‘stability limit’ , the more unstable and ag-gressive the chatter vibration becomes
Referring to the diagrammatic representation of the SLD on the graph in Fig 157c, if a chatter
con-dition arises, such as that found at point ‘a’ , the
ro-tational speed is attuned to the initial recommended
speed (i.e when ‘N=’), resulting in stable machining
at point ‘b’ on this diagram The D OC can be incremen-tally increased until the onset of chatter again – as the
threshold stability is crossed at point ‘c’ By utilising a
hand-held ‘speed analyser’ whilst the chatter
contin-ues – under the previously-selected operating
condi-tions, this will result in the ‘analyser’ giving a modified
speed recommendation that corresponds to point ‘d’
Now, if required, the DOC can be progressively
incre- ‘Speed analysers’ , are normally hand-held devices that
pro-duce dynamically-favoured speed recommendations and are
commercially available Such ‘speed analyser’* equipment
when utilised for a cutting process, can show the relative mo-tion between the tooling and the workpiece and recommends the appropriate speed to avoid chatter-effects
*‘Speed analysers’ can be successfully used for many industrial
applications, such as those involving: High-speed; Thin-chip, hardened-die machining; multi-point cutting operations – milling, etc.; Turning and boring operations These ‘speed anal-ysers’ can also be employed for workpiece compositions rang-ing from ductile metals (i.e aluminium and steel grades) and brittle materials (i.e cast irons and brasses, etc.), together with some non-metallics (plastics, etc.) and composite materials (carbon fibre, etc.)
Trang 3mentally increased to point ‘e’ – this being a
‘safe-limit’ for the optimum machining operation
7.4 Milled Roundness –
Interpolated Diameters
Circular features such as bosses, circular rebates, etc.,
can be CNC milled by utilising a specific
word-ad-dress ‘circular interpolation’ command This CNC
function creates precise and accurate circular control
in two slideways simultaneously, while the milling
cutter mills around the workpiece, as depicted in Fig
158 Here, the milling cutter’s rigidity plays an
impor-tant role in the quality of the final machined feature,
this being based upon the ‘rigidity square rule’ The
deflected milling cutter illustrated in Fig 158-right,
having lack-of-rigidity will produce some unwanted
effects on the final milled part Cutter deflection not
only introduces the potential for chatter vibration,
but if used to mill up to square shoulder, its deflection
distorts the component geometry and introduces
har-monic variation to the circular interpolated feature
So that minimal change takes place in a milled profile,
it is advisable to keep to cutter lengths having short
Generally-speaking, it is not advisable to attempt to maintain
both the DOC and the total cut width at the stability
thresh-old , because any variation in the: workpiece affecting its
cut-ting stiffness ‘K s’; speed errors; or perhaps small changes in
the overall dynamic characteristics of the machining system,
could result in crossing the stability limit, creating severe
chatter For example, in a milling application, the amplitude
of chatter vibration can be limited by a provisional feed per
tooth reduction , until an established and desired speed has
been achieved offering a stable DOC.
‘Circular interpolation’ , is a block of entered information
di-recting the CNC system to cut, either an arc, or a circle, (e.g
G02 – in a clockwise, or G03 anti-clockwise direction).
‘Rigidity square rule’ – for milling cutters states: ‘Cutter
rigid-ity decreases by the ‘square’* of the distance from the holder’
(Smith, 1993, et al.).
*For example, if a cutter ‘stood-out’ from its respective
tool-holder by 50 mm to mill a circular feature (Fig.158 – left), then,
if all other machining conditions remained the same and, then
cutter was replaced by one of 100 mm long (Fig 158 – right),
it would now be 4 times less rigid, causing serious tool
deflec-tion.
stand-off distances, conducive with correct and cur-rent operational practices
There are several distinct problems involved in the milling high-quality circular interpolated features and,
a slight digression into basic machine tool induced-er-rors is necessary to clarify the circumstances for the problems exhibited in Fig 159 Most of today’s
ma-chine tools have what is termed ‘orthogonally-orien-tated axes’ and in the case of the popular three-axis
vertical machining centre configurations, if the axes have not been recently calibrated, then considerable
‘error’8 can be introduced into the final milled part
features It has been well-proven that a machine tool
equipped with three orthogonal sideways: ‘X-axis’;
‘Y-axis’ – in the horizontal plane, together with the
‘Z-axis’ – in the vertical plane, can introduce up to 21
kinematic ‘errors’ into the cutting process The kine-matics for any machine tool are quite complex, when it has the ability to provide motion to all its axes simulta-neously, although these errors are often small, they are
‘Orthogonally-orientated axes’ , (is briefly mentioned in
Foot-note 2) refers to the fact that each axis is positioned at 90° with respect to each other, often situated on top of another axis For example, on a typical 3-axis vertical machining centre,
the ‘Y-axis’ sits on top of the ‘X-axis’ , but at right-angles to it, conversely, the ‘Z-axis’ is situated at 90° to these axes – hence
the term ‘orthogonal’
NB Non-orthogonal machine tools exist, often having
com-plex ‘kinematics’* between five and six axes Therefore with
these machine tools, in order to machine (i.e mill) a
straight-line all the axes must be in synchronised control to achieve
this linear action.
*Kinematics, comes from the Greek word ‘Kinesis’ , which
means ‘Motion’ It can be defined as: ‘The study of motion
with-out regard for the cause‘ (Lombardi, 2001) In machine tool
terminology, it refers to the translational effects of both lin-ear and angular motions It is principally concerned with the
effects of the ‘degrees of freedom’ for a ‘free-body’ in
three-di-mensional space (also see: Footnote 47, in Chapter 3).
‘Error’ is now not considered as an appropriate metrological
term for any form of calibration, the recommended term
to-day, is: ‘uncertainty’*.
*‘Uncertainty’ , has been simply defined as: ‘The doubt that
exists about the result of any measurement’ (Bell/NPL, 1999)
This is why today, uncertainty in measurement is a
combina-tion of many factors, some physical, while others are induced
Hence, another term, along with all of these uncertainty
fac-tors has been coined, which is its ‘Uncertainty budget’ – this
being a simple mathematical calculation, based upon a sum-mary of these uncertainty calculations.
Trang 4Figure 158 The effect of increased milling cutter length on the resultant circular interpolated profile on the workpiece
.
Trang 5Figure 159 The generated errors produced when circular interpolating at high feedrates when high-speed machining
.
Trang 6but significant ‘errors’ , which can be said to be
simplis-tically produced as a result of:
• Linear motions (six) – created by the displacement
of the forward-and-backward motion of the X-, Y-
and Z-axes slideway movements, introducing
par-ticular non-linearities into the slideway
position-ing,
• Rotational motions (three) – yaw, pitch and roll for
each axis All of these partial rotational motions can
be practically-described in the following manner:
• Yaw is the side-to-side ‘crabbing-motion’ along the
slideway,
NB ‘Yaw’ is normally the result of too much
clear-ance (i.e ‘slop’) in the adjacent slideway members
• Pitch introduces a backward-and-forward
rock-ing (pitchrock-ing) action normal to the slideway, as the
moving element traverses along the axis,
NB ‘Pitching’ is probably due to the
‘profile/wavi-ness’ (i.e long-frequency effects) in its respective
slideway
• Roll is the clockwise-and-anticlockwise rotational
motion along the slideway
NB ’Roll’ could be introduced by two ‘adjacent
ways’ situated on each slideway, but not being
coin-cident with respect to each other (i.e laying in the
same respective plane), causing a limited pivoting
action – along the ‘line-of-sight’ of the axis as it
tra-verses along its length
• Squareness (three) – these ‘errors’ occur due to the
fact that each axis may not be at 90° (i.e square) to
one another
These types of 21 ‘kinematic machine-induced
er-rors’ can be appreciably reduced by the application of
calibration through laser-based techniques To a lesser
extent, these ‘errors’ can be minimised via ballbar
ar-tifact-based methods, offering a quick ‘health-check’
by either static, or dynamic assessment techniques
The results of either the laser, or ballbar, can be fed
back into the machine’s CNC controller for dynamic
corrections as cutting takes place, offering a
consid-erable improvement in the machine’s subsequent
ac-curacy and precision The above machine tool
calibra-tion techniques are somewhat beyond the scope of the
present discussion, the same could be said for
‘ther-mally-induced errors’ , however, they can also influence
the machined part surface and the machine tool’s
pro-filing abilities Moreover, ‘error-mapping techniques’
and sophisticated in-process control by an associated
‘dynamic error compensation system’ , have been shown
to extensively reduce the effects of the ‘variety of
er-rors’ that can be present on the machine tool, but once again, these topics are mentioned only for further re-search applications – as necessary
The circular interpolated milled profile shown in Fig 159, shows significant departures from roundness
of the milled workpiece, which is a function of most of the previously discussed kinematically- and thermally-induced machine tool ‘errors’ , together with the possi-bility of some ‘load-induced errors’ This diagrammatic representation (i.e Fig 159), indicates that several
‘errors’ on the milled circular interpolated profile are present At relatively slow simultaneous
feeding-mo-tions of the two axes (‘X-’ and ‘Y-axis’), it will generate
a reasonable facsimile of the required circular feature However, then by somewhat increasing this milled in-terpolation speed, the apparent roundness will appre-ciably degrade, the reasons for this degradation, might
be the result of:
• Servo-spikes – these unwanted effects occur at the
‘axis transition points’ at their respective 90° angu-lar intervals, often termed ‘quadrant-points’ ,
• Back-lash – possibly resulting from any form of
axis reversals, originating from the recirculating ballscrews0, creating a slight ‘off-set’ , or ‘mismatch’
at the axis transition points,
• Servo-errors – when both axes are simultaneously
moving, their respective linear speed should be
‘Axis transition points’ , are where the ‘servo-spikes’ occur
They result from a reversal of one of the axes at this angular position and, its associated motor power-surge creating this
‘spike’ Normally, the ‘spike’ is associated afterward by a cor-responding, but very small localised slack here, as axis take-up
begins once more at these ‘quadrant-points’ on the
circular-interpolated feature (i.e see the inset and magnified diagram
in Fig 159).
0 ‘Recirculating ballscrews’ , are not supposed to have any
ap-preciable back-lash present, as they are normally pre-stressed
by applying loads by the application of either: tension-, or
compression-shimming However, as the pitch of any the screw has minute errors present, these are usually ‘mapped-out’ by
the original machine tool builder – using the recognised
In-ternational Standard laser-calibration techniques Although,
once the machine tool has been operating for sometime and either local ballscrew-wear occurs, or perhaps the machine
has had the occasional ‘tool-crash’ , this can introduce and
af-fect both its pitching- and back-lash-errors.
Trang 7perfectly matched, allowing either a partial arc,
or circular feature to be reproduced If
non-syn-chronised motion occurs, often termed
‘servo-mis-match’ between these two axes, then an elliptical
profile – usually inclined at an 45° angle occurs,
• Squareness – when orthogonal (squareness) is not
maintained between the two interpolating axes,
then the net result will look similar to that of a
milled angular elliptical profile shape, which is
un-affected by the selected circular interpolation
rota-tional direction
Considerably more machine tool-induced factors can
affect a milled circular interpolated profile These
‘er-rors’ can be found, isolated and then reduced by
di-agnostically interrogation by using dynamic artefacts,
such as the ballbar Ballbars and their associated
in-strumentation can not only find the sources of error,
they can prioritise their respective magnitudes – to
show where the main ‘error-sources’ occur, then
in-stigate any feed corrections into the CNC controller
to nullify these ‘machine-induced errors’ As a result
of eliminating such ‘error-sources’ , this enables the
milled circular contouring and overall performance to
be appreciably enhanced
7.5 Machined Surface Texture
Introduction to Surface Texture Parameters
When a designer develops the features for a component
with the requirement to be subsequently machined
utilising a computer-aided design (CAD) system, or
by using a draughting head and its associated
draw-ing board, the designer’s neat lines delineate the
de-sired surface condition, which can be further specified
by the requirement for specific geometric tolerances
In reality, this designed workpiece surface condition
cannot actually exist, as it results from
process-in-duced surface texture modifications Regardless of the
method of manufacture, an engineering surface must
have some form of ‘topography, or texture’ associated
‘Servo-mismatch’ , can often be mistaken for a ‘squareness
er-ror’ , but if the contouring interpolation direction is changed,
from G02 (clockwise) to G03 (anti-clockwise) rotation, then
an elliptical profile will ‘mirror-image’ (‘flip‘) to that of the
op-posite profile – which does not occur in ‘squareness errors’.
with it, resulting from a combination of several inter-related factors, such as the:
• Influence of the workpiece material’s microstruc-ture,
• Surface generation method which includes the cut-ting insert’s action, associated actual cutcut-ting data and the effect of cutting fluid – if any,
• Instability may be present during the production machining process, causing induced chatter, result-ing from poor loop-stiffness between the machine-tooling-workpiece system and chosen cutting data,
• Inherent residual stresses within the workpiece
can occur, promoted by internal ‘stress patterns’ –
causing latent deformations in the machined com-ponent
From the restrictions resulting from a component’s manufacture, a designer must select a functional sur-face condition that will suit the operational constraints for either a ‘rough’ , or ‘smooth’ workpiece surface This then raises the question, posed well-over 25 years ago –
which is still a problem today, namely: ‘How smooth is smooth?’ This question is not as superficial as it might
at first seem, because unless we can quantify a surface accurately, we can only hope that it will function cor-rectly in-service In fact, a machined surface texture condition is a complex state, resulting from a combi-nation of three distinct superimposed topographical conditions (i.e as diagrammatically illustrated Fig 160a), these being:
‘Stress patterns’ , are to be expected in a machined
compo-nent, where: corners, undercuts, large changes in cross-sec-tions from one adjacent workpiece feature to another, etc., produce localised zones of high stress, having the potential outcome for subsequent component distortion ‘Modelling’
a component’s geometry using techniques such as: finite ele-ment analysis (FEA), or employing photo-elastic stress analy-sis* models or similar simulation techniques, will highlight
these potential regions of stress build-up, allowing a designer
to nullify, or at worst, minimise these potential undesirable stress regions in the component’s design.
*Photo-elastic stress analysis displays a stress-field, normally
a duplicate of the part geometry made from a thin two-dimen-sional (planar) nematic liquid crystal, or more robustly from
a three-dimensional Perspex model, which is then observed through polarised light source This polarised condition, will highlight any high-intensity stress-field concentrations in the part , which allows the ‘polarised model’ to be manipulated
by applying either an un-axial tension, or perhaps a bi-axial bending external stress to this model, showing dynamically its potential stress behaviour during its intended in-service con-dition.
Trang 8Figure 160 Surface texture comprises of: ‘long-’, ‘medium-’ and ‘short-components’, together with the
‘direction of the dominant pattern’ – superimposed upon each other [Courtesy of Taylor Hobson]
Trang 91 Roughness – comprising of surface irregularities
occurring due to the mechanism of the machining
production process and its associated cutting insert
geometry,
2 Waviness – that surface texture element upon
which roughness is superimposed, created by
fac-tors such as the: machine tool, or workpiece
deflec-tions, vibrations and chatter, material strain and
other extraneous effects,
3 Profile – represents the overall shape of the
ma-chined surface – ignoring any roughness and
wavi-ness variations present, being the result of perhaps
the long-frequency machine tool slideway errors
The above surface topography distinctions tend to be
qualitative – not expressible as a number – yet have
considerable practical importance, being an
estab-lished procedure that is functionally sound The
com-bination of roughness and waviness surface texture
components, plus the surface’s associated ‘Lay’ are
shown in Fig 160a The ‘Profile’ is not depicted, as it
is a long-frequency component and at best, only its
partial affect would be present here, on this diagram
The ‘Lay’ of a surface tends to be either: anisotropic,
or isotropic in nature on a machined surface
topog-raphy When attempting to characterise the potential
functional performance of a surface, if an anisotropic
‘lay-condition’ occurs, then its presence becomes of
vital importance If the surface texture instrument’s
stylus direction of the trace’s motion over the assessed
topography is not taken into account, then totally
mis-representative readings result for an anisotropic
sur-face condition occur – as depicted in Fig 160b This
is not the case for an isotropic surface topography, as
relatively uniform set of results will be present,
regard-less of the stylus trace direction across the surface (i.e
‘Lay’ , can simply be defined as: The direction of the dominant
pattern’ (Dagnall, 1998).
‘Anisotropic, or isotropic surfaces, either condition can be
in-dividually represented on all machined surfaces Anisotropy,
refers to a surface topography having directional properties,
that is a defined ‘Lay’ , being represented by machined
feed-marks (e.g turned, shaped, planed surfaces, etc.) Conversely,
an isotropic surface is devoid of a predominant ‘Lay’
direc-tion, invariably having identical surface topography
charac-teristics in all directions (e.g shot-peening/-blasting and, to a
lesser extent a multi-directional surface-milling, or a
radially-ground surface, etc.).
see Fig 161a – for an indication of the various clas-sifications for ‘Lay’)
Returning once more to Fig 160b, as the stylus trace obliquity changes from trace ‘A’ , inclining to-ward trace ‘E’ , the surface topography when at ‘E’ has now become flat, giving a totally false impression of the true nature of the actual surface condition If this machined workpiece was to be used in a critical and highly-stressed in-service environment, then the user would have a false sense of the component’s potential
fatigue characteristics, potentially resulting in
ei-ther premature failure, or at worst, catastrophic fail-ure conditions In Fig 162, the numerical data (ISO 1302:2001), has been developed to establish and de-fine relative roughness grades for typical production processes However, some caution should be taken when utilising these values for control of the surface condition, because they can misrepresent the actual state of the surface topography, being based solely on a derived numerical value for height What is more, the
‘N-number’ has been used to ascertain the arithmetic
roughness ‘Ra’ value – with more being mentioned on
this and other parameters shortly The actual ‘N-value’
being just one number to cover a spread of potential
‘Ra’ values for that production process
Neverthe-less, this single numerical value has its merit, in that
it ‘globally-defines’ a roughness value (i.e.‘Ra’) and
its accompanying ‘N-roughness grade’ , which can be
used by a designer to specify in particular a desired surface condition, this being correlated to a specific production process The spread of the roughness for a specific production process has been established from experimental data over the years – covering the
maxi-mum expected ‘variance’ – which can be modified
‘Fatigue’ , can be defined as: ‘The process of repeated load, or
strain application to a specimen, or component’ (Schaffer, et
al., 1999) Hence, any engineering component subjected to repeated loading over a prescribed time-base, will normally undergo either partial, or complete fatigue.
‘Variance’ , is a statistical term this being based upon the
standard deviation, which is normally denoted by the Greek
symbol ‘σ’ Thus, variance can be defined as: ‘The mean of the
squares of the standard deviation’ (Bajpai, et al., 1979)
Thus, σ = √Variance, or more specifically for production
op-erations:
�s=��n − ċ �n
j=(xj− ¯x)
*s = the standard deviation of a sample from a production batch run.
Trang 10depending upon whether a fine, medium, or coarse
surface texture is obligatory Due to the variability in
any production process being one of a ‘stochastic
out-put’ , such surface texture values do not reflect the
likely in-service performance of the part Neither the
surface topography, nor its associated integrity has
been quantified by assigning to a surface
representa-tive numerical parameters In many instances, ‘surface
engineering’ 8 is utilised to enhance specific
compo-nent in-service condition
It was mentioned above that in many in-service
engineering applications the accompanying surface
texture is closely allied to its functional performance,
predominantly when one, or more surfaces are in
mo-tion with respect to an adjacent surface This close
proximity between two mating surfaces, suggests that
the smoother the surface the better, but this is not
nec-essarily true if the surfaces in question are required to
maintain an efficient lubrication film between them
The apparent roughness of one of these surfaces with
respect to the other, enables it to retain a
‘holding-film’ in its associated topographical ‘valleys’ While
another critical factor that might limit the designer’s
choice of the smoothness of an engineering surface’s
selection, is related to its production cost (i.e see
Fig 161b) Therefore, if the designer requires a very
smooth machined surface, it should be recognised that
its manufacturing time is considerably longer – so its
respective cost will be greater to that of a rough
sur-face, this being exacerbated by a very close
dimen-sional tolerance requirement
‘Stochastic processes’ , are defined as: ‘A process which has a
measurable output and operating under a stable set of
condi-tions which causes the output to vary about a central value in a
predictable manner’ (Stout, 1985).
‘Surface engineering’ , is applying suitable discrete
technolo-gies to create surface films (e.g 10 to 100 nm thick), or by
ma-nipulating the surface atomic layers (e.g 2 to 10 atomic layers,
approximately 0.5 to 3 nm), to enhance the ‘engineered’
sur-face condition (i.e Source: Vickerman, 2000).
‘Surfaces’ , are recognised to have topographical features that
mimic the natural world So a regular/irregular engineering
surface can exhibit both peaks and valleys, not unlike
moun-tainous terrain.
7.5.1 Parameters for Machined
Surface Evaluation
In order that a machined workpiece’s surface texture can be determined using stylus-based (two-dimen-sional) instrumentation, three characteristic lengths are associated with this surface’s profile (i.e see Fig 163a), these are:
1 Sampling length0 – is determined from: the length
in the direction of the X-axis used for identifying
the irregularities that characterise the profile under evaluation Therefore, virtually all surface
de-scriptors (i.e parameters) necessitate evaluation over the sampling length Reliability of the data
is enhanced by taking an average of the sampling lengths as depicted by the evaluation length shown
in Fig 162a Most of today’s stylus-based surface texture instruments undertake this calculation au-tomatically,
2 Sampling length – can be established as: the to-tal length in the X-axis used for the assessment of
the profile under evaluation From Fig 163a, this
length may include several sampling lengths – typi-cally five – being the normal practice in evaluating roughness and waviness profiles The evaluation
length measurement is the sum of the individual
sampling lengths (i.e it is common practice to em-ploy a 0.8 mm sampling length for most surface texture assessments),
3 Traverse length – can be defined as: the total
length of the surface traversed by the stylus in mak-ing a measurement The traverse length will
nor-mally be longer than the evaluation length (i.e see Fig 163a), this is due to the necessity of allowing
‘run-up’ and ‘over-travel’ at each end of the
evalua-tion length These addievalua-tional distances ensure that any mechanical and electrical transients, together filter edge effects are excluded from the measure-ment
0 ‘Sampling length’ , is often termed ‘Meter cut-off’ , or simply the ‘cut-off’ length and its units are millimetres The most
common cut-offs are: 0.25, 0.8, 2.5, 8.0, 25.0 mm The 0.8 mm
sampling length will cover most machining production
pro-cesses In any surface texture evaluation, it is essential that the
cut-off is made known to the Inspector/Metrologist reviewing
this surface topographical data.