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
Trang 1perfectly 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 2Figure 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 31 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 4depending 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.
Trang 5The number of two-dimensional surface profile
pa-rameters that have been developed over the years for
just the stylus-based instruments – discounting the
three-dimensional contact and non-contact varieties,
has created a situation where many users simply do
not understand, nor indeed comprehend the intrinsic
differences between them! A term was coined some
years ago to show exasperation by many
metrolo-gists’ with this ever-increasing development of such parameters Many researchers and companies were totally disenchanted with their confunsion and plight,
so they simply called the predicament: ‘parameter-rash’ However, here we need not concern ourselves
with this ‘vast expanse of surface descriptors’ , as only
a few of the well-established parameters and discuss just the most widely-utilised ones It is worth making
Figure 161 ‘Lay’ indicated on drawings, plus the relative cost of manufacture for different production processes
.
Trang 6Figure 162 Anticipated process ‘roughness’ and their respective grades [Source: ISO 1302, 2001]
.
Trang 7Figure 163 Surface texture data-capture, with techniques for the derivation of the arithmetic roughness parameter: Ra
.
Trang 8the point, that all of these two-dimensional surface
pa-rameters can be classified into three distinct groupings
and just some of these parameters are:
1 Amplitude parameters (Ra, Rq, Wa, Wq, Pa, Pq)
– with Ra being universally recognised for the
‘international’ parameter’ for roughness It is: ‘The
arithmetic mean of the absolute departures of the
roughness profile from the mean line’ (i.e see Fig
163b and c) It can be expressed as follows:
Ra = �lr�lr
NB The Ra parameter is often utilised in
appli-cations to monitor a production process, where a
gradual change in the surface finish can be
antici-pated, making it seem to be ‘ideal’ for any form of
machinability trials, but some caution is required
here, as will be seen shortly in further discussion
concerning this ‘surface descriptor’ By way of
com-parison, another previously used amplitude
param-eter is given in Appendix 10 and is the ‘R Z(JIS)’ (i.e
10-point height) parameter
Other useful parameters of the assessed profile, to be
shortly discussed in more detail, include: ‘Skewness’
(Rsk, Wsk, Psk), which is often utilised in association
with ‘Kurtosis’ (Rku, Wku, Pku), producing the
so-called: ‘Manufacturing Process Envelopes’ – as a means
of ‘mapping’ and correlating machined surface
topog-raphies
2 Spacing parameters (Rsm, Wsm, Psm) – can be
de-fined as: ‘The mean spacing between profile peaks at
the mean line, measured within the sampling length’
(i.e depicted along a machined cusp – at differing
The designation of the letters follows the logic that the
pa-rameter symbol’s first capital letter denotes the type of profile
under evaluation For example, the: Ra* – calculated from the
roughness profile; Wa – derives its origin from the waviness
profile; with the latter in this logical sequence, namely the Pa
– being derived from the primary profile Here, in this
discus-sion and for simplicity, only the first term in the series – e.g
‘Ra’ notation – will be used.
*Ra is today shown in the International Standard (i.e ISO
4287: 1997) as being denoted in italics, while in the past, it was
usually shown as follows: ‘R a’ , but even now, many companies
still use this particular notation.
Historically, the classification of the relative roughness of
sur-faces was initially developed in England and was then termed:
‘Centre Line Average’ (CLA), while in the USA its equivalent
term was the ‘Arithmetic Average’ (AA)
feedrates in Fig 169a and b) It can be expressed in the following manner:
Rsm = �n i�=n
i=si= XS +XS+XS +XSn
n
Where:
n = number of peak spacings.
NB The Rsm parameter needs both height and
spac-ing discrimination and, if not specified the default
height bias utilised is 10% of: Rz, Wz, or Pz, – where these are the ‘Maximum height of profile’ As can be
seen from the ‘idealised’ machined surface topog-raphy given in Fig 169a and b, the spacing param-eters are particularly useful in determining the feed
marks Moreover, the Rsm parameter relates very
closely to that of the actual programmed feed rev– of either the cutter, or workpiece – depending on which production process was selected See also, Appendix
10 for a graphical representation of the previously
utilised ‘High Spot Count’ (HSC) parameter.
3 Hybrid parameters (Rmr, Wmr, Pmr, R∆q, W∆q, P∆q, Rpk, Rk, Rvk) – each of these ‘hybrids’ will now be briefly mentioned Rmr, or its alternative notation Mr is the ‘Material ratio curve’ , which can
be defined as: ‘The length of the bearing surface (ex-pressed as a percentage of the evaluation length ‘L’) at
a depth ‘p’ below the highest peak (i.e see Fig 165)
– Rmr:
It is often known as the ‘Abbott-Firestone curve’ ,
the mathematics of this Rmr-curve can be
ex-pressed in the following manner:
Rmr= b +b+b=B +bn
n � =
n
i =n
�
i=bi
NB This ‘Material ratio curve’ represents the
pro-file as a function of level More specifically, by plot-ting the bearing ratio at a range of depths in the profile trace, the manner by which the bearing ratio changes with depth, provides a method of charac-terising differing shapes present on the profile (i.e see Fig 165 and Appendix 10)
– R∆q:
The R∆q parameter, can be defined as: ‘The root
mean square (rms) slope of the profile within the sampling length’ (i.e see how its angle changes at
differing machining feedrate conditions shown
in Fig 169b and c), it can be mathematically ex-pressed as follows:
Trang 9R � q =��lr�lr
[θ(x) − ¯θ]dx
Where:
¯θ = �lr�lr
θ (x)dx
θ = slope of the profile at any given point.
• Rpk, Rk, Rvk:
These parameters (i.e see Appendix 10 for
graphi-cal representations of the parameters), were
origi-nally designed for the control of potential wear in
automotive cylinder bores in volume production
by the manufacturing industry Today, Rpk, Rk and
Rvk are employed across a much more diverse-field
by a range of industries Such hybrid parameters
are an attempt to explain – in numerical terms, the
respective form taken from the profile’s trace of the
‘material ratio curve’ (Rmr), hence:
– Rpk parameter – is the ‘reduced peak height’ ,
il-lustrating that the top portion of a bearing
sur-face will be quickly worn-away when for
exam-ple, an engine initially begins to run,
– Rk parameter – is known as the ‘kernal
rough-ness depth’ , therefore the long-term running –
‘steady-state wear’ of this surface will influence
for example, the performance and life of the
au-tomotive cylinder(s),
– Rvk parameter – is the ‘trough depth’ this
in-dicates that the surface topography has an
oil-retaining capability, specifically via the ‘trough
depths’ which have been purposely
‘cross-honed’ into the bore’s surface
Arithmetic roughness parameter (Ra)
Although the Ra ‘amplitude parameter’ has been
widely quoted ‘Internationally’ , there are a few
provi- ‘Cross-honing’ , uses either: (fine) Abrasives/CBN/Diamond –
‘stones’ , that are fitted into a ‘honing head’ which then rotates
and oscillates within a hole, or an engine’s bore The critical
parameters are the rotational speed (Vr) oscillation speed
(Vo), the length and position of the honing stroke, the
hon-ing stick pressure (Vc) The inclination angle of the
cross-hon-ing action, is a product of the up-/down-ward head motion
(Vo) and the rotational speed for the head (Vo) This complex
action of rotating and linear motion, generates the desired
cross-honed ‘Lay-pattern’ within the bore – for improved oil
retention.
sos, or conditions that must be met, if it is to be utilised satisfactorily, these are:
• The Ra value over one sampling length represents
the average roughness The effect of a spurious
non-typical peak, or valley within the profile’s trace be-ing ‘averaged-out’ so will have only a small
influ-ence on the Ra value obtained;
• The evaluation length contains several sampling
lengths (Fig.163a), this ensures that the Ra value is
representative of the machined surface under test;
• An Ra value alone is meaningless, unless quoted with its associated metre cut-off (λc) length Repeat-ability of the Ra value will only occur at an
identi-cal length of metre cut-off;
• If a dominant surface texture pattern occurs (Lay), then
the Ra readings are taken at 90° to this direction;
• That Ra does not provide information as to the
shape of either the profile, or its surface irregulari-ties Different production processes generate diverse
surface finishes, for this reason its is usual to quote
both the anticipated Ra numerical value along with
the actual manufacturing process;
• Ra offers no distinction between peaks and valleys
on the surface trace
The most confusing argument concerning the use of
an Ra value alone, is that its numerical value is not
only meaningless, but it can have catastrophic conse-quences if interpreted incorrectly These opinions can
be substantiated by close observation of Fig 164a,
where an identical numerical Ra value produces
widely divergent surface topographies In addition, if a
designer’s engineering application called for a ‘bearing surface’ (Fig 164ai), rather than a ‘locking surface’ (Fig
164aiii), then the numerical value of 4.2 µm in isola-tion, becomes pointless, as it tells the designer nothing about the ‘functional’ surface topography This
prob-lem is exacerbated when the wrong surface topography
is selected for a specific engineering application For ex-ample, a ‘locking surface’ applied to a bearing industrial
application in a harsh environment, can be expected to catastrophically fail after very little in-service time
Skewness (Rsk, Wsk, Psk) and Kurtosis (Rku, Wku, Pku) Parameters
These surface descriptors of ‘skewness’ and ‘kurtosis’ are often derided as simply ‘statistical’ amplitude pa-rameters, that can introduce spurious results and as a consequence, having little use in engineering applica-tions However, when used in the correct context, they can provide a valuable insight into the overall shape of