Machinability and Surface Integrity Part 3 pps

(Fig. 153a–c). e inuence of this irregular harmonic
rotational belt-driven rotation can be gained from the
schematic representation shown in Fig. 153d, where
a repeating-series of ‘tumbling three-lobed harmonic’
geometric shapes are reproduced on the workpiece.
e irregular, but periodic nature of the rotational
action of the belt-driven headstock is reproduced on
the workpiece by a series of kinematic combinations
of headstock rotation and linear motion supplied by
the longitudinal feed of the cutting tool along the part
(Fig. 153d). If a direct-drive headstock conguration
is utilised (Fig. 153e), then there is virtually no har-
monic inuence associated from the machine, so more
consistent turned components result.
Returning to Fig. 152, the overall machine-tool-
workpiece system, can be isolated to consider the
simple eect of a cantilevered cutting tool that is in-
adequately supported, or the unlikely occurrence of
too small a cross-sectional area – making it somewhat
‘under-strength’. e main cutting force in turning op
-
erations is the tangential force (i.e. see Fig. 19), it re-
sults from several factors, such as:
•
Resistance to rotation – caused by the workpiece
material’s inherent shear strength,
•
Undeformed chip thickness – resulting from the ra-
dial D
OC
selected,

•
Orientation of cutter rake angle geometry – this
being a combination of either a positive, neutral, or
negative rakes, plus to a lesser degree, the eect of
shape and size of the tool nose radius,
•
Feedrate – in combination with D
OC
, will heavily
inuence the size of the eective chip thickness and
play a dominant role in the resulting surface tex-
ture.
In the upper diagram in Fig. 152, the tangential force
is simplistically shown contacting the cutting insert
at the point. e application of the cutting force here,
causes a large bending moment to occur at the pivot
point – as shown. e resultant dynamic action of this
eect, is depicted in the lower diagram of Fig. 152,
where the tool has been elastically deected in a down-
ward manner by this bending moment. Moreover, as
the resistance to deection increases with the tool’s
downward direction, this intensies the pressure from
the inherent tool-body mechanical strength, enabling
a certain degree of recovery, therefore there is a partial
upward motion of the tool. is cyclical upward, then
downward tool point motion is repeated at a periodic
medium-frequency, causing a sinusoidal motional ef-
fect with this being harmonically reproduced on the
turned surface. High-frequency harmonics can also be
Table 10. The harmonic behaviour related to either the component manufacturing process, or its measurement

Harmonic: Cause:
1
st
(1 upr) Function of measurement – only caused by the setting-up error on the instrument being used to measure the
departures from roundness. The amplitude of this harmonic is equal to the eccentricity of the part, relative to the
spindle axis of the roundness instrument.
2
nd
(2 upr) Function of measurement, or manufacture – this aspect of harmonics is generally termed ovality and can be
caused either by a setting-up error of the roundness instrument, or the part being machined out-of-square to its
axis of rotation.
3
rd
–7
th
Function of manufacture – these harmonics are normally introduced by the work-holding technique during
manufacture. By way of illustration, if a three-jaw chuck were used to hold a relatively delicate part and excessive
clamping force was employed, then upon machining and subsequent workpiece removal a three-lobed part
would be the result.
15
th
-upwards Function of material and manufacture – this aspect of harmonic behaviour is usually introduced to the part by
either machine tool instability (i.e. self-excited vibration – chatter), or by the reaction of the materials used in the
component’s manufacture – cutting insert/toolholder, lubricant – if any used.
Upr: undulations per revolution
NB Higher harmonics may be the result of instrument noise, or vibration.
[Courtesy of Taylor Hobson]
.
Machinability and Surface Integrity 
Figure 153. By utilising turning centre headstocks with direct-drive spindles – for ‘harmonic supres-

sion’, a signicant improvement in machined roundness will result. [Courtesy of Yamazaki Mazak]
.
 Chapter 
superimposed onto the medium-frequency harmon-
ics, this aspect can be shown to good eect by a ‘power
spectrum analysis’
22
of the harmonic behaviour during
machining.
For a simple turning operation, the resultant cutting
forces occur from the consequential combination of:
a workpiece material’s shear strength, its undeformed
chip thickness, the cutting insert geometry and ac-
companying nose radius, which has a signicant aect
on the harmonic ‘departures from roundness’ of the
turned part. So that the eect of these variables in the
cutting generation process can be seen, while simplify-
ing the discussion, only external-diameter operations
will be mentioned concerning these process-based
roundness relationships – in the following section.
.. Turned Roundness – Harmonics
and Geometrics
A typical operation on a either an engine-/centre-lathe,
or a turning centre, is schematically illustrated in Fig.
154. is involves a longitudinal turning process – the
workpiece being shown as partially completed – us-
ing a ‘light-turning and nishing cutting insert’ , as
it progresses along the turned part. Here, the turning
application has a long and slender workpiece this be-
ing held in a work-holding device: chuck, or collet – at

the headstock end, with further support
23
supplied by
22 ‘Power spectrum analysis’ , is a useful aid in process monitor-
ing of the cutting capabilities – giving a good interpretation
of the anticipated surface topography (i.e its ‘micro-terrain’).
A major advantage of utilising the ‘power spectrum’ as a diag-
nostic aid, is that it can separate-out any process-related tool
problems.
NB More details concerning the application of ‘power spec-
trum analysis’ can be obtained from the References by either
Whitehouse (1997), or Smith (2002).
23 ‘Programmable and xed steadies’ are oen used to give addi-
tional support to the long and slender parts, to minimise ‘bar-
relling eects’ – created by increased tool push-o the further
away the tool’s longitudinal distance becomes from the inu-
ence of the tailstock/headstock. Hence, the part has smaller
turned diameters toward its ‘supports’ steadily increasing in
diameter toward its centre, then reducing again – creating a
‘barrel-like prole’ along the entire turned bar’s length.
either a ‘dead-’ , or ‘rotating-centre’
24
– at the tailstock
end. As the orthogonally-oriented cutting insert (i.e.
having a zero-plan approach angle) turns along the
workpiece, a ‘moving step’ is seen to be present as the
‘emerging diameter’ occurs – to its set dimensional size
(Fig. 154). If a very high quality toleranced part is to
be turned, then it is desirable to review the operation
more critically, as some unexpected and unwanted

features may be present in the nal machined com-
ponent. As the turning insert has an orthogonal ori-
entation to the axis of rotation of the part (Fig. 154),
it might be thought that no radial force component
occurs, but this is not the case, as the tool nose radius
can create a radial force aecting the turned surface.
e radial force has little eect on the part’s harmon-
ics when close to the tailstock as shown by the cross-
sectional harmonic eect indicated in section ‘C-C’
(Fig. 154). Once the cutting insert has progressed
some distance along the workpiece, the contributing
and supporting inuence by the tailstock is lessened
and the eect of this radial force component increases,
as exhibited by section ‘B-B’ , this being amplied still
further in section ‘A-A’. Here (i.e. section ‘A-A’), the
harmonic departures from roundness are signicant,
a fact that has been recognised by precision turners
for many years. Experienced machinists when turn-
ing parts having long length-to-diameter ratios, will t
either a ‘xed-steady’ , or more preferably a ‘moving-
steady’ close to the tool cutting zone – on the opposite
side of the workpiece – to counteract ‘push-o’ by the
radial force, while minimising component eccentric-
ity/run-out. If twin-turrets (i.e. upper and lower) are
tted to turning centres, then ‘balanced turning’
25
can
be utilised as an alternative machining strategy.
ere is a direct link between cutting forces and the
geometric shape of the insert, this eect being illus-

24 ‘Rotating centres’ , can introduce their own eccentric error into
the turning process, as they are less rigid than their ‘dead-cen-
tre’ counterparts, but the latter, has a rotational speed restric-
tion – otherwise ‘dead-centre burn-out’ is likely and is there-
fore not practicable for high-production volume demands.
25 ‘Balanced turning’ , situates one cutting edge slightly ahead of
the other in their respective opposing turrets. In this manner,
the radial force components for each cutting insert have the
eect of ‘virtually’ cancelling each other out, allowing long
and slender workpieces to be successfully turned. A produc-
tion bonus being the removal of greater workpiece material
stock per pass.
Machinability and Surface Integrity 
trated in Fig. 155. In these diagrams a simplistic repre-
sentation for a range of cutting insert proles is shown
and for clarity, the tangential force has been excluded,
with just the axial and radial force components indi-
cated for each type of cutting insert shape. Assuming
that the overall cutting data is identical in each case
(i.e. the same: rotational speed, feedrate, D
OC
, insert
rake angle, plus workpiece material), then the only
variable in the longitudinal turning process here will
be the cutting insert shape its orientation. e com-
ponent cutting forces – axial and radial, will vary for
each tool prole in their respective magnitudes, due
Figure 154. Machined roundness is inuenced by a number of factors: unbalanced cutting forces, non-integral headstock and
lack of support on slender/long workpieces
.

 Chapter 
to the variation in plan approach angles. In the case of
the orthogonal insert (0°) plan approach angle, the ax-
ial force dominates with virtually no radial force com-
ponent present, this axial force being directly linked
with the feedrate. e displayed prole chart for this
harmonic roundness trace (i.e. section on ‘A-A’), for
the 0° plan approach angle, shows virtually negligible
harmonic eects. If a triangular-shaped cutting insert
geometry was selected, in this case having an 15° plan
approach, here there is a slight reduction in the axial
force component and a corresponding increase in its
radial counterpart. is slight increase in the radial
Figure 155. Turned roundness can be signicantly aected by the insert shape, its approach angle – which aects cutting forces
– resulting in harmonic out-of-roundness
.
Machinability and Surface Integrity 
force, in combination with a marginally longer cut-
ting edge being in contact with the workpiece’s ‘tran-
sient surface’
26
, leads to a slight increase in the har-
monics on the displayed prole chart (‘B-B’). As the
obliquity of the insert’s plan approach angle increases,
as depicted by the square-shaped cutting insert, being
inclined at an angle of 45°, then the axial and radial
force components equalise. Here, the considerable
radial force component has a signicant eect on the
displayed prole trace, as illustrated by the section at
‘C-C’ , where the harmonics have increased, but with a

notable vibrational tendency superimposed onto this
primary harmonic. is increase in vibration during
turning, is the result of two noteworthy factors: rstly,
the length of the transient surface has increased – with
this square insert’s greater plan approach angle; sec-
ondly, as a result of this rst condition of increased
obliquity, the radial force aects workpiece rigid-
ity, which is compromised, leading to an exacerbated
turned part surface roundness and accompanying
chatter-marks.
Finally, when turning with a round insert geometry,
this will lead to a vast increase in the radial force com-
ponent, which in turn may cause signicant harmonic
out-of-roundness, if a very rigid set-up is not utilised.
In this case, the round insert’s displayed prole trace,
shows evidence of a signicant increase in vibration
– chatter – present, which has a dramatic eect on
the prole of primary harmonic (section on ‘D-D’)
– more will be said on the subject of ‘chatter-marks’ on
the component’s surface shortly. e rationale for this
notable harmonic amplication when using round in-
serts is the product of several interrelated factors. e
transient surface in contact with the round/curved
prole has been markedly increased, together with the
plan approach at the tangency position – with respect
to the workpiece – being at a maximum; thus, the
combination these two factors leads to a momentous
deterioration in workpiece rigidity and as a result here,
the vibration will especially increase.
26 ‘Transient surface’ , can be dened as: ‘e part of the surface

formed on the workpiece by the cutting edge and removed dur-
ing the following cutting stroke, [by the next] revolution of the
tool, or workpiece’ (Boothroyd, 1975).
NB In the case thread-turning operations, the transient ank’s
surface only remains until the next pass of the screw-cutting
insert obliterates it, or until the nal thread depth is reached.
If a large volume of workpiece stock has to be re-
moved in a series of roughing cuts, a strong insert is
necessary, therefore the problem associated with the
harmonic behaviour of cutting insert geometry be-
comes of less importance. is latter fact allows either
a square, or round cutting insert to be utilised due to
their intrinsic strength and if vibration is a problem,
then a ‘nishing cut’ with an insert having an 0° plan
approach geometry would remove any probable sur-
face chatter-marks.
e case of turning with a round insert geometry is
worthy of a closer investigation, as several factors in-
uence the harmonic roundness of a workpiece turned
with its curved prole. For example, let us consider
several conditions for employing a standardised round
insert and its subsequent aect on the harmonics of
the turned part. If one assumes that an identical: ro-
tational speed, feedrate and workpiece material was
used, but having diering D
OC
’s – to isolate variabil-
ity in the turning process. In the rst example using a
small D
OC

, the radial force component is large in com-
parison to the axial force, but the harmonic workpiece
roundness is not compromised here – as these forces
are minute. Conversely, an extreme example of using
a round insert might be when employing a larger D
OC
.
Here, the radial force component has been reduced
in comparison to the axial component, although the
magnitude of these forces will be considerably greater
than in the former case. e pressure exerted on the
very long contact region at the transient curved sur-
face, creates potential harmonics in the turned part as
the resultant force has now signicantly increased. So,
the inuence of an increased D
OC
, in combination with
the round insert prole can create a very long contact
region at the transient surface, thereby causing un-
wanted harmonic eects on the turned surface.
In Appendix 9, a visual impression is given of the
principal techniques for roundness measurement and
its assessment, together with some of the ltering ef-
fects are highlighted.
7.3 Chatter in Machining
Operations
In the machining of metals, chatter (Fig. 156) is a form
of self-excited vibration introduced by the closed-loop
force-displacement response to cutting. e plastic de-
formation during machining operations is always pro-

 Chapter 
ceeded by elastic deformation – the situation is akin to
that of it acting somewhat like a ‘big spring’
27
. More-
over, the mechanism by which a cutting process dissi-
pates energy is termed chatter and vibration, also this
being a function of the workpiece’s rotational speed.
Any chatter/vibration can clearly be heard as an un-
wanted machining noise by an experienced machinist,
who would then modify the speed accordingly. ere
are a wide variety of causes for chatter, including the
process-induced eects from the cutting forces, which
may be the result of changes in: the cutting velocity;
chip cross-sectional area; tool/chip interface friction;
BUE, variations in the workpiece composition; or the
most common factor being process modulation result-
ing in regeneration of vibration. If greater energy is
input into the dynamic ‘machining-loop’ than can be
readily dissipated by the following: mechanical work;
damping, or friction, then an ‘equilibrium status’ is re-
quired and this output is via the somewhat superu-
ous eect of the generation of chatter/vibration.
Vibration is a debilitating process aecting both the
machined surfaces and reducing tool life in any ma-
chining operation, consequently it must be appropri-
ately identied – classied, then one has the potential
to nd the actual cause of this unwanted eect and
resolve it accordingly. In essence, in machining opera-
tions there are three types of vibration that may tran-

spire, these are:
•
Free vibration – this being the response to sudden
change, or to any initial condition, where the vibra-
tional amplitude decreases with time, occurring at
the system’s natural frequency,
NB An interrupted machining operation, or work-
piece feature can create this vibrational eect and it
frequently appears as shadows, or lines following a
surface discontinuity.
•
Forced vibration – can be regarded as a response to
a periodic – repetitive timing – input that occurs at
an identical frequency. At this point, the vibrational
27 ‘Spring-cuts’ , are always present in any ductile component
machining operation, resulting from the relaxation of the
forces and the elastic recovery of the tool and workpiece aer
the cutting insert’s passage along the part. In fact, if the tool
is repositioned once more at the beginning of the original cut,
then simply fed along the component, it will take a minute cut
– assuming that the tool’s edge is still suciently sharp, this is
termed the ‘spring-cut’.
amplitude stays constant for a set of input condi-
tions, being non-linearly related to speed.
NB e most common examples of this eect are
caused by: cutter imbalance, cutting teeth impact-
ing on workpiece, tooling misalignments, plus the
occurrence of any form of rotational system reso-
nance.
•

Self-excitation vibration, or chatter – occurs
through the system’s periodic response to a con-
stant input, which may intensify in amplitude – be-
coming unstable, oen occurring – regardless of
the input, but close to the natural frequency of the
system.
NB Chatter is due to waviness regeneration in
the machined surface, it commonly occurs during
metal cutting operations (i.e. see Fig. 156a).
What is chatter and how might can be characterised?
Degarmo, et al. (2003), has produced a list of the fol-
lowing factors that can indicate the onset of chatter,
these being characterised by:
•
Sudden onset of vibration – whose amplitude will
rapidly increase until a maximum threshold – sat-
uration – is reached (i.e sounding like either: a
screech, whine, or buzz),
•
Chatter frequency is near to that of the machin-
ing system’s natural frequency (i.e critical fre-
quency) – changing only slightly with any process
parameter variations. e largest force-displace-
ment response occurs at ‘resonance’
28
enabling the
maximum dissipation of energy,
•
Chatter produces unacceptable surface texture
(Fig. 156a) – normally highlighted by either an an-

gular, or helical pattern (i.e. the visual appearance
28 ‘Resonance’ , is of practical importance in many engineering
applications, because relatively few oscillatory forces can re-
sult in large vibrational amplitudes that can cause damage, or
interfere with the functioning of the system.
NB e classic example of this phenomenon was found when
ranks of soldiers marched across a bridge in unison (i.e. ‘in-
step’) which, if it coincided with one of the bridge’s resonant
frequencies, could create damage to the structure – despite the
fact that the bridge could safely support their overall weight.
Hence, the order to ‘break-rank’ (i.e randomising both their
pacing and steps) when proceeding over a structure such as a
bridge was mandatory.
Machinability and Surface Integrity 
Figure 156. Vibration and chatter in machining operations, with their machine tool damping characteristics.
 Chapter 
is either ‘pearled’ , or ‘sh-scaled’) superimposed
over the normal cutting insert’s feed marks,
•
Visible surface undulations – these eects are re-
produced in the direction of feed, being the prod-
uct of either serrated, or wavy chip formations, of
variable thicknesses.
.. Chatter and Chip Formation –
Significant Factors Influencing
its Generation
e stability of the cutting process and the onset of re-
generative chatter is inuenced by a range of factors,
such as the: cutting stiness (K
s

)
29
of the workpiece
material – related to its machinability; parameters
of the machining process (e.g. speed, feed, D
OC
, chip
width – total); insert cutting geometry (e.g. rake and
clearance angles, edge preparation, insert shape and
size); cutting process dynamic characteristics (e.g.
machine-tooling-workpiece/xturing). Hence, during
machining operations on the workpiece, the chip is
formed by shearing over the chip area, producing the
cutting, or tangential force (F
T
). e magnitude of this
tangential force is heavily inuenced by the product
of the workpiece material’s stiness (K
s
) and the chip
area, as follows:
F
T
= K
s 
× t × w
Where:
F
T
= tangential force (N),

K
s 
= workpiece material’s stiness (N mm
–2
),
t = chip thickness (mm),
w = chip width (mm).
e direction of the tangential force (F
T
) is predomi-
nantly aected by the cutting insert’s rake and clear-
ance angles, together with the edge preparation on the
insert. In many single-/multi-point machining opera-
tions used to generate for example a milled surface,
there is a requirement to overlap the adjacent cutting
paths (Fig. 84c). For most single-point machining op-
29 ‘Cutting stiness’ (K
s
), is closely associated with that of ‘ow
stress’*, but is more simple to calculate and can be thought
of as a workpiece material property, being dependent on its
hardness.
*‘Flow stress’ , can be dened as: ‘e stress required to sustain
plastic deformation at a particular strain’ ( Kalpakjian, 1997).
erations, this former over-lapping of tool paths does
not take place in the same manner, but will only occur
aer one complete revolution of either the workpiece,
or tool. In operations by either milling (Fig. 85), or
drilling (Fig. 50), an overlap takes place in a fraction
of a revolution, this being dependent upon how many

cutting edges are present on the tool.
In the Degarmo, et al. (2003) machining model
shown in (Fig. 157a), the cutting or tangential force
(F
c
)
30
generation may cause a relative displacement ‘X’
between the cutting insert and the workpiece, aecting
the uncut chip thickness (t), this results in changing
the cutting force. is coupled relationship between
displacement in the ‘Y’ direction – modulation direc-
tion – and the resultant cutting force, creates a closed-
loop response system. Here, the modulation direction
is normally at 90° to the machined surface, so denes
the chip thickness. As a consequence of these inter-
related factors, there is a phase-shi (ε) between the
subsequent overlapping machined surfaces, resulting
in a variable chip thickness and modulation of the
displacement, causing chatter vibration to take place.
Accordingly, this phase-shi between overlapping cut-
ting paths is accountable for the production of chatter
(Fig. 157b). Moreover, there is a favoured speed cor-
responding to a phase-locked condition (e.g. when
‘ε=0’), resulting in a constant chip thickness (t). By
obtaining a constant chip thickness, this results in a
‘steady-state’ cutting force generation with it and, the
eradication of the feed-back mechanism for regenera-
tive chatter. In essence, this is the goal for all machin-
ing operators, as they attempt to achieve this eect by

vary the cutting speeds for a given set of conditions for
a particular machining operation.
.. Chatter – Important Factors
Affecting its Generation
In the previous sections, a brief discussion was made
concerning just some of the causes of regenerative
chatter mechanisms. It is worth looking in greater de-
tail at the reasons why this superuous chatter occurs,
explaining how and why it is generated in the hope of
30 In the Degarmo, et al. (2003) model diagrammatically shown
in Fig. 157a, they use the term and nomenclature of: ‘cutting
force’ and ‘F
c
’ , whereas previously in the text, this has been
referred to as the ‘tangential force’ , denoted by ‘F
T
’.
Machinability and Surface Integrity 
Figure 157. A chatter model, with potential chatter conditions and the application of the ‘stability lobe
diagram’. [Source: Degarmo, Black & Kosher, 2003]
.
 Chapter 