Here, the two edges of the chip consume different quantities of workpiece material, creating dif-fering edge lengths, coupled to the fact that a varia-tion in cutting speed is present, t
Trang 1ponent’s strength and the mechanical strength of the
chip, illustrating that a complex metallurgical and
cut-ting tool geometric relationship exists whilst
machin-ing occurs
In particular for turning operations, a
convention-ally-turned chip is a rather frail product of serrated
appearance (see Figs 25 and 34a and b) In order to
promote good chip-breaking tendencies, thus enabling
short elements to be formed, it is necessary to
encour-age this basic character by causing these serrations to
be as deep as possible and the chip sections in between
to be rigid This chip occurrence causes the chip to be
inflexible, which can then subsequently be broken
There are several distinct ways in which chips can then
be broken, these include:
• Self-breaking – this is when the chip’s mechanical
strength is not great enough to hold the chip
seg-ments together and they consequently break upon
exiting the machining region (Fig 31a),
• Chip collision with the workpiece – as the chip is
steered towards an obstacle such as the workpiece’s
surface this provides the breaking force (see Figs
33 and 34b),
• Chip is stopped by the tool – here the chip-curling
behaviour comes into play, this being a function of
the: tool’s nose radius geometry, depth of cut and
feedrate employed (see Fig 34 bottom left-hand
photograph), the latter two functions affecting the
chip cross-section, or chip thickness
Chip thickness is influenced by the plan approach angle
utilised and the DOC, in association with the selected feedrate
The chip thickness is measured across the cutting edge,
per-pendicular to the cut (i.e along the main cutting edge) The
chip width and thickness are the dimensions that define the
theoretical cut of the edge into the workpiece material Hence,
the chip thickness will vary with the size of the plan approach
angle according to the relationships involving: feedrate, DOC
and the effective cutting depth The chip thickness is related to
the plan approach angle and this affects the amount of pressure
bearing upon the cutting edge Hence, the thinner the chip,
the smaller the distributed pressure along the edge and the less
power consumed, conversely, the thicker the chip, the greater
will be the machine tool’s power consumption A thicker chip
is generally advantageous for an increased tool life, because of
the improved contact between the chip and its cutting edge
Furthermore, if the plan approach angle is too small and chip
thickness is thin, this will reduce tool life, however, this can
be compensated for by increasing the feedrate, to produce a
thicker chip.
NB The helical formation of this chip-curling
behav-iour will shortly be mentioned, but prior to this, chip-breakers/formers will be discussed
2.5.3 Chip-Breakers and Chip-Formers
Chip-breakers have been utilised on turning tools for many years, initially introduced in the 1940’s in the form of an abutment, or step, situated behind the rake face of the tool Hence, with this type of early chip-breaker, as the continuous chip moves across the rake face it collides with this step and breaks This origi-nal form of chip-breaker geomtery was relatively in-efficient as the resultant force direction changed with the programmed tool path, this meant that the step would be approached by the chip from differing di-rections making chip-breaking less controlled Such chip-breakers were superseded in the 1970’s by built ‘wavy-shaped’ chip-breakers sintered into the in-sert’s top face (Fig 34 bottom left-hand photograph) Recent developments in designing chip-breaker geom-etries by computer-generated (i.e CAD) techniques, has shown a significant step-forward in both chip-former design enabling chip control and reduction in frictional forces across the rake face at a range of cut-ting data to be achieved Such ‘automatic’ chip breaker geometry forces the chip to deflect at a narrower angle, causing it to break off, either immediately, or just after the free end of the chip has hit either the tool’s flank or, the workpiece before the first coil has formed If such
a collision does not take place, the result would be a smaller diameter spiral chip and, it can be anticipated that the chip would still break, but only when it be-came slightly longer – this later chip breakage is due to the increasing chip mass and the effect of gravity upon
it, with, or without any further collision
Chip flow direction will depend upon several fac-tors, such as the: chip-breaker profile, back rake and setting angles, nose radius, DOC and feedrate – these latter three factors require further discussion The relationship between the nose radius, DOC and feedrate will often change during vectored tool paths in any machining operation Even though the insert’s nose radius is preset, its influence on the chip direction differs for different DOC’s, depending on how much corner rounding is represented by the total engaged edge length (Fig 34c) Further, the feedrate also af-fects the chip thickness: at different DOC’s and with a constant feedrate, the form of chip cross-section (i.e
Trang 2Figure 34 The principles of chip-breaking and chip-breaking envelopes for ‘coma-shaped swarf’ control and insert
edge preparations
.
Trang 3the ratio of chip width-to-thickness), will change and
this has a deleterious effect on the insert’s
chip-break-ing ability
2.5.4 Helical Chip Formation
Conventional Turning
For the general turning operations, such as sliding (i.e
Z-axis tool feeding) and facing (i.e X-axis tool path
motions), the chip is rolled into a helix, simply because
the chip edges are formed from different rotation radii
(Fig 34d) Here, the two edges of the chip consume
different quantities of workpiece material, creating
dif-fering edge lengths, coupled to the fact that a
varia-tion in cutting speed is present, these relavaria-tionships will
result in a helical chip formation The appearance of
the chip’s helix depends upon the workpiece’s diameter
and its metallurgical specification/condition, which
means the chip helices are extremely difficult to
quan-tify
Most common types of helical chip diameters are
determined either directly by the initial curvature
from its origin, or are the result of additional bending,
introduced by the chip-breaker For example, the
heli-cal chip type shown in Fig 34c (left), has its chip
seg-ments turned inwards, this being a desirable chip form
when not fully developed, that is prior to the first coil
being completed Whether, or not the chip is of this
form will already be determined even before it meets
the chip-breaker, this being the result of its
cross-sec-tion and the natural tendency to bend according to the
‘line of least resistance’ If the chips width is no larger
than its thickness, for example, the resistance to
bend-ing in the segment-stiffened thickness direction is
larger than in the width direction In this case, unless
this kind of chip is broken early, by colliding with
ei-ther part of the tool, or the workpiece whilst it is still
stiff and short – called ‘self-breaking’ – a helical chip
will be formed In this case, the barbed, or serrated
edge is turned outwards causing additional bending,
this being introduced by the chip-breaker For
exam-ple, the helical chip type shown in Fig 34c (right),
becomes difficult and awkward to control This
out-ward-curving helical chip also has weakened sections
in the serrations between the chip segments, but
ap-plied loads on it are readily absorbed by the spring
ac-tion of the chip This type of chip will break as it hits
the insert’s flank face (see Fig 27b) Only today’s very complex chip-breaker designs can reduce these out-ward-curling helical chips Although such chip helices produced by combinations of the feeds and DOC’s that result in the chip width being too small in relation to
its thickness must be avoided
Grooving and Recessing
In conventional turning operations, it is significantly easier to form a manageable chip, than for features re-quiring either grooving, or recessing The chip formed during plunge grooving counter-rotates in relation
to the workpiece, whereby it does not experience the same twisting force as chips produced by either Z-, or X-axes turning operations When grooving, ideally the chip resembles a ‘watch spring’ , indicating that the chip
is curling back onto itself and will ultimately break in several distinct ways: such as at the completion of the grooving cycle, or due to friction between the chip and its groove side walls – as the chip diameter becomes greater In grooving operations, three significant fac-tors affect chip control, these are:
(i) Insert geometry – applied to the rake face, can be
classified into distinct groupings:
• Radial-ground top rake (not shown), producing
the desired ‘watch-spring’ chip formation This grooving insert geometry will not thin the chip, therefore surface finish passes are necessary on both groove side walls
NB For long-chipping materials the chip-former
does not provide enough resistance to produce chip curling, hence, a straight flat chip occurs, that may
One of the problems with this type of chip-breaking, is the
potential for secondary wear on the insert’s non-cutting zone
on the face, caused by the chip helix breaking locally against this face Such an occurrence happens when the chip helix at-tains such a diameter and pitch that its free-end continually strikes the non-cutting portion of the insert’s edge – termed
‘chip-hammering’ – causing the edge to be locally weakened
and to subsequently crumble.
NB Chip-hammering can be alleviated by slightly increasing
the helix diameter (i.e by somewhat modifying the cutting data) causing the chip to break against the tool’s flank – be-low the insert’s cutting edge, this being one of the previously employed and favoured chip-breaking mechanisms, as shown
in Fig 27b.
Trang 4Figure 35 The chip-breaking envelopes related to cutting data and chip-curling behaviour [Courtesy of
Sandvik Coromant]
.
Trang 5wrap itself around either the tool, or workpiece, but
such a geometry is perfect for machining
alumin-ium, or non-ferrous materials
• Radial top rake (illustrated in Fig 4 middle and
to the left – three grooving insert sizes illustrated)
This radial top rake is designed to thin the chip
Such chip thinning, eliminates the need to
under-take finishing passes on the groove’s side walls
Fur-thermore, this type of grooving insert geometry
be-ing on-centre, enables axial turnbe-ing of diameters for
wide shallow grooves, or recesses
• Raised bumps on top rake (see Fig 27a – left)
This sophisticated grooving geometry is utilised for
materials where chip control is difficult, as it
pro-vides an ‘aggressive barrier’ to the curling chip The
raised bumps force the chip back onto itself, either
producing a tightly curled watch-spring chip, or
causes the chip to break
(ii) Surface speed of the workpiece – in order to
ob-tain full advantage of a grooving insert’s
chip-form-ing abilities, the chip must be allowed to flow into the
chip-former This chip-flow can be achieved by either
decreasing the workpiece’s surface speed, or increasing
the feed – more will be said on this shortly The former
technique of decreasing the surface speed, allows the
material to move slower across the top rake of the
cut-ting edge and as a result, has greater contact time to
engage the chip-former This slower workpiece speed,
has the benefit of increasing tool life, through lower
A groove, or recess, can normally be considered as a
straight-walled recessed feature in a workpiece, as illustrated in Fig
40 Typical applications for grooves are to provide thread
re-lief – usually up to a shoulder – so that a mating nut and its
washer can be accurately seated , or for retaining O-rings As
the groove is produced in the workpiece, the tool shears away
the material in a radial manner, via X-axis tool motion The
chip formed with insert geometries having a flat top rake, will
have an identical width as the tool and can be employed to
‘size’ the component’s width feature However, this chip action
– using such a tool geometry, creates high levels of pressure
at the cutting edge as a result of the chip wall friction, which
tends to produce a poor machined surface texture on these
sidewalls Grooving with an advanced chip-former insert
ge-ometry, reduces the chip width and provides an efficient
cut-ting action, this results in decreasing the cutcut-ting edge pressure
somewhat Chip-formers offer longer tool life and improved
sidewall finishes with better chip control, than those top-rakes
that have not incorporated such insert chip-forming
geomet-ric features
tool/chip interface temperatures The negative factors
of such a machining strategy, are that the:
• Part cycle times are increased and as a result, any batch throughput will be lessened,
• As the cutting edge is in contact for a longer du-ration, more heat will be conducted into the tool, than into the chip, which could have a negative im-pact of inconsistent workpiece size control,
• Due to the lower workpiece surface speed, the ben-efits of the insert’s coating will be reduced, as such coating technology tends to operate more effec-tively at higher interface temperatures
(iii) Increasing the feedrate – by increasing the feed
allows it to engage the chip-former more effectively – this being the preferred technique for chip control A
heavier applied feedrate, produces a chip with a thicker
cross-section Further, a thicker chip engages the in-sert’s geometry with higher force, creating a greater tendency to break Hence, by holding a constant work-piece surface speed, allows the faster feedrate to reduce cycle times
Transversal, or Face Grooving
Transversal grooving geometry has a curved tear-shaped blade onto which, the insert is accurately lo-cated and positioned The transversal insert follows the 90° plunged feed into the rotating face of a work-piece These tools are categorised as either right-, or left-hand, with the style adopted depending upon whether the machine tool’s chuck rotates anti-clock-wise (i.e using a right-hand tool), or clockanti-clock-wise (i.e left-hand) The minimum radius of curvature for such transversal grooving tooling is normally about 12mm, with no limit necessary on the maximum radial curva-ture that can be machined For shallow face grooves, off-the-shelf tooling is available, but for deep angular face grooves they require specialised tools from the tooling manufacturers
If a relatively wide face groove requires machining
with respect to the insert’s width, then the key to
suc-cess here, is establishing where in the face to make the
first plunge This initial face plunge should be made within the range of the tool’s diameter, otherwise the tool will not have sufficient clearance and will ulti-mately break Successive plunges to enlarge the face groove should be made by radially moving the insert 0.9 times the insert’s width, for each additional plunge The rotational speed for face grooving is usually about 80% of the speed used for parting-off – soon to be
Trang 6mentioned Feedrates are normally around 50% of
parting-off values, with the proviso that for material
which is subject to work-hardening, minimum feeds
are necessary
In transversal grooving operations, a unique chip
form occurs, because the chip is longer the further
away it is from the workpiece’s centre line of rotation
This results in the chip which no longer flows in a
straight line across the insert’s edge, instead it moves
at an angle Such a naturally curved chip is difficult to
exhaust from the face groove, particularly if it is
bro-ken Hence, no attempt should be made to break the
chip For deep and narrow grooves, the best solution
is to retract the tool at short intervals, to check that
the blade shows no signs of rubbing, this is to guard
against any likely breakage that might occur when
machining outside the blade’s range Due to the fact
that transversal grooving tooling is susceptible to
chat-ter, any excessive overhang of the tool should be
mi-nimised The chip should never be allowed to become
entangled within the transversal groove and should be
ejected speedily, otherwise the tool is likely to break
Chatter is a form of self-excited vibration and such vibrations
are due to the interaction of the dynamics of the chip-removal
process, together with the structural dynamics of the machine
tool Such chatter, tends to be at very high amplitude, which
can result in either damage to the machine tool, or lead to
pre-mature tool failure Typically, chatter is initiated by a
distur-bance in the cutting zone, for several reasons, such as:
Lack of homogeneity – in the workpiece material (i.e
typi-cally a porous component, such as is found in a Powder
Metallurgy compact),
Workpiece surface condition (i.e typically a hard oxide scale
on a hot-rolled steel component, utilsing a shallow DOC),
Workpiece geometry (i.e if the component shape produces
either a variation in the DOC – for example, because of
un-even depth of casting material being machined, or light cuts
on interrupted shapes, such as hexagon, square, or
rectan-gular bar stock),
Frictional conditions (i.e tool/chip interface frictional
variations, whilst machining).
Regenerative chatter is a type of self-excited vibration,
result-ing from the tool cuttresult-ing a workpiece surface that has either
significant roughness, or more likely the result of surface
dis-turbances from the previous cut These disdis-turbances in the
workpiece surface, create fluctuations in the cutting forces,
with the tool being subjected to vibrations with this process
continuously repeating, hence the term ‘regenerative chatter’.
Self-excited vibrations can be alleviated by either
increas-ing the dynamic stiffness of the system, or by increasincreas-ing the
damping.
NB Dynamic stiffness can be defined as the ratio of the
am-plitude of the force to the vibrational amam-plitude
–
–
–
–
For any face grooving of workpiece material that is subject to a continuous chip formation, always use copious amounts of coolant and at high-pressure – if possible, to not only lubricate the cutting zone, but to aid in chip flushing from this groove
Parting-off
The parting-off process is normally considered to be a separate machining operation, but it simply consists of cutting a groove to centre of rotation of the workpiece,
to release it from the bar stock, or to ‘part-off’ to a pre-viously formed internal diameter (shown in Fig 40 for left-hand side operations) Essentially in a parting-off operation, two time-periods are worthy of mention, these are:
(i) At separation from the bar stock – a lower spindle
speed than was previously used on the workpiece, will prevent the ‘released part’ from hitting the machine and potentially damaging its surface Moreover, it al-lows an operator – if present – to hear the change in the lower spindle speed tone, as it is about to separate from the bar stock, avoiding the parting-off tool from getting ‘pinched’ between the stock and the
soon-to-be-released component Often, ‘Part-catchers’ are
utilised to reduce any surface damage to the falling component, once it has been parted-off
NB If the component to be parted-off is held in a
co-axial/sub-spindle, at component release, the additional spindle supports the workpiece and under these con-ditions, the parting-off operation is virtually identical
to that of found in a grooving cycle
(ii) Surface speed reduction – this effectively
oc-curs when the machine’s spindle attains its maximum speed For example, on a machine tool having a maxi-mum speed of 3,000 rpm, 90 m min– would only be achievable until the parting diameter has reached about 8.6 mm When parting to a smaller diameter than 8.6 mm, the surface speed would decrease at a
fixed spindle speed As the parting diameter reaches
5.8 mm the surface speed would be 55 m min–, or 60%
of the ideal, thus significantly increasing the chip load-ing as the tool approaches the workpiece’s centreline
In order to alleviate the increasing tool loading,
lower-ing the feedrate by about 50% until separation is just
about to occur, then finally dropping the surface speed
to almost zero at this point, reduces the tendency for a
‘pip’ to be present on the workpiece On a CNC driven spindle, it is not advisable for parting-off operations,
Trang 7to utilise the ‘canned cycle’ such as the ‘constant surface
speed’ function
NB A more serious parting-off problem has been that
in order to eliminate the pip formed at the centre of the
‘released component’ , some tools have been ground
with the front edge angle of between 3° to 15° Such
a front edge geometry, can introduce an axial cutting
force component, leading to poor chip control, which
in turn, causes the tool to deflect This parting-off tool
deflection, can lead to the component’s face ‘dishing’ ,
creating a convex surface on one face and a concave
surface on the other – so this tool grinding strategy
should be avoided.
Today, parting-off inserts normally consist of two
main types with top rakes that are either of, negative,
or positive cutting edge chip-forming geometries The
negative-style of chip-formers are possibly the most
commonly utilised These inserts have a small
nega-tive land at the front edge which increases the insert’s
strength, giving protection in adverse cutting
condi-tions, such as when interrupted cutting is necessary
during a parting-off operation The land width – often
termed a ‘T-land’ , is relative to the breadth of the
part-ing-off tool This width of the insert’s land has a direct
correlation to the feedrate and its accompanying chip
formation The feedrate must be adequate to force the
workpiece material over the land and into the
chip-former
Notwithstanding the widespread usage of negative
parting-off tooling, positive-style insert geometries
have some distinct advantages The chief one being the
ability to narrow the chip at light feedrates, with
mini- ‘Constant surface speed’ CNC capability as its name implies,
allows the machine tool to maintain a constant surface speed
as the diameter is reduced The main problem with using
this ‘canned cycle’ , is that as the maximum spindle speed is
reached, the chip load will also increase This is not a
prob-lem, so long as the maximum speed has not occurred, such as
when parting-off a component with a large hole at its centre.
Parting-off operations that employ a negative-style insert (i.e
with a land and accompanying chip-former), normally have
the feedrate determined in the following manner: by
multi-plying the width of the insert by a constant of 0.04 For
ex-ample, for a 4 mm wide tool, it is necessary to multiply the
insert’s width of 4 mm by 0.04 to obtain a feedrate of 0.16 mm
rev– This will give a ‘start-point’ for any parting-off
opera-tions, although it might be prudent to check this feedrate is
valid, from the tooling manufacturer’s recommendations
mal tool pressure If excessive tool pressure occurs,
this can promote work-hardening of the ‘transient
surface’ of the workpiece These abilities are
impor-tant points when machining relatively low mechanical strength components, which might otherwise buckle
if machined with negative-style inserts when subse-quently parted-off
Positive cutting edge parting-off tooling having chip-formers, are ideal for applications on machine tools when either low fixed feedrates are utilised, or
if the workpiece material necessitates lower cutting speeds This positive-style of parting-off tooling, oper-ates efficiently when machining softer workpiece mate-rials, such as: aluminium-or, cooper-based alloys and many non-metallic materials, typically plastics Feed-rates can be very low with these positive-type part-ing tools, down to 0.0254 mm rev– with exceptional chip control and consistent tool life One major dis-advantage of using these positive tooling geometries
for parting-off, is that the tool is much weaker than its
equivalent negative geometry type
The concept of insert self-grip in its respective
tool-holder, was developed by the cutting tool manufac-turer Iscar tools in the early 1970’s and has now been adopted by many other tooling manufacturers (Fig 40 top left-hand side) These ‘self-grip’ tooling designs, rely on the rotation of the part and subsequent tool pressure to keep the ‘keyed and wedged’ insert seated
in its respective toolholder pocket Previously,
double-ended inserts termed ‘dogbones’ , were often used but
were limited to low DOC’s – due to the length of the secondary cutting edge, so have been somewhat over-shadowed by the ‘self-grip’ varieties of parting-off tooling
2.5.5 Chip Morphology
The Characterisation of Chip Forms (Appendix 2)
In the now withdrawn ISO 3685 Standard on Ma-chinability Testing Assessment, of some interest was the fact that this Standard had visually characterised
Transient surfaces are those machined surfaces that will be
removed upon the next revolution of either the:
Workpiece (i.e in rotating part operations), or Cutter (i.e for rotating tooling – drilling, milling, reaming,
etc.).
– –
Trang 8chip forms under eight headings, with several
varia-tions appearing in each groups (i.e see Appendix 2
for an extract showing these chip form classifications)
Although in the main, the chip forms were related to
turning, some of these chip morphologies could be
ex-trapolated to other manufacturing processes The chip
type that will be formed when any machining
opera-tion is undertaken is the product of many interrelated
factors, such as:
• Workpiece material characteristics – will the
mate-rial that forms the chip significantly work-harden?,
• Cutting tool geometry – changing, or modifying the
cutting insert geometries and its plan approach
angles will have a major influence on the type of
chip formed,
• Temperatures within the cutting zone – if high, or
low temperatures occur as the chip is formed, this
will have an impact on the type of chip formed,
• Machine tool/workpiece/cutting tool set-up – if
this ‘loop’ is not too rigid, then vibrations are likely
to be present, which will destabilise the cutting
process and affect the type and formation of chips
produced,
• Cutting data utilised – by modifying the cutting
data: feeds and speeds and DOC’s, with the insert
ge-ometry maintained, this can play a significant role
in the chip formed during machining operations
NB Chip formation has become a technology in
its own right, which has shown significant
devel-opment over the last few decades of machining
ap-plications
As has been previously mentioned, chip formation
should always be controlled, with the resultant chips
formed being broken into suitable shape formation,
such as ‘spirals and commas’ , as indicated by the
re-sultant chip morphology shown in Fig 35a
Uncon-trolled chip-steaming (i.e long continuous workpiece
strands), must be avoided, being a significant
risk-fac-tor to both the: machine tool’s operation and its CNC
setter/operator alike
Chip-breaking envelopes (see Fig 34 middle right), are the
product of plotting both the feedrate and DOC on two axes,
with their relative size and position within the graphical area
being significantly affected by the cutting insert’s geometry
– as depicted by the three cutting insert geometric versions
shown by types: A, B and C (Fig 34).
For every cutting insert geometry, there is a recom-mended application area – termed its ‘chip-breaking
envelope’ (i.e see footnote 38 below) – with regard to
its range of feedrates and DOC’s Within this ‘envelope’ , chips of acceptable form are produced by the
cut-ting insert’s geometry Conversely, any chips that are formed outside this ‘envelope’ are not acceptable,
be-cause they are either formed as unbroken strands, or are too thick and over-compressed When component profiling operations are necessary (Fig 31a), this nor-mally involves several machining-related parameters: variations in DOC’s, together with path vectoring of the feeds and as a result of this latter point, changes to the resultant chip’s path on the rake face These factors are important as they can modify the chip morphology when profiling operations include: recessed/undercut shoulders, tapers and partial arcs, facing and sliding operations with the same tool, together with many other combined profiled features All of these opera-tions make significant demands on the adaptability of the cutting insert’s geometry to efficiently break the chip
In general, the cutting insert’s chip formation prin-ciples are concerned with the chip-breaker’s ability to create a chip form that is neither not too tight a curl, nor too open
If chip curling is too tight for the specific
machin-ing application, the likely consequences are for a chip form creating:
• ‘Chip-streaming’ – producing long chip strands
that are undesirable, wrapping itself around the machined surface of the workpiece with work-hardened swarf and possibly degrading this ma-chined surface, or may become entangled around the various parts of the machine tool, which could impede its operation,
• Excessive heat generation – this can decrease tool
life, or be conducted into the machined part and consequently may affect specific part tolerances for the individual part, or could lead to modifications
in the statistical variability of a batch of parts,
Statistical variability in component production can cause
variations from one part to another, as the standard deviation and mean changes, these important factors will be mentioned
later in the text.
Trang 9• Increased built-up edge (BUE) formation – which
through ‘attrition wear’ 0 may cause the risk of
pre-mature cutting edge failure
When the chip curling is too open, this may result in
the following negative tendencies:
• Poor chip control – creating an inefficient
chip-breaking ability by the cutting insert,
• Chip hammering – breaking down the edge and
causing it to crumble and as a result creating the
likelihood of prematurely failing,
• Vibrational tendencies – affecting both the
ma-chined surface texture and shortening tool life
Chip formation and its resultant morphology, is not
only affected by the cutting data selected, but will be
influenced by the plan approach (i.e entering) angle
of the insert In most machining operations, they are
usually not of the orthogonal, but oblique cutting
in-sert orientation, so the affect is for the entering angle
to modify the chip formation process The insert’s
en-tering angle affects the chip formation by reducing the
chip thickness and having its width increased with a
smaller angle With oblique cutting geometry, the chip
formation is both ‘smoother and softer’ in operation as
the plan approach angle tends toward say, 10° to 60°,
furthermore, the chip flow direction will also
advanta-geously change with the spiral pitch increasing
As the nose radius is changed with different cutting
inserts, this has the effect of changing both the
direc-tion and shape of the chips produced This nose radius
geometry is a fundamental aspect in the development
of chips during the machining process – as depicted by
Fig 35b Here, an identical nose radius and feedrate
is utilised, but the difference being the DOC’s, with a
shallow DOC in Fig 35b (left), giving rise to a slow chip
helix, whereas in Fig 35b (right) the DOC is somewhat
deeper, creating a tighter chip helix which is
benefi-cial to enhanced chip-breaking ability Shallow cutting
depths produce ‘comma-shaped’ chip cross-sections,
0 Attrition wear is an unusual aspect of tool wear, in that it is
the result of high cutting forces, sterile surfaces, together with
chip/tool affinity, creating ‘ideal’ conditions for a pressure
welding situation Hence, the BUE develops, which builds-up
rapidly and is the ‘swept away’ by the chip flow streaming over
the top rake’s surface, taking with it minute atomic surface
lay-ers from the tool’s face This continuous renewal and
destruc-tion of the BUE, enhances crater wear formadestruc-tion, eventually
leading to premature cutting edge failure.
having a small angle when compared to the cutting edge Equally, a larger depth means that the nose ra-dius has somewhat less affect from its rara-dius and greater influence by the entering angle of the cutting edge, producing an outward directed spiral Feedrate also affects the width of the chip’s cross-section and its ensuing chip flow
Chip formation begins by the chip curving, this be-ing significantly affected by combinations of the cut-ting data employed, most notably: feedrate, DOC, rake angle, nose radius dimensions and workpiece condi-tion A relatively ‘square’ cross-sectional chip nor-mally indicates that an excessively hard chip compres-sion has occurred, whilst a wide and thin band-like chip formation is usually indicative of long ribbon-like chips producing unmanageable swarf If the chip curve
is tight helix, coupled to a thick chip cross-section, this means that the length of the chip/tool contact has increased, creating higher pressure and deformation
It should be noted that excessive chip cross-sectional thickness, has a debilitating effect on any machining process By careful use of CAD techniques coupled
to FEA to construct the insert’s cutting edge, comma-shaped chips are the likely product of any machining, providing that the appropriate cutting data has been selected In some machining operations, chip forma-tion can be superior using a slightly negative insert rake angle, thereby introducing harder chip compres-sion and self-breaking of the chip, particularly if utilis-ing small feeds Conversely, positive rakes can be give other important machining advantages, depending which chip form and cutting data would be the most advantageous to the part’s ensuing manufacture
Usu- Chip-flow is the result of a compound angle between the chip’s
side- and back-flow The chip’s side-flow being a measure of the flow over the tool face (i.e for a flat-faced tool), whilst back-flow establishes the amount of streaming into the chip-breaker groove Detailed analysis of chip side-flow (i.e via high-speed photography), has indicated that it is influenced
by a combination of groove dimensions and cutting data If the feedrate is increased, this results in a higher chip back-flow angle, promoting chip-streaming into the chip-breaker groove The ratio of feed-to-length of restricted contact has been shown to be an important parameter in the determina-tion of chip- back-flow Typically with low feedrates the cor-responding chip back-flow is going to be somewhat lessened, resulting in poor chip-breaker utilisation When the restricted contact between the chip and the tool is small – due to low feed – the chip-flow does not fully engage the chip-breaker and will as a result curve upward, with minimal ‘automatic’ chip-breaking effect.