Turning and Chip-breaking Technology Part 3 pdf

So, as the tool’s rake face moves rela-tive to that of the workpiece, it ‘engages’ one card at a time, causing it to slide over its adjacent neighbour, this process then repeats itself ‘

and later, by Herbert (1928) Around this time, the

cut-ting speeds were steadily improving with the arrival of

new cutting tool materials, such as cemented carbide

In 1937, Piispanen introduced his so-called ‘Deck of

Cards’ principle as an explanation of the cutting

pro-cess (see Fig 24 for Piispanen’s idealised model, with

Fig 25 depicting sheared chips at a range of cutting

speeds) Here, Piispanen’s model depicts the workpiece

material being cut in a somewhat similar manner to that of a pack of cards sliding over one another, with the free surface an angle, which corresponded to the shear angle (ϕ) So, as the tool’s rake face moves rela-tive to that of the workpiece, it ‘engages’ one card at

a time, causing it to slide over its adjacent neighbour, this process then repeats itself ‘ad finitum’ – during the remainder of the cutting process Some important

Figure 25 Variations in chip morphological surfaces at different cutting speeds, giving an indication

of the various shearing mechanisms [Source: Watson & Murphy, 1979]

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limitations are present with Piispanen’s model, namely

that it:

• exaggerates strain in homogeneity,

• shows tool face friction as elastic rather than plastic

in nature,

• considers shearing takes place on a completely flat

plane,

• assumes that BUE does not occur,

• takes an subjectively assumed shear angle,

• takes no account of either chip curling, or

predic-tion of chip/tool length

NB Piispanen’s model is easily understood and does

contain the major concepts in the chip-forming

process – admittedly for simple shear in the main

By way of further information concerning chip

mor-phology: the micrographs of chip surfaces illustrated

in Fig 26 show in these cases, that the morphology

indicates a semi-continuous chip form These chip

forms point towards the fact that noticeable periodic

variations have occurred, perhaps as the result of the

stress becoming unstable, rather than resulting from

any vibrational effects produced by the machine tool

Any such instability, has the effect of causing minute

oscillations (i.e backward and forward motion) in

the shear zone, while the machining takes place The

differences in segment shapes shown and their

fre-quency occurring at differing cutting data in these

micrographs, are thought to be dependent upon the

frequency of the shear plane’s oscillation relative to the

cutting speed

A considerable volume of fundamental work on

machining research has been undertaken over the

last few years, but during World War Two (i.e from

a European perspective), Ernst and Merchant (1941)

produced another significant paper dealing with the

mechanics of the machining process – some of these

research findings will be briefly dealt with in the

chap-ter on Machinability and Surface Integrity, along with

other contributions to this subject

2.3 Chip-Development

Most metallic materials can be considered as

rela-tively hard to machine and this is evident from all of

the reported literature on the subject of metal cutting,

indicating that shearing occurs in a concentrated

re-gion between the chip and tool, this effect being

de-picted schematically in Fig 26 The overall machining process is well concealed behind a amalgamation of: workpiece material, high speeds and feeds, elevated temperatures and enormous pressures The actual cutting dynamics in contemporary machining opera-tions, utilises just a few millimetres of physical contact between the tool and the chip of a precisely-shaped cutting edge geometry in an exotic mixture of tool ma-terial to efficiently machine the workpiece – this being

an impressive occurrence worthy of note

In the early work on machining, it was thought that the chip was formed by deformation along a shear plane, elastically in the first instance, then plastically

as the evolving chip passed through a stress concentra-tion The Piispanen model (i.e Fig 24) illustrates this point, where workpiece material is being cut by pro-gressive slip relative to the tool point, an angle which corresponded to that of the shear plane Here (i.e Fig 24), it shows how each chip segment forms a small, but very thin parallelogram, with slippage occurring along its shear plane

In an orthogonal cutting process, as the workpiece material approaches this ‘shear plane’ it will not

be-gin to deform until it reaches the ‘shear plane’ Here,

it is transformed from that of simple shear, as it moves across a thin shear zone, with the minute amount of secondary shear being virtually ignored, as is the case for tertiary shear – this being the equivalent of a slid-ing friction but havslid-ing a constant coefficient of fric-tion Chip deformation in reality, is produced over a zone of finite width, usually termed the ‘primary shear zone’ (see Fig 26) As the chip evolves, the back of the chip tends to be roughened, due to the plastic strain being inhomogeneous in nature (see Fig 25) This shearing action creates a particular chip morphology

as a result of the either, stress concentrations, or by presence of points of weakness in the workpiece

be- Interface pressures between the chip and the tool are

nor-mally exceedingly high, typically of the order of 1,000 to 2,000

N mm–, with temperatures in certain instances at the tool’s face reaching approximately 1100°C.

 Orthogonal machining, is when the cutting tool’s edge (i.e rake

face – see Fig 19b) is presented ‘normal’ to the evolving chip and thus, to the workpiece, at 90° to the relative cutting motion That

is, little if any, side shearing action occurs, while the chip is be-ing formed as it progresses up the tool’s rake face – effectively created by two distinct cutting forces: tangential and axial.

Figure 26 Schematic representation of a sing-point stock removal process, during the continuous cutting of ductile metals

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ing machined0 Once the chip deformation begins, it

will continue within this ‘zone’ , as though here in this

vicinity, the workpiece material is exhibiting a form of

negative strain-hardening

The oblique cutting process presents a different

and much more complex analytical problem, which has

been the subject of a lot of academic interest over the

years Even here, the whole cutting dynamics change,

when the tool’s top rake surface is not flat, which is the

normal status today, with the complex contoured

chip-breaker geometries nowadays employed (typically

il-lustrated in Figs 4, 10 and 27a)

Actual chips are normally severely work-hardened,

in particular with any strain-hardening materials (for

example: high-strength exotic alloys employed for

heat-resistance/aerospace applications) as they evolve,

by the combined action of: elevated interface

tempera-tures, great pressures and high frictional effects Such

machined action of the combined effects of mechanical

and physical work, produce a ‘compressive chip

thick-ness’ , which is on average, dimensionally wider than

the original undeformed chip thickness (see Fig 26)

The rake angle depicted in Fig 26 is shown as

posi-tive, but its geometry can tend to the neutral, right

through to the negative in its inclination As the rake

angle changes, so will the complete dynamic cutting

behaviour also change, modifying the mechanical and

0 As the shear plane passes through a particular stress

concen-tration point, it will deform more readily and at a lower stress

value, than when one of these ‘points’ is not present.

 Oblique machining, is when the rake face has a compound

an-gle, that is it is inclined in two planes relative to the workpiece,

having both a top and side rake to the face, creating a

three-force model (see Fig 19a), where the cutting three-force

mathemati-cal dynamics are extremely complex and are often produced

by either highly involved equations, or by cutting simulations

This latter simulated treatment is only briefly mentioned later

and is outside the remit of this current book However, this

information on dynamic oblique cutting behaviour can be

gleaned, from some of the more academic treatment given in

some of the selected books and papers listed at the end of this

chapter.

 Compressive chip thickness is sometimes known as the: chip

thickness ratio (r)* – being the difference between the

unde-formed chip thickness (h) and the width/chip thickness of the

chip (h)

*Chip thickness ratio (r) = h/h ** (i.e illustrated in Fig 26)

** h = W/ρwl

Where: W = weight of chip, ρ = density of (original)

work-piece material – prior to machining, w = chip width (i.e DOC),

l = length of chip specimen.

physical properties within the chip/tool region, as the various deformation zones are distinctly altered In ef-fect, due to rake angle modification (i.e changing the rake’s inclination), this can have a profound affect on the: cutting forces, frictional effects, power require-ments and machined surface texture/integrity The chips formed during machining operations can vary enormously in their size and shape (see Fig 35a) Chip formation involves workpiece material shearing, from the vicinity of the shear zone extending from the tool point across the ‘shear plane’ to the ‘free surface’

at the angle (ϕ) – see Fig 26 In this region a consider-able amount of strain occurs in a very short time in-terval, with some materials being unable to withstand this strain without fracture For example, grey cast iron being somewhat brittle, produces machined chips

that are fragmented (i.e termed ‘discontinuous’),

con-versely, more ductile workpiece materials and alloys such as steels and aluminium grades, tend to produce chips that do not fracture along the ‘shear plane’ , as

a result they are continuous A continuous chip form

may adopt many shapes, either: straight, tangled, or with different types of curvature (i.e helices – see Fig

35a) As such, continuous chips have been significantly

worked, they now have considerable mechanical strength, therefore efficiently controlling and dealing with these chips is a problem that must be overcome

(see the section on Chip-breaking Technology) Chip

formation can be classified in a number of distinct ways, these chip froms will now be briefly reviewed:

• Continuous chips – are normally the result of high cutting speeds and/or, large rake angles (see Figs

26 and 27b) The deformation of workpiece mate-rial occurs along a relatively narrow primary shear zone, with the probability that these chips may de-velop a secondary shear zone at the tool/chip inter-face, caused in the main, by frictional effects This secondary zone is likely to deepen, as the tool/chip friction increases in magnitude Deformation can also occur across a wide primary shear zone with

 One of the major cutting tool manufacturer classifies chips in

seven basic types of material-related chip formations, these

are: Continuous, long-chipping – mostly steel derivatives, La-mellar chipping – typically most stainless steels, Short-chip-ping – such as many cast irons, Varying, high-force chipShort-chip-ping – many super alloys, Soft, low-force chipping – such as alu-minium grades, High pressure/temperture chipping – typified

by hardened materials, Segmental chipping – mostly titanium and titanium-based alloys.

Figure 27 Chip-breaking inserts and chip control whilst turning – in action [Courtesy of Iscar Tools]

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curved boundaries, with the lower boundary being

below the machined surface (Fig 26), which may

distort a softer workpiece’s machined surface –

par-ticularly with small rake angles and at low speeds

Strain-hardening of this type of chip, results in it

becoming harder than the bulk hardness of the

original workpiece material (see Fig 28c, where the

bulk workpiece hardness is 230 HK and the

work-hardened chip is ≈350 HK) This increase in the

chip’s strength and hardness will depend upon the

shear strain (see Table 4 for details of the

Rheololog-ical status, related to the inclination of the tool’s

rake angle) Therefore as the rake angle decreases,

the shear strain will increase, causing this

con-tinuous chip to become both harder and stronger

– behaving in a similar manner to that of a rigid,

perfectly plastic body In order to satisfactorily deal

with long continuous work-hardened chips, that

could either wrap around the machined workpiece,

potentially spoiling the surface texture, or become

ensnarled around tooling, or even, reduce efficient

coolant delivery to the cutting edge, with integrated

tool chip-breakers having been designed and

devel-oped – see Fig 27b

 Rheology is a branch of science dealing with both the flow

and deformation of materials, with the shear strain rate, often

termed just the shear rate (i.e usually quoted in

Pascals-sec-onds ‘Pa-s’)

• Continuous chips with a built-up edge (BUE) – when machining ductile workpiece materials, a built-up edge (BUE) can form on the tool’s tip This BUE con-sists of gradually deposited material layers from the workpiece, hence the term ‘built-up’ (see Fig 28)

As cutting continues, the BUE becomes larger and more unstable, eventually partially breaking away, with some fragments being removed by the under-side of the chip, while the remainder is randomly deposited on the workpiece’s surface (Fig 28a) This process of BUE formation, shortly followed by its destruction, is continuously repeated during the whole cutting operation The BUE deposited on the workpiece will adversely affect the machined sur-face texture The BUE modifies the cutting geom-etry, creating a large cutting tip radius (Fig 28a and b) Due to the BUE being severely work-hardened

by the action of successive deposits of workpiece material, the BUE’s hardness significantly increases

by around 300% over the bulk component hardness (Fig 28c) At this severely work-hardened level, the BUE becomes in effect a modified cutting tool

Nor-mally, an unstable BUE is undesirable, conversely, a thin stable BUE is as a rule, regarded as desirable,

as it protects the top rake surface The formation

mechanism for the BUE is thought to be one of ad-hesion of workpiece material to the tool’s rake face, with the bond strength being a function of the af-finity of the workpiece to that of the tool material

This adhesion, is followed by the successive

build-up of adhered layers forming the BUE Yet another factor that contributes to the formation of a BUE,

Table 4 Strength and hardness of chips when turning mild steel.

[Source: Nakayama and Kalpakjian 1997]

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Figure 28 The development of a continuous chip with Built-Up Edge (BUE), its typical hardness distribution and its affect

on the machined surface

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is the strain-hardening tendency of the workpiece

material Therefore, the greater the

strain-harden-ing exponent, the higher will be its BUE formation

From experiments conducted on BUE formation, it

would seem that the higher the cutting speed, the

less is the tendency for BUE to form Whether this

lack of BUE formation at higher cutting data is the

consequence of increased strain rate, or the result

of higher interface temperatures is somewhat open

to debate However, it would seem that a paradox

exists, because as the speed increases, the

tempera-ture will also increase, but the BUE decreases The

propensity for BUE formation can be lessened by:

I Changing the geometry of the cutting edge –

by either increasing the tool’s rake angle, or

de-creasing the DOC, or both,

II Utilising a smaller cutting tip radius,

III Using an effective cutting fluid, or

IV Any combination of these factors.

• Discontinuous chips – consist of adjacent

work-piece chip segments that are usually either loosely

attached to each other, or totally fragment as they

are cut (Fig 29) The formation of discontinuous

chips usually occur under the following machining

conditions:

I Brittle workpiece materials – these materials

do not have the machining capability to

un-dergo the high shear strains,

II Hard particles and impurities – materials with

these in their matrix, will act as ‘stress-raisers’

and actively encourage chip breakage,

III Very high, or low cutting speeds – chip

veloc-ity at both ends of the cutting spectrum, will

result in lack of adherence/fragmentation of

the chip segments,

IV Low rake angles/large D OC’s – either small top

rakes and heavy DOC’s will decrease the

adher-ence of the adjacent chip segments,

V Ineffective cutting fluid – poor lubricity,

com-bined with a meagre wetting ability, will

en-courage discontinuity of chip segments,

VI Inadequate machine tool stiffness – creating

vibrational tendencies and cutting instability,

leading to disruption of the machining

dynam-ics and loosening of chip segments

As mentioned in ‘Roman II’ above, the hard particles

and impurities tend to act as crack nucleation sites,

therefore creating discontinuous chips Large DOC’s

increase the probability that such defects occur in the cutting zone, thereby aiding discontinuous chip for-mation While, faster cutting speeds result in higher localised temperatures, causing greater ductility in the chip, lessening the tendency for the formation of dis-continuous chips If the magnitude of the compressive stresses in the both the primary and secondary shear zones significantly increase, the applied forces aid in discontinuous chip formation, this is because of the fact that the maximum shear strain will increase, due

to the presence of an increased compressive stress

NB Due to the nature of discontinuous chip

forma-tion, if the workpiece-tool-machine loop is not

suffi-ciently stiff, this will generate vibrational and chatter tendencies, which can result in an excessive tool wear regime, or machined component surface damage

• Segmented chips – are sometimes termed: in-, or

non-homogeneous chips, or serrated chips This

chip form has the characteristic saw-toothed pro-file which is noted by zones of low and high shear strain (Fig 30) These workpiece materials possess low thermal conductivity, as such, when machined their mechanical strength will drastically decrease with higher temperatures This continuous thermal cycle of both fracture and rewelding in a very nar-row region, creates the saw-toothed profile, being particular relevant for titanium and its alloys and certain stainless steel grades For example, to ex-plain what happens in realistic machining situation, the specimen Fig 30a is displayed, for an austenitic stainless steel quick-stop micrograph This micro-graph being the result of a less than continuous ma-chining process (1), utilising a 5° top rake-angled turning insert Here, variations in the cutting pro-cess have created fluctuations in the cutting forces, resulting in waviness of the machined surface (2) Prior to the material yielding, then the shearing process occurring, the workpiece material has de-formed against the cutting edge (3) To explain how changing the top rake angle influences the resul-tant chip formation for an identical stainless steel workpiece material, Fig 30b is shown Machining has now been undertaken with a 15° top rake, pro-moting a more continuous machining process than was apparent with the 5° tool (i.e illustrated in Fig 30a) This more efficient cutting process, results in smaller variations in the cutting forces (1 and 2) The chip is seen to flow over the rake face in a more

Figure 29 Discontinuous chip formation [Courtesy of Sumitomo Electric Hardmetal Ltd.]

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consistent manner (3) It was found with this

work-piece material in an experimental cutting

proce-dure, that the tangential cutting force component,

was closer to the actual cutting edge than when

similar machining was undertaken on unalloyed

steel specimens

NB The cutting data for machining the

stain-less steel specimens in Figs 30 a and b, were:

180 m min– cutting speed, 0.3 mm rev– feedrate,

3 mm DOC

2.4 Tool Nose Radius

The insert’s nose radius has been previously mentioned

in Section 2.1.6, concerning: Cutting Tool holder/In-sert Selection Moreover, the top rake geometry of the

cutting insert will significantly affect the chip

forma-tion process, particularly when profile turning In Fig 31a, a spherically-shaped component is being ‘profile machined’ using a large nose-radiused turning insert Here, as the component nears its true geometric cur-vature, the cutting insert forces will fluctuate

continu-Figure 30 Segmented chip formation, resulting from machining stainless steel and the work-hardening zone

– which is affected by the sharpness of the insert’s edge [Courtesy of Sandvik Coromant]

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