Turning and Chip-breaking Technology Part 1 pdf

The rake angle is the inclination of the top face of the cutting edge and can vary according to the work- Forming can be achieved in a number of ways, ranging from complex free-form fea

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Turning and Chip-breaking

Technology

‘Machines are the produce of the mind of Man;

and their existence distinguishes the civilized man from the savage.’

WILLIAM COBBETT (1762–1835) [Letter to the Luddites of Nottingham]

2.1 Cutting Tool Technology

In the following sections a review of a range of

Turn-ing-related technologies and the importance of

chip-breaking technology will be discussed

2.1.1 Turning – Basic Operations

Turning can be broken-down into a number of basic

cutting operations and in effect, there are basically

four such operations, these are:

1 Longitudinal turning (Fig 16a),

2 Facing (Fig 16b),

3 Taper turning – not shown,

4 Profiling – not shown.

NB These turning operations will now be very

briefly reviewed

In its most simple form, turning generates cylindrical

forms using a single-point tool (Fig 1.16a) Here, a tool

is fed along the Z-axis slideway of the lathe (CNC), or

a turning centre, while the headstock rotates the

work-piece (i.e the part is held in either: a chuck, on a

man-drel, face-plate, or between centres – when overhang

is too long), machining the component and thereby

generating a circular and cylindrical form of consistent

diameter to the turned part Facing is another basic

machining operation that is undertaken (Fig 16b) and

in this case, the tool is fed across the X-axis slideway

while the part rotates, again, generating a flat face to

the part, or a sharp corner at a shoulder, alternatively it

can be cutting the partial, or finished part to length (i.e

facing-off) Taper turning can be utilised to produce

short, or long tapers having either a fast taper (i.e with

a large included angle), or slow taper (i.e having a

small included angle – often a ‘self-holding taper’ , such

as a Morse taper) There are many different operations

that can be achieved on a CNC lathe/turning centre,

 The range of turning operations is vast, feedrates can be

var-ied, as can rotational speeds

 Facing operations can also be used to produce either curved

convex, or concave surface features to the machined part –

here the surface is both generated and formed, requiring

si-multaneous programmed feeding motions to the Z- and

X-axes.

including: forming, while others such as drilling, bor-ing, screw-cuttbor-ing, of internal features, and forming and screw-cutting of external features, to name just a few of the traditional operations undertaken

With the advent of mill/turn centres, by hav-ing CNC control of the headstock and rotational, or

‘driven-/live-tooling’ to the machine’s turret, this al-lows prismatic features to be produced (i.e flats, slots, splines, keyways, etc.), as well as drilled and tapped holes across and at angles to the major axis of the work-piece, or off-axis Even this explanation of mill/turn centres is far from complete, with regard to today’s sophisticated machine tools As machine tool builders today, can offer a vast array of machine configurations, including: co-axial spindles (ie twin synchronised in-line headstocks), fitted with twin turrets with X- and Y-axes simultaneous, but separate control, having pro-grammable steadies (i.e for supporting long slender workpieces), plus part-catchers , or overhead gantries for either component load/unload capacity, to multi-axes robots feeding the machine tool This type of ma-chine tool exists and has multi-axes CNC controllers

to enable the machine’s down-time to be drastically reduced and in this manner achieving high productive output virtually continuously

2.1.2 Turning – Rake and Clearance

Angles on Single-point Tools

In order for a turning tool to effectively cut and pro-duce satisfactory chips, it must have both a rake and clearance angle to the tool point (Fig 17) Today’s sin-gle-point cutting tools and inserts are based upon de-cades of: past experience, research and development, looking into all aspects of the tool’s micro-geometry

at the cutting edge Other important aspects are an ef-ficient chip-breaking technology, in certain instances critical control of the flexure (i.e elastic behaviour) of the actual tool insert/toolholder combination for the latest multi-functional tooling is essential – more will

be said on some of these topics later in the chapter The rake angle is the inclination of the top face of the cutting edge and can vary according to the

work- Forming can be achieved in a number of ways, ranging from

complex free-form features (externally/internally) on the ma-chined part, to simply plunging a form tool to the required depth.

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Figure 16 Typical turning operations with the workpiece orientation shown in relation to the cutting insert, for either: (a)

cylin-drical turning, (b) facing [Source: Boothroyd 1975]

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piece material being machined In general, for ductile

materials, the rake inclination is a positive angle, as

the shearing characteristics of these materials tends

to be low, so a weaker wedge angle (i.e the angle

be-tween the top face and the clearance angle) will suffice

For less ductile, or brittle workpiece materials, the top

rake inclination will tend toward neutral geometry,

whereas for high-strength materials the inclination

will be negative (see Fig 17), thereby increasing the

wedge angle and creating a stronger cutting edge This

stronger cutting edge has the disadvantage of

requir-ing greater power consumption and needrequir-ing a robust

tool-workpiece set-up Machining high-strength

mate-rials requires considerable power to separate the chip

from the workpiece, with a direct relationship existing

between the power required for the cutting operation

and the cutting forces involved Cutting forces can

be calculated theoretically, or measured with a

dyna-mometer – more will be said on this subject later in

the text Both side and front clearances are provided

to the cutting edge, to ensure that it does not rub on

the workpiece surface (see Fig 17) If the tool’s

clear-ance is too large it will weaken the wedge angle of the

tool, whereas if too small, it will tend to rub on the

machined surface Most tools, or inserts have a nose

radius incorporated between the major and minor

cut-ting edges to create strength here, while reducing the

height of machined cusps, with some inserts having a

‘wiper’ designed-in to improve the machined surface

finish still further – more will be mentioned on these

insert integrated features later

2.1.3 Cutting Insert Edge Preparations

Often, a minute edge preparation (see Figs 17 and

18b, c and d) is created onto the sharp cutting edge of

the insert, this imparts additional strength to the

out-ermost corners of the cutting edge, where the rake and

clearance faces coincide There are four basic manners

in which the honed edge preparation is fashioned,

these are:

 Machined cusps result from a combination of the feedrate and

the nosed radius of the tool If a large feedrate occurs with a

small nose radius then the resultant cusp height will be high

and well-defined, conversely, if a small feedrate is utilised in

conjunction with a large nose radius, then cusp height is

mini-mised, hence the surface texture is improved.

1 Chamfer – which simply breaks the corner – not

illustrated,

2 Land – stretching back negatively from the

clear-ance side to various lengths on the rake face (see Fig 18b),

3 Radius – around the actual corner (see Fig 18c) ,

4 Parabolic – has unequal levels of honing on two

faces (see Fig 18d)

Even here, more often than not, certain combinations

of these four edge preparations are utilised, so that the cutting forces are redirected onto the body of the rake’s face, rather than directed down against the more frag-ile cross-section of the edge The T-lands and hones are often actually incorporated into the insert geom-etry of the contoured surface Typical T-lands range in size from 0.07 to 0.50 mm, having angles varying from

5 to 25° off of the rake face (Fig 18b)

Honing which is the ‘rounding’ of the cutting edge, can be performed in one of several ways Probably the oldest technique for honing, utilises mechanical means, which employs a vibrating tub filled with an abrasive media, such as aluminium oxide – to ‘break’ the corner on these inserts A variation in this de-sign, uses an identical abrasive, except here the inserts are held by centrifugal force to the inside of a rotat-ing tank While yet another method of honrotat-ing usrotat-ing

an abrasive media, involves spraying the inserts with fine abrasive particles – to hone the edges of the in-serts Probably the most popular method for obtaining cutting insert honed edges, uses brushes made from extruded nylon impregnated with diamond (see Fig 18a) The inserts to be honed pass by these brushes in individual carriers and rotate as they all revolve under the brushes, thereby applying equal hones to all insert edges Depending upon the amount of desired honing, these brushes can be either raised, or lowered, or alter-natively, the inserts can make multiple passes through the machine All of the above honing techniques pro-duce a hone that is roughly equal on both the flank and rake faces – what is termed a ‘round hone’ (Fig 18c) Yet another honing profile termed the parabolic hone (i.e sometimes this honed edge is known as:

 The radius is sometimes termed ‘edge rounding’ (i.e denoted

by the letters ‘ER’) – often applied to most edge preparations,

enabling the cutting forces to be directed on to the stronger part of the insert.

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Figure 17 Typical turning ‘finishing’ insert/toolholder geometry and the insert’s edge chamfering, in relation to the workpiece

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P-hone, oval, or waterfall), is produced by a machine

with a soft, diamond-charged rotating rubber wheel

Therefore, as the abrasive material rubs across the

in-serts, it tends to extend slightly over the inserts sides,

producing a hone of uneven proportions between the

two insert faces (Fig 18d) As in the case of the T-land

cutting insert edge preparation, the P-hone directs the cutting forces into the body of the insert

Honing can be specified in a number of sizes, usu-ally being determined by the amount of time these insert spend in the honing device The original Stan-dard for honing was established in the United States by

Figure 18 A honing machine (i.e brush-style) and several types of honing edge preparations [Courtesy of Ingersoll]

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the American National Standards Institute (ANSI) in

1981, which included dimensions and expected

toler-ances for these three basic hones Today, many cutting

tool manufacturers have expanded upon this

Stan-dard, or adopted their own – specifying hone

manu-facturing and identification methods Hones must be

applied prior to the application of coatings Inserts

that are destined to receive a CVD coating, must have

a minimum hone to strengthen the edge, in order to

counteract the effects of this high temperature coating

process Conversely, PVD coatings, can be equally

ap-plied either over fully-honed insert edges, or on an

un-honed cutting edge In recent years, the cutting tool

manufacturers have an emphasis toward providing

honed edges of greater consistency and repeatability

2.1.4 Tool Forces – Orthogonal

and Oblique

The cutting forces are largely the result of chip

separa-tion, its removal and chip-breaking actions, with the

immense pressure and friction in this process

produc-ing forces actproduc-ing in various directions Stresses at the

rake face tend to be mainly compressive in nature,

al-though some shear stress will be present (see Table 2,

by way of illustration of the machining shear stresses

for various materials), this is due to the fact that the

rake is rarely ‘normal’ to the main cutting direction

This compressive stress tends to be at its greatest

clos-est to the cutting edge, with the area of contact between

the chip and rake face being directly related to the

ge-ometry here, hence the need for tooling manufacturers

to optimise the geometry in this region

There are two distinct types of forces present in

machining operations concerning single-point cutting

tools/inserts (see Fig 19), these are:

1 Orthogonal cutting forces – two forces (ie

tangen-tial and axial – see Fig 19b),

2 Oblique cutting forces – three forces (i.e

tangen-tial, axial and radial – see Fig 19a)

 As well as the tool/chip interface temperatures being up to

1,000°C, the interface pressures can reach a maximum of

3,000 MPa, these being sterile smooth surfaces makes them

‘ideal’ conditions for the occurrence of

‘pressure-welding’/sei-zure.

NB Both of these cutting force models are

heav-ily influenced by the: cutting tool/insert orientation

to workpiece, tool’s direction of cut and its applied feedrate

Oblique Cutting Forces

Fig.1.19a, can be seen a model of the three-dimen-sional cutting force components in an oblique turn-ing operation, when the principal cuttturn-ing edge is at an

angle to the main workpiece axis (i.e Z-axis) These

component forces can be separated into the:

• Tangential force (F T) – which is greatly influenced

by the contact and friction between both the work-piece and tool, as well as the contact conditions between the chip and the rake face of the cutting edge The magnitude of the tangential cutting force

is the greatest of these three component forces and

contributes to the torque, which in turn, influences

 Feedrates play a major role in determining the axial force in

single-point cutting operations, in association with the tool’s orientation to the part being machined.

Table 2 Typical in-cut shear strengths of various materials

Material: Shear yield strength in cutting

(N mm –2 )

0.13% C steel 480 Ni-Cr-V steel 690 Austenitic stainless steel 630

Copper (annealed) 250 Copper (cold-worked) 270 Cartridge brass (70/30) 370 Aluminium (99.9% pure) 97

[Source: Trent ( 1984)]

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Figure 19 The two- and three-force models of orthogonal and oblique cutting actions,

with the component forces approximately scaled to give an indication of their respective magnitudes

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40 Chapter 2

the power requirement for cutting Fundamentally,

the product of the tangential force and the cutting

speed represent the power required for machining

The specific cutting force  is a unit expression for

the tangential cutting force, being closely related to

the material’s undeformed chip thickness and

se-lected feedrate,

• Axial force (F A) – the magnitude of this force will

vary depending on the selected feedrate and the

chosen tool geometry and in particular, the ‘plan

approach angle’ , or ‘entering angle’ , – more will

be said on this topic later Its direction is from the

feeding of the tool, along the direction of workpiece

machining,

• Radial force (F R) – is directed at right angles to the

tangential force from the cutting point The ‘plan

approach angle’ and the size of the nose radius, will

influence this force

NB These three component forces are significantly

influenced by the rake angle, with positive rakes

producing in general, lower cutting forces The

re-sultant force, its magnitude and angle, will be

af-fected by all three component forces, in

conjunc-tion with the tool’s geometry and the workpiece

material to be cut

Orthogonal Cutting Forces

In Fig 19b the two-dimensional model for orthogonal

cutting is depicted, once again, for comparison to the

oblique cutting model, in a single-point turning

op-eration For simplicity, if one assumes that the point of

the tool is infinitely sharp and that the tool is at right

angles to the workpiece axis having no deflection

pres-ent, then the two component forces are the tangential

force and axial force (i.e previously mentioned above)

In this case, this tool geometry-workpiece

configura-tion, allows long slender bars to be turned, as there is

less likelihood of tool ‘push-off’ (i.e as the radial force

 In reality, the specific cutting force is a better indication of

the power requirement, as it is the force needed to actually

deform the material prior to any chip formation It will vary

and is influenced by the: undeformed chip thickness, feedrate,

and yield strength of the workpiece material For example, if

the cutting conditions are kept the same and only the material

changed, then if a nickel-based alloy is machined, the initial

chip forming force (i.e specific cutting force) will be more

than ten times greater than when cutting a pure aluminium

workpiece

has been neutralised – as indicated by the fact that the

resultant force shows no X-axis offset) If any radial

force was present, this would create either a ‘candle-stick effect’ , or ‘barrelling’ to the overall turned length

In reality, there will always be some form of nose

ra-dius, or chamfer to the tool point, which will have some degree of ‘push-off’ , depending upon the size of this incorporated nose feature – creating a ‘certain de-gree’ of radial component force affect

2.1.5 Plan Approach Angles

The manner in which the cutting edge contacts the

workpiece is termed the ‘plan approach angle’ (Fig 20a), being composed of the entering and lead angles for the selected tool geometry In effect for single-point turning operations, the tool’s orientation of its plan approach, is the angle between the cutting edge and feeding direction When selecting a tool geometry for turning specific workpiece feature – such as a 90° shoulder – it is important as it will not only affect the machined part geometry, but has an influence on con-sequent chip formation and the direction and magni-tude of the component cutting forces, together with the length of engagement of the cutting edge (see Fig 20b) In single-point turning (Fig 20b), the depth of cut (DOC), or ‘cutting depth’ is the difference between

an un-cut and cut surface, this being half the differ-ence in the un-cut and cut diameter (i.e the diame-ter is reduced by twice the DOC in one pass along the workpiece) This DOC is always measured at 90° to the tool’s feed direction, not the cutting edge The manner

in which the cutting edge approaches the workpiece is termed the ‘entering angle’ (i.e plan approach angle), this being the angle between the cutting edge and feed direction (Fig 20a – shown here in a cylindrical turn-ing operation) Moreover, the plan approach angle not only influences the workpiece features that can be produced with this cutting geometry, it also affects the formation of chips and the magnitude of the compo-nent forces (Fig 20b)

The ‘entering angle’ affects the length of the cut-ting edge engaged in-cut, normally varying from 45°

to 90°, as illustrated in the four cases of differing plan approach angles shown in Fig 20b Here, in ‘case I’ an

 In single-point turning operations, the depth of cut (D OC) is

sometimes referred to by the term:

‘undeformed chip thick-ness’.

Figure 20 Insert approach angle geometry for turning operations

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