Drilling and Associated Cutting Tool Technology Industrial Handbook_5 potx

The insert’s substrate – if cemented carbide – re-quires some thought, as if it is too hard, this type of insert may chip via the effects of machining vibrations, this is particularly so

The insert’s substrate – if cemented carbide –

re-quires some thought, as if it is too hard, this type of

insert may chip via the effects of machining vibrations,

this is particularly so, if the tool geometry has an

ex-tra-positive and sharp insert cutting edge It might be

more prudent to initially choose a medium-hard

ce-mented carbide grade, as it tends to cope with a

poten-tial edge-chipping condition more readily, then, if this proves successful, a harder grade may be selected

Cutting Parameters – Decisions

Two complementary cutting parameters are the insert’s nose radius and the influence it has on the DOC For

Figure 62 Interchangeable cutting heads for machining internal features [Courtesy of Sandvik Coromant]

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example, when a finish boring operation is required,

then it is recommended that both a small nose radius

and DOC is used This smaller boring insert nose

ra-dius, minimises contact between the workpiece and

insert, resulting in lower tangential and radial cutting

forces For fine-boring applications, a good start point

is to choose an insert with a 0.4 mm nose radius, with

a 0.5 mm DOC It should be noted that the DOC ought to

be larger than the nose radius, this is because if it was

the other way around, cutting forces would be directed

in a radial direction – increasing potential vibrational/

bar-bending (i.e push-off ) problems

Feedrates should be identical regardless of tool’s

overhang, as any feed selection is normally based upon

the insert’s chip-breaking capabilities Avoidance of

very high feedrates when rough boring is necessary, as

it can significantly increase the tangential cutting force

component For finish boring operations, it is normally

the workpiece’s surface texture requirement that

dic-tates the maximum feedrate that can be utilised More

will be mentioned on the machined cusp height’s effect

on surface texture, this being created by the remnants

of the partial nose arc (i.e radius) of the cutting insert

and the periodic nature of the selected feedrate on the

bored workpiece’s surface, later on in the relevant

sec-tion in the book

A mistake often made by setters/machinists in

order to attempt to minimise vibrational tendencies, is

to reduce the rpm This strategy will not only decrease

productivity, but the lower rotational speed can lead

to BUE formation, which in turn, modifies the insert’s

cutting geometry and could change the cutting force

directions Instead of rpm reductions, modification

of other cutting data variables is suggested, in order

to improve these adverse vibrational/chatter effects

Sometimes even increasing the rotational speed, can

eliminate unwanted chatter

Although it is not a specific cutting performance

parameter, an often disregarded measure is that of

boring bar tool clamping In many circumstances,

cy-lindrical boring bars are simply clamped with several

setscrews, this is a poor choice of clamping method, as

at best, setscrews only contact about 10% of the boring

bar Conversely, a split-tool block, clamps along almost

 ‘Tool push-off’ – often termed ‘spring-cuts’ , are the result of

tool deflection, particularly when light cuts are used To

mini-mise the ‘push-off’ , very rigid workpiece-machine-tool setup

with a smaller nose radius to that of the D is recommended.

all of the boring bar’s periphery in the toolpost, allow-ing much greater tool rigidity and cuttallow-ing stability, al-leviating many of the potential problematic in-service machining conditions

3.2.3 Multiple-Boring Tools

Twin cutting insert tooling, usually consists of a cy-lindrical shank with slides mounted at the front (Fig 63a), or a U-shaped bar with cartridges (Fig 63b) The slides and cartridges can be radially adjusted, allow-ing for a range of various bored diameters to be ma-chined Normally, such tooling has a 7 mm maximum cutting depth recommended – for both edges simul-taneously in-cut With Twin-edged boring tools the

cartridges can be so arranged, that ‘Step-boring’  can

be utilised

When large diameter component features require

a boring operation, then the ‘Divided-version boring’

tooling can be exploited, but diametral accuracy is not

as good as for some of the other types of boring tool designs An advantage of the Divided-version’ boring tools, is the fact that a large diameter range can be

cov-ered, with this single tool If a ‘Universal fine-boring’

tool is utilised (Fig 63b), either internal (Fig 63b-top),

or external machining (Fig 63b – bottom), can be un-dertaken In this case, the fine-bore cartridges (1) are mounted on a radially-moveable slide (2), which is mounted on a bar (3) In the latter case of external com-ponent feature boring, there is a physical limit to the minimum diameter that can be machined – this being controlled by the bar’s actual size (i.e Here, it should be said that this particular tooling ‘setup’ can be thought

of as virtually a Trepanning operation with a boring tool) Moreover, with this external finishing operation,

the spindle must rotate in a left-hand rotation

Tri-bore tooling often having individual micro-bore

cartridge adjustment (i.e not shown), as its name im-plies, uses three cutting inserts equally-spaced at 120° apart This boring tool arrangement of cutting inserts, offers very high quality bored diametral accuracy and

 ‘Step-boring’ , refers to using special shims with one of the

cutting inserts axially situated a little way in front of the other, while at the same time, the cartridges are radially adjusted en-abling the front insert to cut a slightly smaller diameter to that

of the rear one It should be noted that when ‘Step-boring’ , the maximum DOC is normally 14 mm, with an associated feedrate

of 0.2 mm rev–.

Figure 63 Twin-edged boring tooling [Courtesy of Sandvik Coromant]

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precision to the machined hole, but such tooling can

be somewhat more costly than when utilising a

single-insert tool

3.2.4 Boring Bar Damping

For boring bars that have an L/D ratio of <5:1, then

relatively stable cutting conditions with controllable

vibrational influences can be tolerated However, if L/D

ratios utilised are larger than this limiting value, then

potentially disastrous vibrational tendencies could

oc-cur, leading to a variety of unwanted machining and

workpiece characteristics, these include:

• Limited tool life – caused by forced and self-excited

vibrations, restricting both cutting efficiency and

tool life,

• Unacceptable machined surface texture –

vibra-tions in the form of workpiece surface chatter, can

be the cause for component rejection,

• Substandard machined roundness – vibration/

chatter effects creating high-frequency harmonic

effects on the roundness profile

Stiffness can be expressed in terms of either static, or

dynamic stiffness Static stiffness of a bar is its ability to

resist a bending force in a static condition, conversely,

dynamic stiffness is the bar’s ability to withstand

os-cillating forces (i.e vibrations) Dynamic stiffness is an

essential property for a boring bar, as it is a measure of

its capacity to dampen the vibrations occurring during

machining, being greatly dependent of its overhang As

one would expect in testing for dynamic stiffness, with

 ‘Harmonics’ – on a machined component are the product

of complex interactions, including method of manufacture:

component geometry, cutting data utilised, any vibrational

influences encountered and material composition and its

manufacture (e.g Powder Metallurgy parts can vary in both

porosity and density throughout the part, which may affect, or

locally destabilised the cutting edge).

NB Harmonics on the machined workpiece, can be thought

of as a uniform waveform (i.e sinewave) that is

superim-posed onto the part’s surface The part’s low frequency

harmo-nicoften has higher frequency harmonics superimposed onto

the roundness For example, a 15 undulation per revolution

(upr) harmonic, could have a 500 upr harmonic superimposed

onto it, requiring suitable a Roundness Testing Machine with

Gaussian filters to separate out the respective harmonic

con-ditions – for metrological inspection and further analysis.

a boring bar’s overhang increasing under standardised machining conditions, the amplitude will also increase However, if the boring bar was dampened in some way, perhaps by utilising a ‘shock-absorber effect’ , ma-chining could be undertaken at longer overhangs This

‘damping effect’ is indicated by the highly centralised amplitude of oscillatory movements quickly reducing with time, indicating a high level of dynamic stiffness, this being crucial for long L/D ratios Obviously, the boring bar’s cutting edge deflection at its tool tip, is directly related to the amount of bar overhang, this de-flection being the result of the applied cutting forces The magnitude of a boring bar’s deflection being de-pendent upon: bar composition, diameter, overhang and the extent and magnitude of tangential and radial cutting forces The rigidly clamped and cantilevered boring bar’s ‘free-end’ will deflect/deform by forces acting upon it and, some idea of the magnitude of this deflection can be gleaned by the simple application of

‘mechanics of materials’ , using the following formula:

Where:

∆ = Boring bar deflection (mm),

F = Cutting force (N),

L = Boring bar overhang (mm),

E = Bar material’s coefficient of elasticity (N mm–),

*I = Moment of Inertia (mm)

* For a boring bar of circular cross-section, the Mo-ment of inertia will be:

For example, assuming that if a φ25 mm steel boring bar has an L/D overhang of 4:1, with an applied cut-ting force of 100 kP, then the magnitude of bar deflec-tion, using the above formula, would be:

∆L = D = 0.083 mm

If the overhang of this boring bar was now increased

to L/D ratios of 7:1 and 10:1, respectively, this would produce tool tip deflections of:

∆L = D = 0.444 mm

∆L = 0D = 1.293 mm

Hence, these deflection values emphasise the impor-tance of reducing overhang as it increases by approxi-mately ‘cube’ of the distance Moreover, deflection can

also be reduced by utilising a different boring bar

ma-terial, as this will improve its coefficient of elasticity

In boring-out roughing operations, any vibrations

present are only a problem if they lead to insert

dam-age For finish-boring operations, vibrational

condi-tions that may occur could be the difference between

success and failure for the finished machined part

So, the boring bar’s ability to dampen any vibrational

source becomes imperative, once a fine-boring

opera-tion is necessary Vibraopera-tions can occur in any number

of ways that could affect the boring operation, from

the constructional elements of the machine tool,

through to slideways, or their recirculating ball

bear-ings, etc Hence, the joints in a machine tool can be

regarded as a complicated dynamic system, with any

slideway motion of vibrating contact faces,

necessitat-ing lubricatnecessitat-ing oil to not only reduce any stiction and

frictional effects, but to help dampen these structural

elements Machine tool builders are acutely aware that

certain machine tool materials ‘damp’ more readily

than others Cast iron and in particular ‘Granitan’ (i.e

a product of crushed granite and epoxy resin), can

pre-dominantly act as built-in dampening media for any

vibrational sources present The main source for any

vibrations in boring, results from the long overhangs,

necessary to machine the hole depth of the

compo-nent’s feature Therefore, the magnitude of vibrations

in the overall system result from the dampening

capa-bilities of the actual boring bar

Tuned Boring Bars

A boring bar that has been ‘tuned’ , has the ability to

dampen any generated vibrations between the

work-piece and the cutting edge while machining The

‘dampening effect’ is achieved through a vibration

ab-sorbing device (i.e see Figs 61a and 62b), this has the

consequence of increasing the bar’s dynamic stiffness,

giving it the ability to withstand oscillating forces The

 Coefficient of elasticity, for a steel boring bar composition,

E = 21 × 10 (N mm–), conversely, using a cemented carbide

material for an identical boring bar, E = 63 × 10 (N mm–),

giv-ing three times greater stiffness, allowgiv-ing much greater borgiv-ing

bar overhangs.

NB In reality, the boring bar’s deflection will be higher than

the values given in these examples, as the formula is based

upon the assumption that the bar is absolutely rigidly clamped,

which is impossible to achieve.

method of achieving this bar damping has already been mentioned in Section 3.2.1, with the relationships be-tween the size of the bar’s body, suspension, viscosity

of the liquid media, being carefully designed by the tooling manufacturer During the boring operation, the vibrations set the body in oscillation Hence, the body and the liquid alternate, taking each others place

in the space within the actual boring bar A pattern is established during boring, where the oscillations of the body are not in harmony with the vibrations resulting from machining This out-of-harmony, means that the

vibrations are virtually neutralised – to an acceptable

level – via the kinetic energy being transformed by the

‘system damping’ Any vibrations present during bor-ing, are relative to the amount of bar overhang, there-fore on longer boring bar lengths, they are normally fitted with some means of adjustment, so that they can

be ‘tuned’ to the frequency occurring within its range The simplest manner of achieving adjustment, is by

a rotation of a lockable set screw, which when either tightened, or slackened, affects the suspension of the body in the liquid, thus ‘tuning’ the boring bar to the actual machining conditions present

3.2.5 ‘Active-suppression’

of Vibrations

As has been stated at the beginning of Section 3.2.4, if boring bars have an L/D ratio >5:1, then vibrational ef-fects may result in tool chatter It has been observed in experimental work, that the boring bar’s tip produces

a vibration motion that follows an elliptical path in the plane normal to the longitudinal axis of the bar The ratio of the amplitude of vibration along the major and minor axes varies with cutting conditions, further-more, the inclination of these axes to the ‘radial line’

of the tool also varies Of significance, is the fact that the build-up of chatter will begin almost immediately, even before one revolution of the workpiece has oc-curred This build-up continues almost evenly until some limiting amplitude occurs, which suggests that the well-known ‘Orthogonal mode coupling’ is pres-ent, further, with the phase difference between the vi-brations causing an elliptical tool tip path, the vibra-tional energy is fed into the tool-workpiece system, promoting self-excitation

As has been suggested, the dynamic stability of the boring bar is of prime importance, with the onset of self-excited chatter, being governed by the ‘Multiple regenerative effect’ , which is a function of the so-called

‘space phase’ This ‘space phase’ condition, is the phase

of vibration around respective turns of work,

fluctu-ating between 90° and 180° and is equal to the phase

between the inner and outer modulation Moreover,

it has been shown that by modifying the workpiece’s

rotational speed, this disturbs the ‘space phase’ and,

consequently influences the ‘time phase’ , leading to

a reduction in self-excited chatter It has been

practi-cally demonstrated that by modifying the peripheral

speed of the workpiece, this technique is only partially

successful in alleviating chatter More success can be

made by utilising damped boring bars, such as the

‘Lanchester’ type, with dynamic vibration absorbers

(DVA’s), to really suppress vibrational influences

dur-ing the bordur-ing process

Some progress has been made on the development

of DVA techniques, but the potential ‘step-change’

will occur in vibrational suppression for boring bars,

when the improvement of production versions of

‘ac-tive’ dampers for such tooling becomes a reality Just

such a potential ‘active’ boring bar is shown

schemati-cally in Fig 64 Invariably, the boring bar has a supply

of energy to it – via an external source, that controls

the cutting edge’s position by monitoring the feedback

of the relative displacement of tool’s edge with respect

to the workpiece In later research work by Matsubara

et al (1987), chatter suppression was analysed for the

boring bar using ‘feed-forward’ control of the cutting

force Further, the cutting edge was positioned in

re-sponse to this force, with these type of ‘active’ control

systems being known as: ‘Cutting edge positional

con-trol systems’

Typical of a vibrational control approach is

illus-trated by the ‘active’ boring bar already mentioned and

depicted in Fig 64, where the forces are damped in

re-sponse to the vibrational velocity of the cutting edge,

which has been termed a: ‘Vibrational velocity control

system’ In this damping technique, the boring bar

sup-pression is by a series of piezo-electric elements that

act as ‘active dampers’ Such a ‘damper’ responds to

onset of chatter vibration (i.e the high-energy

com-ponents) Moreover, the damping force achieves

opti-mal phase difference, since the phases between both

 ‘Lanchester boring bars’ , normally utilise an internal metal

slug which is usually surrounded by some form of: liquid/fluid

medium, DVA’s, or more primitively, sprung-loaded and as

such, the slug is free to move out-of-phase with the cutting

conditions, dictated by the boring bar’s applied cutting forces,

thereby the onset of chatter will be potentially ‘cancelled out’.

the ‘damping’ and vibrational forces are controllable This type of ‘active’ boring bar arrangement, achieves directional damping characteristics via its ‘dampers’ ,

here they control two ‘degrees of freedom’  via the ‘Re-generative feedback loop’ , which diminishes oscillatory

motion (i.e harmonics), by careful control of energy losses

In recent years with the advent of artificial intelli-gence (AI) applications to major industrial engineering problems, and more specifically, in the performance

and robustness of certain types of ‘Neural networks’ ,

the goal of obtaining some form of real-time monitor-ing and control in the machinmonitor-ing process is now closer

to reality These AI systems have been successfully utilised for applied research applications to tool wear monitoring in turning tool operations – after suitable

‘training’ of a pre-selected neural network architecture These ‘networks’ could be successfully applied to bor-ing bar vibrational monitorbor-ing and control situations More detailed information will be said on how, where and when Neural network decision-making and, why these cutting tool monitoring applications should be utilised in the production environment, later in the text

3.2.6 Hard-part Machining,

Using Boring Bars

Although ‘hard-part’ turning has been utilised for some considerable time, with the advent of polycrys-talline cubic boron nitride (PCBN) tooling, etc., it has seen little in the way of exploitation for boring opera-tions, to date One of the major reasons for this lack

of tooling application, is because most hardened parts are in the region of hardness values ranging from 42

to 66 HRC Such high component hardness, requires considerable shearing capability by the tooling to suc-cessfully machine the excess stock from the workpiece Generally, the robust nature of toolholding for turning

 ‘Degrees of Freedom’ , the ‘free-body kinematics’ , exhibit 6

de-grees of translatory (i.e linear) motions in space, these are: back-ward/forward, upward/downward and leftward/rightward.

NB Of some interest but in the main, to machine tool

build-ers for the purposes of volumetric calibration, are the rotary motions of: yaw, pitch and roll, giving 18 degrees of freedom, together with the 3 squareness errors, totalling 21 possible de-grees of freedom.

Figure 64 An ‘active’ boring bar and their capacity to suppress vibrational effects on boring holes [After

Mat-subara; Yamamoto and Mizumoto; 1987]

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tools with their modest overhangs, does not present

in-surmountable difficulties during machining, however

for the much longer overhangs associated with boring

operations (i.e see Figs 62a and 65a), then the cutting

forces generally dictate, short L/D ratios of <5:1 and

relatively large and robust boring bars (Fig 65b)

There are considerable difficulties to be

over-come when any form of hard-part machining is

required – particularly for boring operations, when

the components have been either case- or

through-hardened, these are:

• High temperatures in the cutting zone –

necessitat-ing high temperature resistant and

thermally-sta-bility of cutting insert materials,

• Cutting force magnitudes are both higher and more

variable – robust cutting edge geometry is

neces-sary to withstand these increased shearing/cutting

force demands on the insert,

• Small chip cross sections – these exert high

pres-sure near the insert’s cutting edge, often necessitat-ing an edge preparation to the insert’s corner,

• Greater tool wear rates – often more rapid cutting

edge wear, or the tendency to catastrophic break-down of the insert,

• Workpiece stresses during cutting – these stresses

are released during machining and may present localised geometric variations to the final shape of the part,

• Poor homogeneity in the workpiece material

– hardness variations across and through the part (e.g differential case hardened depths), can lead to significant and variable cutting force loadings on the boring insert,

• Insufficient stability – if the

‘machine-tool-work-piece loop’ is not sufficiently robust, then due to the greater cutting forces when hard-part machining,

Figure 65 Boring bar operational limitations and hard part boring at relatively high speed

[Cour-tesy of Sandvik Coromant]

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this creates potential tool deflection which could

become a major problem

Boring Bar Deflection

When any boring operations take place, even with a

very rigid tool mounting and a small boring bar

over-hang, some vibration and tool tip deflection will

in-evitably occur, this is exacerbated by machining

hard-parts The former problem of vibration has previously

been mentioned and methods of minimising it are

possible However, tool deflections are more difficult,

if not impossible to completely eliminate, with these

longer cantilevered tools Of note regarding

overhang-ing tool deflections, are that a tool tip deflects in two

directions (i.e see Fig 66a), these are:

• Radial deflection (∆T) – affects the machined (i.e

bored) diameter,

• Tangential deflection (∆R) – causes the tip to move

downward for the centreline

In each of these tool tip deflections, both the size and

direction of the cutting forces are influenced by the

chip thickness and insert geometry selected (i.e

illus-trated in Fig 66b) The radial deflection will be equal

to the difference between the diameter which was

orig-inally set and the actual bored diameter, this can be

easily found by the simple expedient of measuring it,

then adjustment can be made for this apparent

deflec-tion The tangential deflection of the boring bar’s tip

can be established by either ‘direct’ , or ‘indirect’

met-rological techniques at the tool’s tip In Fig 66a, the

graph depicts deflections ‘∆’ (i.e both the tangential

‘∆T’ and radial deflection ‘∆R’), as a function of the cutting depth ‘aP’ Due to the fact that the tangential deflection (∆T) linearly increases with increasing DOC

(aP), it is usually recommended that machining passes are divided into a number of cuts when close toler-ances are needed (i.e in the region of IT7) – see Table

549 for an abridged version of the IT tolerances, with

*Rmax values in µm

The magnitude of radial deflection as a function

of the cutting depth, is also influenced by the ratio between the insert’s nose radius and the DOC (aP), to-gether with the boring insert’s entering angle In some

cases, a boring bar is situated slightly above the

work-piece centreline, so that when it enters the cut at full depth it will have tangentially-deflected to the actual

 ‘IT’ (i.e in units of µm) – represents the average value of the

basic tolerance for the ‘diameter range’ in question Hence, it will vary according to the choice of diameter range selected.

 These values are related to surface texture expression of:

*Rmax (µm), which is: The maximum individual peak-to-val-ley height The Rmax values (i.e in Table 5) can be calculated

from the IT value, using the following equation, rather than

the conventional equation: Rmax = (fn/rε ) 125

this equation tends to give excessively high surface texture va-lues, thus more practical values related to IT are to be found

from: 

  �Rmax= �n � IT IT (µm)

Where: n = The number of IT’s.

Table 5: IT values related to the basic tolerance for various diameter ranges

-/3 Over/up to 3/10 Over/up to 10/50 Over/up to 50/180 Over/up to 180/400 Over/up to 400/800

[Source: Sandvik Coromant (1995)]

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