Drilling and Associated Cutting Tool Technology Industrial Handbook_4 pot

Typically, cemented carbide heads, have an external V-shaped chip-flute which extends along the shank, the angle of this chip-flute has been experimentally-deter-mined to be 110°, provi

Figure 58 Deep-hole drilling operations, such as: (a) gundrilling, (b) double tube ejector drilling and (c) single tube ejector

drill-ing [Courtesy of Sandvik Coromant]

.

• Minimal fluid-flow disturbance – giving

consis-tent/regular flow-rate to drill-head,

• Minimum of fluid-turbulence – allowing chips to

be easily evacuated from the cutting region

Typically, cemented carbide heads, have an external

V-shaped chip-flute which extends along the shank, the

angle of this chip-flute has been

experimentally-deter-mined to be 110°, providing the following advantages:

• Optimum flute cross-section – allowing the most

rapid cutting fluid return and chip transportation,

• Facilitates an extra support pad – this is necessary

when drilling through crossing holes,

• Provides optimal torsional strength – important

for workpieces having very long length-to-diameter

ratios,

• Facilitates tool clamping – enabling the tool to be

held in a three-jaw chuck for convenient regrinding

on a suitable cutter-grinder

Gun-Drill Failure

One of the main reasons for Gun-drills to fail in

op-eration, is through an excessive misalignment of the

drill bushing and this will be in relation to the drill’s

rotational axis (i.e see Fig 58a) This type of

align-ment failure mode is termed a ‘balk-crash’ –

caus-ing the tool to fracture into numerous pieces If the

drill is rotated rather than the workpiece, the stress is

re-applied to differing portions of the tip and, at the

weakest point, namely the drill’s corner, the tip will

most likely fracture in this region A potential failure

mode is related to the Gun-drill’s length, which has its

rigidity decreased with increased length The shank

of a longer Gun-drill will not transmit a large amount

of bending force to the cutting tip – when misaligned

– however, the tip does not fracture, but instead, any

axis misalignment causes the shank to flex with each

revolution, a situation that is ideal for a fatigue

fail- ‘Balk-failure’ of Gun-drills is the result of the ‘brittle’ carbide

tip being unable to withstand the bending stresses created by

its unintentional axis misalignment.

 Gun-drill ‘rigidity rule’: as the drill’s length increases, its

ri-gidity decreases by the ‘cube’ of the distance For example, if

two identical Gun-drill diameters are employed for drilling

the same workpiece material, then if one drill is twice as long

as the other, then its rigidity will 8 times less rigid than its

counterpart (i.e namely: 2).

ure mode Yet another Gun-drill failure situation may

arise if there is excessive clearance between the drill

bush and the drill’s tip Under these circumstances, the Gun-drill’s edge cuts a significant volume of workpiece material and, as this edge is not designed to cut – hav-ing a zero clearance angle (i.e created by the circular margin at this edge) – the excessive cutting forces cause the edge to prematurely fracture

If insufficient coolant flow occurs, this is also a typ-ical factor in subsequent Gun-drill failure This lack of

coolant causes the chips to pack in the V-flute, forming

a plug, which then creates excessive torque in the Gun-drill and, this plug allows the tip to separate away from

the shank Occasionally, end-users blame the Gun-drill

tooling manufacturer for poor brazing, if the tool’s tip

separates from the shank However, when analysis of

the brazed fractured surfaces occurs, invariably, small carbide particles are adhered to the shank, this being evidence of the fact that the braze was stronger than

the tip, clearly demonstrating that the brazing was not

at fault

In many circumstances, the Gun-drill tool manu-facturer is blamed by the customer for its failure dur-ing machindur-ing, but when investigated, it is usually premature failure being the result of a poor tooling installation and operation One of the major causes of Gun-drill failure, is via the coolant distribution sys-tem, where inconsistent delivery of the fluid can either

‘starve’ the Gun-drill’s cutting edge, or ‘over-flood’ the system One of the major factors contributing to this over-/under-supply of coolant delivery, is due to the fact that in the main, coolant pressure is being moni-tored, rather than the measurement of coolant flow-rate If holes are Gun-drilled < φ4 mm, then high-pres-sure coolant flow-rate to the point is essential, but in many cases of coolant systems fitted to ‘standard’ ma-chines, they are of relatively low-pressure delivery Re-cently, one machine tool manufacturer, has designed and developed a coolant intensifier pump coupled to

a special high-pressure union, which gives variable pump pressures of over 200 bar, with special-purpose couplings to overcome the problems of poor coolant flow-rates to the cutting vicinity

3.1.8

Double-Tube Ejector/Single-Tube System Drills

Double-tube Ejector drills (i.e often just termed

‘Ejec-tor Drills’), are designed around a twin tube system

(i.e see Fig 58b – for the schematic and inset a photo

of the drill head) Here, the self-contained system (i.e

not requiring specific sealing arrangements), of the

cutting fluid, is externally pumped along the space

be-tween the inner and outer tubes The major portion of

the cutting fluid is fed forward to the drill head, while

the remainder is forced through a groove in the rear

section of the inner tube A ‘negative pressure’ occurs

in the front portion of the inner tube, which causes the

cutting fluid at the drill head to be sucked out through

the inner tube along with the chips As is the case for

Gun-drilling coolant supply, it must be of sufficient

pressure and volume, to overcome any likelihood of

‘starvation’

The ‘ejector head’ of the drill comprises of: a

con-nector, outer and inner tubes, a collet and sealing

sleeve, together with a drill head Disposable heads

with cemented carbide tips are utilised for diameters

ranging from 18.5 to 65 mm, normally supplied with

two types of cutting edge geometries, with the carbide

cutting tips precisely located on either side of the drill

head The asymmetric design of these ‘Ejector Drills’

has support pads provided, to absorb the radial

cut-ting forces and guide while supporcut-ting the tool as it

penetrates into the workpiece At the commencement

of the deep-drilling operation, the drill bushing’s main

function (i.e shown in Fig 58b), is to guide and

sup-port the drill at initial workpiece entry and until drill

penetration allows the support pads to bear on the

partially-drilled hole surface and thereupon

remain-ing in contact throughout the drillremain-ing operation

Whilst deep-hole drilling, the drill and workpiece

centrelines must not deviate by > 0.02 mm, so any

sub-sequent drill bush wear needs to be carefully

moni-tored and controlled It is usual practice to have a

ro- Asymmetric Drill Head design, refers to the fact that the

cutting inserts are not only radially, but are angularly offset

Therefore, they normally require two support pads to

counter-act and sustain the radial cutting forces generated while

dril-ling deep holes By locating the cutting inserts on both sides

of the drill head, the greater percentage of radial forces are

negated at these pads.

 Drill bushing tolerances between the drill and bush for both

the ‘Ejector’ and Single-tube Systems, require a fit of ISO G6/

h6, equating to a minimum play of 0.006 mm This drill bush

is usually manufactured from a hardened material (i.e 60 to

62 HRC) such as cemented carbide, as it has a longer service

life, with bush wear normally limited to 0.03 mm.

tating workpiece and a stationary tool, with any centre divergence resulting in bell-mouthing at the hole’s en-trance and a wavy hole surface Once the support pads

in the drill head have moved x5 their length down the drilled hole, then any further waviness is negligible, as they begin to press down on the hole’s curvature Many deep-drilled hole profile and tolerance abnormalities

result from centre divergence, which needs special

at-tention to minimise such effects

Single-tube [Ejector] System drills (i.e commonly

referred to and abbreviated as simply ‘SST’) are

sche-matically depicted in Fig 58c With this SST tooling assembly, the cutting fluid is pumped under pressure

between the drill and the hole wall (i.e normally this

width of space is approximately 1 mm) and it exits

with chips through the inside of the drill tube (Fig

58c) The quantity of cutting fluid passing through the

drill is twice as great and with higher pressure, than

for an equivalent ‘Ejector’ tooling assembly Hence, the SST set-up provides improved chip-breaking and mi-nimises any potential chip-jamming, even when vary-ing chip lengths occur

The drill head arrangement of cutting inserts will vary from two, three, or more, depending on the drill’s diameter, usually made of cemented carbide, often as brazed over-lapping tips, although disposable index-able pocketed inserts with chip-breakers are often utilised for larger diameter holes SST tools can be used

to drill small diameter holes, ranging from φ12.5 mm upward, with 100:1 depth-to-diameter ratios The SST tooling system copes with difficult-to-machine work-piece materials, such as Monel, Inconel and Hastel-loy and other ‘exotic materials’ In actual production machining trials, it has been found that SST tools can produce deep-drilled holes up to 15 times faster than

is achievable by conventional Gun-drilling This high production output level gives an 80% improvement

in machining rates for this SST Deep-drilled hole production output and, it has been shown in several instances, to give a ‘Return on Investment’ (ROI) in about 6 months

 Return on Investment (ROI), for Deep-hole drilling

operati-ons (i.e in % terms), is given (i.e in simplistic terms) by the following formula:

% ROI=Cost of a -to- productivity gain

Total conversion cost

3.1.9 Deep-Hole Drilling –

Cutting Forces and Power

In Deep-hole drilling operations, the underlying

the-ory for the calculation of cutting forces and for torque

are similar to that utilised for ‘conventional’ drilling

operations The major difference between the hole

production calculations for Deep-hole drilling to that

of ‘conventional hole-making’ techniques, lies in the

fact that support pads create a sizeable level of

fric-tional forces, that cannot be ignored These increased

frictional effect contributions – by the pads – to the

overall Deep-hole drilling cutting forces and torque

values are somewhat difficult to precisely establish,

however, an approximate formulae can be used to

esti-mate them, as follows:

Feed force (N):

Fp + Fpµ = 0.65 × kc × ap × f × sinκr

Where:

Fp = Feed force, or drilling pressure (N),

Fpµ = Force and Frictional effects (N),

kc = Specific cutting force (N mm–),

ap = Depth of cut (mm),

f = Feed per revolution (mm rev–),

sinκr = Entering angle (°)

Torque, or Moment (Nm):

Mc+ M µ = kc� ap� f � D (. − ap�D)

Where:

Mc = Torque cutting (Nm),

Mµ = Torque and Frictional effects (Nm),

kc = Specific cutting force (N mm–),

ap = Depth of cut (mm),

f = Feed per revolution (mm rev–),

Relatively high speeds are utilised for Deep-hole

Drill-ing operations, in order to achieve satisfactory

chip-breaking, this necessitates having a machine tool with

a reasonable power availability

The underpinning theory for calculating the power

requirements, corresponds with that of ‘conventional’

drilling operations However, the friction forces that are

present, due to the employment of support pads, gives

rise to a torque contribution (Mµ), which in turn

pro-duces an associated contribution ‘Pµ’ to the total

Deep-hole drilling power Therefore, in order to estimate the machine tool’s power requirement (i.e ‘P’ in kW ), an allowance must be made for any power losses in the machine tool Hence, the gross power required can be established by dividing the Deep-hole drilling power (i.e Pc + Pµ), by the machine tool’s efficiency ‘η’ This efficiency indicates what percentage of the power sup-plied by the machine tool, that can be utilised, while Deep-hole drilling

Power (kW):

(Pc+ P µ) = kc� a, p� f � vc(. − ap�D)

Where:

Pc + Pµ = Power contributions of: cutting and friction

respectively (kW),

vc = Cutting speed (m min–)

∴P = Pc + Pµ/η Where:

η = Machine tool efficiency

3.2 Boring Tool Technology –

Introduction

The technology of boring has shown some important advances in recent years, from advanced chip-break-ing control toolchip-break-ing (i.e see Fig 59, this photograph illustrates just some of the boring cutting insert

ge-ometries that can be utilised), through to the ‘active

suppression of chatter’  – more will be mentioned on the topic and reasons why chatter occurs and its sup-pression later in the text Probably the most popular type of boring tooling is of the cantilever type (Fig

59), although the popularity of either ‘twin-bore-’ , or

 ‘Chatter’ , is one of the two basic types of vibration (i.e

namely, ‘forced’ and ‘self-excited’) that may be present

dur-ing machindur-ing In the main, chatter is a form of self-excita-tion vibraself-excita-tion.‘[It is]… due to the interacself-excita-tion of the dynamics

of the chip-removal process and the structural dynamics of the machine tool The excited vibrations are usually very high in amplitude and cause damage to the machine tool, as well as lead to premature tool failure’ [After: Kalpakjian, 1984].

‘tri-bore-heads’ , with ‘micro-bore adjustment’ of the

ei-ther the individual inserts, or having a simultaneous

adjustment of all of the actual cutting inserts, is

be-coming quite common of late

Boring operations invariably utilise cantilevered

(i.e overhung) tooling, these in turn are somewhat

less rigid than tooling used for turning operations

Boring, in a similar manner to Deep-hole drilling and

Gun-drilling operations, has its rigidity decreased by

the ‘cube’ of the distance (i.e its overhang), as the

fol-lowing equation predicts:

fo= π

�

� EI

L(Mt+ .Mb)

Where:

fo = normal force acting on the ‘free end’ of the

can-tilever (i.e boring tool overhang),

*EI = flexural stiffness (i.e I = cross-sectional moment

of Inertia) (Nm),

Mt = boring bar mass (kg),

L = length of cantilever (mm),

Mb = Modulus of elasticity of the boring bar

(N mm–)

* E, relates to the boring bar’s ‘Young’s modulus’.

Boring a hole will achieve several distinct production

criteria:

• Enlargement of holes – a boring operation can

en-large either a single, or multiple series of diameters,

to be either concentric to its outside diameter (i.e

O.D.), or machined eccentric (i.e offset) to the

O.D.,

• Correction of hole abnormalities0 – the boring

process does not follow the previously produced

 ‘Eccentric machining’ of the bore of a component with respect

to its O.D., was in the past accurately achieved by

‘Button-bo-ring’ – using ‘Toolmaker’s buttons’ (i.e accurately ground and

hardened buttons of ‘known diameter’) that were precisely

off-set using gauge blocks (i.e ‘Slip-gauges’) This technique might

still be employed in some Toolrooms, but normally today, on

CNC-controlled slideways, a simple ‘CNC offset’ will achieve

the desired amount of bored eccentricity

0 Correction of hole abnormalities, as Fig 60 schematically

il-lustrates, how boring can correct for ‘helical wandering’ of the

drill as it had previously progressed through the workpiece

The drill’s helical progression would cause undesirable hole

eccentricity, resulting from minute variations in its geometry,

hole’s contour, but generates its own path and will therefore eliminate drill-induced hole errors by the subsequent machining operation (i.e see the sche-matic representation shown in Fig 60),

• Improvement of surface texture – the boring tool

can impart a high quality machined surface texture

to the enlarged bored hole

NB  In this latter case, boring operations to

previ-ously drilled, or to any cored holes in castings, can be

adjusted to give exactly the desired machined surface texture to the final hole’s dimensions, by careful

ad-justment of the tool’s feedrate and the selection of an

appropriate boring tool cutting insert geometry

3.2.1 Single-Point Boring Tooling

‘Traditional’ boring bars were manufactured as solid one-piece tools, where the cutting edge was ground

to the desired geometry by the skilled setter/operator, which meant that their useful life was to some extent restricted Later boring bar versions, utilised indexable cutting inserts, or replaceable heads (Fig 61) Boring bars having replaceable heads are versatile, with the same bar allowing different cutting head designs and cutting inserts (Fig 61a) Here, the insert is rigidly

clamped to the tool post, with replaceable ‘modular

tooling’ heads with the necessary mechanical coupling

to be utilised (i.e Fig 61b), offering ‘qualified tooling’ 

dimensions

necessitating correction by a boring operation This ‘correc-tion’ is necessary, because the drill’s centreline follows the path indicated, ‘visiting’ the four quadrant points as it spirally progresses through the part Hence, hole eccentricity along with harmonic departures from roundness can be excessive,

if the drill’s lip lengths and drill point angles are off-centre The cross-hatched circular regions represent the excess stock material to be removed by the boring bar, where it corrects these hole form errors, while machined surface texture is also considerably improved

 ‘Qualified Tooling’ , refers to setting the tool’s offsets, with all

the known dimensional data for that tool, allowing for ease of tool presetting and efficient tool-changing – more will be said

on this subject later in the text.

Figure 59 A selection of some tooling that can be employed for boring-out internal rotational features [Courtesy

of Seco Tools]

.

Figure 60 The harmonic and geometric corrections by a boring operation, to correct the previous helical drift, resulting from

the drill’s path through the workpiece

.

In the case of the boring bar’s mechanical interface

(i.e coupling) example shown in Fig 61a- top, the

ser-rated V-grooves across the interface along with the

four clamping screws provide an accurate and secure fitment for the replaceable head, with internal tension adjustment via the interior mechanism illustrated

Figure 61 Interchangeable cutting heads for boring bars utilised in machining internal features [Courtesy of

Sandvik Coromant]

.

Possibly a more adaptable modular system to the

‘ser-rated and clamped’ version, is illust‘ser-rated in Fig 61b,

where the cutting head is held in place by a single

rear-mounted bolt and grub screws around the periphery

of the clamped portion of the boring bar securely lock

the replaceable head in-situ, enabling the cutting head

to be speedily replaced Some of these boring bar’s

have a dovetail slide mechanical interface, with the

dovetail coupling providing radial adjustment of the

cutting insert’s edge This ‘universal system’ (Fig 61b),

is normally used for larger bored diameters, that would

range from 80 to 300 mm Furthermore, it is possible

to add spacers/shims to precisely control the boring

bars overall length, this is particularly important when

medium-to-long production batches are necessary, in

order to minimise cycle time and its non-productive

setting-up times

In Fig 62a and b, are illustrated single-point

inter-changeable boring insert tooling, with Fig 62a giving

typical length-to-diameter (i.e L/D) ratios for actual

boring and clamping lengths The amount of boring

bar-overhang will determine from what type of

ma-terial the boring bar will be manufactured The most

common tool shank materials are alloy steel, or

ce-mented carbide, for L/D ratios of <4:1, with the

for-mer tool material in the main, being used here For

L/D ratios of between 4: to 7:1, steel boring bars do not

have adequate static, or dynamic stiffness, so in this

case cemented carbide is preferred One limitation of

utilising cemented carbide tool shanks, is its greater

brittleness when compared to steel, so careful tool

design is necessary to minimise this problem

‘Com-pound’ boring bar tool shanks have been exploited

to reduce both problems associated with either steel,

or cemented carbide tools A successful compound

tool used in cutting trials by the author, featured a

ce-mented carbide core surrounded by alloy steel, which

proved to be quite efficient in damping performance

and machining characteristics Fig 62b, illustrates the

internal mechanism of the boring bar, for potential

‘bar-tuning/damping’ – to reduce vibrational

influ-ences whilst machining Here, the mechanism consists

of a heavy slug of metal, held at each end by rubber

grommets, in a chamber filled with silicon oil

There-fore, as the boring operation commences the slug

vi-brates at a different frequency to the steel bar, which

counteracts the vibration, rather than intensifying

vi-brational effects Such ‘damped’ boring bars, have been

utilised with large overhangs, of between 10: to 14:1

L/D ratios More information on ‘damping effects will

be mentioned in Section 3.2.4

3.2.2 Boring Bar Selection of:

Toolholders, Inserts and Cutting Parameters

Boring Bar Toolholder – Decisions

Whatever the material chosen for the boring bar, its is

always preferable to use a cylindrical shank whenever

possible, as it offers greater general cross-sectional ri-gidity, to other boring bar geometric cross-sections Once the bar cross-section has been selected, the next decision to be taken concerns the tool’s lead angle Usually the first choice for lead angle would be a 0° lead, as the radial cutting forces are minimised, with the resultant forces being directed axially along the bar, toward the tool’s clamping point – which is ideal

If, a 45° lead angle is selected, then the cutting forces are split between the axial and radial directions This latter radial cutting force, can increase the probabil-ity of increased bar deflection and be a source for un-wanted vibrational effects

NB  For more information concerning boring bar

se-lection, see Appendix 1b, for the ISO ‘code key’ for

‘solid’ boring bars.

Insert Selection – Decisions

Apart form the boring bar’s lead angle, an insert’s ge-ometry will also affect vibration during machining The two main types of insert inclination (i.e rake) an-gles are either positive, or negative – referring to their angular position in the bar’s pockets It is well known, that a positive insert shears workpiece material more readily than a negative style insert, as a result, the positive insert will generate a lower tangential cutting force This positive rake angle, is at the expense of de-creased flank clearance and, if too small, the insert’s flank will rub against the workpiece creating friction, causing potential vibrations to occur

Assuming that the insert’s edge strength will be adequate for the machining application, then when selecting an insert for boring, selection of a positive geometry with a small amount of edge preparation, having a suitable coating (i.e PVD, rather than CVD),

is a good start point Furthermore, the choice of a pe-ripherally-ground insert having a sharper cutting edge

in comparison to that of a directly-pressed and sin-tered insert, is to be recommended