Drilling and Associated Technologies Part 2 ppt

The thrust force is just one of the cutting resistances in a drilling operation, contributions to drill resistance are from the: • Lips – equal lip lengths and angles are important for

Figure 48 Symmetrical twist drill cross-sectional profiles [After: Spur and Masuha, 1981]

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workpiece (see Fig 49) The thrust force (Fig 49) is

the result of the selected penetration rate (i.e feed), in

combination with the bulk hardness of the workpiece

and its work-hardening ability and the efficiency of the

coolant supply – if any – to the cutting edges (i.e lips)

The resolution of the cutting resistance into their

vari-ous components when twist drilling, is shown in Fig

49 at a mid-point along the lips The thrust force is just

one of the cutting resistances in a drilling operation, contributions to drill resistance are from the:

• Lips – equal lip lengths and angles are important for a ‘balanced cutting action’ , this being consid-ered an efficient cutting process,

• Chisel edge – is highly negatively skewed and as it acts like a ‘blunt wedge-shaped indentor’ , extrud-ing the workpiece material from this vicinity,

Figure 49 The balanced cutting forces resulting from drilling holes utilising twist drill geometries [After: Kaczmarek, 1976]

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• Land, or margin – via a rubbing, or frictional

ac-tion

NB  The latter two are relatively inefficient processes,

moreover, the resistance components of the lips and

chisel edge are the product of resistance of the

unde-formed chip to plastic strain, in combination with

re-sistance due to external friction The land rere-sistance

occurs from the friction (i.e rubbing) against the side

of the drill’s hole

When symmetrical twist drilling (illustrated in Fig

50), the undeformed chip can be characterised by its:

• Cutting depth (a) – where ‘a’ = d/2, with ‘d’ being

the drill’s diameter (mm),

• Feedrate (s) – this being the distance the cutting

edge moves in the drilling axis direction during

one revolution Normally two rigidly joined

cut-ting edges are cutcut-ting at any instant, each one in its

travel corresponds to feed ‘sz’ , which removes an

undeformed chip whose size – in the direction of

the drilling axis is: ‘s’ and ‘s’ respectively (i.e see

Fig 50a) and, as most drills are symmetrical in

de-sign, then:

• Undeformed chip thickness (hz) – to be removed by

each of the drill’s cutting lips, which can be

deter-mined from the following relationship:

NB  With a symmetrical drill, then: Hz = h = h

• Undeformed chip thickness (b) – can be found from

the following relationship:

∴ It follows from these expressions, that the

trans-verse cross-sectional area of the undeformed chip at

each of the twist drill’s cutting lips, can be shown by

the following relationship:

Az = szd/2 = sd/4 = hzb (mm)

Hence, the total transverse cross-sectional area when

drilling of the undeformed chip will be:

A = 2Az = szd = sd/2 = 2hzb (mm) Conversely, in the case of ‘Pilot’ hole drilling (Fig 50b), the undeformed chip elements are identical to

‘Solid’ drilling, but for the exception of the DOC, which can be expressed in the following manner:

Where:

d = diameter of final hole (mm),

do = diameter of primary hole (mm)

Thus, for example in the case of ‘Pilot’ hole drilling, the total cross-sectional area of the undeformed chip, will be:

A = 2Az = sz(d – do) = s(d – do)/2 = 2hzb (mm)

The calculation of cutting forces in ‘Solid’ hole drilling

(Fig 50a), can be found from the general formulae for axial force (F) and torque (M), in the following man-ner, respectively:

M = CM d bM suM KH (kg mm) Where:

CF and CM  = constants (i.e derived from

Kacz-marek‘s findings),

d = nominal drill diameter (mm),

bF and bM = exponents characterising the

influ-ence of the drill diameter,

s = feed rate (mm rev–),

uF and uM = exponents characterising influence of

feedrate,

KH = workpiece material’s correction

co-efficient (i.e concerning mechanical properties)

 CF is derived from experimental data, typically: Carbon steel (construction) 84.7, Grey CI 60.5, Malleable CI 52.5, Bronze (medium hardness) 31.5 – with HSS drills, ranging from φ10

to 60 mm.

 CM is derived from experimental data, typically: Carbon steel (construction) 33.8, Grey CI 23.3, Malleable CI 20.3, Bronze (medium hardness) 12.2 – with HSS drills, ranging from φ10

to 60 mm.

Figure 50 Drilling a hole with/without a ‘pilot’ hole and the cutting, rubbing and extrusion mechanism [After: Kaczmarek,

1976]

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Conversely, for ‘Pilot’ hole drilling (Fig 50b), these

mathematical formulae are modified in the following

manner:

F = CF d bF aeF suF KH (kg)

M = CM d bM aeM suM KH (kg mm)

Where:

a = DOC (mm),

eF and eM = exponents indicating the DOC’s

influ-ence

These axial force and torque formulae derived in the

work by Kaczmarek, are concerned with ‘so-called’

av-erage twist drilling values These ‘avav-erages’ are related

to drill diameters between 15 to 35 mm, having feed

ranges in the vicinity of 0.2 to 0.4 mm rev– Therefore,

the entire axial force (F) and torque (M), comprises of

contributions of the lips, land and chisel point, in the

following manner:

• Axial force (F) – lips (50%), land (10%) and chisel

point (40%),

• Torque (M) – lips (80%), land (12%) and chisel

point (8%)

NB These contributing factors to axial force and

torque are for drill depths that do not exceed 2.5d

If the drilling force is significantly increased, then this

has the effect of distorting the drill shaft Such

distor-tion, causes the drill’s cutting edge to move forward

into the workpiece material, in this manner it jointly

increases the DOC and the drilling force

Correspond-ingly, if the drilling force is reduced, the twist drill will

recover its shape, with the cutting edge moving back

from the workpiece, thus reducing both the DOC and

cutting force This stretching and compression of the

drill’s shaft – somewhat like a spring – is unique to

twist drilling, being an unstable element in the

cut-ting process By way of comparison, most cutcut-ting tool

 ‘Lengthening effect’ is associated with the twist drill’s shaft

being twisted by the application of torque, with elastically

springs-back upon release of the drilling torque Not only

will the twist drill ‘spring’ , but it can also ‘bend’ due to the

increased thrust loads produced by high penetration rates.

edges are normally deflected away from increases in

the load

A common form of failure of twist drills in opera-tion is from shattering, with such catastrophic failure being related to the dynamic nature of twist drilling

By way of illustration, a φ4.5 mm long-series twist drill

is capable of withstanding a torque of approximately

6 Nm before it catastrophically fails Normally, the torque for most drilling operations is around 1 Nm

Temperatures in Twist Drilling

The accumulation of heat in the vicinity of cutting is

an important factor in the cutting process, with much

of the mechanical energy necessary for machining be-ing converted into heat, then conducted into the chip, workpiece and tool (Fig 51) The consequential ther-mal phenomena are important, as they can affect the:

• Mode of deformation – elastic/plastic behaviour of the chip,

• Machined surface – for metals the ultimate metal-lurgical state of the material,

• Tool wear rate – which depends upon a number

of criteria, such as the tool’s coating, cutting data employed, work-hardening ability of the workpiece and coolant delivery and its efficiency

It is imperative to comprehend the factors that control both heat generation and its dissipation, together with the tool and work’s temperature distribution in and near the cutting zone

A drilling operation can be considered as a complex machining process, with specific and unique charac-teristics, not least of which, are the production of chips when drilling These chips are in continuous contact with the drill flutes and the generated hole’s surface Hence, any minute changes in the drill’s geometry, can cause enormous modifications to the either the drill’s wear rate and its predicted life Heat generated whilst drilling will be transformed by a range of ‘states’ , in-cluding:

• Conduction – through the chips, workpiece and drill,

• Convection and radiation – via the ‘air-spaces’

in the hole as the drill penetrates deeper into the workpiece

The drilling temperature during a prolonged operation can approximate steady-state conditions, with the heat generated whilst cutting when employing a new drill

Figure 51 The drilling process and the asociated zones of heat generation whilst hole-making [After:

Trigger and Chao, 1951]

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is associated with two distinct regions (i.e see Fig 51

– Section on X-X) at the:

• Primary shear zone – where

plas-tic deformation occurs, this being the

ma-jor source of heat generation,

• Secondary shear zone – from within the tool/chip

interface, where pronounced friction takes place

NB The drill clearance surface temperature, is

sig-nificantly affected by the rake face interface

tem-perature

The bulk rise in the drill’s temperature is multifarious,

due to the necessity to consider a range of factors,

in-cluding: heat flow distribution, the geometric shape of

the conducting bodies, together with any variation of

thermal properties of both the drill and workpiece

ma-terials with temperature changes The generated heat

distribution when drilling depends upon the thermal

properties of the tool, workpiece and chip Therefore,

the thermal diffusivity (K/ρc), will determine the rate

at which heat transfers through the material, while

also controlling the penetration depth of the surface

temperature While the absorption coefficient (Kρc),

determines the quantity of heat being absorbed by a

given mass of material Drilling temperatures vary

considerably in the research work undertaken over

the years, being heavily influenced by a wide range of

cutting-related parameters, making it extremely

diffi-cult to obtain meaningful comparisons of local

tem-peratures in a real-time drilling operation For

exam-ple, the scatter of ‘bulk’ temperature values for say, a

φ6mm twist drill, can vary between approximately 200

to 380°C, under steady-state drilling conditions, for

comparable workpiece materials, making it very

dif-ficult to obtain meaningful drill life comparisons

Coolant delivery is imperative when drilling and to

this end, through-the-nose coolant operation enables

the lubrication and cooling of the drill’s point (Fig 52c

– illustrating the coolant holes behind the lips) This

efficient technique of ensuring that the coolant gets to

the action of drilling, gives better chip control, helping

 Twist drill interface temperatures have been reported to be

over 870°C in the workpiece’s ‘plasticity region’ , which

some-what contradicts the ‘bulk’ temperatures, although in

mitiga-tion, it should be said that these very high temperatures at the

interface at somewhat localised

to reduce machining temperatures significantly and aid drill penetration rates, while increasing tool life Coolant holes through-the-nose are not restricted to twist drills, as Spade- and Gun-drills, together with Indexable drills also often incorporate this coolant de-livery feature, to remove heat and lubricate the cutting edges

3.1.4 Indexable Drills

Indexable drills have some significant advantages over their twist drilling counterparts (i.e a range of both indexable and twist drills are depicted in Fig.52a) These indexable drills – allowing the cutting inserts to

be changed (see Fig 52c), permit faster cutting speeds and enable a wider range of workpiece materials to be successfully drilled than when utilising conventional twist drills Normally, indexable drills are limited to shorter hole depths of around ‘4D’ , than equivalent diameter twist drills

Indexable drills must be set up with care and in the correct relationship to the machine tool’s headstock/ spindle, ensuring that both the drill’s and the spindle’s centrelines are coincident, otherwise over-, or under-sized holes may be produced (see Fig 53a – top) Yet another problem that needs to be addressed when

employing these indexable drills, is termed ‘radial runout’, which affects the inserts centre height and should be limited to <0.127 mm One advantage of be-ing able to manipulate the indexable drill’s axis, is that

it can be used to adjust the drilled hole’s diameter, by parallel adjustment of the drill’s and spindle’s respec-tive centrelines – this being very useful for controlling

 Spade drills are twist drills (i.e bodies are normally

manu-factured from either 1018, or 1020 low-carbon steel) with a standard blade being inserted at the drill’s point, enabling holes to be generated up to 8D deep Blades are usually coated micrograin HSS with high-cobalt content, or coated cemented carbides, ranging from stub drills to extra-long lengths, with either straight, or spiral flutes

 Gun-, or Deep-hole drills will be mentioned in some detail later

in this chapter, but they allow considerable length-to-diameter ratios to be drilled in workpieces, necessitating high-pressure coolant delivery, with efficient chip-flushing capabilities

 Radial runout refers to misalignment in the radial direction,

which should be minimised, as it alters the position of the

drill’s cutting inserts.

Figure 52 Short hole drilling [Courtesy of Sandvik Coromant]

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drilled hole tolerances On turning centres this can be

readily achieved by modifying the CNC cutting

pro-gram, to offset the drill with respect to the machine’s

centreline Moreover, for turning operations

employ-ing drilled features, then the indexable insert’s top

sur-faces must remain parallel to the X-axis of the machine

tool The arrangement of the inner and outer cutting

edges of an indexable insert drill relative to each other,

together with the drill’s position to the axis of rotation

are vital for perfect drilling operations (i.e see Fig

53a) The cutting inserts positions by possible X-axis

adjustments, are critical for the: smooth running,

re-sultant cutting forces and, will influence the drilled

hole’s alignment Preferably, the cutting edges are

ar-ranged in such a manner that the inner- (SBI) and

outer-inserts (SBA) have identical cutting widths (Fig

53a – bottom left) When new insert cutting edges are

utilised, this results in a balance of the cutting forces in

the Y-axis, guaranteeing drilled holes of accurate size

and surface texture without ‘retraction striae’ 0 When

selecting the appropriate adjustment angles (χi) and

(χA), the lines of force via feed forces (Fa and Fi) will

co-incide with the drill’s axis in the centre of the clamping

shaft (Fig 53a – top) Hence, the clamping shaft must

only transmit torque resulting from the cutting forces

and the bending moment of the resultant cutting force

which will be present Typically, the outer insert’s

re-sultant cutting force FA (i.e Fig 53b) is comprised of

the following forces:

• Remaining cutting force (∆Fc) – generated through

greater wear rates at the periphery of the outer

cut-ting insert,

• Passive force (Fp) – generated by the corner radius

of the outer cutting insert

With indexable drills, the chip flute is selected so that

the drill’s profile from the tip up to the chip flute, has

its runout twisted by between 65° to 85° In the

vicin-ity of the chip flute the runout (i.e the longest ‘lever

arm’ of the force), is where the maximum resistance

 Some tooling manufacturers recommend that an indexable

drill’s inner insert is positioned slightly below the spindle’s

cen-treline, as this allows a small core of uncut material to pass

over the top surface of the insert and break off – being carried

away with the rest of the chips.

0 ‘Retraction striae’ refers to the ‘trail-lines’ resulting from the

outer insert’s gouging, or ploughing the previously drilled

sur-face as it is withdrawn from the hole.

moment to the resultant cutting force FA is found The bending strength attained in this manner can be greatly increased by employing round profiled chip flutes This rounded chip flute cross-section, does not significantly weaken the drill’s body and provides op-timum chip-flow – even when drilling long-chipping workpieces A taper of the tool holder behind the in-sert seats, prevents a ‘squeezing’ of the chips between the drill and the drill-hole wall

Due to the design of the indexable drill, the two cutting inserts are subjected to very dissimilar stresses when drilling For example, the indexable drill (Fig 53a – bottom), has the outer insert being subjected

to greater stress than its inner counterpart, typically having both thermal and abrasive stresses, while the inner insert must have high toughness characteris-tics Some cutting tool manufacturers recommend

so-called ‘mixed-tipping’  of inserts, where a toughened

grade is used for the inner insert and a wear-resistant grade for the outer insert However, some discretion should be used when utilising indexable drills with mixed cutting inserts, so perhaps reference back to the tooling manufacturer may be advisable if produc-tion quantities are sufficient in order to optimise this potential ‘mixed grade strategy’ Typically, by exploit-ing ‘mixed-tippexploit-ing’ , for example when machinexploit-ing free-cutting steel grades cutting speeds of up to 400 m min– are possible, whereas when drilling low-silicon aluminium grades cutting speeds of 600 m min– can

be achieved with tool lives of up to 45 min of cutting time per edge

Several design factors will influence an indexable drill’s performance, these include:

• Sintered cutting insert chip-breakers – these will improve chip control and enable high penetration rates to be utilised,

• Advanced flute design – allowing deeper chip gul-lets, thus minimising chip-jamming tendencies,

• Faster-, slower-, or straight-fluted designs – with wider flute profiles reduce chip-binding and degra-dation of the drilled hole surface, whilst also im-proving penetration rates,

• Cutting insert shape – utilising square (i.e having

4 cutting edges), rectangular (i.e with two edges),

or triangular inserts (i.e having three edges) – the

 ‘Mixed-tipping’ , refers to having dissimilar grades of inserts

for the outer and inner cutting edges, as they fulfil different mechanical working criteria whilst drilling

Figure 53 Indexable insert drills – insert position and flute geometry [Courtesy of Kennametal Hertel]

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latter being the most popular version for

general-purpose drilling operations

NB Double-sided cutting inserts are available, but

are mainly used for milling operations

One major advantage that indexable drills have over

twist drills, is that they can be offset to produce

differ-ent hole diameters This offset for turning cdiffer-entres can

be up to 3.8 mm, or 7.6 mm on a diameter, which

in reality, amounts to a ‘fine-boring’ operation,

giv- On machining centres this offset is somewhat less, with the

max-imum radial offset being approximately 1 mm, or 2 mm on dia-meter Of note, is that when an indexable drill is offset, then the maximum feedrate should be no greater than 0.15 mm rev–.