Figure 48. Symmetrical twist drill cross-sectional proles [After: Spur and Masuha, 1981]. Drilling and Associated Technologies workpiece (see Fig. 49). e 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 eciency of the coolant supply – if any – to the cutting edges (i.e lips). e 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. e 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 ecient 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]. Chapter • Land, or margin – via a rubbing, or frictional ac- tion. NB e latter two are relatively inecient 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. e land resistance 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 cutting at any instant, each one in its travel corresponds to feed ‘s z ’ , which removes an undeformed chip whose size – in the direction of the drilling axis is: ‘s 1 ’ and ‘s 2 ’ respectively (i.e see Fig. 50a) and, as most drills are symmetrical in de- sign, then: s = s1 = s2 = s/2 (mm), • Undeformed chip thickness (h z ) – to be removed by each of the drill’s cutting lips, which can be deter- mined from the following relationship: h z = s z sinθ (mm). NB With a symmetrical drill, then: H z = h 1 = h 2 . • Undeformed chip thickness (b) – can be found from the following relationship: b = a 1/sinθ = d/2sinθ (mm). ∴ 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: A z = s z d/2 = sd/4 = h z b (mm 2 ). Hence, the total transverse cross-sectional area when drilling of the undeformed chip will be: A = 2A z = s z d = sd/2 = 2h z b (mm 2 ). 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 D OC , which can be expressed in the following manner: a = d-d o /2 (mm). Where: d = diameter of nal hole (mm), d o = diameter of primary hole (mm). us, for example in the case of ‘Pilot’ hole drilling, the total cross-sectional area of the undeformed chip, will be: A = 2A z = s z (d – d o ) = s(d – d o )/2 = 2h z b (mm 2 ). e 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: F = C F d bF s uF K H (kg) M = C M d bM s uM K H (kg mm) Where: C F 12 and C M 13 = constants (i.e. derived from Kacz- marek‘s ndings), d = nominal drill diameter (mm), bF and bM = exponents characterising the inu- ence of the drill diameter, s = feed rate (mm rev –1 ), uF and uM = exponents characterising inuence of feedrate, K H = workpiece material’s correction co- ecient (i.e. concerning mechanical properties). 12 C F 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. 13 C M 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. Drilling and Associated Technologies Figure 50. Drilling a hole with/without a ‘pilot’ hole and the cutting, rubbing and extrusion mechanism. [After: Kaczmarek, 1976] . Chapter Conversely, for ‘Pilot’ hole drilling (Fig. 50b), these mathematical formulae are modied in the following manner: F = C F d bF a eF s uF K H (kg) M = C M d bM a eM s uM K H (kg mm) Where: a = D OC (mm), eF and eM = exponents indicating the D OC ’s inu- ence. ese axial force and torque formulae derived in the work by Kaczmarek, are concerned with ‘so-called’ av- erage twist drilling values. ese ‘averages’ are related to drill diameters between 15 to 35 mm, having feed ranges in the vicinity of 0.2 to 0.4 mm rev –1 . erefore, 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 ese contributing factors to axial force and torque are for drill depths that do not exceed 2.5d. If the drilling force is signicantly increased, then this has the eect of distorting the drill sha. Such distor- tion, causes the drill’s cutting edge to move forward into the workpiece material, in this manner it jointly increases the D OC 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 D OC and cutting force. is stretching and compression of the drill’s sha – somewhat like a spring 14 – is unique to twist drilling, being an unstable element in the cut- ting process. By way of comparison, most cutting tool 14 ‘Lengthening eect’ is associated with the twist drill’s sha 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 deected 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 e 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). e consequential ther- mal phenomena are important, as they can aect 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 eciency. 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 specic and unique charac- teristics, not least of which, are the production of chips when drilling. ese chips are in continuous contact with the drill utes and the generated hole’s surface. Hence, any minute changes in the drill’s geometry, can cause enormous modications 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. e drilling temperature during a prolonged operation can approximate steady-state conditions, with the heat generated whilst cutting when employing a new drill Drilling and Associated Technologies Figure 51. The drilling process and the asociated zones of heat generation whilst hole-making. [After: Trigger and Chao, 1951] . Chapter 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 e drill clearance surface temperature, is sig- nicantly aected by the rake face interface tem- perature. e bulk rise in the drill’s temperature is multifarious, due to the necessity to consider a range of factors, in- cluding: heat ow 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. e generated heat distribution when drilling depends upon the thermal properties of the tool, workpiece and chip. erefore, the thermal diusivity (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 coecient (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 inuenced by a wide range of cutting-related parameters, making it extremely di- 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 15 , for comparable workpiece materials, making it very dif- cult 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). is ecient technique of ensuring that the coolant gets to the action of drilling, gives better chip control, helping 15 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 signicantly and aid drill penetration rates, while increasing tool life. Coolant holes through-the-nose are not restricted to twist drills, as Spade- 16 and Gun-drills 17 , together with Indexable drills also oen incorporate this coolant de- livery feature, to remove heat and lubricate the cutting edges. .. Indexable Drills Indexable drills have some signicant advantages over their twist drilling counterparts (i.e. a range of both indexable and twist drills are depicted in Fig.52a). ese 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’ 18 , which aects 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 16 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 utes. 17 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 ecient chip-ushing capabilities. 18 Radial runout refers to misalignment in the radial direction, which should be minimised, as it alters the position of the drill’s cutting inserts. Drilling and Associated Technologies Figure 52. Short hole drilling. [Courtesy of Sandvik Coromant]. Chapter drilled hole tolerances. On turning centres this can be readily achieved by modifying the CNC cutting pro- gram, to oset 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. e 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 19 (i.e. see Fig. 53a). e cutting inserts positions by possible X-axis adjustments, are critical for the: smooth running, re- sultant cutting forces and, will inuence 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 le). 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’ 20 . When selecting the appropriate adjustment angles (χ i ) and (χ A ), the lines of force via feed forces (F a and F i ) will co- incide with the drill’s axis in the centre of the clamping sha (Fig. 53a – top). Hence, the clamping sha 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 F A (i.e. Fig. 53b) is comprised of the following forces: • Remaining cutting force (∆F c ) – generated through greater wear rates at the periphery of the outer cut- ting insert, • Passive force (F p ) – generated by the corner radius of the outer cutting insert. With indexable drills, the chip ute is selected so that the drill’s prole from the tip up to the chip ute, has its runout twisted by between 65° to 85°. In the vicin- ity of the chip ute the runout (i.e. the longest ‘lever arm’ of the force), is where the maximum resistance 19 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 o – being carried away with the rest of the chips. 20 ‘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 F A is found. e bending strength attained in this manner can be greatly increased by employing round proled chip utes. is rounded chip ute cross-section, does not signicantly weaken the drill’s body and provides op- timum chip-ow – 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’ 21 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 sucient in order to optimise this potential ‘mixed grade strategy’. Typically, by exploit- ing ‘mixed-tipping’ , for example when machining free-cutting steel grades cutting speeds of up to 400 m min –1 are possible, whereas when drilling low-silicon aluminium grades cutting speeds of 600 m min –1 can be achieved with tool lives of up to 45 min of cutting time per edge. Several design factors will inuence 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 ute design – allowing deeper chip gul- lets, thus minimising chip-jamming tendencies, • Faster-, slower-, or straight-uted designs – with wider ute proles 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 21 ‘Mixed-tipping’ , refers to having dissimilar grades of inserts for the outer and inner cutting edges, as they full dierent mechanical working criteria whilst drilling. Drilling and Associated Technologies Figure 53. Indexable insert drills – insert position and ute geometry. [Courtesy of Kennametal Hertel]. 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 oset to produce dier- ent hole diameters. is oset for turning centres can be up to 3.8 mm, or 7.6 mm on a diameter 22 , which in reality, amounts to a ‘ne-boring’ operation, giv- 22 On machining centres this oset is somewhat less, with the max- imum radial oset being approximately 1 mm, or 2 mm on dia- meter. Of note, is that when an indexable drill is oset, then the maximum feedrate should be no greater than 0.15 mm rev –1 . Chapter . (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. Drilling and Associated Technologies Figure 50. Drilling a hole with/without. relationship: A z = s z d /2 = sd/4 = h z b (mm 2 ). Hence, the total transverse cross-sectional area when drilling of the undeformed chip will be: A = 2A z = s z d = sd /2 = 2h z b (mm 2 ). Conversely,. alters the position of the drill’s cutting inserts. Drilling and Associated Technologies Figure 52. Short hole drilling. [Courtesy of Sandvik Coromant]. Chapter drilled hole tolerances.