Figure 58. Deep-hole drilling operations, such as: (a) gundrilling, (b) double tube ejector drillingand (c) single tube ejector drill- ing. [Courtesy of Sandvik Coromant] . Chapter • Minimal uid-ow disturbance – giving consis- tent/regular ow-rate to drill-head, • Minimum of uid-turbulence – allowing chips to be easily evacuated from the cutting region. Typically, cemented carbide heads, have an external V- shaped chip-ute which extends along the shank, the angle of this chip-ute has been experimentally-deter- mined to be 110°, providing the following advantages: • Optimum ute cross-section – allowing the most rapid cutting uid 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). is type of align- ment failure mode is termed a ‘balk-crash’ – caus- ing the tool to fracture into numerous pieces 33 . If the drill is rotated rather than the workpiece, the stress is re-applied to diering 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 34 . e 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 ex with each revolution, a situation that is ideal for a fatigue fail- 33 ‘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. 34 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 3 ). 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 signicant 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 insucient coolant ow occurs, this is also a typ- ical factor in subsequent Gun-drill failure. is lack of coolant causes the chips to pack in the V-ute, 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 machining, 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 uid can either ‘starve’ the Gun-drill’s cutting edge, or ‘over-ood’ 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 ow- rate. If holes are Gun-drilled < φ4 mm, then high-pres- sure coolant ow-rate to the point is essential, but in many cases of coolant systems tted to ‘standard’ ma- chines, they are of relatively low-pressure delivery. Re- cently, one machine tool manufacturer, has designed and developed a coolant intensier 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 ow-rates to the cutting vicinity. .. Double-Tube Ejector/Single- Tube System Drills Double-tube Ejector drills (i.e. oen just termed ‘Ejec- tor Drills’), are designed around a twin tube system DrillingandAssociated Technologies (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 specic sealing arrangements), of the cutting uid, is externally pumped along the space be- tween the inner and outer tubes. e major portion of the cutting uid 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 uid 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 sucient pressure and volume, to overcome any likelihood of ‘starvation’. e ‘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. e asymmetric design 35 of these ‘Ejector Drills’ has support pads provided, to absorb the radial cut- ting forces and guide while supporting the tool as it penetrates into the workpiece. At the commencement of the deep-drilling operation, the drill bushing’s main function 36 (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 drilling 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- 35 Asymmetric Drill Head design, refers to the fact that the cutting inserts are not only radially, but are angularly oset. erefore, 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. 36 Drill bushing tolerances between the drill and bush for both the ‘Ejector’ and Single-tube Systems, require a t of ISO G6/ h6, equating to a minimum play of 0.006 mm. is drill bush is usually manufactured from a hardened material (i.e. 60 to 62 HR C ) 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 prole and tolerance abnormalities result from centre divergence, which needs special at- tention to minimise such eects. 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 uid 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). e quantity of cutting uid 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. e drill head arrangement of cutting inserts will vary from two, three, or more, depending on the drill’s diameter, usually made of cemented carbide, oen as brazed over-lapping tips, although disposable index- able pocketed inserts with chip-breakers are oen 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. e SST tooling system copes with dicult-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. is 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) 37 in about 6 months. 37 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 Chapter .. 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. e major dierence 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. ese increased frictional eect contributions – by the pads – to the overall Deep-hole drillingcutting forces and torque values are somewhat dicult to precisely establish, however, an approximate formulae can be used to esti- mate them, as follows: Feed force (N): F p + F pµ = 0.65 × k c × a p × f × sinκ r Where: F p = Feed force, or drilling pressure (N), F pµ = Force and Frictional eects (N), k c = Specic cutting force (N mm –1 ), a p = Depth of cut (mm), f = Feed per revolution (mm rev –1 ), sinκ r = Entering angle (°). Torque, or Moment (Nm): M c M µ k c a p f D . a p D Where: M c = Torque cutting (Nm), M µ = Torque and Frictional eects (Nm), k c = Specic cutting force (N mm –1 ), a p = Depth of cut (mm), f = Feed per revolution (mm rev –1 ), D = Hole diameter (mm). 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. e 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 (M µ ), which in turn pro- duces an associated contribution ‘P µ ’ to the total Deep- hole drilling power. erefore, 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. P c + P µ ), by the machine tool’s eciency ‘η’. is eciency indicates what percentage of the power sup- plied by the machine tool, that can be utilised, while Deep-hole drilling. Power (kW): P c P µ k c a p f v c , . a p D Where: P c + P µ = Power contributions of: cuttingand friction respectively (kW), v c = Cutting speed (m min –1 ). ∴P = P c + P µ /η Where: η = Machine tool eciency. 3.2 Boring ToolTechnology – Introduction e technology of boring has shown some important advances in recent years, from advanced chip-break- ing control tooling (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’ 38 – 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 38 ‘Chatter’ , is one of the two basic types of vibration (i.e. namely, ‘forced’ and ‘self-excited’) that may be present dur- ing machining. In the main, chatter is a form of self-excita- tion vibration.‘[It is]… due to the interaction of the dynamics of the chip-removal process and the structural dynamics of the machine tool. e excited vibrations are usually very high in amplitude and cause damage to the machine tool, as well as lead to premature tool failure’. [Aer: Kalpakjian, 1984]. DrillingandAssociated Technologies ‘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 drillingand Gun-drilling operations, has its rigidity decreased by the ‘cube’ of the distance (i.e. its overhang), as the fol- lowing equation predicts: f o π EI L M t .M b Where: f o = normal force acting on the ‘free end’ of the can- tilever (i.e boring tool overhang), *EI = exural stiness (i.e. I = cross-sectional moment of Inertia) (Nm 2 ), M t = boring bar mass (kg), L = length of cantilever (mm), M b = Modulus of elasticity of the boring bar (N mm –2 ). * 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 39 (i.e. oset) to the O.D., • Correction of hole abnormalities 40 – the boring process does not follow the previously produced 39 ‘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 o- set using gauge blocks (i.e ‘Slip-gauges’). is technique might still be employed in some Toolrooms, but normally today, on CNC-controlled slideways, a simple ‘CNC oset’ will achieve the desired amount of bored eccentricity. 40 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. e 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 nal hole’s dimensions, by careful ad- justment of the tool’s feedrate and the selection of an appropriate boring toolcutting insert geometry. .. 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 dierent cutting head designs andcutting 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), oering ‘qualied tooling’ 41 dimensions. necessitating correction by a boring operation. is ‘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 o-centre. e 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. 41 ‘Qualied Tooling’ , refers to setting the tool’s osets, with all the known dimensional data for that tool, allowing for ease of tool presetting and ecient tool-changing – more will be said on this subject later in the text. Chapter Figure 59. A selection of some tooling that can be employed for boring-out internal rotational features. [Courtesy of Seco Tools] . DrillingandAssociated Technologies 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 tment for the replaceable head, with internal tension adjustment via the interior mechanism illustrated. Chapter Figure 61. Interchangeable cutting heads for boring bars utilised in machining internal features. [Courtesy of Sandvik Coromant] . DrillingandAssociated Technologies Possibly a more adaptable modular system to the ‘ser- rated and clamped’ version, is illustrated 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. is ‘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. e amount of boring bar-overhang will determine from what type of ma- terial the boring bar will be manufactured. e 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 stiness, 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 ecient in damping performance and machining characteristics. Fig. 62b, illustrates the internal mechanism of the boring bar, for potential ‘bar-tuning/damping’ – to reduce vibrational inu- ences whilst machining. Here, the mechanism consists of a heavy slug of metal, held at each end by rubber grommets, in a chamber lled with silicon oil. ere- fore, as the boring operation commences the slug vi- brates at a dierent frequency to the steel bar, which counteracts the vibration, rather than intensifying vi- brational eects. Such ‘damped’ boring bars, have been utilised with large overhangs, of between 10: to 14:1 L/D ratios. More information on ‘damping eects will be mentioned in Section 3.2.4. .. Boring Bar Selection of: Toolholders, Inserts andCutting 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 oers 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 rst 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. is latter radial cutting force, can increase the probabil- ity of increased bar deection and be a source for un- wanted vibrational eects. 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 aect vibration during machining. e 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. is positive rake angle, is at the expense of de- creased ank clearance and, if too small, the insert’s ank 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. Chapter . considerably improved. 41 ‘Qualied Tooling’ , refers to setting the tool s osets, with all the known dimensional data for that tool, allowing for ease of tool presetting and ecient tool- changing. the tool s feedrate and the selection of an appropriate boring tool cutting insert geometry. .. Single-Point Boring Tooling ‘Traditional’ boring bars were manufactured as solid one-piece tools,. cost Chapter .. 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