.. 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 dierence 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 eect contributions – by the pads – to the overall Deep-hole drilling cutting forces and torque values are somewhat dicult 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 eects (N), k c = Specic 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 eects (Nm), k c = Specic 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 eciency ‘η’. is eciency 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: cutting and friction respectively (kW), v c = Cutting speed (m min –1 ). ∴P = P c + P µ /η Where: η = Machine tool eciency. 3.2 Boring Tool Technology – 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’. [Aer: Kalpakjian, 1984]. Drilling and Associated 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 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: 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 stiness (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. oset) 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 oset’ 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 tool cutting 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 dierent 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), oering ‘qualied 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 ‘Qualied Tooling’ , refers to setting the tool’s osets, with all the known dimensional data for that tool, allowing for ease of tool presetting and ecient 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] . Drilling and Associated 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] . Drilling and Associated 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 stiness, 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 ecient in damping performance and machining characteristics. Fig. 62b, illustrates the internal mechanism of the boring bar, for potential ‘bar-tuning/damping’ – to reduce vibrational inu- 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 dierent frequency to the steel bar, which counteracts the vibration, rather than intensifying vi- brational eects. Such ‘damped’ boring bars, have been utilised with large overhangs, of between 10: to 14:1 L/D ratios. More information on ‘damping eects will be mentioned in Section 3.2.4. .. 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 oers 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 deection and be a source for un- wanted vibrational eects. NB  For more information concerning boring bar se- lection, see Appendix 1b, for the ISO ‘code key’ for ‘solid’ boring bars. Inser t Selection – Decisions Apart form the boring bar’s lead angle, an insert’s ge- ometry will also aect 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  e insert’s substrate – if cemented carbide – re- quires some thought, as if it is too hard, this type of insert may chip via the eects 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 inuence it has on the D OC . For Figure 62. Interchangeable cutting heads for machining internal features. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies  example, when a nish boring operation is required, then it is recommended that both a small nose radius and D OC is used. is smaller boring insert nose ra- dius, minimises contact between the workpiece and insert, resulting in lower tangential and radial cutting forces. For ne-boring applications, a good start point is to choose an insert with a 0.4 mm nose radius, with a 0.5 mm D OC . It should be noted that the D OC 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-o 42 ) 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 signicantly increase the tangential cutting force component. For nish 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 eect 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 oen made by setters/machinists in order to attempt to minimise vibrational tendencies, is to reduce the rpm. is strategy will not only decrease productivity, but the lower rotational speed can lead to BUE formation, which in turn, modies the insert’s cutting geometry and could change the cutting force directions. Instead of rpm reductions, modication of other cutting data variables is suggested, in order to improve these adverse vibrational/chatter eects. Sometimes even increasing the rotational speed, can eliminate unwanted chatter. Although it is not a specic cutting performance parameter, an oen 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 42 ‘Tool push-o’ – oen termed ‘spring-cuts’ , are the result of tool deection, particularly when light cuts are used. To mini- mise the ‘push-o ’ , very rigid workpiece-machine-tool setup with a smaller nose radius to that of the D OC is recommended. all of the boring bar’s periphery in the toolpost, allow- ing much greater tool rigidity and cutting stability, al- leviating many of the potential problematic in-service machining conditions. .. 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). e 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’ 43 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 ne-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 ne-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 nishing operation, the spindle must rotate in a le-hand rotation. Tri-bore tooling oen having individual micro-bore cartridge adjustment (i.e. not shown), as its name im- plies, uses three cutting inserts equally-spaced at 120° apart. is boring tool arrangement of cutting inserts, oers very high quality bored diametral accuracy and 43 ‘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 D OC is normally 14 mm, with an associated feedrate of 0.2 mm rev –1 .  Chapter  Figure 63. Twin-edged boring tooling. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies  precision to the machined hole, but such tooling can be somewhat more costly than when utilising a single- insert tool. .. Boring Bar Damping For boring bars that have an L/D ratio of <5:1, then relatively stable cutting conditions with controllable vibrational inuences 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 eciency 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 eects creating high-frequency harmonic 44 eects on the roundness prole. Stiness can be expressed in terms of either static, or dynamic stiness. Static stiness of a bar is its ability to resist a bending force in a static condition, conversely, dynamic stiness is the bar’s ability to withstand os- cillating forces (i.e. vibrations). Dynamic stiness 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 stiness, with 44 ‘Harmonics’ – on a machined component are the product of complex interactions, including method of manufacture: component geometry, cutting data utilised, any vibrational inuences encountered and material composition and its manufacture (e.g. Powder Metallurgy parts can vary in both porosity and density throughout the part, which may aect, 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. e part’s low frequency harmo- nicoen 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 lters 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 eect’ , ma - chining could be undertaken at longer overhangs. is ‘damping eect’ is indicated by the highly centralised amplitude of oscillatory movements quickly reducing with time, indicating a high level of dynamic stiness, this being crucial for long L/D ratios. Obviously, the boring bar’s cutting edge deection at its tool tip, is directly related to the amount of bar overhang, this de- ection being the result of the applied cutting forces. e magnitude of a boring bar’s deection being de- pendent upon: bar composition, diameter, overhang and the extent and magnitude of tangential and radial cutting forces. e rigidly clamped and cantilevered boring bar’s ‘free-end’ will deect/deform by forces acting upon it and, some idea of the magnitude of this deection can be gleaned by the simple application of ‘mechanics of materials’ , using the following formula:   F  L    E  I (mm) Where: ∆ = Boring bar deection (mm), F = Cutting force (N), L = Boring bar overhang (mm), E = Bar material’s coecient of elasticity (N mm –2 ), *I = Moment of Inertia (mm 4 ). * For a boring bar of circular cross-section, the Mo- ment of inertia will be: I = π × D 4 /64 (mm 4 ). 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 deec- tion, using the above formula, would be: ∆ L = 4D = 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 deections of: ∆ L = 7D = 0.444 mm ∆ L = 10D = 1.293 mm. Hence, these deection values emphasise the impor- tance of reducing overhang as it increases by approxi- mately ‘cube’ of the distance. Moreover, deection can  Chapter  . D OC is normally 14 mm, with an associated feedrate of 0.2 mm rev –1 .  Chapter  Figure 63. Twin-edged boring tooling. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies  precision. nose radius and the inuence it has on the D OC . For Figure 62. Interchangeable cutting heads for machining internal features. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies. machining internal features. [Courtesy of Sandvik Coromant] . Drilling and Associated Technologies  Possibly a more adaptable modular system to the ‘ser- rated and clamped’ version, is illustrated