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ing considerable scope in diametric sizing of holes. ese drills should be selected for the minimum overhang and need to have both the drill’s and ma- chine tool’s centrelines parallel 23 to within 0.076 mm, or better. By utilising an indexable drill on a turning opera- tion, they can be benecial when attempting to start drilling angled, or uneven workpiece surfaces, as illus- trated by some of the surface faces depicted in Fig. 54. Indexable drills, employed for such non-at workpiece faces, obviates the need for either a previous counter boring, or spotfacing operation. e major advantage of indexable drills over either their HSS, or carbide twist drill counterparts, is their ability to run at much higher rates. For example and, by way of simplistic comparison, if a φ18 mm hole has to be drilled in free-cutting stainless steel, then an: • HSS twist drill – can be run at 16.8 m min –1 with a 0.2 mm rev –1 feed would produce 280 rpm at 57 mm min –1 , • Solid carbide twist drill – can be run at 61 m min –1 with a 0.1 mm rev –1 feed would produce 1,019 rpm at 103 mm min –1 , • Indexable drill – can be run at 170 m min –1 with a 0.76 mm rev –1 feed would produce 2,852 rpm at 217 mm min –1 . NB e indexable drill can be run up to 0.8 mm rev –1 and if one were to attempt to utilise a twist drill with the cutting data shown above, then it would either burn-out, or catastrophically fail in the endeavour. An important safety note when using these indexable drills in turning operations, is that the high penetra- tion rates, coupled to the exit of a through hole in the part, creates a slug which is thrown clear from either the chuck, or from the rear of the bore of the machine’s headstock at tremendous force. If the machine tool is not guarded appropriately, it could prove to be hazard- ous to any operator in the local vicinity. 23 Axis oset/misalignment: for example, if there is a 1° mis- alignment between these centrelines, then the centrelines will be 0.43 mm apart at 25 mm from the point at which the cen- trelines cross at the tool’s tip. .. Counter-Boring/Trepanning A counter-boring operation is an oen utilised tech- nique for the enlargement of previously manufac- tured holes, normally to provide them with accurate dimensions and/or improved surface nish (i.e. see Fig. 55a). Counter-bores are also produced to register a larger-faced shouldered sha, or to sink a precision cap-head bolt below the clamped part’s surface. In this latter case, oen the previously-drilled hole is used to align the bolt’s axis, by using a counter-boring tool with a ‘pilot-bush’ of the same drilled diameter to act as a guide to allow machining to a correct counter-bored depth. Counter-boring heads are employed to open-out existing holes (Fig. 55a). e heads were in the past, oen of a single insert design, but latterly, they are produced with multiple inserts – particularly when the working clearance is such, that it cannot cope with by the single insert variety. e multi-insert counter-bor- ing heads, can have their inserts nely diametrically- adjusted by means of precision wedge and radial ad- justing screws. A trepanning operation is undertaken in one op- eration, but instead of machining the complete hole, only part of the hole is cut, leaving a core (Fig. 55b). For large workpiece dimension machining, a trepan- ning operation uses less power and axial pressure than other equivalent manufacturing processes. However, one major problem occurs with the trepanning opera- tion, which is that the core produced as the trepanning tool penetrates into the workpiece, becomes quite dif- cult to handle (i.e. see Fig. 55b -lower diagram). Trepanning heads are normally utilised on: • Large workpiece diameters – greater than 120 mm, • Limited machine tool power – alternatively, if it is not prudent to switch to another machine tool and/ or lose part orientation and register 24 , • Core utilisation – large cores can be usefully utilised as precision material stock. 24 Part orientation and register, refers to the initial setup, where once the workpiece has been clamped and partially machined, it cannot be satisfactorily removed then reset without a loss of both accuracy and precision. Drilling and Associated Technologies Figure 54. A wide range of drilling/boring operations can be undertaken using indexable insert drills. [Courtesy of Seco Tools]. Chapter Figure 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies .. Special-Purpose, or Customised Drilling and Multi-Spindle Drilling Most of today’s Special-purpose, or Customised Drills and Multi-spindle drills are normally designed and manufactured to meet the following criteria: • Long production runs 25 – these enable the extra cost of this purpose-designed and built tooling to amortised 26 over the cost of the production period, • Short cycle times 27 – when time is the ‘essence of importance’ in the production of the component, but it is not necessarily related to the overall quan- tity of the production run, • Tooling accuracy is reected in the part’s manufac- ture – if for example the precision part must have all its component features in accurate alignment, or in a specic relationship to a particular datum 28 : face, plane or point. Special-purpose, or Customised tooling is normally required if one, or several of the criteria mentioned above are to be met. To have a tooling manufacturer design special-purpose tooling to meet the production demands of manufacture, is not undertaken lightly, as for complex tooling, its: design, build and prove-out, prior to use, could prove to be expensive. However, many companies resort to this type of custom-built tooling, because it is the only way that the product can be manufactured economically. Oen, multiple fea- tures are incorporated into just one tool, typically for hole- and post-hole making operations, such as those depicted in Fig. 56a. A relatively simple example of this special-purpose tooling is illustrated in Fig. 56b, where three production operations for the manufac- ture of the female part of the pull-stud mechanism for a milling cutter toolholder is depicted, namely, drilling, 25 Long production runs usually refers to some form of continu- ous production , or large batch sizes, of > 5,000 components, to make the cost of the Special-purpose tooling viable. 26 Amortisation, refers to the ‘pay-back’ of the tooling over the ‘life’ of the production of the parts produced. 27 Short cycle times, are considered to be the quickest time that the part can be produced, under ‘standard’ machining conditions. 28 Datum – the term refers to origin of the measurements for the particular component feature, which could be from a face, plane, or point. chamfering and counter-boring. In this case, not only does the usage of special-purpose tooling here seem the obvious solution, as it combines these individual operations in one, it has the advantage of meeting all three of the production criteria listed above, with the added advantages of both using fewer tools and utilis- ing less space in the tool magazine. Some special-pur- pose tools are very complex in their design and quite sophisticated in operation, but their supplementary cost more than outweighs this by the production gains oered by their consequent implementation. Multi-spindle drilling 29 tooling is ideal when a series of hole patterns are required on a component, such as for specic congurations of: pitch circle diameters, hole grid patterns, line of holes, or a combination of these (i.e. see examples of specic patterns in Fig. 57 – top right). Hole pitch circle diameters can easily be accommodated, for large and small pitch diameters on the same tooling, Likewise, hole line and grid patterns can be quite diverse, within the diametral area of the ‘cluster plate’ (i.e. see Fig. 57 – top le). Multi-spindle drilling heads utilise a main drive gear which is engaged with an idler and then onto the drill spindle gear, this being attached to the individual drill (i.e. see Fig. 57 – exploded view of a typical sys- tem). e cluster plate orientates the individual drill spindles and their rotational speeds can be margin- ally increased, or decreased, by changing the driver- to-driven gear ratio, moreover, their rotational direc- tion can be changed by the introduction of another idler into the gear train. erefore, if additional idlers are present, to change the drill’s rotation, then the an appropriate le-hand drill would be required here. By purposefully modifying each drill’s rotational di- rection, this has the advantage of minimising overall torque eects on the multi-spindle drilling head, al- lowing a large number of drills to be utilised for one particular operation (i.e. see Fig. 57 – lower le-hand photograph). An important point in utilising multi- spindle drilling heads, is presetting their respective drill lengths, so that they engage with the workpiece’s surface at the correct height. By the correct production application of both Spe- cial-purpose and Multi-spindle drilling tooling, then 29 Multi-spindle tools, refer to more than one individual tool ro- tating in its respective toolholder, enabling several holes to be manufactured in just one operation. Chapter Figure 56. Special-purpose multi-functional tooling can be designed and manufactured to machine many part features simultaneously . Drilling and Associated Technologies Figure 57. The application of multi-spindle drilling heads to increase productive throughput. Chapter signicant economic savings can be made and their initial capital outlay will have been worthwhile. How- ever, such complex and expensive tooling used inap- propriately can be counter-productive, so consider- able thought and care should be made into any future implementation of these tools. .. Deep-Hole Drilling/ Gun-Drilling Deep-Hole Drilling – an Introduction Deep-hole drilling can be characterised by, high mate- rial removal rates, having excellent: hole straightness, dimensional tolerances and machined surface texture. Deep-hole drilling applications are utilised across di- verse industrial applications, including: aerospace, nuclear power, oil and gas, as well as for steel and chemical processing industries. ese industries place a high demand on all aspects of drilled hole quality and reliability, with components being very expensive, any failures will have severe economic consequences. e name Deep-hole drilling implies the machin- ing of holes with a relatively long hole depth to its diameter. Typically at the lower length-to-diameter ratios they can be as short as x5 the diameter, con- versely at the other end of the scale, ‘ratios’ of > x100 the diameter can be successfully generated, with close tolerances and a surface texture approaching 0.1 µm (Ra). ere are a considerable number of deep-drill- ing production techniques, with each one having an appropriate usage for a particular hole generation method. A typical deep-drilling tooling assembly is essentially ‘self-piloting’ , in that the cutting forces generated are balanced, not with respect to the cut- ting edges – as is the situation with Twist drills – but invariably, by pads that are situated at 90° and 180° to that of the cutting edge. ese pads rub against the bore’s surface being generated and therefore support the head, while burnishing 30 the surface. e machine tools enabling these deep-drilled holes to be generated can be expensive, along with the appropriate tooling, but the production costs can be dramatically reduced, by employing such a machining strategy. One of the 30 Burnishing will improve the surface nish and dimensional ac- curacy, by plastically deforming the machined surface layers – cusps – without removing any additional workpiece material. major problems of utilising Deep-hole drilling, is chip disposal, as the deeper the hole is drilled, the further the chip must travel from the cutting edge to the hole’s exit. is chip evacuation distance, can increase the probability of chip-jamming, or binding in the ute as the chip attempts to exit the deep-drilled hole. Notwithstanding the problems associated with BUE, which hinders the tool’s ability to break chips. Coolant control and its operational usage is important in any the Deep-hole drilling technique, as one of its main functions is – apart from lubricating/cooling the cut- ting edge and chip ushing – is to restrict frictional ef- fects between the: drill, chip and hole wall. Moreover, if friction builds-up due to poor coolant delivery, this can result in higher torsional eects, which may cause the drill to snap. Gun-Drills Gun-drills (i.e. see Fig. 58a), are normally utilised to machine small, straight diameters to high tolerances and having excellent nishes in a single operation. Drilled hole sizes can range from as small as φ1.5 mm to φ75 mm in a single pass, with depths equating to 100 times the tool’s diameter 31 . e ‘drilling system’ is a highly developed and ecient technique for produc- ing deep holes in wide variety of workpiece materials, ranging from: plastics, breglass, to high-strength ma- terials such as Inconel. is tooling usually consists of either a cemented carbide, or cemented carbide-tipped drill head tted to a tube-shaped shank 32 . e former solid carbide drill head version allows the tooling to be reground as necessary, while the latter version is normally employed for larger diameter hole drilling operations. e drill head has two distinct designs, either having a ‘kidney-shaped’ , or a cylindrical hole present, for the delivery of cutting uid, which pro- vides: • Flow of cutting uid – to create the maximum ow rate and chip-ushing, 31 ‘Special-purpose’ Gun-drills can be produced to generate drilled holes up to φ150 mm having 200:1 length-to-diameter ratios, at penetration rates of better than equivalent diameter Twist drills. 32 Gun-drills would as a rule, have their drill head’s brazed – via silver soldering – onto a tube-shaped shank, these in turn, are also brazed onto a ‘driver’ (i.e. of various designs) of the re- quired length for the successful drilling of long slender holes in the workpiece. Drilling and Associated Technologies 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] . 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 Drilling and Associated 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 . 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies .. Special-Purpose, or Customised Drilling and Multi-Spindle Drilling Most of today’s. clamped and partially machined, it cannot be satisfactorily removed then reset without a loss of both accuracy and precision. Drilling and Associated Technologies Figure 54. A wide range of drilling/ boring. successful drilling of long slender holes in the workpiece. Drilling and Associated Technologies Figure 58. Deep-hole drilling operations, such as: (a) gundrilling, (b) double tube ejector drilling
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