Drilling and Associated Technologies Part 5 pps

also be reduced by utilising a dierent boring bar ma- terial, as this will improve its coecient of elasticity 45 . In boring-out roughing operations, any vibrations present are only a problem if they lead to insert dam- age. For nish-boring operations, vibrational condi- tions that may occur could be the dierence between success and failure for the nished machined part. So, the boring bar’s ability to dampen any vibrational source becomes imperative, once a ne-boring opera- tion is necessary. Vibrations can occur in any number of ways that could aect the boring operation, from the constructional elements of the machine tool, through to slideways, or their recirculating ball bear- ings, etc Hence, the joints in a machine tool can be regarded as a complicated dynamic system, with any slideway motion of vibrating contact faces, necessitat- ing lubricating oil to not only reduce any stiction and frictional eects, but to help dampen these structural elements. Machine tool builders are acutely aware that certain machine tool materials ‘damp’ more readily than others. Cast iron and in particular ‘Granitan’ (i.e. a product of crushed granite and epoxy resin), can pre- dominantly act as built-in dampening media for any vibrational sources present. e main source for any vibrations in boring, results from the long overhangs, necessary to machine the hole depth of the compo- nent’s feature. erefore, the magnitude of vibrations in the overall system result from the dampening capa- bilities of the actual boring bar. Tuned Boring Bars A boring bar that has been ‘tuned’ , has the ability to dampen any generated vibrations between the work- piece and the cutting edge while machining. e ‘dampening eect’ is achieved through a vibration ab- sorbing device (i.e see Figs. 61a and 62b), this has the consequence of increasing the bar’s dynamic stiness, giving it the ability to withstand oscillating forces. e 45 Coecient of elasticity, for a steel boring bar composition, E = 21 × 10 4 (N mm –2 ), conversely, using a cemented carbide material for an identical boring bar, E = 63 × 10 4 (N mm –2 ), giv- ing three times greater stiness, allowing much greater boring bar overhangs. NB In reality, the boring bar’s deection will be higher than the values given in these examples, as the formula is based upon the assumption that the bar is absolutely rigidly clamped, which is impossible to achieve. method of achieving this bar damping has already been mentioned in Section 3.2.1, with the relationships be- tween the size of the bar’s body, suspension, viscosity of the liquid media, being carefully designed by the tooling manufacturer. During the boring operation, the vibrations set the body in oscillation. Hence, the body and the liquid alternate, taking each others place in the space within the actual boring bar. A pattern is established during boring, where the oscillations of the body are not in harmony with the vibrations resulting from machining. is out-of-harmony, means that the vibrations are virtually neutralised – to an acceptable level – via the kinetic energy being transformed by the ‘system damping’. Any vibrations present during bor- ing, are relative to the amount of bar overhang, there- fore on longer boring bar lengths, they are normally tted with some means of adjustment, so that they can be ‘tuned’ to the frequency occurring within its range. e simplest manner of achieving adjustment, is by a rotation of a lockable set screw, which when either tightened, or slackened, aects the suspension of the body in the liquid, thus ‘tuning’ the boring bar to the actual machining conditions present. .. ‘Active-suppression’ of Vibrations As has been stated at the beginning of Section 3.2.4, if boring bars have an L/D ratio >5:1, then vibrational ef- fects may result in tool chatter. It has been observed in experimental work, that the boring bar’s tip produces a vibration motion that follows an elliptical path in the plane normal to the longitudinal axis of the bar. e ratio of the amplitude of vibration along the major and minor axes varies with cutting conditions, further- more, the inclination of these axes to the ‘radial line’ of the tool also varies. Of signicance, is the fact that the build-up of chatter will begin almost immediately, even before one revolution of the workpiece has oc- curred. is build-up continues almost evenly until some limiting amplitude occurs, which suggests that the well-known ‘Orthogonal mode coupling’ is pres- ent, further, with the phase dierence between the vi- brations causing an elliptical tool tip path, the vibra- tional energy is fed into the tool-workpiece system, promoting self-excitation. As has been suggested, the dynamic stability of the boring bar is of prime importance, with the onset of self-excited chatter, being governed by the ‘Multiple regenerative eect’ , which is a function of the so-called Drilling and Associated Technologies  ‘space phase’. is ‘space phase’ condition, is the phase of vibration around respective turns of work, uctu- ating between 90° and 180° and is equal to the phase between the inner and outer modulation. Moreover, it has been shown that by modifying the workpiece’s rotational speed, this disturbs the ‘space phase’ and, consequently inuences the ‘time phase’ , leading to a reduction in self-excited chatter. It has been practi- cally demonstrated that by modifying the peripheral speed of the workpiece, this technique is only partially successful in alleviating chatter. More success can be made by utilising damped boring bars, such as the ‘Lanchester’ type 46 , with dynamic vibration absorbers (DVA’s), to really suppress vibrational inuences dur- ing the boring process. Some progress has been made on the development of DVA techniques, but the potential ‘step-change’ will occur in vibrational suppression for boring bars, when the improvement of production versions of ‘ac- tive’ dampers for such tooling becomes a reality. Just such a potential ‘active’ boring bar is shown schemati- cally in Fig. 64. Invariably, the boring bar has a supply of energy to it – via an external source, that controls the cutting edge’s position by monitoring the feedback of the relative displacement of tool’s edge with respect to the workpiece. In later research work by Matsubara et al. (1987), chatter suppression was analysed for the boring bar using ‘feed-forward’ control of the cutting force. Further, the cutting edge was positioned in re- sponse to this force, with these type of ‘active’ control systems being known as: ‘Cutting edge positional con- trol systems’. Typical of a vibrational control approach is illus- trated by the ‘active’ boring bar already mentioned and depicted in Fig. 64, where the forces are damped in re- sponse to the vibrational velocity of the cutting edge, which has been termed a: ‘Vibrational velocity control system’. In this damping technique, the boring bar sup- pression is by a series of piezo-electric elements that act as ‘active dampers’. Such a ‘damper’ responds to onset of chatter vibration (i.e. the high-energy com- ponents). Moreover, the damping force achieves opti- mal phase dierence, since the phases between both 46 ‘Lanchester boring bars’ , normally utilise an internal metal slug which is usually surrounded by some form of: liquid/uid medium, DVA’s, or more primitively, sprung-loaded and as such, the slug is free to move out-of-phase with the cutting conditions, dictated by the boring bar’s applied cutting forces, thereby the onset of chatter will be potentially ‘cancelled out’. the ‘damping’ and vibrational forces are controllable. is type of ‘active’ boring bar arrangement, achieves directional damping characteristics via its ‘dampers’ , here they control two ‘degrees of freedom’ 47 via the ‘Re- generative feedback loop’ , which diminishes oscillatory motion (i.e. harmonics), by careful control of energy losses. In recent years with the advent of articial intelli- gence (AI) applications to major industrial engineering problems, and more specically, in the performance and robustness of certain types of ‘Neural networks’ , the goal of obtaining some form of real-time monitor- ing and control in the machining process is now closer to reality. ese AI systems have been successfully utilised for applied research applications to tool wear monitoring in turning tool operations – aer suitable ‘training’ of a pre-selected neural network architecture. ese ‘networks’ could be successfully applied to bor- ing bar vibrational monitoring and control situations. More detailed information will be said on how, where and when Neural network decision-making and, why these cutting tool monitoring applications should be utilised in the production environment, later in the text. .. Hard-part Machining, Using Boring Bars Although ‘hard-part’ turning has been utilised for some considerable time, with the advent of polycrys- talline cubic boron nitride (PCBN) tooling, etc., it has seen little in the way of exploitation for boring opera- tions, to date. One of the major reasons for this lack of tooling application, is because most hardened parts are in the region of hardness values ranging from 42 to 66 HR C . Such high component hardness, requires considerable shearing capability by the tooling to suc- cessfully machine the excess stock from the workpiece. Generally, the robust nature of toolholding for turning 47 ‘Degrees of Freedom’ , the ‘free-body kinematics’ , exhibit 6 de- grees of translatory (i.e. linear) motions in space, these are: back- ward/forward, upward/downward and leward/rightward. NB Of some interest but in the main, to machine tool build- ers for the purposes of volumetric calibration, are the rotary motions of: yaw, pitch and roll, giving 18 degrees of freedom, together with the 3 squareness errors, totalling 21 possible de- grees of freedom.  Chapter  Figure 64. An ‘active’ boring bar and their capacity to suppress vibrational eects on boring holes [After. Mat- subara; Yamamoto and Mizumoto; 1987] . Drilling and Associated Technologies  tools with their modest overhangs, does not present in- surmountable diculties during machining, however for the much longer overhangs associated with boring operations (i.e. see Figs. 62a and 65a), then the cutting forces generally dictate, short L/D ratios of <5:1 and relatively large and robust boring bars (Fig. 65b). ere are considerable diculties to be over- come when any form of hard-part machining is required – particularly for boring operations, when the components have been either case- or through- hardened, these are: • High temperatures in the cutting zone – necessitat- ing high temperature resistant and thermally-sta- bility of cutting insert materials, • Cutting force magnitudes are both higher and more variable – robust cutting edge geometry is neces- sary to withstand these increased shearing/cutting force demands on the insert, • Small chip cross sections – these exert high pres- sure near the insert’s cutting edge, oen necessitat- ing an edge preparation to the insert’s corner, • Greater tool wear rates – oen more rapid cutting edge wear, or the tendency to catastrophic break- down of the insert, • Workpiece stresses during cutting – these stresses are released during machining and may present localised geometric variations to the nal shape of the part, • Poor homogeneity in the workpiece material – hardness variations across and through the part (e.g. dierential case hardened depths), can lead to signicant and variable cutting force loadings on the boring insert, • Insucient stability – if the ‘machine-tool-work- piece loop’ is not suciently robust, then due to the greater cutting forces when hard-part machining, Figure 65. Boring bar operational limitations and hard part boring at relatively high speed. [Cour- tesy of Sandvik Coromant] .  Chapter  this creates potential tool deection which could become a major problem. Boring Bar Deflection When any boring operations take place, even with a very rigid tool mounting and a small boring bar over- hang, some vibration and tool tip deection will in- evitably occur, this is exacerbated by machining hard- parts. e former problem of vibration has previously been mentioned and methods of minimising it are possible. However, tool deections are more dicult, if not impossible to completely eliminate, with these longer cantilevered tools. Of note regarding overhang- ing tool deections, are that a tool tip deects in two directions (i.e. see Fig. 66a), these are: • Radial deection (∆ T ) – aects the machined (i.e. bored) diameter, • Tangential deection (∆ R ) – causes the tip to move downward for the centreline. In each of these tool tip deections, both the size and direction of the cutting forces are inuenced by the chip thickness and insert geometry selected (i.e. illus- trated in Fig. 66b). e radial deection will be equal to the dierence between the diameter which was orig- inally set and the actual bored diameter, this can be easily found by the simple expedient of measuring it, then adjustment can be made for this apparent deec- tion. e tangential deection of the boring bar’s tip can be established by either ‘direct’ , or ‘indirect’ met - rological techniques at the tool’s tip. In Fig. 66a, the graph depicts deections ‘∆’ (i.e. both the tangential ‘∆ T ’ and radial deection ‘∆ R ’), as a function of the cutting depth ‘a P ’. Due to the fact that the tangential deection (∆ T ) linearly increases with increasing D OC (a P ), it is usually recommended that machining passes are divided into a number of cuts when close toler- ances are needed (i.e. in the region of IT7 48 ) – see Table 5 49 for an abridged version of the IT tolerances, with *Rmax values in µm. e magnitude of radial deection as a function of the cutting depth, is also inuenced by the ratio between the insert’s nose radius and the D OC (a P ), to- gether with the boring insert’s entering angle. In some cases, a boring bar is situated slightly above the work- piece centreline, so that when it enters the cut at full depth it will have tangentially-deected to the actual 48 ‘IT’ (i.e. in units of µm) – represents the average value of the basic tolerance for the ‘diameter range’ in question. Hence, it will vary according to the choice of diameter range selected. 49 ese values are related to surface texture expression of: *Rmax (µm), which is: e maximum individual peak-to-val- ley height. e Rmax values (i.e. in Table 5) can be calculated from the IT value, using the following equation, rather than the conventional equation: Rmax = (fn 2 /r ε ) 125 this equation tends to give excessively high surface texture va- lues, thus more practical values related to IT are to be found from:    � Rmax   IT n  IT (µm) Where: n = e number of IT’s. Table 5: IT values related to the basic tolerance for various diameter ranges Dc (mm): Over /up to -/3 Over/up to 3/10 Over/up to 10/50 Over/up to 50/180 Over/up to 180/400 Over/up to 400/800 IT5 0.6 0.8 1.3 2.2 3.2 4.5 IT6 0.9 1.2 1.9 3.1 4.6 6.4 IT7 1.4 1.9 3.1 5.0 7.4 10.1 IT8 2.0 2.9 4.7 7.8 11.5 15.8 IT9 3.6 7.5 9.4 12.4 18.3 25.2 IT10 5.7 7.6 12.1 20.0 29.8 40.5 IT11 8.6 11.8 19.1 31.4 46.2 63.8 [Source: Sandvik Coromant (1995)] . Drilling and Associated Technologies  Figure 66. Hard-part boring, can create excessive boring bar deections and potential vibrational problems – if not carefully controlled. [Courtesy of Sandvik Coromant] .  Chapter  workpiece’s centreline. Boring bar overhang is not a problem when ‘Line-boring’ 50 as the tool is supported at both ends, or in the case of the novel ‘Telescopic line- boring tooling’ 51 . e chip area (i.e. illustrated in Fig. 66b – right), has an eect on the load on the insert’s cutting edge, par- ticularly when hard-part boring, although with small chip areas, this may not create a vibration problem, unless high friction is present between the insert and workpiece. However, the cutting forces substantially in- crease if a large chip area is utilised, necessitating some means ‘damping stability’ to the boring tool. 3.3 Reaming Technology – Introduction e reamer is the most commonly utilised tool for the production of accurate and precise holes, having high surface quality being true to form and tolerance. Ma- chine reamers can have either a single-blade design (Figs. 67 and 68), or are produced with a multiple series of cutting edges – of constant diameter (Fig. 69) or, ta- pered (Fig.73b) across a diverse range of diameters and lengths. e surface texture quality obtainable by ream- 50 ‘Line-boring’ , as its name implies is utilised for boring part’s with concentric and oen varying diameters throughout the overall component’s length. Normally, a ‘Line-boring tool’ is supported by a steady with suitable bushing and a mating ex- tension bar, some distance from the cutting edge and its re- spective rotating toolholder. is additional support enabling long bored features to be precisely machined to the part’s cen- treline in-situ. 51 ‘Telescopic line-boring tool’ , One major machine tool builder in association with a tooling manufacturer, produced a rather novel and clever ‘Telescopic line boring tool‘, for the machining of quite long cranksha bearing housings on both automo- tive engine blocks and bored cam-seatings for cylinder heads. is uniquely-designed ‘Telescopic line-boring tool’ , machined the rst bore, then continued to extend (i.e. telescopically feed-forward), whilst supporting its progress by mating with each automotive-machined bore, as it progressed through the large automotive component, thereby supporting the machin- ing operation throughout its boring cycle, then retracting on completion, allowing the tool to be held in the machine tool’s magazine, allowing/facilitating an ecient and speedy multi- ple in-line boring operation to be executed. ing ranges from approximately ‘Ra’ 52 0.2 to 6.5 µm, ac- cording to recommendations of DIN 4766. Normally, reamed nishes of about Ra 0.5 µm can be regarded as satisfactory. In general, reaming achieves tolerances of IT7, but if the reamer has been carefully ground, it can achieve tolerances of IT6, or even to IT5. 52 Arithmetic roughness ‘Ra’ parameter – it is the arithmetic mean of the absolute ordinate values Z(x) within the sampling length. It is the most frequently quoted international surface texture (i.e. amplitude) parameter, expressed in the following manner: Ra   lr l r    Zx  dx NB In the past and specically in the USA, its equivalent term was known as the ‘Arithmetic Average’ , denoted by sym- bols: ‘AA’. Figure 67. A sample of indexable insert reamer technology – for solid and oating reamer applications. [Courtesy of Seco Tools] . Drilling and Associated Technologies  Figure 68. Single-blade reamers oer superior hole geometry over conventional reamers. [Courtesy of Shefcut Tool & Eng’g Ltd.] .  Chapter  Prior to beginning the reaming process 53 , holes have to be either pre-drilled, or holes cored-drilled 54 . Due to the nature of the role of the burnishing pads on the hole’s machined and highly-compressed surface in Gun-drilling operations, it is not particularly suitable for reaming. Machine reamers can be divided into several cat- egories, these are: multi-point reamers with either a straight, or Morse taper 55 shank, these reamers are usually either manufactured from: HSS, Tungsten carbide (Solid), or with carbide tips. Typically, the Tungsten carbide (solid) reamers can be run at 10% higher feedrates, to their HSS equivalents and can ream workpiece materials up to a tensile strength of 1200 N mm –2 . Machine reamers are available with: straight utes, le-hand (LH) spirals, or 45° LH ‘quick’ spirals this lat- ter reamer version is oen termed a ‘Roughing reamer’ and is oen used for ‘long-chipping’ workpiece mate- rials. Reamers with straight utes are usually utilised to ream blind holes, but with the absence of chip space at the bottom, this means that swarf must be evacuated by the utes. For virtually all other machining tasks, such as holes with keyways, or intersecting holes, etc., 53 ‘Hand-reamers’ , are available for the reaming both cylindrical and tapered holes. NB A basic rule to be observed when hand-reaming, is to only turn the tool in the cutting direction and, never reverse it (e.g. is is the standard practice in cutting a thread with hand taps), as the reamer’s cutting edges will immediately be- come blunt. 54 ‘Core-drilling’ , this is normally undertaken with a multi- uted drill, as the hole already exists in the cast component and in the main, the drill cuts on its periphery, so needs more cutting edges in contact with the cored hole. Coring is result of employing a core, prior to casting and it stays in the cavity as the molten metal is gently poured to cast the part (i.e. cores are normally made from an appropriate sand and binder, or another suitable material, that can be removed at the ‘fettling stage’ – leaving the hole), hence, its name: cored hole. 55 ‘Morse taper’ , was developed in the USA in the mid-to-late 1800’s by Steven Morse (i.e famed for his design and develop- ment of the original geometry for the Twist drill). e Morse taper is a ‘self-holding taper’ , which can be suitable sleeved ei- ther upward, or downward in ‘ioned diameter’ to t the inter- nal taper for the machine tool’s spindle/tailstock, requiring a ‘dri’ to separate the matching tapers upon completion of the work. e Morse taper’s included angle varies marginally, de- pending upon its Number (i.e ranging from 0 to 6). Typically, a ‘No. 1’ is: 2° 58´ 54´ ´, with a ‘No. 6’ being: 2° 59´ 12´ ´. LH spiral reamers are employed. e chip direction is always in the feed direction and, for this reason, the spiral ute geometry is virtually exclusively used for through hole reaming operations. .. Reaming – Correction of Hole’s Roundness Profiles Machine Reaming In the ‘classical’ reaming operation, it is centre-drilled, then the hole is through-drilled possibly producing a variety of hole form harmonic out-of-roundness errors present (i.e. see Fig. 70 ‘polar plots’ – bottom le), including ‘bell-mouthing’ 56 at the entry and exit of through drilled holes. Not only is there a possibil- ity of ‘bell-mouthing’ , but a serious likelihood of the drill following a helical path through the part, this is termed: ‘helical-wandering’ (i.e. see ‘Footnote No. 3’ , for an explanation of this drilling condition). By a fol- lowing boring operation, this will correct for any prole errors, while improving both the part’s overall out-of- roundness 57 as exhibited by the ‘polar plots’ (ie. as il- lustrated in Fig. 70 middle-le), but the hole’s ‘cylin- dricity’ 58 . Finally, the machine reamer is used to full several functions: improve both the harmonic out-of- 56 ‘Bell-mouthing’ , is the result of the unsupported drill (i.e. the margins as yet, not in contact with the drilled hole’s side walls), producing the so-called ‘bell-mouth prole’ , upon hole entry. At exit, if the drill is allowed to feed too far past the un- derside of the hole, the drill has a ‘whipping-tendency’ , which could introduce a smaller ‘bell-mouthing eect’ beneath the part’s lower face. 57 ‘Out-of-roundness’ , was a term previously utilised, but today, the term used has been changed to: ‘Departures from round- ness’ , moreover, the term ‘polar plot’ has also been super- seded by the term ‘displayed prole’ , however, in the current context the former terms will be used. 58 ‘Cylindricity’ , is the term dened as: ‘Two, or more roundness planes used to produce a cylinder where the radial dierences are at a minimum’. NB A more easily-understood appreciation of what ‘cylindric- ity’ is, can hopefully be gained by the following ‘working ex- planation’: If a perfectly at plate is inclined at a shallow angle and, a parallel cylindrical component is rolled down this plate, then if it is ‘truly round’ as it rolls there should be no discern- ible radial/longitudinal motion apparent. In other words, the component is a truly round cylinder, which can be equated to a hole, or indeed, to a turned, or ground diameter. Drilling and Associated Technologies  roundness (Fig. 70 top-le) and surface texture, while ‘sizing’ the hole’s diameter. To further emphasise the point that drilling does not produce a consistent harmonic out-of-roundness, nor even a straight hole, Fig 71a, illustrates how the ‘polar plots’ are fundamentally modied at dierent hole depths, here the ‘plots’ are shown near the top, in the middle and close to the bottom of the drilled hole. Correction of these roundness and diametrical errors by machine reaming is not always the case, here (i.e. shown in Fig. 71b), if the reamer is either not set up correctly, or is slightly axially bent, in this case a Figure 69. Types of solid reamer and their associated geometry. [Courtesy of Guhring Ltd.] .  Chapter  . 0.6 0.8 1.3 2.2 3.2 4 .5 IT6 0.9 1.2 1.9 3.1 4.6 6.4 IT7 1.4 1.9 3.1 5. 0 7.4 10.1 IT8 2.0 2.9 4.7 7.8 11 .5 15. 8 IT9 3.6 7 .5 9.4 12.4 18.3 25. 2 IT10 5. 7 7.6 12.1 20.0 29.8 40 .5 IT11 8.6 11.8 19.1. 19.1 31.4 46.2 63.8 [Source: Sandvik Coromant (19 95) ] . Drilling and Associated Technologies  Figure 66. Hard -part boring, can create excessive boring bar deections and potential vibrational. overhangs associated with boring operations (i.e. see Figs. 62a and 65a), then the cutting forces generally dictate, short L/D ratios of < ;5: 1 and relatively large and robust boring bars (Fig. 65b).