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  Figure 70. The exaggerated hole errors caused by an incorrect drill point geometry and the manufacturing techniques for its subsequent correction . Drilling and Associated Technologies  Figure 71. Reaming can correct an assymetrically drilled hole – when correctly adjusted.  Chapter  large harmonic variation in the ‘plots’ is depicted, as is the case when a ‘Floating reamer’ with roller drive has been used inappropriately. Floating Reaming Solid machine reamers can be ‘oated-down’ 59 a pre- drilled hole, to produce a much straighter reamed hole, than would otherwise be the case. When ‘oat- ing’ reamers within their specially-located toolhold- ers, two techniques are used to ‘oat reamers’ (i.e see Fig. 72), these are: 1. Radial play – where the machine reamer has lim- ited movement laterally with respect to the princi- pal axis, 2. Composed radial and pendulum play – this has both radial play, together with a degree of limited angular movement (i.e. this motion is similar to that of a Grandfather clock’s timing mechanism, via its pendulum motion). NB  is latter ‘oating’ technique has the potential for a combination of both radial and pendulum motions to the machine reamer. ese unrestrained kinematic motions gives it free motion without lateral and angu- lar constraint, to simply follow the ‘line of least resis- tance’ along the spindle axis, as the reamer progres- sively feeds down through the predrilled workpiece. .. Radially-Adjustable Machine Reamers Special-purpose machine drill/reamers (Fig. 74a) are oen utilised in high-volume production envi- ronments such as in the automotive sector, for util- ity engines which can account for >55,000 complex- reaming operations per week. Conversely, for defence vehicle engines the production volumes are quite low, accounting for <300 operations per month. Typical operations on such automotive components, using a machining centre include the reaming of: 59 ‘Floated-reaming’ , relates to the reamer’s ability to have some degree of lateral compliance, namely limited motion, allowing it some ‘play’ to follow the hole’s path, but still correcting for any previous ‘helical wandering’ by the drill. • Cylinder head tappet rail drill-reaming – in a sin- gle operation, • Cylinder head valve seats and guides – machining both features, in the parent bore and nish machin- ing, Figure 72. Solid machine reamers can be ‘oated-down’ a pre-drilled hole, by two distinct ‘oating techniques’: (I) radial play, (II) composed radial pendulum play. [Courtesy of Guhring Ltd.] . Drilling and Associated Technologies  • Engine block and crank bores and cheek faces – nish machining, with this latter feature requiring controlled ‘radial infeed’ of the cutting/reaming in- sert. NB  e special-purpose ‘radial-infeed’ tooling neces- sary for the satisfactory machining of the cheek faces of this latter low-volume production engine block, will now be briey discussed. Case-Study of Engine Block Bore Features In this novel, but interesting automotive ‘case-study’ , all of the challenges facing such special-purpose reamers are present. Here, the machining application consisted of the following: a six-cylinder diesel cast iron engine block for an armoured personnel car- rier, reaming at 70 m min –1 , requiring a bore straight- ness of 0.02 mm/m, tolerance on the bore diameter of 0.025 mm, with >0.003 mm tolerance between the individual journals. e solution to this demanding industrial problem, was the machining with two tools and three operations of the crank bore and the genera- tion of two cheek faces – this latter operation was nec- essary to minimise the tted cranksha’s end oat. is particular special-purpose reamer had a ra- dial feed-out/retract cutting insert requirement for the nal-machining of the cheek faces. erefore, the base-tool holder contained a thrust and feed-out mechanism, in addition to the whole tooling assem- bly ‘running-true’ , so that it could be ‘datum-out’ and precisely and axially-set with respect to its potential engine block machining features. e radial mecha- nism would incorporate an actuator sha mechanism which can be pulled-/pushed-back, thereby resulting in either a radial infeed, or retraction, respectively, of the cutting insert. is bi-directional control of the feed-out/-in of the radial mechanism is achieved in conjunction with the CNC feed spindle of the machin- ing centre. In general, these special purpose reamers, have two guide pads and a blade (Fig. 74a) with the reaming blade set with a back-taper, producing the well-known characteristic ‘saw-toothed prole’ to the reamed sur- face (Fig. 74b – right). Such reamed surface texture to- pography has been highly magnied in the schematic diagram (Fig. 74b – right) and, requires very high vertical magnication of the surface topography (i.e. x50,000) to see any trace prole details at all! e posi- tion of the cemented carbide guide pads, with respect to the blade is critical to the reamer’s performance, as is the residual stiness of the whole cantilevered tool- ing assembly. For many automotive industrial reaming applica- tions, the components are oen cast from high-silicon aluminium materials, as the addition of the element silicon, creates a micro-grained and harder cast struc- ture, than would otherwise be the case. However, the disadvantage from a machining viewpoint, is that the resultant cast matrix is highly abrasive to the cutting edge. Under these circumstances, the reamer’s blade Figure 73. Reamers in action, reaming automotive parts. [Courtesy of Shefcut Tool & Eng’g Ltd.] .  Chapter  . 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 ),. [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. 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,