with the rotational axis, but intersects it at the cen- tre of gravity of the ‘assembly’s body’. Under such conditions the force vectors equalise, but are 180° apart. 3. ‘Dynamic unbalance’ – dual-plane. Such a condi- tion of the toolholder assembly arises when the axis does not coincide with the rotational axis and is not either parallel to, nor intersecting this axis (i.e. see Fig. 232). For any rotating tooling assembly, estimating the cut- ter unbalance is possible using the following variables: M = cutter/holder mass, S = mass centre, e = displacement of mass centre, r = distance from centre of tooling, to the centre of gravity of mass (m), ω = angular velocity, m = mass unbalance, U = cutter unbalance, 9549 = a constant. Determining the relative unbalance (U) of a rotat- ing tooling assembly, can be found by the following expression(s): U = M × e or, alternatively: U = m × r (i). It is usual to express unbalance in terms of the product of the mass times distance, typically using the units: ‘g-mm’. Finding the magnitude of centrifugal force produced by the rotating tooling assembly with a given unbal- ance, can be established as follows: F = U × ω 2 (ii). Where: ‘ω’ is the angular velocity in units of radians sec –1 . e formula to nd ‘ω’ is expressed by: ω = � π � r pm (iii). erefore, by combining formulae: (i) and (iii), in (ii), we can obtain the magnitude of centrifugal force ‘F’ , as follows: F = m × r × ( 2 × π × rpm/60) 2 (iv). As established in equation (iv), the centrifugal force caused by tooling unbalance will increase by the ‘square of the speed’ , in a similar manner to the spin - dle nose taper swelling (i.e. growth) previously men- tioned. Nonetheless, assuming that this specic tool- holder initially has a low unbalance, this will become a problem if the rotational speeds are increased beyond 10,000 rev min –1 . For example, with most toolholders exhibiting single-plane unbalance 25 , research experi- mentation has shown that the initial unbalance of a typical tooling assembly will be of the order: 250 g- mm. When such tooling is rotated at 15,000 rev min –1 , this 250 g-mm of out-of-balance develops a continu- ous radial force of 642.6 N. Unbalanced tooling can introduce considerable detrimental eects on not only the machine tool – this high centrifugal force causing internal bearing stresses leading to premature spindle failure, but aects cut- ter life and degrades workpiece surface texture. Much of the principal tooling unbalance problems can be traced-back to several sources, such as: • Toolholders of the V-ange type, which might have dierent depth of drive/slots, these toolholder fea- tures being part of the inherent design, • Toolholders for some end mills and slot-drills, hav- ing set screws for locking the cutter securely in place, so due to necessary clearance and the radial application of the set screw, this creates minute cut- ter eccentricity – causing unbalance, • Out-of-balance caused by an unground V-ange base, • Collet and its collet nut tend to be recurring sources of unbalance in HSM tool holders. NB Most of these tool holding-related issues can be eliminated by simply modifying the tooling de- sign. As can be seen from Fig. 232, the marginally eccen- tric adjustable balance rings can be rotated to adjust the degree of single-plane balance, with several of the tooling manufacturers oering diering adjustment methods for HSM toolholders. Finally, consideration needs to be given to the level of balance-quality required and in HSM applications for example, a milling cutter is expected to withstand 25 ‘Single-plane unbalance’ , relates to the type of unbalance that occurs in either one of two planes. Namely, the tooling assem- bly’s single-plane unbalance will be in either its axial, or radial directional plane. Machining and Monitoring Strategies both high rotational speeds and associated cutting forces, thus here it can be considered as a ‘rigid ro- tating body’. is assumption allows one to use the ANSI S2.19-1989 Standard, for achieving balance – see Fig. 233, which denes the permissible residual unbalance of a rotating body relative to its maximum speed. is Standard and its equivalents (e.g. ISO: 1940:1; ISO: 1290 G), assigns dierent balance-quality grades termed: ‘G-numbers’ , related to the grouping of rotating bodies (i.e. not shown), these groupings be- ing based upon the experienced gained with a variety of: sizes; speeds; and types. us, the balance-quality grade ‘G’ , equals the specic unbalance ‘e’ times the rotational speed ‘ω’ , as follows: Balance-quality G = e × ω (mm sec –1 ). Furthermore, the equation was described earlier, thus: e = U M (i) ∴ solving for ‘U’ , we obtain: U = � M � G rpm (v). From the Standard, the balance-quality for machine tool drives is given as: G2.5, although in many in- stances the value utilised should ideally approach that of G1.0 – this being the specication for grinding ma- chine tool drives, as today in HSM applications they are compatible. However, if for the purposes of clari- cation of the unbalance tooling condition the value of G2.5 is utilised, then the following worked example illustrates the balance-quality necessary using a tool- holder weighing 3 kg, rotating at 25,000 rev min –1 : U (higher) = � � . , (g-mm) ∴ U (higher) = 2.85 g-mm. As alluded to previously, this unbalance condition is the ‘worst case’ and the tooling should ideally ap- proach G1.0, this balance-quality value, gives: U (lower) = � � . , (g-mm) ∴ U (lower) = 1.14 g-mm. is then follows that the balance is between 1.14 and 2.85 g-mm, which is toward the ‘upper-end’ for the maximum residual specic unbalance for the G2.5, while approaching this level for the G1.0 (i.e shown by the graph in Fig. 233). Even when the tooling assembly has been dynami- cally balanced in both planes (i.e. see Fig. 234a – more to be said on this topic shortly in Section 9.5.2), prob- lems still exist, particularly in the t of the spindle ta- per connection (Fig. 232). is is a result of the taper rate accuracy requirements between both the shank and taper socket. In fact, the situation is quite a con- fusing one, due to the relative cone ‘Angle Tolerance’ grades: AT-1 to AT-6, that are employed using the con- ventional tment of: 7:24 taper. Not only do dierent countries oen have their own connection Standards, but previously, even individual machine tool manu- facturers within each country had adopted diering Standards! Today, many machine tool companies tend to utilise taper spindle connections that are compat- ible to an appropriate Standard and complement those of the tooling manufacturers. .. HSM – Dynamic Balancing Machine Application It has been discussed in the previous sections that cut- ting tool assemblies when combined with an HSM strategy, can be a large contributor to dynamic unbal- ance. For instance, in the production and manufacture of say, the geometry of a face-mill, the tooling stock material is: externally/internally turned on one side; unclamped; ipped-over and rechecked; then turned on the other side; then located onto a milling machine tool for operations on the individual insert pockets that must be milled; and indexed 26 – as appropriate for the number of cutting edges; this necessary clamp- ing/reclamping workpiece (i.e. face-mill) procedure, will create a tool that is marginally-unbalanced. With HSM, the otherwise unnoticeable unbalance at con- ventional rotational speeds, becomes intolerable in these high-speed ranges. Oen, the most economical technique for achieving balanced tooling for tooling 26 ‘Insert pockets’ , are sometimes ‘dierentially-pitched’ which means they have unequal spacing of teeth around the cutter’s periphery. is pitching technique for cutting insert pockets, is quite eective as a means of reducing machining vibrational eects oen encountered with coarse-pitched face-mills. Chapter Figure 233. A graph to determine high-speed cutter unbalance ‘U’ (ANSI S2.19–1989). Machining and Monitoring Strategies designated for the HSM ranges, is by dynamically bal- ancing the tools in an appropriate machine (Fig. 234a). us, during tool balancing, the cutter is clamped in a xture that rotates in very rigidly supported bear- ings (Fig. 234a). Any unbalance in the rotating cut- ter is directly measured as centrifugal force, which is transmitted along with its actual rotational position to a specially-congured computer. e computer calculates the amount and location of the material to be removed from the tool’s body, in order to properly adjust the mass distribution. is unwanted tool ma- terial can then be removed, by either drilling holes, or milling ats (i.e. see Fig. 234b – illustrating that a small amount of material has been removed – milled – from its ange and is termed ‘Hard-balancing’). Usu- ally, ats are preferred for removing larger amounts of tool material stock, because at high rotational speeds a tool with a hole can generate a unacceptable ‘whistling noise’: see Fig. 235 to gain an appreciation of the eect of these high peripheral speeds, here, a large face-mill is shown in an HSM automotive application. Since the ‘balancing operation’ can compromise the tool’s strength and performance, it is important to establish where any superuous tool material can be safely re- moved from on the cutter’s body. Balance is a ‘zero-quantity’ , so it is customary to measure balance in the absence – within acceptable limits – of unbalance. us, the ‘balance tolerance’ is the maximum residual unbalance (g-mm) allowed for a particular tool’s weight and rotational speed. For ex- ample, the ANSI Standard quality-grades for balance tolerance range from G0.4 to G6.30, with the lower the ‘G-number’ the closer the balance tolerance. It should be emphasised, that only when a balanced tool and its balanced toolholder are balanced together as a com- plete unit, are they truly dynamically-balanced 27 . 27 ‘Dynamic-balancing’ postscript: the cutting inserts, screws wedges that are retained in the cutter’s pockets must be se- curely locked into position. If these small items become de- tached when HSM, this may cause disastrous consequences for any operator in the vicinity. erefore, machine tool guard- ing of more than adequate protection is vital here, to minimise potential safety hazards to the personnel when in use. 9.6 HSM – Research Applications Introduction Possibly the foremost reason for conducting applied re- search programmes at various universities and similar research-based organisations, is to ‘push the boundar- ies’ of our theoretical and practical understanding of current, or novel machining concepts. Rather than at- tempt to give a perhaps less-than-informed account of what is transpiring at other ‘learned organisations’. It was decided just to deal with some recent work that was undertaken by the author, in association with in- dustrial and academic colleagues, both in the UK and abroad. As these particular research projects were ei- ther undertaken, or instigated by the author, oen in close union with others, mainly concerned with indus- trial-sponsored doctoral programmes, it was felt that here at least, some continuity concerning HSM-related research themes could be achieved. .. Ultra-High Speed: Face-Milling Design and Development Introduction In order to achieve a cutting speed of say, 1,000 m min –1 , a φ10 mm milling cutter has to rotate at ap- proximately 32,000 rev min –1 . In fact, this is quite fea- sible for smaller diameter cutters, as there are quite a few of today’s machine tools having HSM spindles that can exploit these speeds. However, it is worth a sight digression here, prior to continuing with the theme concerned with special-purpose UHSM by face-mill- ing, to ask the pertinent question: ‘What do we mean by HSM?’ Is it: • High rotational speed machining? • High cutting speed machining? • High feed machining? • High speed and feed machining? • High productivity machining? Even these ve potential contenders for what amounts to HSM, are by no means an exhaustive list, one could also add more obscure factors such as: the power de- mand on the spindle; tool assembly balance speeds; Chapter Figure 234. Dynamic dual-plane (i.e. radial and axial) cutter balancing. [Courtesy of Ingersoll]. Machining and Monitoring Strategies Figure 235. High-speed large diameter face (nish) milling on a grey cast iron engine block. [Courtesy of Sumitomo Electric Hardmetal Ltd.] . Chapter the taper size in relation to its rotational speed; thin- walled machining capability; etc.; the list will grow, depending upon what we consider to constitute as go- ing either very fast, or what speed allows us to machine certain types of part features! Although even here, the major benets of an ultra-high speed machining strat- egy are somewhat lost if a ‘working denition’ is not clearly stated. In this current discussion on the sub- ject, one measure of HSM could be if the peripheral speed of the cutter, or workpiece is >1,500 m min –1 . Hence, this could represent a base-line for the tran- sition from conventional to HSM, so for comparison, as in the case for previously mentioned small diam- eter milling cutter of φ10 mm, it would need to rotate >47,750 rev min –1 , or conversely, a larger face-mill of say, φ300 mm, would have to rotate at approximately 1,600 rev min –1 – in order to sustain a peripheral cutter speed of at least 1,500 m min –1 . is latter rotational speed although considerably slower, is a much greater problem that that presented for the former small diam- eter cutter. e reasons for this are three-fold: rstly, has the machine tool got enough spindle power to achieve the necessary stock removal rates required, or will it be likely to stall? Secondly, is the spindle taper tment robust enough to cope with the torque eects and bending moments imparted during machining? irdly, will the cutting inserts still be retained at the high centrifugal forces generated in association with and exacerbated by the imparted cutting forces? ese and other lesser important questions and decisions need to be addressed, if the large diameter face-mill is to successfully mill across a wide workpiece surface with any degree of eciency. is former point of the manner in how workpiece surface stock is removed is important, for example, two markedly diering ma- chining strategies could be adopted, such as: I. Shallow depths of cut combined with rapid tra - verse rates and small step-overs, utilising smaller diameter cutters at high peripheral speeds, II. Deep and wide cuts with a large diameter face- mill with slower traverse rates, having much lower rotational speeds. NB Both machining strategies will remove simi- lar amounts of part stock! UHSM: Face-Milling Cutter Design When designing large diameter face-milling cutter assemblies for production applications in the UHSM range, a number of critical features need consideration, such as: cutter-body material; its weight and rigidity; taper tment; dual-plane balancing; as well as its aero- dynamic behaviour – at fast peripheral velocities. If one attempts to design and develop a large face- milling cutter with an insert cutting circumference designed to rotate at 3,000 m min –1 , which at rst glance, may not seem that fast. However, if we equate this cutting speed to that of the same φ10 mm mill- ing cutter previously mentioned, then this smaller tool would have to rotate at ≈95,500 rev min –1 , but for the larger diameter cutter, it would also require to be dual-plane balanced 28 . Without dynamic bal- ancing, the large cutter may be prone to a disastrous series of vibrational problems, which may ultimately lead to premature cutter failure – with all its attendant safety hazards. e cutter design in Fig. 236 for this applied research programme, was dual-plane balanced to Standard dened in ISO:1940/1, being to a specic ‘G-number’. is Standard was initially conceived for the rotational balancing of impellers and similar high-speed equipment, across a large speed range. e large face-mills had been dual-plane balanced to G2.5 @ 10,000 rev min –1 . As mentioned in Section 9.5.1, this ‘G-number’ refers to the maximum tolerable imbal- ance for the complete tooling assembly, being based upon the previously described formula (i.e given be- low again for clarication), resulting in the following calculations: Unbalance: U = � M � G N (g-mm) [or] Force: F U N N Where: U = allowable unbalance (g-mm); 9549 = a constant; M = mass, or weight of the total cutter assembly (kg); G = pre-selected balance tolerance number from ISO: 1940/1; 28 ‘Dual-plane balanced cutters’ , are those cutters that have been dynamically balanced in two planes: having both radial and axial balance. Machining and Monitoring Strategies N = maximum rotational speed of tooling assembly (rev min –1 ); F = force (N). When the above equation for cutter unbalance is utilised, for these tooling assemblies, utilising the fol- lowing values: G = 2.5; M = 4.294; N = 6,000; which then gave an allowable imbalance U = 17.9 g-mm. is level of imbalance means that the cutter’s mass can- not rotationally shi by more than 17.9 g-mm, if it is to maintain dual-plane balance at a peripheral speed of at least 3,000 m min –1 . In fact, two identical cutter assemblies were designed and manufactured, hav- ing a BT40 taper tment – for the vertical machining centre (i.e. Cincinnati Milacron Sabre 500). In order to maintain both structural rigidity and integrity, the complete cutter bodies and their associated ta- pers were each produced from single stock of EN24T steel. Aer precision turning and milling the complete bodies and insert pockets, these cutters were nitride- hardened 29 to HR C 52, prior to a ‘very light-grinding’ process and then balancing. e four cutting insert pockets were equally-spaced (pitched) and the but- ton-style cemented carbide inserts were: φ12 mm by 4mm thick, single-sided and TiN-coated (Stellram: RPET 1204 DFZ). e insert pocket geometry had a 11° toe angle with a neutral geometry. Button inserts were selected as they give the strongest shape cutting geometry available (see Fig. 23), producing an innite approach angle to the workpiece (see Fig. 83b), thus minimising impact load at entry to the cut while of- fering multiple cutting edges – when subsequently turned in their seatings. As one might expect, insert security is vitally important, due to the great centrifu- gal eects and applied cutting forces. Due to the pre- vious nitride-hardening process, hardened insert seats were unnecessary, once the retaining screws had been ‘torqued-up’ locking and then sealing them – for se- 29 ‘Nitride-hardening’ , produces a very hard surface with a soer and tougher matrix. e UHSM cutters were held in a pressure-tight furnace and heated to between 500-550°C for some hours in an ammonia gas*, allowing the nitrogen atoms to diuse into the surface and to form ne stable nitride pre- cipitates with aluminium constituents, allowing the nitrided- steel surface to be precipitation- hardened. No subsequent heat-treatment is necessary. *Approximately 30% of the ammonia disassociates ( NH 3 → ← 3H+N) and part of the nascent nitrogen is absorbed by the surface layers of the steel. (Source: Cotrell et al., 1979) curity. e actual seatings for the inserts had consid- erable body-support around their periphery, just hav- ing a working-clearance at the insert’s cutting region. ese UHSM face-mills were extremely compact, with the minimum stand-o height from the cutter’s gauge line (i.e. see Fig. 236), which reduced the eects of the previously mentioned ‘rigidity rule’. Due to the relatively large diameter and weight of these face-mills and the fact that the machining centre had limited spindle power, these cutters could, if used appropriately, exploit the ‘mass’ , or ‘ywheel-eect’ of their weight in conjunction with rotational speed to ‘store inertia’. So, when the spindle power is restricted, cutters with high mass must be taken up to their de- sired rotational speed in a progressive manner, other- wise they are likely to ‘trip’ a ‘spindle over-load’ in the CNC controller. is steady and progressive increase in the cutter’s rotational speed occurred at 500 rev min –1 increments – dwelling for several seconds to minimise inertial power overload, between increases to the de- sired peripheral speed. Due to the machine tool hav- ing a maximum spindle speed of 6,000 m min –1 , this equated to a peripheral cutting speed of 3,000 m min –1 , with the face-mill having 0.5 m cutting circumference. Once the cutter has reached its top speed, it can then be rapidly progressed (i.e. fed) across the workpiece at a rate of 20 m min –1 . In this case the workpiece ma- terials were a range of stainless steel alloy testpieces (Fig. 236). Aer rapidly face-milling these ‘stainless testpieces’ , the cutter’s rotation was decremented in 500 rev min –1 intervals until stationary. e cutter once stationary, could have its edge wear (inserts) assused and workpiece milled surface texture and surface in- tegrity could be inspected and investigated. One factor to bear in mind concerning UHSM with large face-milling cutters being utilised for their ‘in- ertial eect’ , is to design them without driving dogs. If these ‘dogs’ were tted, not only can they introduce out-of-balance eects, but tend to signicantly disrupt the air-ow and introduce alarming and high noise factors. is aspect of cutter design is important, if the cutter cannot ‘cleave through the air’ with aerody- namic eciency, turbulent air ow will result and op- erational noise becomes excessive. One problem that these particular cutters did not suer from (i.e despite the conventional taper-cone angle) – unlike many of their higher rotational speed counterparts, was ‘spin- dle nose swelling’ , which can cause a lack of regis - ter if the taper tment connection is not of either the double-, or triple-contact face-and-cone types. One unexpected aspect of employing such large face-mills Chapter Figure 236. A specially-designed dual-plane (radial and axial) face mill, for ultra-high-speed milling. [Source: Smith, Wyatt & Hope, 1998] . Machining and Monitoring Strategies in UHSM, was found to be due to the very high peri- pheral speed. A ‘suction-eect’ resulted from the un- derside clearance of the cutter’s body, created a ‘low- pressure region’. is ‘virtual vacuum’ here, meant that conventional-pressure ood coolant application was not possible, as it simply vaporised to a mist! UHSM: Cutting Trials e neutral geometry enabled the cutting inserts to present a strong cutting edge to these stainless steel testpieces, enabling an undistorted cut path and ma- chined cusp to be generated. is insert geometry feature, allowed the milled surface topography to be unaected by insert inclination angles. A stringent test for any cutter is to machine stainless steel by UHSM (ie. being least × 10 faster than any work previously undertaken) and here, tests were conducted on vari- ous grades (Fig. 236). e subsequent milled surface analysis showed little in the way of sub-surface plas- tic deformation – aer UHSM. e surface layers exhibiting only marginal increases in the vicinity of the surface when tapered sections were micro-Knoop ‘foot-printed’ , over these stainless steel’s substrate (i.e. see Footnotes: 15, Chapter 1; 85, Chapter 7; and Fig. 187c – concerning Knoop indentors usage). Milled surface topography can be viewed visually and surface parameters taken by either: 3-Dimensional contact; or non-contact instruments; but in this case, by utilising an SEM 30 with its unique ‘Stereo-imaging and topog- 30 ‘Scanning Electron Microscopes’ (SEM’s), operational princi- ple is relatively simple. In that, at the top of the SEM’s column an electron gun resides having a tungsten lament held in a strong electrical eld. is results in the electron gun emitting electrons (i.e. negatively-charged atomic particles), which ac- celerate to very high speeds. ese high speed electrons – held in a vacuum – travel down the column, being inuenced by lenses lower in the column, which squeeze them together to form an electron beam of very small diameter. is minute diameter electron beam is then focussed prior to colliding with the test specimen in the microscope’s specimen chamber, now as a diminutive spot. is minuscule spot will then scan both to the le and to right as well as up and down over the test surface, the information from which is then brought to a screen as an image. Prior to this, as the electron beam strikes the test sample’s surface, many dierent processes occur, such as: secondary electrons; backscattered electrons; Auger elec- trons; X-rays; Cathodo-luminescence; etc.; these being emit- ted, collected and counted, then utilised for further analyses. raphy soware’ – for 3-D visual assessment coupled to its height-to-depth proling application. For the milled testpieces produced from 316-aus- tenitic stainless steel, subjected to UHSM by this face- mill at 3,000 m min –1 , the surface topography showed the inuence of the wear land at produced by the four φ12 mm TiN-coated inserts, although the remain- der of periodic surface oered little sign of any surface modication. e 303 stainless steel grade testpieces, indicating a slight improvement over the former 316 grade. While, 416 martensitic stainless steel testpieces under identical cutting data generated no appreciable surface blemishes, with the additional benets of: an extended cutting insert life; signicant reductions in both cutting forces and power requirements, over the 303 and 316 stainless steel grades. .. Ultra-High Speed: Turning Operations Introduction As has been shown in the previous sections with ref- erence to HSM by milling, considerable applied and fundamental research eort has occurred, conversely, little endeavour has been made regarding high-speed turning operations. Possibly the major reason for the lack of interest here into HSM by turning operations, is because a dierent approach to the workholding is- sues needs to be taken. In that, on a CNC turning cen- tre, or lathe, the ‘bursting-pressures’ 31 resulting from signicant centrifugal forces with conventional work- NB e depth of elds from an SEM are considerably deeper than that produced by conventional microscopes, allowing some exacting surface topography analysis to be undertaken. (Source: Smith et al., 2002) 31 ‘Bursting-pressure’ problems, have been well-known in tra- ditional turning activities for many years. is aspect of safe-working practice, was particularly relevant for large cast iron face-plate work, where a rotational speed limitation is imposed by a machine tool builder. If this restricted speed is exceeded, then the cast iron – being poor in terms of tensile strength, will literally fragment (i.e. ‘burst’), due to the exces- sive centrifugal forces imposed. While, the problem is not as severe for chuck and collet work, the lack of gripping-pressure on the part – at high rotations, will aect the workpiece if long slender parts supported on one end only are turned – possibly causing a ‘whipping-eect’ and attendant safety hazard. Chapter . HSM?’ Is it: • High rotational speed machining? • High cutting speed machining? • High feed machining? • High speed and feed machining? • High productivity machining? Even these ve potential. de- mand on the spindle; tool assembly balance speeds; Chapter Figure 234. Dynamic dual-plane (i.e. radial and axial) cutter balancing. [Courtesy of Ingersoll]. Machining and Monitoring Strategies. unbalance will be in either its axial, or radial directional plane. Machining and Monitoring Strategies both high rotational speeds and associated cutting forces, thus here it can be considered