Figure 228. High-speed milling toolholders (chucks) utilising tapered sleeves with needle rollers for elastic deection, when tightened – to precisely hold cutter’s shank along its whole gripping length. [Courtesy of Diashowa Tooling] . Machining and Monitoring Strategies quickly heats up the holder and in so doing, expands the bore allowing the tool’s shank to be placed in-situ in the toolholder whereupon it contracts around the shank – taking about 7 seconds to initially expand the bore by ≈5 µm. is thermal expansion gives enough clearance to allow the speedy withdrawal of an old tool and its replacement by the new one. e heat is lo- calised at the tool’s sleeve enabling the user to remove it wearing protective gloves. Having removed the new tooling assembly from the induction machine the user places this tooling in a solid aluminium cooling-block, which eciently dissipates the heat from the tooling assembly. us, just a few seconds aer the assembly has resided in the cooling-block, the bore shrinks back, but is constrained from reaching its equilibrium diameter by the slightly larger diameter of the tool’s shank. is thermal/mechanical restriction creates the high, but uniform gripping pressure around and along the tool’s shank which rmly secures it in place. e entire shrink-tting process from start-to-nish, only takes about 30 seconds to complete. e heating control can be adjusted by the user to accommodate large, or small diameter toolhold- ers, requiring diering thermal expansion cycles and with experience, the optimum settings can be quickly established. So, once the correct power and tempera- ture cycles have been created these parameters can be employed to regulate the induction machine, thus avoiding energy wastage. If even tighter control is de- manded of the heating process, an infra-red sensing device can detect the amount of heat in the toolholder, so that once it reaches a preset temperature, the in- duction heating is automatically shut o. Many other features and protection devices are incorporated into such induction machines allowing them to be speci- cally-tailored to the company’s requirements. Typical HSM shrink-t tool holding assemblies can be dynamically balanced to G2.5 at 20,000 rev min –1 , while special-purpose toolholders can be purchased that are balanced to G1.0 t 40,000 rev min –1 . More will be mentioned concerning this critical factor of dy- namic tool balancing for HSM applications shortly, in Section 9.5. Cryogenic methods (i.e not shown) are the direct opposite of thermal techniques. Here, instead of the toolholder’s sleeve being thermally-expanded, the tool is now the subject. As the tool’s shank is inverted and held at the desired depth beneath the cryogenic liquid which is held in a suitable container, it is cooled down by the required ratio of a mixture of liquid nitrogen- to-methanol. is immersion of the tool’s shank in the cryogenic uid, shrinks the shank’s diameter. us, the contracted tool’s diameter is then placed into the tool- holder (i.e. held at its ambient temperature), which has its bore as an interference t 23 with respect to the tool shank’s diameter. Here, the tool shank nally expands and causes an increased gripping pressure as it is re- stricted by the toolholder’s smaller bore. As a safety note, the user should always wear cryogenically-pro- tective gloves, otherwise a serious case of prospective ‘frost-bite’ will normally be the likely outcome! HSM Chucks – for Turning Operations Although in this section the principal concern has been with the design concepts and techniques of se- curely and accurately locking the tool in its respective toolholder. It is worth deviating somewhat here, to consider the major problems encountered when at- tempting an HSM strategy in turning operations, prior to discussing how and by what mechanical methods, the rotating toolholders – previously alluded to, are held in the spindle’s taper. In HSM turning operations, the grip, or clamping force exerted by a chuck on the workpiece changes when the chuck is rotated at speed. is change be- ing the direct consequence of centrifugal force that acts on the chuck’s jaw assembly. e magnitude of this clamping force is very signicant, as a standard power chuck loses 66% of its ‘static clamping force’ (i.e. the force exerted by the chuck when it is not rotating) at its maximum rated speed (i.e. these maximum oper- ating chuck speeds are dened in European Standard: EN1550). 23 ‘Interference ts’*, can be best dened, relating to the ‘Limits and Fits’ of a mating hole and sha, as follows: ‘e minimum permitted diameter of the sha is larger than the maximum allowable diameter of the hole.’ (Source: Galyer and Shotbolt, 1990)* In practice, interference ts are those for which, prior to assembly, the inside (male) component being larger than the outside (female) component, requires either some form of: deformation (i.e. pressure); thermal contraction – for a sha; or otherwise, thermal expansion – for the hole; to obtain a ‘permanent and secure tment’. us, either plastic deform- ation – for press-ts, or elastic pressure – by thermal eects, have been exerted between these mechanically-mating sur- faces, in order to achieve the desired component assembly. (Sources: BS 4500A and B; Childs, 2004; Griths et al., 2003) Chapter e capacity for the workpiece, or even a chuck component to cause damage, or personal injury is di- rectly related to its kinetic energy (KE). us, the kin- etic energy of a body is proportional to the square of its speed. As a consequence, the capacity of a fast ro- tating body (i.e. chuck assembly) such as a 1 kg work - piece rotating at 2,000 rev min –1 would have a KE of ≈ 7 Joules , whereas at 6,000 rev min –1 the value would be 62 Joules – approximately the same as a small cali- bre (0.25) handgun bullet ballistically hitting a target at 100 m range! Even though the KE of the workpiece represents a serious safety hazard, the energy stored in the clamping jaws is far greater. For example, a chuck assembly tted with its jaws (i.e. shown in partial sec- tion in Fig. 229b) of φ200 mm rotating at 6,000 rev min –1 would produce a KE of 1460 Joules (Fig. 229c), which is about the same as a 0.44 Magnum handgun bullet as it ballistically-issues from the muzzle! With only 33% of the static clamping force remaining at the maximum chuck rotational speed, there is con- siderable energy being stored in the rotating chuck assembly. It is important that operators understand how to determine safe operational speeds when using non-standard jaws, coupled to its residual grip, while having an appreciation of the energy stored in these rotating parts. As one might surmise from the application of HSM turning chucks, friction plays an important role in terms of the chuck’s actual performance in-service. Of note, is that a freshly-assembled and greased power chuck will oen exceed its maximum clamping force by almost 20%, despite this fact, aer a few weeks of use, or indeed non-usage, the maximum-rated clamp- ing force may be reduced to as little as 30%. is considerable drop-o in clamping performance is at- tributable to the absence of eective lubricant and the presence of particulates on the chuck’s sliding surfaces. Substantial losses can result even when the chuck has been frequently lubricated, resulting from the wrong grease, or deposits that are not periodically removed. In essence, grease comprises of oils and solids that are bound together with soap. us, the oils lubricate the sliding surfaces and the purpose of the solids are two- fold: rstly, they impede the escape of oils when the pressure between the sliding surfaces would usually squeeze them away; secondly, they directly lubricate by a shearing mechanism at higher pressures. So, greases having a higher solid content would normally produce optimum clamping performance. However, when em- ploying the chuck in the regions HSM, the centrifugal eects act to separate out the solid and oil constitu- ents of the grease, causing the oils to be thrown out of the chuck and leaving solid matter remnants. Over an extended time period, these solids from the grease collect in various: gaps; recesses; and cavities; combin- ing with small amounts of debris (e.g. nes and parti- cles) generated during previous machining operations, which impair the chuck’s performance. While the ap- plication of coolant exacerbates this situation still fur- ther, as it tends to leach-away the grease and accelerate its break-down, possibly causing corrosive damage to a very expensive chuck. In order to combat these un- desirable eects, additional compounds are necessary, such as polymers that can improve the lubricant’s: co- hesion; adhesion; and water-resistance. Special care must be utilised to ensure that a chuck’s reliable performance occurs when HSM operations are employed, as they are especially susceptible to cen- trifugal separation and leaching of grease – by coolant application. Under such circumstances, it is probably advisable to monitor the chuck performance over a period of time and apply grease, or service the chuck once a certain wear pattern emerges allowing one to create a specic maintenance schedule. Measurement of chucking performance is vital and simply applying grease at regular intervals is no guarantee of its per- formance. When a chuck becomes congested with solid mat- ter – separated grease constituents, it prevents fresh grease from reaching the critical surfaces, so in eect provides little, or no improvement. Clamping force measurements should be taken both before and aer the application of grease, so ensuring that it is evident as to whether, or not, the chuck needs to be serviced/ cleaned. Measurements of the static and dynamic clamping forces can be simply determined using a ‘Ra- dio Frequency Gripmeter’ (RFG). e RFG essentially comprises of just a load cell and handset. e load cell is clamped in the chuck’s jaws and the handset displays the measured clamping forces (i.e. the ‘grip‘ being accurate to 1kN per jaw), thus avoiding any lengthy and time-consuming calculations. ese RFG’s are available from reputable chuck manufacturers, with a typical handset being able to store up to 120 separate readings, having a PC-link to Windows © compatible soware, allowing graphical trends and further ana- lyses to be undertaken – as necessary. In any HSM applications for turning, ‘centrifugally- balanced chucks’ can be utilised as they incorporate a counter-balance mass that equalises the centrifu- Machining and Monitoring Strategies Figure 229. Thermal expansion tooling its operation and high-speed turning chuck details. Chapter gal loss of the jaws – when rotating at typically high turning speeds (i.e see Fig. 229b for a diagrammatic cutaway assembly of an HSM quick-change chuck). ese quick-change chucks incorporate a traditional wedge-style and lever mechanism, that instead of dir- ectly acting on the jaws, the radial force acts through the actuator and lever mechanism, prior to transfer- ring the eort to the jaws. So, when the chuck rotates at high-speed, the actuators are thrown outward by centrifugal force, but are restrained from moving by the lever, which pivots about the central connected sphere. At the opposite end of the lever, the jaws are also thrown outward and act to move the pivot in the opposing direction (i.e. to that of the actuators) – ef- fectively balancing each other. e performance of the counter-balanced chuck depends upon the accuracy of the balance achieved, as the actuator mass is constant and the top jaw mass being variable depending upon the particular top jaws in use, thus the state of the balance will also vary. As a consequence, the clamp- ing force may fall with rotational speed, or actually increase with heavy and light jaws, respectively. With standard hardtop jaws, the clamping force remains al- most constant across the range of the operating speed, making it unnecessary to calculate the clamping force losses. An additional feature is that the static clamping force can be much lower, since there is no centrifugal loss. is lower static clamping force application, has the benet that when turning either thin-walled, or more delicate workpieces that may otherwise distort with higher clamping forces, such chucks are unlikely to aect these components, when an HSM turning strategy is utilised. Much more could be said concerning HSM turn- ing operations, particularly relating to the calculations and working practices, but it was not the intention here, to give a comprehensive account of such tech- nical aspects, simply a concise account of the antici- pated problems and possible solutions when turning at high rotational speeds. In the following section, a discussion concerning toolholder coupling to the ma- chine tool’s spindle will be briey reviewed. .. Toolholder Design and Spindle Taper Introduc tion In the past, the taper cone and its associated driving dogs and pull-stud, provided adequate location and torque for the cutter assembly when mounted into the machine tool’s spindle. e tool’s cone taper an- gle was adequately manufactured so that it perfectly ‘wedged’ into its mating spindle taper and the prob- lem of the single-contact mechanical interface was not really exposed as decient, until very high rotational speeds were being utilised, coupled to much greater feedrates that the newly-developed tooling geometries and tool materials could now exploit. In recent years, both dual- and triple-contact tooling systems have been introduced, these designs will now be briey re- viewed. Dual-Contact Tool/Spindle Design One of the most signicant developments in maintain- ing a complete mechanical interface between the tool- holder and the machine’s spindle was the dual-contact 7/24 taper system 24 . e CAT Standard incorporates this 7/24 taper, but also allows simultaneous contact on both the toolholder’s ange and taper, when HSM machining is the requirement. By achieving this dual- contact, the CAT-shank toolholders minimise any form inherent imbalance at say, 2,000 rev min –1 . How- ever, if the cutter assembly is to be rotated at 10,000 rev min –1 , the toolholder must cope with a × 25 increase in centrifugal force, which may compound any unbal- ance present in the tooling assembly. Further, if the ro- tational speed is increased still further, into the HSM range, then here, the centrifugal force is × 100 greater and the onset of considerable imbalance may create chattering conditions. At such high rotational speeds, if coolant is utilised in the machining process, the HSM conditions could develop a vortex around the cutting tool, that conventional ood coolant pressures cannot penetrate. In these circumstances, possibly the only realistic option is to utilise a through-the-spindle coolant delivery application at pressures of >690 kPa (i.e. 1,000 psi), coupled to perhaps, micro-ltration of the coolant with special pipes and couplings. e CAT system of dual-contact oers reasonable rotational control of the tooling assembly at moderate-to-high rotational speeds, as the mechanical interface system of face-and-cone provides a certain security against 24 ‘Dual-contact 7/24 taper system’ , refers to the taper being to the 7 inches of taper per 24 inches of length. is 7/24 system incorporates several Standards: CAT and BT 40- and 50-taper tooling. Machining and Monitoring Strategies the onset of imbalance. Typical applications for these HSM dual-contact systems include: aerospace part production; precision die and mould making; automo- tive component production; as well as medical compo- nent manufacturing. It is worth digressing somewhat, to explain the situ- ation of why the single-cone mechanical interface is simply not eective for HSM production applications. When rotational speeds begin to approach 20,000 rev min –1 , it is not an unusual occurrence for the single- contact conventional, or standard CAT V-ange tool- ing assembly to be eectively sucked into the spindle (i.e. as there is no mechanical contact at the ange), this being the result of a combination of the pull-stud pressure and the machine’s spindle ‘taper swelling’ – due to the very high centrifugal force acting at such high rotational speeds. In fact, this minute amount of ‘taper swelling’ can cause the tool holder to separate from the spindle’s surface and as a result cause considerable damage to both the cone’s male and female surfaces. In order to alleviate this HSM problem and run the tooling assemblies at even faster rotational speeds, the HSK dual-contact toolholders were developed, which will now be briey mentioned. Hsk Dual-Contact Tooling ere are a number of toolholder designs that are al- ternatives to the conventional steep-taper spindle con- nection. Probably the most popular version for HSM is the HSK-designed tooling connection (i.e see perti- nent HSK tooling details in Fig. 126c). HSK toolholder connections oer simultaneous tment on both the taper and face, at the front of the spindle. e reason for their acknowledged popularity amongst the HSM machining companies, is because the increased rigid- ity of the joint, coupled with their inherent reduction in dimensions, compared to the equivalent conven- tional steep-taper connection. In Fig. 126c, the HSK 8° (included angle) short taper with its gauge face con- tact and simultaneous taper interference can be seen, which was designed in Germany to Standard: DIN 69893, being introduced in 1993. HSK is a German acronym that translates into English as: ‘Hollow short taper’. us, the HSK connection provides: • both high static and dynamic stiness, • oering great axial and radial repeatable accuracy, • with low mass and stroke, • having inner clamping. erefore, with all these proven design advantages over conventional spindle connections, it allows the HSK tooling assemblies to utilise the increased rota- tional speeds necessary for an HSM strategy. Triple-Contact Tool/Spindle Design e triple-contact connection is being oered by a few toolholder manufacturers (i.e. shown in Fig. 230). e triple-contact design relies on an inner expand- ing sleeve which maintains uniform contact between the machine tool spindle and the: toolholder’s top ta- per; bottom taper; and ange; this being regardless of the spindle speed employed. Of particular note is the inner expanding sleeve which functions particularly well at high spindle speeds. So, as the centrifugal forces increase – with higher rotational speeds, it causes the spindle to grow (i.e. ‘swell’), the toolholder’s spring mechanism forces the split-cone sleeve to proportion- ally-expand with the spindle. Further, the expanding sleeve also acts as a vibration-dampening device. e expanding sleeve extends the tool’s life on average by between 300 to 500%, by virtually eliminating vibra- tion. As a result of this ‘vibration-free interface’ be- tween the tool and workpiece, it provides smoother machining of: tool steels; aluminium alloys; plus other metallic alloys. is triple-contact connection system, also performs eciently with extra-long tools (i.e see Fig. 231), notably when utilised on horizontal machin- ing centres. e main reason for the enhanced triple- contact tool’s cutting performance with extended tooling assemblies, is the result of the ‘oating’ inner sleeve (Fig. 230) which acts to minimise any potential Z-axis deection, thus maintaining its rotational con- centricity. Such triple-contact tooling is not inexpensive to purchase, but these toolholders really do amortise their cost, by signicantly extending cutter life, while improving part production rates. Further, it is claimed by the tooling manufacturer that the toolholder is ‘maintenance-free’ , while its spring-mechanism in ‘life-testing’ has achieved upward of one million tool changes. With the advent of either the double- and triple-contact systems, enabling contact between the machine tool’s spindle and the toolholder’s mechanical interface: top-taper; bottom-taper; plus ange; while ‘eliminating vibration’; this has been achieved under the unique conditions that arise with today’s HSM and high-accuracy and precision manufacturing needs. Chapter 9.5 Dynamic Balance of Toolholding Assemblies Introduc tion Balancing tools that are intended for HSM applica- tions is vitally important and there are quite a few In- dustrial/Manufacturing engineers and users who do not really understand the concept of how to achieve balanced tooling, or why it is really necessary. Either very long extended tooling required for say, for deep- pocketing (Fig. 231), or tooling that is out-of-balance, will more than likely produce: chattering eects; goug- ing of a step, or face; loss of workpiece accuracy and precision; not to mention uneven and premature cut- ter wear. Whenever a new tooling assembly is destined Figure 230. Triple-contact tool connection system is ideal for any potential HSM operations. [Courtesy of Heartech Precision Inc. (HPI)] . Machining and Monitoring Strategies Figure 231. Tool runout (≥10 µm) should be of prime importance when machining deep pockets. [Courtesy of Sandvik Coromant] . Chapter for HSM applications on a workpiece, a balancing operation needs to be undertaken, this statement is also true for many sub-HSM applications, particularly when extended tooling is used for whatever reason (Fig. 231). In fact, every rotating object (i.e. chuck, or tooling assembly, etc.), will generate vibration. As has been explained in the previous section, this vibration results from a number of sources, but princi- pally here, from centrifugal forces produced by the ro- tation of an unbalanced mass. ere are several types of unbalance that could arise, but here, we are mainly concerned with what is termed dynamic unbalance, which increases by the square of the rotational vel- ocity. For example, any vibration produced by a tool- ing assembly at 3,000 rev min –1 , is × 100 greater than an identical tooling conguration that is rotating at 300 rev min –1 . Moreover, what is oen either misun- derstood, or indeed overlooked, is that any change to the tooling assembly – no matter how small it might seem, requires re-balancing! ese tooling modica- tions include any occasion when a cutting tool is ad- justed, or changed, or similarly if the toolholder is also either adjusted, or changed. Such changes to the ‘sta- tus quo’ of the tooling, will directly aect its ensuing balance, even minutely when just a ‘few microns’! So that, these miniscule changes to the tooling’s dynamic condition, causes a degree of tooling oscillation, hence an out-of-balance condition – with the likely problems that this creates. With the wide variety of tooling that is held in: tool storage carousels; magazines; turrets; etc.; they must all be ‘balanceable’ by some means. A range of balanc- ing techniques can be employed here for either single-, or dual-plane balancing – more will be said concerning these eects will be made in the following section. e techniques utilised in achieving tool balance could in- clude: • ‘Hard-balancing’ (i.e. see Fig. 234b) – when the complete assembly either has to have material re- moved, or added at a certain part of its assembly. NB e major problem associated with ‘hard-bal- ancing’ is that if the tooling setup changes, so will the likely rotating mass change, which will mean modifying the amount of material to be either added, or subtracted from this newly-distributed mass, • ‘Adjustable balancing rings’ (i.e. see Fig. 232) – by rotating the twin lower and higher balance rings either clockwise, or anti-clockwise they minutely modify the balance-condition, allowing single- plane balance to be achieved. NB ese matched pair of balance rings are in a symmetrical state of unbalance (i.e. they are both ‘unbalanced’ to the same degree). Letting the user adjust the pair to counter any unbalance in the cut- ting tool/toolholder assembly and locking them into place – usually achieved on commercially- available balancing machines (i.e. see Fig. 234a). e state of unbalance is not merely a subject to the ‘caprice’ of the machine tool operator, a tool assembly’s balance is given by various quality Standards, such as ISO 1940/1, or ANSI S2.19 – being basically exact reections of each other. In the following related sec- tions, they deal with how and in what manner rotat- ing cutter assembly balance is achieved, utilising such HSM balance calculations and associated graphical details as necessary, from these Standards. .. HSM – Problem of Tool Balance Unbalance of a rotating body (i.e. here we are con- cerned with a complete tooling assembly), can be dened as: ‘e condition existing when the principal mass – axis of inertia – does not coincide with its ro- tational axis’ (i.e. shown schematically in Fig. 232). For example, such an undesirable state of aairs can be comprehended by considering the following situ- ation: if a φ50 mm face mill assembly is rotated at 15,000 rev min –1 , it will produce a peripheral speed >240 km hr –1 , which may prove to be disastrous if it is unbalanced! Basically there exists, three types of unbalance con- ditions for rotating assemblies – such as tooling, these are: 1. ‘Static unbalance’ – single-plane. is type of un- balance occurs when the mass does not coincide with the rotational axis, but is parallel to it and the force created by such unbalancing, is equal to the magnitude at both ends of the rotating body. us, if some relief – metal removal (i.e see Fig. 234b) – on the toolholder body equal to the out-of-balance mass that occurs, then a nominal static unbalance is achieved, 2. ‘Couple unbalance’ – Under these circumstances, the cutter assembly – mass axis – does not coincide Machining and Monitoring Strategies Figure 232. The taper tment against runout/eccentricity for a milling cutter and its associated balanced tool- holder . Chapter . Standards: CAT and BT 40 - and 50-taper tooling. Machining and Monitoring Strategies the onset of imbalance. Typical applications for these HSM dual-contact systems include: aerospace part. tool- holder and the machine’s spindle was the dual-contact 7/ 24 taper system 24 . e CAT Standard incorporates this 7/ 24 taper, but also allows simultaneous contact on both the toolholder’s ange and. whole gripping length. [Courtesy of Diashowa Tooling] . Machining and Monitoring Strategies quickly heats up the holder and in so doing, expands the bore allowing the tool’s shank to be placed