• Insert wear uneven – possibly resulting from inad- equate pre-setting of the cutting inserts – allowing some to ‘stand-proud’ of the rest and as a conse- quence, being subjected to higher wear than the others, • Insert shape irregularities – possibly the result of poorly manufactured cutting insert geometries, creating diering heights once secured and accu- rately positioned in their respective milling insert seatings, • Irregular chip-ow – possibly the result of either the insert chip-breakers operating inconsistently, or the workpiece material having matrix inconsistencies. 4.2 Pocketing, Closed-Angle Faces, Thin-Walled and Thin-Based Milling Strategies Pocket Milling In particular and in the aerospace industries, alumin- ium machining from: wrought, extruded stock and forged parts is a regular practice and, to a lesser extent, this also occurs in many precision machining environ- ments. Oen both for shallow and deep pockets and for ribs, it is necessary to relieve weight at critical sec- tions on components. One of the oldest established techniques for achiev- ing pocket features, is to drill a hole at the centre of the pocket to a pre-set depth. en change tools and plac- ing the milling cutter in this hole clear-out the pocket, repeating this cycle until the pocket is ‘roughed-out’. Perhaps changing cutters and taking nishing cuts to complete the feature (Fig. 87ai). Rather than simply plunging to depth with the cutter, ramping-down into pockets is an eective way of reaching the ‘rst-level’ for the pocket’s area clearance. Both ramping and ‘double-ramping’ (i.e. this latter technique is particu- larly ecient for smaller pocket dimensions), are ways of removing stock via a ‘diagonal plunge’ , while tak - ing the milling cutter to its required depth (Fig. 87aii). is technique is an ecient machining strategy for the milling of square and rectangular pockets – for high stock removal. If the pocket is of non-uniform dimensions, then perhaps a ‘lace’ , or ‘non-lace’ 26 cutter path clearance technique might be the preferred option, when having to machine these type of component features. Milling Closed-Angle Faces For the machining of so-called ‘closed angle features’ such as a re-entrant pocket 27 , or ‘dovetail’ 28 , these latter features are typically utilised for drop-forging inserts. In Fig. 87bi, the pocket has a land (i.e. to impart ad- ditional mechanical strength to the corner), this land which would run around the base of the enclosed pocket, requiring a 5-axis machining centre 29 to com- plete the milling operation. In Fig. 87bii, a normal end mill cannot remove the excess material le in the base of the re-entrant angle, necessitating either a ball-nosed, or tapered ball-nosed cutter to reach in and mill the desired feature (Fig. 87biii – in this case, the illustra- tion shows a tapered ball-nosed milling cutter). 26 ‘Lace’ , or ‘non-lace’ cutter path, a ‘lace-cut’ is where the cutter clears (i.e. machines) an area with cutter paths that step-over at regular pre-dened intervals, normally used when a sur- face has regular dimensions, such as a square, or rectangular feature. Conversely, a ‘non-lace’ cut is normally reserved for the machining of irregular surfaces with the tool paths being non-linear in their step-over paths, for example, when milling a triangular-shaped pocket/feature, or similar. NB With the advent of sophisticated Computer-aided Manu- facture (CAM) programming capabilities, much of the auto- mated generation of cutter paths, decision-making is under- taken by the soware, to optimise area clearances for these component features. 27 ‘Re-entrant pocket’ , is one where the base of the pocketed fea- ture is somewhat larger than its top, meaning that the pocket faces slope inward. 28 ‘Dovetails’ , are normally open at both ends allowing the male dovetail on the part to be held and its tapered key to easily inserted, positioned and locked in-situ. Usually, such features are generated and formed on machine tools such as: Plano- mills, Shapers, etc., but where such equipment is not available, then a ‘closed-angle milling’ operation is necessary. 29 5-axis Machining Centre, normally has 3 linear (i.e. X, Y and Z) and 2 rotary (i.e. A and B) axes. e relationship of the rotary axis will depend upon the machine tool’s congura- tion, but they allow axis of a milling cutter into an otherwise closed-feature, negating the possibility of any cutter/spindle fouling on the workpiece. Milling Cutters and Associated Technologies  Milling Thin-Walls For the machining of thin-walls (Fig. 88), such as when milling rib-sections on aerospace components, the machining strategy will vary, depending upon the respective height and wall thickness. In every case of thin-walled machining, the number of passes will be determined by the component’s wall dimensions and axial depth of cut, in the following manner: • Height-to-thickness ratios of <15:1 – then possibly the most favoured milling strategy is to machine one side of the wall in non-overlapping passes, fol- lowed by a repetition on the remaining side – as depicted in Fig. 88a. In all cases of thin-walled ma- chining a ‘nishing allowance’ is le on both sides and the base for subsequent machining, • Height-to-thickness ratios of <30:1 – there are two basic milling techniques that are usually employed, these are: • ‘Waterline milling’ (Fig. 88b-le) – this is where either side of the thin-wall feature is milled to pre- determined depths, in non-overlapping passes, Figure 87. Pocket and closed-angle feature milling. [Courtesy of Sandvik Coromant].  Chapter  Figure 88. Thin-walled machining strategies. [Courtesy of Sandvik Coromant]. Milling Cutters and Associated Technologies  • ‘Step-support milling’ (Fig. 88b – right) – this tech- nique utilises a similar approach to the previous method, but in this case, there is an overlap between passes on opposite sides of the wall. is strategy gives more support at the vicinity where machin- ing occurs and the cutting forces are less likely to distort the wall as it height increases. NB For very large height-to-thickness ratios of >30:1, an alternative milling strategy, is to alter- natively mill either side of the wall – approaching the desired wall thickness in stages in a so-called: ‘Christmas tree routine’ 30 (i.e. not shown), so that the thinner sections are always supported by thicker sections below them. is method is then repeated as the step-wise milling operation moves down the wall. Milling Thin Bases Unsupported thin-base features, such as the one il- lustrated in Fig. 89, are dicult to produce once the previous side has been machined, because of the lack of support, particularly at the base’s central region. One milling approach in the production of this unsup- ported thin-base, is to ‘helically mill’ the feature (i.e. shown in cross-section in the small inset diagram in Fig. 89). is usually necessitates milling at the cen- tre of the base region, spiralling-down to the required depth, then milling outward in a ‘attened helical manner’ from that point (Fig. 89 – main illustration and plan view). Occasionally, one of the faces has al- ready been machined and under these conditions it must be ensured that the cutter’s ank makes minimal contact with this face, for this operation it is usual to employ tooling with the minimum number of utes. Sometimes a component to be thin-based milled, has a hole at its base’s centre, in such a situation it is prudent to leave a support leg in place when milling the rst side. en machine the second side, nally re- moving (i.e. milling) this support leg aer both sides have been completed, thereby minimising any base de- viation due to the presence of the cutting forces whilst milling the feature. 30 ‘Christmas tree routine’ , is so-called, because as it is being step-wise milled and progressively develops, the silhouette re- sembles the prole of the Christmas tree – hence its name. 4.3 Rotary and Frustum- Based Milling Cutters – Design and Operation Rotating Insert Face-Mills One of the novel face-milling cutters which is cur- rently available includes rotating round inserts that are self-propelled as they cut (Fig. 90a), promoted by the chip-ow over the insert’s face. It has been claimed by the tooling manufacturers of these interchangeable rotating insert cutters, that their unique cutting ac- tion provides greater cutting eciency and is less de- structive to the inserts, than the conventional ‘locked’ milling inserts. e term that is used for this rotating cutting action is ‘roll shearing’ 31 . e rotation of the inserts continually introduces a ‘fresh’ cutting edge to the workpiece, this, it is claimed, minimises any heat build-up in the cutting zone, with much of the heat being transferred to the milled chips. Any remaining heat being easily dissipated along the entire length of these round inserts (i.e. the insert circumference has an eective total cutting edge length of approximately 85 mm). is rotating insert has an almost innite ef- fective cutting edge length, enabling around a 10-to-1 improvement of insert life. Due to the increased tool life, less down-time for changing cutting edges is re- quired, thereby improving cutting eciency and im- pacting on actual overall cycle-times, because faster cutting speeds 32 can be utilise. It has further been claimed by the tooling manu- facturer, that with the very high cutting speeds the lo- calised heat is of benet, as the heat within the cutting zone is concentrated in the chip and not in the work- piece, or the insert. is local heating of the metallic workpiece, allows it to reach its plastic deformation stage, causing the chip to ow freely away from the 31 ‘Roll shearing’ , is a combination of the rotating action of the round cutting inserts, in combination with the angled axis which slices, or shears through the workpiece. 32 Rotating insert cutting speeds – with these rotary insert cut- ters has been increased dramatically, when compared to the more conventional ‘locked insert’ face-mills. For example, when the silicon nitride cutting inserts are face-milling cast iron components 1,000 m min –1 is possible, conversely, when face-milling aluminium workpieces cutting speeds of 2,300 m min –1 have been successfully employed.  Chapter  Figure 89. A strategy for the milling of thin-bases. [Courtesy of Sandvik Coromant]. Milling Cutters and Associated Technologies  Figure 90. A range of rotary milling cutters. [Courtesy of Rotary Technologies Corp.].  Chapter  milled surface. is localised plasticity allows the en- ergy to be maximised and the cutting eciency to be increased. Moreover, lower workpiece heat, results in less component distortion. Yet another benet of this ‘roll shearing’ action, is that when conventional cutters are used the tangential force component is high and it is one of the primary causes for spindle bearing wear, because of the side load it imparts into the spindle’s bearings. Due to the rotary motion of these inserts, they minimise tangential forces and as such, reduce side loads on the machine’s spindle bearings. Frustum-Based Face-Mills When compared to some other milling cutter insert geometries, the round inserts have two advantages: • Inherent strength – no sharp edges, minimising potential points of weakness in the geometry, im- parting high shock resistance and fracture tough- ness. Hence, ‘frustums-based’ face mills have up to 10 times longer tool life, in comparison to conven- tional milling insert geometries, • More cutting edges – they can be turned and locked in their seatings, creating approximately twice as many cutting edges per insert – giving up to 24 in- dexes per insert – when compared to conventional milling inserts. NB Like conventional insert geometries, normal round inserts oer the user two choices, whether to choose a high eciency positive insert, or longer insert life using a negative geometry round milling insert. e frustum-shaped (round) face-milling insert (Fig. 91), has a cutting edge which is reinforced by addi- tional mass while at the same time oering a 60° posi- tive shearing action (Fig. 91a). is frustum-designed insert geometry, eliminates angles and straight lines, allowing high stock removal rates to be utilised. Typi- cally, these frustum-based insert designs, when mill- ing grey cast iron can use peripheral speeds of >700 m min –1 at feedrates of 6.4 m min –1 , whereas, for alu- minium milling, the surface speed can be increased to 1,650 m min –1 , but with a feedrate of >8 m min –1 . Figure 91. A frustum-based milling cutter. [Courtesy of Rotary Technologies Corp.] . Milling Cutters and Associated Technologies  Figure 92. A range of special tools (i.e. customised), catering for specic company production needs. [Courtesy of Ingersoll].  Chapter  4.4 Customised Milling Cutter Tooling Custom-built tooling is as its name implies, oers quite considerably diverse tool designs (i.e. see Fig. 92 for just ‘snap-shot’ of a small range of these types of tools). Some of this customised tooling can be rela- tively simple, perhaps just manufactured to mill only one particular feature, while others are very complex and sophisticated in both their design and operation. Of this latter type are the numerically-controlled, or ‘feed-out’ facing and boring heads (not shown). ese programmable heads allow the machining of features such as large bores with intricate proles, typically: multiple diameters, grooves, tapers and even threads – on a range of prismatic parts. Until such heads became available, these workpiece features would have required the knowledge by either a CNC programmer, or more likely they would have been ‘routed’ to a conventional jig-boring machine for a highly-skilled technician known as a jig-borer to complete the complex machin- ing task. ese numerically-controlled heads have a programmable U-axis tool-slide that can be co-or- dinated to that of the Z-axis, enabling it to produce tapers and contoured bores, or even outside diameter features. Once the head is located in the spindle, its powered tool slide via a compact auxiliary d.c. servo- drive motor (i.e. being a closed-loop system with feed- back – to monitor its relative position at all times), will control the radial motion as it rotates down a bore, or around the outside diameter of a component. Tooling can be designed to create virtually any component feature on a workpiece and, with the CAD/CAM soware available today, tooling designers have a vast array of computing power to allow them to eciently produce customised tooling within very short lead-times. However, a word of caution here, these customised tools are not inexpensive and should only be purchased if the alternative tooling approach is such, that cycle-times are otherwise lengthy, or there is simply no other technique that will enable these part features to be produced at economic cost. 4.5 Mill/Turn Operations On many hybrid machine tools today, the traditional operations associated with one particular type of ma- chine tool, are now being produced on others. Take for example a turning centre, in the past it would simply have been employed in the production of workpieces with rotational features. Now, with the addition of a turret equipped with live/driven tooling (i.e. rotating spindles in some, or all of the turret’s pockets), it is possible to lock the headstock spindle, mill a feature: at, keyway, gear tooth, or spline, then angular index the spindle and repeat, until all of the so-called ats – oen known as ‘prismatic features’ – are completed. Moreover, it is possible to purchase a ‘mill/turn centre’ with ‘full’ C-axis headstock spindle control giving it the capability to generate contoured surfaces, or faces on the previously turned part (i.e. see Fig. 93). is diversity in the machining operations that can be un- dertaken by simply one machine tool, means that the so-called ‘one-hit machining’ 33 operations are possible, thereby reducing the risk of loosing the accurate da- tum initially set when the part was turned, so increas- ing production consistency due to its more repeatable machining precision and accuracy. Some companies in England and elsewhere, are now producing rotational features on prismatic parts, these being produced on machining centres. is un- usual reversal technique is achieved by tting turning tools in a suitable xture on the machine tool’s bed and rotating the part held in a appropriate manner in the machine’s spindle. Moreover, it is now possible to not only purchase a machine tool that can: turn, mill, bore, thread, but it can even cylindrically grind as well – truly showing the diversity over a range of machin- ing operational processes. For further information on the possible problems that may be encountered in milling operations and the anticipated solutions, these are given in the Trouble- shooting Guide for Milling Operations, in Appendix 6. 33 ‘One-hit machining’ , refers machining parts from wrought stock, etc., in one complete operation. Milling Cutters and Associated Technologies  Figure 93. Driven/live tooling: milling a spiral groove (top) and face-contouring (bottom) under ‘full’ C-axis control, on a mill/turn centre. [Courtesy of DMG (UK) Ltd.] .  Chapter  . strategy for the milling of thin-bases. [Courtesy of Sandvik Coromant]. Milling Cutters and Associated Technologies  Figure 90. A range of rotary milling cutters. [Courtesy of Rotary Technologies. Pocket and closed-angle feature milling. [Courtesy of Sandvik Coromant].  Chapter  Figure 88. Thin-walled machining strategies. [Courtesy of Sandvik Coromant]. Milling Cutters and Associated Technologies. and Thin-Based Milling Strategies Pocket Milling In particular and in the aerospace industries, alumin- ium machining from: wrought, extruded stock and forged parts is a regular practice and,