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Figure 82. Face-milling cutters, having inclined approach angles, with either high-shear (i.e. tangentially-mounted) inserts, or cutter insert density variations. [Courtesy of Ingersoll, Courtesy of Sendrik Coromant] . Milling Cutters and Associated Technologies • Chip ow may be hampered – the vectored angle for exhausting chips may be compromised, • High radial force component 16 – i.e. in relation to the axial force, produces unfavourable loads on the spindle, creating vibration tendencies, hence the feeds must be restricted, • Positive geometry triangular inserts should not be used – these weaken insert corners, whereas rhom- boid-shaped insert geometries, or similar, oer much stronger insert cutting edges. For general-purpose milling operations, intermedi- ate approach angles such as inserts having an 75° ap- proach are common (i.e see Fig. 82a), as they provide good edge strength in combination with a favourable relationship between insert size and cutting depth. Moreover, if these 75° approach-angled inserts are tangentially-mounted (i.e. as depicted in Fig. 82a), the edges are even stronger because of the ‘body’ of the in- sert is more fully-supported, than is generally the case for most of the radially-mounted variants. Equally, an insert with an 45° approach angle, will spread the load over a longer cutting edge (Fig. 83b). is 45° insert approach geometry, provides good chip-ow for long- chipping materials, with a low radial force component in comparison to that of the axial force, what is more a strong insert edge allows higher feedrates to be utilised (Fig 83a). When roughing-cuts are necessary, or dicult-to- machine workpiece materials must be milled requir- ing strong insert edges, then a round insert might be the answer. In general, round inserts have a positive geometry with no sharp edges and as a result, oer very strong cutting edges and chip-loads are relatively evenly distributed along the rounded contact region (Fig. 83b – right). Furthermore, a round insert usu- ally has a positive insert geometry, which can then be turned in its seating to simply provide additional cut- ting edges. 16 ‘Square-shoulder cutters’ , can produce a high radial force component, which means that feedrates must be limited, as they may cause ‘edge frittering’ (i.e. see Fig. 81 – bottom le, illustrating an edge break-out condition), this unacceptable machining condition is particularly prevalent on brittle-types of workpiece materials, such as: (most) Brasses, (many) Cast irons, (some) Powder Metallurgy compacts, together with (many) non-metallic materials – Plastics, Perspex, Tufnol, Carbon-bre, etc. .. Face-Milling Engagement – Angles and Insert Density Face-Milling Engagement In any milling machining operation involving the po- sitioning of the cutter in relation to a workpiece, some thought should be given to not only the cutter’s: diam- eter; number of teeth, or cutting inserts; width of the workpiece; but also how the resultant cutting force(s) might inuence the overall eectiveness of the pro- duction process. is latter point, not only inuences method of component clamping, dictating: how, where and what will be the optimum method of ‘location and restraint’ 17 of the workpiece, but on exit from cut, the sudden disengagement and release of cutting forces will potentially create not only a exit-burr on ductile materials, or frittered edge on a brittle material. e cutter’s exit can inuence the type of stress induced into the workpiece surface – a compressive stress be- ing preferred (more will be said on this topic later, in the section dealing with ‘Machined Surface Integrity’). In most milling operations the term: ‘engagement’ con- cerns the relationship of cutter-to-workpiece position- ing and, in virtually all face-milling operations, one tries to prevent at the exit of the cut, the chip being at its thickest, as this is an unfavourable machining strat- egy. e objective when milling, is to always try to get the thinnest possible chip at exit from the cut. Some of these engagement positioning relationships are de- picted in Fig. 84, indicating where the most favourable cutter/workpiece relationships are present. Also in Fig. 84, are depicted some unfavourable engagements that should be avoided, this may be possible by either changing the milling cutter’s diameter, or its tool path if possible, to avoid such engagements. e milled cut length is inuenced by the position of the cutter with respect to the workpiece, with tool life being related to each cutting insert’s amount of time engaged in the actual cut. For example, in Fig. 84g, a cutter has been positioned centrally over the workpiece, this produces the shortest possible time 17 ‘Location and restraint’ , are important factors when work- holding. A component needs to be not only accurately located with respect to either a ‘datum’ , or held on a ‘grid-plate’ in a known relationship to that of the cutter’s position, but it must also be properly restrained – to prevent any, compliance of its ‘degrees of freedom’ while it is clamped during machining. Hence the term: ‘Location and restraint’. Chapter in-cut, conversely, in Fig. 84h the cutter has been moved just o-centre, causing a longer arc of cut for each insert, which is likely to reduce the tool’s ‘cutting life’ somewhat, but this is only part of the problem of o-centre cutting. Returning to the cutter positioned centrally (i.e. Fig. 84g), here the direction of the radial component cutting forces will uctuate, with respect to the cutting edges start and nish cutting, which may create potential vibrational problems, or prema- ture edge breakdown. However, with o-centre mill- ing (i.e. Fig. 84h), this machining strategy introduces a constant force direction, moreover, as the cutter is positioned not quite centrally over the workpiece, this central region produces the largest average chip thick- ness. Just to complicate matters still further, if the cut- ter is positioned even further o-centre, this will allow even more inserts to be simultaneously brought into cut (i.e. shown as ‘α’ , in Fig. 85). ere are oen many Figure 83. The importance of cutting insert approach angle inclination on the resultant chip shape. [Sources: Fig. a: Tooling University, 2003; Fig. b: Heuwinkel & Richter, 2005] . Milling Cutters and Associated Technologies milling strategy decisions and frequently some com- promises that must be made, in order to obtain the op- timum cutter/workpiece engagement for a particular machining situation. Milling Cutter Density In any face-milling operations the number of inserts in cut (i.e. see Fig. 85), is a function of the quantity of Figure 84. Face-milling cutter positioning over the workpiece – indicating favourable/unfavourable cut- ter and workpiece placement – together with other important factors. [Courtesy of Sandvik Coromant] . Chapter inserts around the cutter’s periphery (Z) and the en- gagement angle (α). An expression for these milling cutter inserts and the cutter diameter’s relationship is derived [Source: Isakov – Kennametal Inc. and pub- lished in American Machinist 1996)] from Fig. 85, as follows: α = 90° + α 1 sin α = AB OA = W − .D .D = (W − .D) D = W − D D α = arcsin W − D D Z c = Z( � + arcsin W − D�D) � Where: Z c (i.e. see the following footnote 18 ) D = Cutter diameter (mm), W = Radial width of cut (mm), α = Engagement angle (°), α 1 = Angle between cutter centreline and cutter ra- dius to the peripheral point of either exit, or entry (°). is above formula, can be simplied to the following relationship: Z c = Zα/360 is engagement angle is dependent upon the radial width of the cut (W) and the face-milling cutter’s di- ameter (D). erefore, if the radial width of the cut equals the cutter diameter (W/D = 1.0) and, the en- gagement angle is 180°, then: Z c = 180Z/360° = 0.5Z 18 ‘Z c ’ represents the number of inserts in-cut, which can be found for any cut width (W), by applying the above formula, derived from the schematic diagram, illustrated in Fig. 85. e values of ‘Z c ’ can be obtained from the Table 7, for various ‘W/D’ ratios, given below: e face-milling cutter density must be such that it allows the chip to correctly form and exhaust from the cut. If inadequate chip space is provided, this will result in the chip remaining in the chip gullet. is lodged chip is then carried around and merging with the succeeding chip and as a result, welding itself to it, potentially causing cutting edge breakage and possi- bly damage to the workpiece. In any cutter/workpiece engagement, it is necessary to provide a cutting edge density with at least one insert in-cut at all times. Fail- ure to achieve this cutter density could result in severe edge hammering, leading to one, or more of the fol- lowing conditions: chipped cutting edges; a damaged cutter; or excessive machine tool wear. For ‘coarse-pitched’ milling cutters, having between 1-to-1.5 inserts per 25 mm of diameter, this will allow for larger chip gullet spaces and as such, can be recom- mended to be used on either: so workpiece materials that produce continuous chips; or, for wide cuts with a long insert engagement. Conversely, ‘ne-pitched’ milling cutters, with approximately 4-to-5 inserts per 25 mm of diameter, are normally utilised where lack of insert engagement is a problem. ese milling cutters having ‘ne-pitches’ , will allow at least one insert to be in-cut at all times, even when machining very thin workpiece cross-sectional areas. ese high-insert den- sity milling cutters, are usually recommended when machining high-temperature exotic alloys, or hard steels – where light chip loads are taken. As a result of the smaller chips, less chip gullet space is necessary, al- lowing more inserts around the cutter’s periphery. .. Peripheral Milling Cutter Approach Angles – Their Affect on Chip Thickness As has been previously discussed, a multi-point tool such as a milling cutter, will cut intermittently, as its cutting edges repeatedly enter and exit the workpiece’s arc of cut (i.e. engagement). It was suggested in the previous section, that at least one, but preferably two, Table 7: The ratio of cut width-to-diameter (W/D). Number of inserts in-cut Z c W/D: 0.88 0.80 0.75 0.67 0.56 0.38 0.33 0.19 0.125 Z c : 0.38 Z 0.35 Z 0.33 Z 0.30 Z 0.27 Z 0.21 Z 0.20 Z 0.14 Z 0.14 Z [Source: Isakov – Kennametal Inc./pub. in American Machinist, 1996] . Milling Cutters and Associated Technologies Figure 85. Typical facemilling cutters and their inserts, with a schematic representation of a milling cutter engagement angle and the number of inserts in-cut. [Source: Isakov/American Machinist, 1996] . Chapter cutting edges should be in-cut at all times. Tooling manufacturers design and test their cutters during engagement, carefully determining both the feed and speed of a milling operation, ensuring that the cutting forces are eectively balanced-out around all of the teeth. e machining objective here, is to discover the optimal chip thickness. As has been previously shown, milling cutter utes can be either helical, or straight, with the replaceable cutting inserts being located and secured by either a wedge, or screw clamp. With each adjacent cutting edge around the cutter’s periphery be- ing referred to as its pitch. During a face-milling oper- ation, a chip is formed at the two cutting edges, where- upon, it slides up the tooth face and into the ute, striking the llet, or rounded corner of this ute. e approach angle is a key milling geometry fac- tor, being formed the tool’s axis and by the peripheral edges of either a solid cutter, or its cutting inserts. is approach angle describes how far the top of the insert inclines away, from that of being parallel to the cutter’s axis (i.e. as shown in Fig. 83a). In most general milling operations, the ‘approach’ ranges from 0° for creating square shoulders (Fig. 83a-le), to 45° in nish-mill- ing (Fig. 83a-right). Usually, milling cutter approach angles ranging between 15° to 45° are the norm, with a 15° approach enabling deeper cuts to be taken. As the approach angle of the cutting edge inclination in- creases, the chip becomes both longer and thinner for the same D OC , or ‘a e ’ (Fig. 83b), with the load being spread over longer edge length – resulting in smoother cutting. Larger insert inclination, enable higher fee- drates to be employed, although it must be empha- sised, with shallow cuts (Fig. 83a-bottom). Taking a dierent milling operational premise, the objective when ‘rough-milling’ , is to remove the maxi - mum workpiece stock in the shortest possible time. e material removal rate being limited by the ‘avail- able’ spindle power, although this condition can be op- timised by ‘radial chip-thinning’ 19 . e chip thickness is based upon the calculated feed per tooth (f z ) and it diminishes as the radial width decreases and in reality, creating a lighter actual ‘f z ’. is ‘lessening eect’ of the chip thickness, causes the cutting edges to rub, rather than cut the workpiece material, as a result, the feed per tooth (f z ) should be increased as the radial depth 19 ‘Radial chip-thinning’ , is the eect of taking a radial D OC (a e ) of less than 25% of the milling cutter’s diameter. decreases. is cutting strategy ‘boost’ in the eective feed per tooth, provides the twin benets of longer tool life, with shorter cycle-times. For any operation in milling involving a ‘chip-thin- ning exercise’ , of paramount importance is the cutter’s approach/inclination angle (χ). erefore, as the ap- proach angle (χ) become more inclined from say, 90° to 45°, the chip thickness, its ‘h-value’ 20 decreases (i.e. as schematically-demonstrated in Fig. 83b). e optimal chip thickness for a given set of cutting data, can be entered into a machine tool’s CNC program, by utilis- ing the following formula: f z = h m /sinχ e chip thickness (f z ) is always constant, regardless of the approach angle inclination, be it operated at 90°, or down to 30°, or indeed, at a atter approach (see Fig. 83b). e exception to this ‘chip thickness rule’ be- ing when utilising a round, or button-type insert (Fig. 83b-right), as it does not have either a top geometry, or an edge chamfer, thereby creating the strongest type of cutting edge. Round inserts without the straight cut- ting edges associated with other milling inserts, cre- ate chips that increase in thickness as the D OC becomes deeper. Hence, for round inserts, the average chip thickness (i.e. its ‘h m ’ – value 21 ), relates to the thickness of cut this being based upon the insert’s radial engage- ment of the workpiece via the milling cutter’s diam- eter. If a comparison is made between a round milling insert to that of an insert with a 90° approach (i.e. Fig. 83b-right and Fig. 83b-le, respectively), an identical volume of chips will be removed for both at a set feed 20 ‘h-values’ , for a material group are represented as a range, with a lower number being the starting value. For example, if utilising a machining centre with a 35 kW spindle power avail- ability for the milling of non-ferrous, or aluminium alloys, the ‘h-value’ , or chip thickness ranges between 0.050 mm to 0.076 mm. Alternatively, using this same machine tool to mill, either: stainless steels, titanium alloys, or heat-resistant super- alloys, the ‘h-values’ will range from 0.076 mm to 0.152 mm, whereas, for: plain carbon steels, cast-/nodular-cast irons the range will be between 0.152 mm to 0.254 mm. NB Do not attempt to mill thicker chips than is recommended in the literature, as this action could result in over-loading the cutting inserts and breaking their edges. 21 ‘h m value-ranges’ for various workpiece materials are identical to those ‘h-value ranges’ previously mentioned. Milling Cutters and Associated Technologies per tooth and D OC . Although, if the D OC is half that of the round insert’s inscribed circle, this round geom- etry creates chips that are 30% thinner to that of the 90° approach inserts. is reduction in chip thickness is the result of the round insert having a longer cutting edge, which engages radially with the workpiece (Fig. 83b-right). Alternatively, if chip volume for both the round and 90° approach inserts were identical, then the chip length generated by the round insert is ap- proximately 50% longer and it is much thinner than its counterpart. Moreover, with the same feed for the round insert, but the D OC is reduced so that it is 25% of it’s inscribed circle, the chip thickness produced is now 50% less for an identical chip volume. Hence, to achieve the desired productivity benets that will ac- crue from utilising a ‘chip-thinning strategy’ , the D OC needs to be <25% of the round insert’s inscribed cir- cle. As the D OC has now become more shallow using a round insert, the chip has now been ‘thinned’ , so in order to compensate for this loss of stock removal, the feedrate needs to be increased. erefore, as the round insert’s D OC becomes more shallow the approach angle attens-out to almost ‘innite length’. So, when the average chip thickness and approach angle variables are entered into the formula for ‘feed per tooth’ 22 in the CNC program they can be signicantly higher – al- most up to 100% greater. In order to establish either round, or button-style geometries for their ‘eective approach angles’ , the fol- lowing formula has been derived: ‘Eective approach angle’ Tan χ = a e /(IC e /2)χ Where: χ = Approach angle (°), a e = D OC (mm), IC e = Inscribed circle – ‘eective’ (mm). If a 90° approach angle is used, the rate of advance per tooth equals the chip thickness. When there is a decrease in the approach angle inclination, the chip volume stays the same, but the length of cutting edge engagement with the workpiece will increase. is re- sults in a chip which is both smaller and longer than that programmed, hence it is necessary to raise the 22 ‘Feed per tooth’ (f z ), calculations will aect the chip-loading for the milling cutter and be inuenced by the spindle power availability – see previous equation for this relationship. feedrate to increase the chip thickness to its required level, when the D OC is less than the round insert’s ra- dius. It can be said that although the chip created by the round insert geometry has an almost identical chip thickness to that of a 90° approach angled insert, the button-style insert geometry removes workpiece ma- terial at a considerably faster stock removal rate. e only ‘down-side’ to that of utilising button-style milling inserts, is the spindle power requirement is greater. With increasing insert inclination of the approach angles the chip thins, causing the cutting forces to be re-directed. By way of an illustration of this eect, if a 45° insert approach is used, the axial force component will be identical to that of the radial force. is radial force component, tends to make the tool deect and may generate chatter, conversely, the axial component force is toward the direction of the spindle thereby re- ducing the potential risk of its damage via vibrational eects. erefore, if the insert inclination is such that the approach angle is almost at, this has the advan- tage of the axial force component being in the spindle’s direction, this will minimise the likelihood of tool de- ection. us the primary objective here, is to remove workpiece stock at high rates and speedily, keeping the approach angle low, with a light D OC , in this manner allowing chips to be thin and as a result the cutter will ‘y’! .. Spindle Camber/Tilt – when Face-Milling On some conventional and CNC milling machines, the spindle can be tilted slightly 23 , this small inclina- tion in the direction of the feed, ensures that the cut- ter does not lie completely at to the workpiece’s sur- face. is small spindle tilting technique avoids the so-called ‘re-cutting eect’ 24 that normally if present when utilising a large face-milling cutter. In reality, the spindle camber is very slight and generally amounting 23 On many machining centres it is not always possible to tilt the spindle and, in such situations, back-, recutting is an unavoid- able milling surface texture condition. 24 ‘Re-cutting eect’ , this is the product of the cutting inserts on the ‘back-edge’ of a large face-milling cutter scoring the recently machined surface, thereby aecting and slightly degrading the milled surface texture, while simultaneously avoiding additional ank wear on the inserts – prolonging the cutter’s life. Chapter to between 0.1 to 0.3 mm over a length of 1,000 mm. When this is converted to angular measurements, this equates to a value of between 20 to 60 seconds of arc respectivly – as shown in the exaggerated diagram in Fig. 86. Oen, when it is not possible to slightly tilt the spindle, back-cutting problems can arise through such factors as spindle, or workpiece deections. is problem can be minimised by: Figure 86. The inuence of spindle camber (tilt) on the milled workpiece surface. [Courtesy of Kennametal Hertel] . Milling Cutters and Associated Technologies • Improvements in workpiece support – ensuring the both packing and clamping are sucient to support the component, • Modifying the cutter to a positive geometry – this has the eect of reducing any back-cutting milling by the cutting inserts, • Reduction in cutting forces – though: feedrate re- ductions, lighter D OC ’s/cut widths, or by increasing the cutting speed, • Modications to approach angles – this will have the eect of reducing the axial force component, • Reducing spindle overhang – will decrease cutter deection, • Inspection of cutter milling mounting – this will ensure that any burrs, debris, or misalignments are minimised. If the spindle is slightly cambered when face-mill- ing, a plain workpiece surface will not normally be produced. Under these conditions of a slight camber, the machined surface is normally concave, due to the angular tilt of the milling cutter (i.e. shown in Fig. 86 top/middle schematic diagrams). e surface concav- ity generated by this camber, depends upon the rela- tionship between the: cutter’s diameter; width of the workpiece surface being cut; together with the D OC . e milled workpiece concavity ‘f’ , can be calculated using the well-established Kirchner–Schulz formula, as follows: Milled concavity f = q [D e � − (D e � − e �) ] Where: f = Milled concavity (mm), q = 1000tanθ where θ is the spindle camber (°), D e = Eective diameter of the cutting circle (mm), e = Width of workpiece surface being milled (mm). Alternatively, a reasonable estimate of the milled con- cavity ‘f’ can be obtained from the graph in Fig. 86 (bottom), that illustrates the variation in the concave shape, for a variety of spindle cambers and face-mill- ing diameters. ese concave surface modications produced by the spindle camber are never large devia- tions from the ‘true’ plane surface. For example, even under the extreme conditions of employing a relatively small diameter milling cutter of: φ100 mm; together with a large spindle camber ‘q’ value of 0.05 mm, the deviation in milled surface concavity only amounts to 25 µm over a workpiece width of 100 mm – this being within the accepted tolerances for many commercial situations. However, for the generation of high-pre- cision milled surfaces, this minute level of concavity would not be tolerated. In fact, when a milled surface is metrologically inspected at high levels of magnica- tion, the ‘surface topography’ 25 consists of both form and surface variations. ese form and surface varia- tions are related directly to either the cutter-spindle ac- curacy, or the axial displacement of the cutting inserts, with the distance between wave crests (i.e. sometimes termed the asperities – high points – on the machined cusps) frequently coinciding with the feed per tooth. It is possible to establish the reasons why a surface might deviate from the ‘true’ plane, with some of the possible factors being caused by: • Machine tool condition – possibly resulting from the fact that the spindle bearings are in poor con- dition, the slideways have appreciable ‘back-lash’ present, or poor ‘damping’ generating vibrational tendencies – showing-up as ‘chatter-marks’ on the milled surface, • Workpiece clamping/stability – if the workpiece is not suciently and correctly clamped, then it could ex, or move on the xture/pallet/table, whilst be- ing machined, creating unwanted surface devia- tions/uctuations, • Axial insert displacement – possibly created by cutting inserts not precisely located in their respec- tive pockets, or resulting from movement during milling – due possibly to inadequate locking of the insert in-situ during pre-setting, 25 ‘Surface topography’ , is a term that is oen used to describe the form, waviness and surface texture uctuations from the ‘true’ plane. A machined surface may exhibit some, or all of these variables, together with its ‘lay’. NB Form errors are long-frequency components of a surface, with waviness being medium-frequency components, while surface texture is normally associated with short-frequency components. Depending upon the relative size of the work- piece, these variables are superimposed onto each other, but each one can be ‘ltered-out’ by suitable magnication on a Surface Texture Machine – for future analysis. e ‘lay’ is the direction of the dominant surface pattern, created by the passage of the cutter over the surface. When assessing a ma- chined surface with a anisotropic lay condition – this being a surface that has a signicant lay (i.e. clearly visible machining marks), it is normal procedure to assess the surface’s condi- tion at 90° to the lay. Chapter . [Sources: Fig. a: Tooling University, 20 03; Fig. b: Heuwinkel & Richter, 20 05] . Milling Cutters and Associated Technologies milling strategy decisions and frequently some com- promises that. 0.33 0.19 0. 125 Z c : 0.38 Z 0.35 Z 0.33 Z 0.30 Z 0 .27 Z 0 .21 Z 0 .20 Z 0.14 Z 0.14 Z [Source: Isakov – Kennametal Inc./pub. in American Machinist, 1996] . Milling Cutters and Associated Technologies. inserts and breaking their edges. 21 ‘h m value-ranges’ for various workpiece materials are identical to those ‘h-value ranges’ previously mentioned. Milling Cutters and Associated Technologies
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