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Figure 20. Insert approach angle geometry for turning operations. Chapter entering angle of 45° and lead angle of 45° is utilised, giving rise to equal axial and radial component forces. In ‘case II’ , the entering angle has changed to 75° and lead angle is now 15°, these altered angles change the component forces, with an increase in the axial force while reducing the radial force. In ‘case III’ , an or - thogonal cutting action occurs, with only a 90° enter- ing angle (i.e. the lead angle reduces to zero), showing a large increase in the axial force component at the expense of the radial force component which is now zero 10 . In ‘case IV’ , an oblique cutting action has re- turned (i.e. as in ‘cases I and II’), but here the entering angle has changed to -15°, with the lead angle 75°, this produces a large axial component force, but the radial component force direction has now reversed. is last tool plan approach angle geometry (i.e. ‘case IV’), is similar to the geometry of a light turning and facing tool, allowing cylindrical and facing operations to be usefully undertaken – but the tool’s point is somewhat weaker that the others, with the tool points becoming of increased strength from right to le. erefore, in ‘case I’ , for a given feedrate and constant D OC , the cut length/area is greater than the other ‘cases’ shown and with this geometry, it enables the tool to be employed for heavy roughing cuts. Returning to ‘case III’ , if this tool is utilised for nish turning brittle-based work- piece materials, then upon approaching the exit from a cut, if the diameter is not supported by a larger shoul- der diameter, then the axial component force /pressure, will be likely to cause edge break-out (i.e. sometimes termed ‘edge frittering’), below the machined surface diameter at this corner (i.e. potentially scrapping the machined part). In mitigation for this orthogonal cut- ting tool geometry, if longer slender workpieces re- quire cylindrical turning along their length, then with the radial force component equating to zero, it does not create signicant ‘push-o ’ and allows the part to be successfully machined 11 . A single-point turning geometry is subject to very complex interactions and, as one geometric feature is modied such as changing the entering angle, or in- 10 In all of these cases, it is assumed – for simplicity – that there is no nose radius/chamfer on the tool and it is innitely sharp. 11 In order to minimise the eects of the radial force component when cylindrically turning long slender workpieces with ‘Case I and II’ tool geometries, the use of a programmable steady, or a ‘balanced turning operation’ (i.e. utilising twin separately programmable turrets on a turning centre, with tools situ- ated virtually opposite each other running parallel during the turning operation – see Fig. 41), will reduce this ‘push-o ’. creasing the tool’s nose radius, this will inuence other factors, which in turn could have a great impact on the: type of machined surface nish produced, expected tool life and the overall power consumption during the operation. In fact, the main factors that inuence the application of tooling for a specic turning operation are: I. Workpiece material – machinability, condition (i.e. internal/external), mechanical and physical properties, etc., II. Workpiece design – shape, dimensions and ma- chining allowance, III. Limitations – accuracy and precision require- ments, surface texture/integrity, etc., IV. Machine tool – type, power, its condition and specications, V. Stability – loop stiness/rigidity (i.e. from the cutting edge to its foundations), VI. Set-up – tool accessibility, workpiece clamping and toolholding, tool changing, VII. Tool programme – the correct/specied tool and its tool osets, etc., VIII. Performance – cutting data, anticipated tool-life and economics, IX. Quality – tool delivery system and service. In order to gain an insight into the complex and im- portant decisions that have to be made when select- ing tooling for the optimum production of either part batch sizes, or for continuous production runs, then the following section has been incorporated. .. Cutting Toolholder/Insert Selection When deciding upon the correct selection of a tool- holder/cutting insert for a given application, a range of diverse factors must be considered, as indicated in Fig. 21. As can be seen by the diagram (Fig. 21) and associated text and captions, there are many other variables that need to be considered prior to selection of the optimum toolholder/insert. Generally, the xed conditions cannot be modied, but by ‘juggling’ with the variable conditions it is possible to accomplish the best compromise toolholder/insert geometry, to opti- mise these cutting conditions for the manufacture of a specic workpiece and its intended production re- quirements. Whenever toolholders and cutting inserts are required for a specic manufacturing process, it is important to view the tooling selection procedure as a logical progression, in order to optimise the best Turning and Chip-breaking Technology Figure 21. The factors that must be considered prior to commencing a turning operation, when utilising indexable inserts . Chapter possible tools/inserts for the job in hand. Perhaps the following selection strategy for a ‘start point’ in choice and application of turning tools, can be undertaken according by the following step-by-step approach: Start Point → Edge clamping system, ↓ Toolholder size and type, ↓ Insert shape, ↓ Insert size, ↓ Nose radius, ↓ Insert type, ↓ Tool material, ↓ Cutting data → Final Tool- holder and Insert Selection Edge Clamping System Initially, the tool holder clamping system should be selected to provide optimum performance in dier- ent applications over a wide range of workpiece geom- etries. e type of machining operation and to a lesser extent, the workpiece size determines tool holder se- lection. For example, roughing-out operations on big components will make considerably dierent demands, to that of nishing passes on small components. NB Pin, clamp and lever are just three of the insert clamping systems available – consultation with the tool suppliers at this point might be benecial. Toolholder Size and Type Once the clamping system has been selected, the size and type of toolholder must be determined, with its selection being inuenced by: feed directions (i.e. see Fig. 22 for turning insert shapes and feed directions), size of cuts, workpiece and toolholder situated in the machine for accessibility requirements. e work- piece’s shape plays a decisive role if surface contouring is necessary, this is particularly relevant for machining part access, as a toolholder is dened by its: eective entering and point angles 12 , together with the insert’s shape (see Fig. 22). Toolholders should be the largest possible size for the turning centre’s tool turret, this requirement is vi- tal, as it reduces the ‘tool overhang ratio’ – providing rigidity and integrity to stabilise the insert’s cutting edge. NB Appendix 1a shows the ISO ‘Code Key’ – for Ex- ternal Toolholders. Appendix 1b shows the ISO ‘Code Key’ – for Solid Boring Bars. Appendix 1c shows the ISO ‘Code Key’ – for Car- tridges. Insert Shape e insert shape should be selected relative to the en- tering angle needed for the tool’s accessibility, or ver- satility. Here, the largest suitable point angle should be chosen for strength and economy (see Fig. 23). In Fig. 23, is illustrated a practical example of how chang- ing only one variable – insert geometry (shape) – can inuence an insert’s turning application. e shape of an insert will determine its inherent weakness, or strength, which is of particular relevance if rough- turning operations are necessary. Furthermore, insert shape will inuence whether it is prone to vibration, or not and its predictable tool life. Hence, if one is con- cerned about vibrations of either the tool, workpiece, or both, then a weaker insert such as a light turning and facing geometry with less cutting edge length ex- posed in-cut, might be more suitable. Variable condi- tions such as the selection of insert’s geometric shape can aect other machining parameters and, this is valid for other insert factors, so a compromise will al- ways occur in any machining application. 12 Eective entering angles (κ 1 ) must be carefully selected when the operation involves proling, or copying. e maximum proling angle (β) is recommended for each tool type – if ‘workpiece fouling’ is to be avoided. NB κ 1 = κ + β (for plunging into a surface), whereas κ 1 = κ – β (for ramping-out of a surface), κ 1 = κ (β = 0°) for cylindrical turn- ing, Where: eective entering angle (κ 1 ), entering angle (κ), maximum in-copy angle (β). Always select the smallest enter- ing angle that the part geometry will allow. Turning and Chip-breaking Technology Figure 22. Tool paths in nish turning operations. [Courtesy of Sandvik Coromant]. Chapter NB Appendix 1d shows the ISO ‘Code Key’ – for In- dexable Inserts. Insert Size An indexable insert size is directly related to the tool- holder selected for the operation, with the entering angle and insert shape having previously been estab- lished. Only the matching-shaped insert can be tted into the seat of a particular toolholder, as its shape and size are predetermined by the seating dimensions. In roughing-out operations, the largest cutting depth for a given toolholder, will inuence the insert size. For any insert, the eective cutting length has to be determined (see Fig. 20b), as the entering angle will inuence the size of the insert selected. If the eective cutting edge length is less than the depth of cut (D OC ), a larger insert should be chosen, or the D OC should be reduced. Sometimes in more demanding turning operations, a thicker insert – of the same geometric shape – gives extra reliability. Figure 23. Selecting indexable inserts for turning operations. [Courtesy of Stellram]. Turning and Chip-breaking Technology Nose Radius Of particular relevance in any turning operation is the insert’s tool nose radius (r ε – see Fig. 17), as it is the key factor with regard to: • inherent strength in roughing operations, • the resulting surface texture from nishing opera- tions. Further, the size of the nose radius aects vibrational tendencies (see Fig. 23) and in certain instances, the feedrates. e nose radius is the transition between the major and minor cutting edges, which determines the strength, or weakness of the point angle (see Figs. 16a and 17), therefore it is an imperative factor to get right. In general, roughing-out should be undertaken with the largest possible nose radius, as it is the strongest tool point (see Fig. 23). Further, a larger tool nose ra- dius permits higher feedrates, although it is important to monitor any possible vibrational tendencies. Later in the relevant section, more will be said on the inu- ence that the insert’s tool nose radius plays in the nal machined surface texture, but it is worth mentioning here that the feedrate for roughing operations should be set to approximately half the size of the nose radius utilised. e size of the nose radius has an aect on the power consumed in turning in conjunction with the material’s yield strength and chip-forming ability, par- ticularly in rough-turning operations. e maximum material removal rate (MMR) can be obtained by a combination of high feedrate, together with a moder- ate cutting speed, with other limiting factors, such as depth of cut (D OC ), tool’s nose radius, under consider- ation. Oen, the machine tool’s power (P) availability c an sometimes be a limiting factor when mmR is the requirement and, in such circumstances the cutting speed is usually lowered somewhat. For a given nose radius and cutting insert geometry, the power can be derived, to ensure that the machine tool will be able t o cope with this pre-selected mmR, in the following manner: Machine tool’s power requirement (P): P = tangential force (F T ) x cutting speed (V C ) P = F T × V C P = k C × A × V C ∴ P = k C × f × a P × V C (kW) Where: f = feed/rev (mm/rev) a P = depth of cut (mm) Cutting speed (V C ) V C = πDN/1000 (m/min) Where: D = workpiece diameter (mm) N = workpiece rotational speed (rpm) Specic cutting force (k C ): k C = F T /A (N/mm 2 ) Where: A = cutting area (mm 2 ) For example, for nishing operations, with the nose radius in combination with the feedrate (i.e. pre-se- lected), this will aect the surface texture and part ac- curacy, in the following manner: Machined surface texture (Rt): (Rt, this parameter being: maximum prole height) Rt = f 2 /8 × r ε x 1000 (µm) Where: f 2 = feedrate per revolution (mm/rev) r ε = nose radius (mm) NB e surface texture parameter ‘Rt’ , can be con- verted into other surface texture parameters – as nec- essary. By utilising either: larger turning insert tool nose ra- dius, ‘wiper insert’ (yet to be discussed), a more posi- tive plan approach angle, or in certain circumstances, a higher cutting speed, the surface texture can be im- proved. In general, the coordination of the tool’s nose radius and the pre-selected feedrate in nishing op- erations, indicates that the feed should be kept below a certain level to achieve an acceptable machined sur- face texture value. Insert Type e cutting insert type is for the most part determined by the previously selected geometry – see Appendix 1d for the selection of indexable inserts. In reality, vari- ous cutting conditions and workpiece materials make dierent demands on the insert’s cutting edge. For ex- ample, when machining hardened steel parts, this will be completely dierent from that to the machining of aluminium components. 48 Chapter 2 Once the insert shape has been established in con- nection with its plan approach angle together with the nose radius dimension, this just leaves the type of ge- ometry to be found. In this instance, the type of insert geometry refers to the ‘working area’ (i.e. nominally found by its depth of cut and feedrate – more will be said concerning this topic later, when ‘chip-breaking envelopes’ will be discussed). Additional factors can inuence the type of cutting geometry choice, such as: machine tool’s condition, its power, the stability of the workpiece-tool-machine set-up, other factors that could aect geometry selection include: whether con- tinuous, or intermittent cutting occurs, any tendency toward vibration while machining. Turning operations can be separated into a number of ‘working areas’ , be - ing based upon the removal of workpiece material and the generation of accurate machined component di- mensions, in combination with specic surface texture requirements – as shown in Table 3. When establishing an insert type, the feedrate and depth of cut should be identied with one of the ‘working ranges’ (i.e. from Table 3), as the various in- sert types to be chosen relate to this chart. It should be borne in mind that the most suitable ‘working area’ selected, will vary, in combination with such factors as the insert’s: size, shape and nose radius. Tool Material e penultimate evaluation to be made concerning tooling decision-making is the choice of insert mate- rial, or combination of materials that constitute the cutter’s tool edge. Today, manufacturers of tooling have a strategy for continuous improvement with varia- tions in both tool matrices and coatings being consid- erable. Not only are cutting tool material research and development an on-going intensive activity, but their application for wider ranges of machining applica- tions are being considerably enhanced. A brief review of just some of the current tool materials and coatings have been previously mentioned in Section 1.2, with the main range of cutting tool materials being: ce- mented carbides, coated cemented carbides, ceramics, cermets, cubic boron nitride, polycrystalline diamond and monolithic (i.e. natural) diamond. NB A good ‘start-point’ for most machining opera- tions, is to consider coated carbides initially, then if these grades prove unsatisfactory, for whatever reason, select one of the other materials – perhaps aer con- sultation with a cutting tool manufacturer, or aer a machinability testing procedure. Cutting Data Once all of the physical, metallurgical and geometrical factors for the cutting tool have been established for the machining operation, then it is necessary to set, or calculate the cutting data – oen these criteria can be found from tooling manufacturers recommendations and cutting data tables. Certain variable factors such as feedrate should have already been made, allowing the cutting speed to be calculated, from the well-known expression (below): V C = πDN/1000 (m min –1 ) Where: V C = cutting speed (m min –1 ) D = Workpiece diameter (mm) 13 N = rotational speed (rpm) 13 In the case of drilling, reaming and tapping operations, it is the diameter of the cutting tool that is used in the calculation. For any other internal machining operations – such as in bor- ing, it is the initial hole diameter that is employed in the cut- ting speed calculation. Table 3. Typical working areas for external turning opera- tions Type of machining operation: Feedrate (f): Depth of cut (D OC ): Extreme nishing 0.05 to 0.15 0.25 to 2.0 Finishing 0.1 to 0.3 0.5 to 2.0 Light roughing 0.2 to 0.5 2.0 to 4.0 Roughing 0.4 to 1.0 4.0 to 10.0 Heavy roughing >1.0 6.0 to 20 Extremely heavy roughing >0.7 8 to 20 (mm) (mm) [Courtesy of Sandvik (UK) Ltd] . Turning and Chip-breaking Technology Once again, manufacturers data tables are oen useful ‘starting-points’ for estimating the initial cutting pa- rameter information. Considerable care must be taken if the material has either a high work-hardening ten- dency, or intrinsic bulk (i.e. workpiece material) hard- ness, as this can inuence the numerical data selected. Moreover, the plan approach angle also has an eect on the numerical value for the parameter, for example, oblique machining allows a higher value than for or- thogonal machining. 2.2 History of Machine Tool Development and Some Pioneers in Metal Cutting .. Concise Historical Perspective of the Development of Machine Tools Toward the end of the 1700’s, any high-quality machin- ing at the time meant tolerances of 0.1mm being con- sidered as ‘ultra-precision’ , with this level of tolerance having steadily improved from the beginning of the Industrial Revolution. Pioneers in machine tool devel- opment such as John Wilkinson (1774), developed the rst boring machine, this being capable of generating a bored hole of 1270 mm in diameter, with a error of about 1 mm. A contemporary of Wilkinson, namely Henry Maudslay (1771–1831), invented many preci- sion machine tools, but he was particularly noted for the design and development of the rst engine lathe. Slightly later, Sir Joseph Whitworth (1803–1887), de- veloped the rst modern-day Vee-form screwthread and nut (i.e. 55° included angle – ‘Whitworth thread’), thereby enabling precision feed-motion to be achieved via suitable gear trains on such machine tools. ese early fundamental advances in machine design, al- lowed others and in particular, Joseph R. Brown (1852) to design the ‘dividing engine’. is newly-de- veloped equipment, allowed precision engraving of the hand dials on machine tool axes, enhancing them with much better machinist’s judgment in both rotary and linear control, in combination with consistent repeatability by the skilled operative. Shortly aer these developments, Eli Whitney produced the origi- nal milling machine, which was rened still further by the Cincinnati Screw and Tap Company in 1884. is ‘Cincinnati machine’ was a direct forerunner of today’s manual controlled knee-type milling machine tools. Of particular note was the ergonomic grouping of the controls centrally for a more ecient hand con- trol by the skilled operator. At this time the machine tool still utilised the Vee-form screw thread, with the Acme-form (ie having the ability to take-up backlash) still someway o development. Steady development and renement of a range of machine tools continued into the the rst half of the 20 th century until the next major ‘milestone’ oc- curred. is signicant development was the ‘modern’ numerically-controlled (NC) machine. Around the late 1940’s, the ‘recirculating ballscrew’ 14 was designed so that it could take-up backlash in both directions of rotation for machine tool axes. ese early ‘ballscrews’ were tted to a converted Cincinnati Milling Machine Company’s ‘Hydro-Tel’ die-sinking machine tool, at MIT (Massachusetts Institute of Technology). is military research-funded project having been commissioned by the United States Air force – who required complex free-form aeronautical parts to be automatically machined for the latest aircra. is research was undertaken by MIT, in association with ‘Cincinnati’ and the Parsons Tool Company. e binary-coded punched-paper tape, controlled the simultaneous machine tool axes using alpha-numeri- cal characters (ie the forerunner of today’s programs using ‘G- and M-coded’ CNC controllers), through a 14 Who, when and where ‘recirculating ballscrew’ design and development took place is open to some debate. As propo- nents in the UK say it was Alfred Herbert and Sons, whereas in the United States, the Parsons Tool Company are oen quoted as the originators. However, what is not in question, is that with its unique ‘Gothic’ arch’ (i.e. Ogival geometry), having point contacts between the screw and the adjacent re- circulating balls, allows the assembly to be pre-loaded in-situ, thereby eliminating any appreciable backlash allowing accu- rate control of these axes. NB e previous Acme taper thread (i.e. 29° included angle) tted to ‘conventional’ machine tools had an eciency of no better that 40% – with backlash present, whereas today’s hardened ‘ballscrews’ have eciencies of ~90%, coupled to an impressive rigidity (~900 N µm –1 ) and minimal ‘stick-slip’ , therefore minimising the so-called ‘ballscrew wind-up’ due to the action of torque-eects in combination with the cutting forces. Chapter . con- sultation with a cutting tool manufacturer, or aer a machinability testing procedure. Cutting Data Once all of the physical, metallurgical and geometrical factors for the cutting tool have. concerning tooling decision-making is the choice of insert mate- rial, or combination of materials that constitute the cutter’s tool edge. Today, manufacturers of tooling have a strategy for continuous. roughing-out operations, the largest cutting depth for a given toolholder, will inuence the insert size. For any insert, the eective cutting length has to be determined (see Fig. 20 b), as the entering
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