Cutting Tool Technology Episode 3 potx

3 Drilling and Associated Technologies ‘In all things, success depends upon previous preparation and without such preparation……there is sure to be failure.’  (c550–c487BC) [Analects] 3.1 Drilling Technology .. Introduction to the Twist Drill’s Development Drilling operations are perhaps the most popular ma- chining process being undertaken today, with their origins being traced back to cutting tool develop- ments in North America in the 19 th century. In 1864 toward the latter part of the American Civil War, Ste- ven Morse (i.e. later to design the signicant ‘Morse taper’ – for accurate location of the ‘sleeved drills’ into their mating machine tool spindles) founded the Morse Twist Drill and Machine Company in the ‘North’. Morse then proceeded to develop probably the most important cutting tool advance to date, namely, the ubiquitous twist drill. In Fig. 42, several of today’s twist drills are illustrated along with just a small range of ‘solid’ contemporary designs. Morse’s originally-de- signed twist drill has changed very little over the last 150 years – since its conception. In comparison to the somewhat cruder-designed contemporary drills of that time, Morse stated: ‘e common drill scrapes metal to be drilled, while mine cuts the metal and discharges the chips and borings without clogging’. Morse’s statement was at best, to some extent optimistic, whereas the ‘cold reality’ tells a dierent story, as a drill’s perfor- mance is inuenced by a considerable number of fac- tors, most of which are listed in Fig. 43. .. Twist Drill Fundamentals e basic construction of a conventional twist drill is depicted in Fig. 44a. From this illustration two dis- tinct cutting regions can be established: rstly, the main cutting edge, or lips; secondly at the intersection of the clearance and main cutting edge – termed the chisel edge. In fact for a twist drill, the cutting process can be equated to that of a le-hand oblique turning tool, where the rake and clearance face geometries are identical and the correlation between these two ma- chining processes have been validated in the experi- mental work by Witte in 1982. Both of these regions remove material, with the cutting lips producing ef- cient material removal, while the chisel edge’s con- tribution is both inecient and is mainly responsible for geometric errors in drilling, coupled to high thrust loads. e main cutting edges are accountable for a rela- tively conventional chip formation, as shown in the ‘quick-stop’ photomicrograph in Fig. 44b. An oblique cutting action occurs to the direction of motion, being the result of an oset of the lips that are parallel to a radial line – ahead of centre – which is approximately equal to half the drill point’s web thickness and in- creases toward the centre of the drill. is obliquity is responsible for inducing chip ow in a direction nor- mal to the lips in accordance with Stabler’s Law 1 . e increasing chip ow obliquity can be seen in Fig. 45a, by observing the ow lines emanating from the chip’s interface along the lips and up the ute face. Such an oblique cutting action serves to increase the twist drill’s eective rake angle geometry. With the advent of ‘Spherical trigonometric computer soware’ for ob- taining direct three-dimensional calculations – previ- ously described by Witte (1982) in two-dimensional formulae for cutting edge performance – these calcu- lations have been enhanced. Under the chisel point, or web, the material re- moval mechanism is quite complex. Near the bottom of the utes where the radii intersect with the chisel edge, the drill’s clearance surfaces form a cutting rake surface that is highly negative in nature. As the centre of the drill is approached, the drill’s action resembles that of a ‘blunt wedge-shaped indentor’ , as illustrated in Fig. 45b. An indication of the inecient material removal process is evident by the severe workpiece deformation occurring under the chisel point, where such deformed products must be ejected by the drill to produce the hole. ese ‘products’ are extruded, then wiped into the drill ute whereupon they intermingle with the main cutting edge chips. is fact has been substantiated by force and energy analysis, based on a combination of cutting and extruding behaviour under the chisel point, where agreement has been conrmed with experimental torque and thrust measurements. e chisel edge in a conventionally ground twist drill has no ‘true’ point, which is one of the major sources for a drilled hole’s dimensional inaccuracy. 1 Stabler’s Law – for oblique cutting, can be formulated, as be- low: Chip ow (cos η) = cos I (b c /b) Where: I = inclination of cutting edge, b c = chip ow vector, b = direction of cutting vector.  Chapter  e conventional twist drill chisel point geometry can be seen in Fig. 46, together with associated no- menclature for critical features and tolerance bound- aries. From the relatively complex geometry and dimensional characteristics shown in Fig. 46, the ob- tainable accuracy of holes generated whilst drilling is dependent upon grinding the drill to certain limits. Any variations in geometry and dimensions, such as: dissimilar lips and angles, chisel point not centralised, and so on, have a profound eect on both the hole di- Figure 42. A selection of just some of the many ‘solid’ and ‘through-spindle’ drilling varieties and ‘inserted-edge’ insert geometries currently available. [Courtesy of Seco Tools] . Drilling and Associated Technologies  Figure 43. The principal technical drill performance criteria and factors associated with drilling operations in this case for ex- ample, on castings .  Chapter  Figure 44. The twist drill geometry and associated chip shearing mechanism. [Source: C.J. Oxford Jr., 1955]. Drilling and Associated Technologies  Figure 45. The twist drill shearing and extrusion mechanism at the bottom of a hole. [Source: C.J. Oxford Jr., 1955] .  Chapter  Figure 46. Twist drill geometry. Drilling and Associated Technologies  mensional accuracy and roundness, with some ‘helical wandering’ 2 as the drill passes through the workpiece. Hole accuracy and in particular the ‘bell-mouthing ef- fect’ 3 , is minimised by previously centre-drilling prior to drilling to ‘size’. e main cause of such this ‘bell- mouthing’ is probably the inconsistency in the drill geometry. Such eects are exacerbated using Jobber drills 4 , or even worse, by utilising longer-series drills, which tend to either slightly ‘unwind’ , or bend as a re- sult of lessening rigidity promoting some drill bend- ing/deection. It is worth noting that the rigidity of a tool such as a drill will decrease by the ‘square of the distance’ 5 . erefore it follows that the greater the drill penetra- tion into the workpiece, the progressively larger the deection and, the further from the ‘true axis of rota- tion’ will be the subsequent drill’s path. is deected drilled hole slope angle ‘ϕ’ , can be dened in the fol- lowing manner: Drilled hole slope angle ‘φ’ = 3/2 l × R/T (1 – I/k × tan k l) Where: l = length of deected tool, 2 ‘Helical wandering’ is the result of the drill’s geometry be- ing ‘unbalanced’ , resulting from of diering lip lengths, or an oset chisel point, causing the drill to ‘spiral-down’ through the workpiece, as it progresses through the part (see Fig. 70). ‘Bell-mouthing’ of the drilled hole is attributable to the chisel point and is produced by the line-of-contact, as the drill point initially touches the component’s surface, causing it to ‘walk’ until the feed/penetration stabilises itself at the outer corners (i.e. margins) entering the workpiece, whereupon, these mar- gins guide the drill into the part. 3 ‘Bell-mouthing eect’ is produced by the drill chisel point’s eccentric behaviour as it attempts to centralise its rotational motion as it enters, or exit’s the workpiece. 4 ‘Jobber drills’ are considered to be ‘standardised drills’ that are normally utilised for most drilling general operations, un- less otherwise specied. 5 ‘Rigidity rule’: a drill, reamer, tap, or a milling cutter held in a spindle will have its rigidity decreased by the ‘square of the distance’ , namely, if a drill is twice as long it is four times less rigid. NB A cantilevered tool such as a boring bar has its rigidity de- creased by the ‘cube’ or the distance – meaning that too much tool overhang, will seriously reduce tooling rigidity. R = ratio of the transverse reaction at the drill point, T = thrust force, I = system’s ‘moment of inertia’ , k = √T/E I. As suggested above, this ‘axis slope error’ is initiated when the chisel edge begins to penetrate the workpiece and unless the feed is discontinued, or in some man- ner the error is corrected, the magnitude of deection will increase as drill penetration continues. e drill’s magnitude of deection can reach up to 60 µm, under exaggerated drilling conditions. e geometry of the point has been the subject of considerable research and development for many years, with some unusual departures from the ‘stan- dard’ 118° drill point included angle. Typical of these extreme approaches were the so-called ‘Volvo point’ , having a negative 185° included angle – primarily utilised to avoid ‘frittering’ 6 of drilled holes, or the highly positive geometries such as 80° included an- gle used for drilling some plastics. Not only can the point angle be modied, but the shape and prole of the chisel point, or web 7 oers numerously-ground opportunities for detailed geometric modications, with only some of which being shown in Fig. 47. Four of the most commonly-ground drill point geometries being: • Conventional – the ‘original’ Morse geometry, hav- ing a straight chisel edge, with poor self-centring drilling action (Fig. 46a), • Split-point 8 – there are a range of point-splitting techniques available to alter the point prole, which has the eect of modifying the chisel point to allow a reasonable self-centring action (Fig. 47b), 6 ‘Frittering’ refers to the break-out at the hole’s edge as the drill exit’s the part, on some brittle materials, such as on several Powder Metallurgy compacts. 7 ‘Web’ refers to the internal core of the drill – which imparts mechanical strength to the drill. e web increases in thick- ness the further one gets from the chisel edge (i.e. shown in Fig. 47 – in lower diagrams and with cross-sections). Hence, if the drill is reground many times, the chisel point width will obviously increase, this necessitates that the chisel point must be ‘thinned’ , otherwise too high a thrust force occurs and an inecient drilling action will result. 8 ‘Split-point’ ground drills are sometimes referred to as ‘Multi- facet drills’.  Chapter  Figure 47. A range of typically ground twist drill points. Drilling and Associated Technologies  • Web-thinning – as its name implies, the chisel point is web-thinned/notched, by regrinding to reduce the width of the chisel point, while slightly modify- ing the prole, giving a partial self-centring action (Fig. 47c), • Helical – the chisel point is ground to an ‘S-shape’ , which modies both the chisel point and its pro- led shape, improving the drilling performance and self-centring action (Fig.47d). NB  On some drills a sophisticated grinding action has imparted drills without a chisel point, which sig- nicantly improves their drill penetration rates into the workpiece, but requires a complex drill regrinding operation to re-sharpen them when the edge becomes ‘dulled’. Not only are drills supplied with appropriate point geometries, but for twist drills the twin spiral utes of the drill can also be specied – from the tooling manufacturer, as this gives the drill its ‘equivalent of the rake angle’ as found on a single-point turning tool. On conventional jobber drills, the normal ute angle is 29° – giving a relatively ‘slow’ helix (Fig. 47a) and in the past, typically being utilised for drilling most plain carbon steel grades. Conversely, a drill with a ‘quick’ helix angle (Fig. 47d), might be employed to drill so materials such as certain plastics. Brittle materials on the other hand, which might be utilised typically when drilling Cartridge Brass (i.e. 70Cu 30Zn composition), require a zero, or slightly negative helix. NB  It is possible to temporarily modify the drill’s he- lix angle by re-grinding, termed ‘drill dubbing’ , which refers to lightly ‘ash-grinding’ the utes at the lips to decrease the eective ute helix angle. e main strength of a drill is via its web, or its cross- section which can be changed and as a result, will modify the ute’s geometric prole (i.e. see Fig. 48). In general, drill cross-section are classied in three groups, namely: • Axe-shaped – having well-dened margins (Fig. 48 –top), • Rounded heel – with increased web, but small mar- gins (Fig. 48 – middle), • Rhombic – incorporating a large web, with wide margins (Fig. 48 – bottom). NB  Some twist drills feature oil/coolant holes to allow cutting uid to reach right down toward the cutting edges of the drill, increasing both tool life and improv- ing the hole’s ‘Surface Integrity’ 9 . .. The Dynamics of Twist Drilling Holes Introduction e term ‘drilling’ refers to all production techniques for the manufacture of cylindrical holes in workpieces using chip-making cutting tools, for short-hole 10 and deep-hole drilling operations. e expression ‘solid drilling’ has been introduced in recent years – which is hole-making generation undertaken in a single opera- tion, to dierentiate it from that of the previous tech- niques of either: centre-drilling or, pilot-hole drilling (i.e, see Fig. 50b) prior to drilling to size. Drill technol- ogy includes a range of specialised hole-making tool- ing, including: twist drills, solid drills, counter-boring and trepanning tools and deep-hole drills 11 . In the rst instance, mention will be made of twist drilling opera- tions, then a review of these other drilling production methods will occur. Twist Drills Twist drilling operations have been carried out for around 150 years, with a twist drill imparting ‘bal- anced cutting conditions’ , assuming that the drill’s geometry is symmetrical. It has been suggested that the work of drilling may be considered as two single- point lathe tools engaged in an internal straight turn- ing operation. A twist drill produces both torque and thrust as it rotates and is fed into the workpiece. e main contribution to torque is through the lips, with a small amount of torque being generated by the chisel point as the drill rotates against the resistance of the 9 ‘Surface Integrity’ has been coined to describe the ‘altered ma- terial zone’ (AMZ), for localised sub-surface layers that dier from those of the bulk material – considerably more will be said on this subject in Chapter 7. 10 ‘Short-hole drilling’ operations cover depth-to-hole-diam- eter-ratios of up to 6D (i.e. for diameters up to 30 mm), whilst larger drilled holes are limited to depths of 2.5D. Where: D = nominal drill diameter. 11 ‘Deep-hole drilling’ and ‘Gun-drilling’ operations are virtu- ally the same, with the term Deep-hole drilling being the pre- ferred term in this text.  Chapter  [...]... Malleable CI 52.5, Bronze (medium hardness) 31 .5 – with HSS drills, ranging from φ10 to 60 mm 13 CM is derived from experimental data, typically: Carbon steel (construction) 33 .8, Grey CI 23. 3, Malleable CI 20 .3, Bronze (medium hardness) 12.2 – with HSS drills, ranging from φ10 to 60 mm 100 Chapter 3 Figure 50.  Drilling a hole with/without a ‘pilot’ hole and the cutting, rubbing and extrusion mechanism...  ower contributions of: cutting and friction P respectively (kW), vc = Cutting speed (m min–1) ∴P = Pc + Pµ/η Where: η = Machine tool efficiency 3. 2 Boring Tool Technology – Introduction The technology of boring has shown some important advances in recent years, from advanced chip-breaking control tooling (i.e see Fig 59, this photograph illustrates just some of the boring cutting insert geometries... shaft (Fig 53a – top) Hence, the clamping shaft must only transmit torque resulting from the cutting forces and the bending moment of the resultant cutting force which will be present Typically, the outer insert’s resultant cutting force FA (i.e Fig 53b) is comprised of the following forces: • Remaining cutting force (∆Fc) – generated through greater wear rates at the periphery of the outer cutting insert,... ‘Qualified Tooling’ , refers to setting the tool s offsets, with all the known dimensional data for that tool, allowing for ease of tool presetting and efficient tool- changing – more will be said on this subject later in the text Drilling and Associated Technologies Figure 59.  A selection of some tooling that can be employed for boring-out internal rotational features [Courtesy of Seco Tools] 119... tooling assembly is essentially ‘self-piloting’ , in that the cutting forces generated are balanced, not with respect to the cutting edges – as is the situation with Twist drills – but invariably, by pads that are situated at 90° and 180° to that of the cutting edge These pads rub against the bore’s surface being generated and therefore support the head, while burnishing30 the surface The machine tools... two types of cutting edge geometries, with the carbide cutting tips precisely located on either side of the drill head The asymmetric design35 of these ‘Ejector Drills’ has support pads provided, to absorb the radial cutting forces and guide while supporting the tool as it penetrates into the workpiece At the commencement of the deep-drilling operation, the drill bushing’s main function36 (i.e shown... preferred One limitation of utilising cemented carbide tool shanks, is its greater brittleness when compared to steel, so careful tool design is necessary to minimise this problem ‘Compound’ boring bar tool shanks have been exploited to reduce both problems associated with either steel, or cemented carbide tools A successful compound tool used in cutting trials by the author, featured a cemented carbide... drill with the cutting data shown above, then it would either burn-out, or catastrophically fail in the endeavour 108 Chapter 3 Figure 54.  A wide range of drilling/boring operations can be undertaken using indexable insert drills [Courtesy of Seco Tools] Drilling and Associated Technologies Figure 55.  Counterboring and trepanning [Courtesy of Sandvik Coromant] 109 110 Chapter 3 3.1.6 Special-Purpose,... Customised tooling is normally required if one, or several of the criteria mentioned above are to be met To have a tooling manufacturer design special-purpose tooling to meet the production demands of manufacture, is not undertaken lightly, as for complex tooling, its: design, build and prove-out, prior to use, could prove to be expensive However, many companies resort to this type of custom-built tooling,... to the X-axis of the machine tool The arrangement of the inner and outer cutting edges of an indexable insert drill relative to each other, together with the drill’s position to the axis of rotation are vital for perfect drilling operations19 (i.e see Fig 53a) The cutting inserts positions by possible X-axis adjustments, are critical for the: smooth running, resultant cutting forces and, will influence . hardness) 31 .5 – with HSS drills, ranging from φ10 to 60 mm. 13 C M is derived from experimental data, typically: Carbon steel (construction) 33 .8, Grey CI 23. 3, Malleable CI 20 .3, Bronze. Preferably, the cutting edges are ar- ranged in such a manner that the inner- (SBI) and outer-inserts (SBA) have identical cutting widths (Fig. 53a – bottom le). When new insert cutting edges. free -cutting steel grades cutting speeds of up to 400 m min –1 are possible, whereas when drilling low-silicon aluminium grades cutting speeds of 600 m min –1 can be achieved with tool lives