Drilling and Associated Cutting Tool Technology Industrial Handbook_1 pdf

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  . 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. C.J. Oxford Jr., 19 55]. Drilling and Associated Technologies  Figure 45. The twist drill shearing and extrusion mechanism at the bottom of a hole. [Source: C.J. Oxford Jr., 19 55] .  Chapter. 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