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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 19th century. In 1864 toward the latter part of the American Civil War, Ste-ven Morse (i.e. later to design the significant ‘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. 35693
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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: ‘The 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 different story, as a drill’s perfor-mance is influenced by a considerable number of fac-tors, most of which are listed in Fig. 43. 3.1.2 Twist Drill FundamentalsThe basic construction of a conventional twist drill is depicted in Fig. 44a. From this illustration two dis-tinct cutting regions can be established: firstly, 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 left-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-ficient material removal, while the chisel edge’s con-tribution is both inefficient and is mainly responsible for geometric errors in drilling, coupled to high thrust loads. The 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 offset 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. This obliquity is responsible for inducing chip flow in a direction nor-mal to the lips in accordance with Stabler’s Law . The increasing chip flow obliquity can be seen in Fig. 45a, by observing the flow lines emanating from the chip’s interface along the lips and up the flute face. Such an oblique cutting action serves to increase the twist drill’s effective rake angle geometry. With the advent of
‘Spherical trigonometric computer software’ 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 flutes 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 inefficient 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. These ‘products’ are extruded, then wiped into the drill flute whereupon they intermingle with the main cutting edge chips. This 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 confirmed with experimental torque and thrust measurements. The 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. The 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 effect on both the hole di- mensional accuracy and roundness, with some ‘helical wandering’ as the drill passes through the workpiece. Hole accuracy and in particular the ‘bell-mouthing ef-fect’ , is minimised by previously centre-drilling prior to drilling to ‘size’. The main cause of such this ‘bell-mouthing’ is probably the inconsistency in the drill geometry. Such effects are exacerbated using Jobber drills , 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/deflection. It is worth noting that the rigidity of a tool such as a drill will decrease by the ‘square of the distance’ . Therefore it follows that the greater the drill penetra-tion into the workpiece, the progressively larger the deflection and, the further from the ‘true axis of rota-tion’ will be the subsequent drill’s path. This deflected drilled hole slope angle ‘ϕ’ , can be defined 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 deflected tool, ‘Helical wandering’ is the result of the drill’s geometry be-ing ‘unbalanced’ ,