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The Effects of Cutting Parameters on Cutting Forces and Heat Generation when Drilling Animal Bone and Biomechanical Test MaterialsAkos Cseke, Robert HeinemannSchool of Mechanical Aerospace and Civil Engineering, The University of Manchester, Pariser Building, Manchester M1 3BB, United KingdomAbstractThe research presented in this paper investigated the effects of spindle speed and feed rate on the resultant cutting forces (thrust force and torque) and temperatures while drilling SawBones? biomechanical test materials and cadaveric cortical bone (bovine and porcine femur) specimens. It also investigated cortical bone anisotropy on the cutting forces, when drilling in axial and radial directions. The cutting forces are only affected by the feed rate, whereas the cutting temperature in contrast is affected by both spindle speed and feed rate. The temperature distribution indicates friction as the primary heat source, which is caused by the rubbing of the tool margins and the already cut chips over the borehole wall. Cutting forces were considerably higher when drilling animal cortical bone, in comparison to cortical test material. Drilling direction, and therewith anisotropy, appears to have a negligible effect on the cutting forces. The results suggest that this can be attributed to the osteons being cut at an angle rather than in purely axial or radial direction, as a result of a twist drill’s point angle.Keywords: Bone drilling, Biomechanical test material, Bovine, Porcine, Cutting forces, Cutting temperaturesCorresponding authorEmail addresses: akos.cseke@manchester.ac.uk (Akos Cseke),robert.heinemann@manchester.ac.uk (Robert Heinemann)1. IntroductionThe number of orthopaedic operations has been steadily increasing over the last few years. According to statistical data provided by the National Joint Registry in their 12th annual report [1], 226,871 operations were performed in the United Kingdom in 2014 alone, marking this year with a 9.3% increase compared to 2013. Academic and industrial research has been responding to this trend, by developing more effective surgical devices and efficient procedures, making the field of orthopaedics a major area of research. Performing the necessary validation procedures on these novel devices is a time-consuming and tedious process, mainly because surgical conditions (e.g. working with actual human bones with an intact vascular system) are hard to recreate. As the availability of live specimens (i.e. bone stock) is sparse, whatever sample size and quality required to be statistically relevant, reliable data is hard to obtain within a reasonable time-frame, before environmental deterioration like dehydration or biological decay takes effect and alters the specimens [2, 3]. To somewhat mitigate these limitations, biomechanical test materials are used instead of genuine human tissues, not only because access to these is not constrained due to their commercial availability, but also because they maintain stable (bio-)mechanical properties.While a number of researchers have investigated the physical and mechanical characteristics of these substitute materials (e.g. density, tensile and compressive strength) [4, 5], there is still a lack of consistent information available regarding machining characteristics, i.e. cutting forces and cutting temperatures, which are crucial for the assessment of orthopaedic and other procedures involving bone and bone substitutes. Because one cannot derive reliable information on machining characteristics from the materials properties already available, the aim of this research was to fill the gap by providing information on the machining characteristics of commonly used biomechanical test materials and compare them to cadaveric animal specimens. In these experiments cutting forces (torque and feed force) and the resulting temperatures were measured for different kinds of bone and bone substitute materials, using discrete spindle speed and feed rate combinations. Cortical bone is highly anisotropic, due to the orientation of microscopic fibers called osteons, which are cylindrical stems of about 0.2-0.25 mm in diameter and 1-3 mm in length.These are responsible for the high tensile and compressive strength of cortical bone tissue parallel to the main axis of the osteons [6, 7]. As a result of the anisotropy of cortical bone, reaction forces during plastic deformation, i.e. fracture, seem to be also affected, so much so, that cutting forces in the case of single blade penetration were reported to exhibit a significant (up to 124%) increase in reaction force during radial cutting as compared to axial cutting [8]. Unfortunately, a comparison between axial and radial approach in drilling of cortical bone tissue has not been provided by the literature. As a consequence, tests on both radial and axial approach were included, in order to establish whether the drilling direction actually has an impact.2. Materials and Methods2.1. SpecimensFour SawBones? biomechanical test materials with different densities were chosen for the experiment: a high porosity solid rigid polyurethane (PU) foam for simulating trabecular bone (SawBones 1522-01); two lower-porosity PU foams (SawBones 1522-03 and 05) for the more compact trabecular bone, and a high density epoxy and microfiber composite (SawBones 3401-06) for simulating human cortical bone. For testing animal specimens, bovine and porcine femur were chosen, as these are often used in experiments where a close resemblance to human bone structure is required [9–11]. An overview of the test materials and their corresponding properties can be found in Table 1. Unfortunately, mechanical properties for porcine bone in the radial direction could not be ascertained. Neither was it possible to obtain information on the thermal properties of the SawBones materials.Table 1: Materials and their mechanical properties used in the experiment a[12], b[13], c[14], d[2], e[15]MaterialDensity(g/cm3)Tensile strength(MPa)Modulus of elasticity(GPa)Axial(Longitudinal)Radial(Traverse)Axial(Longitudinal)Radial(Traverse)SawBones 1522-010.16±10%a2.1a0.086aSawBones 1522-030.32±10%a5.6a0.284aSawBones 1522-050.64±10%a19a1aSawBones 3401-061.64±2.5%a106a93a16a10aBovine cortical bone1.92b97.41c-113d40.18±9c20.22c-25d12.43cPorcine cortical bone1.96-2.38e88±1.5dn/a14.9dn/aThe cortical bone samples were acquired from a local butcher and were kept in a freezer at 15°C for not more than five days. The specimens were prepared by slicing the diaphysis (mid-section) of the femur into disks of approximately 20 mm thickness, and milled flat using a machining center, to ensure that the surfaces were parallel. Prior to the experiments the cortical bone samples were thawed in 25°C water to minimise dehydration.2.2. ToolsDrilling was the machining method chosen for this research, since it is an often practised material removal method in orthopaedic surgery [16] and, therefore, allows the results to be utilised in real life applications. In view of the width and thickness of the material layers in the available bone specimens, a hole diameter of 3 mm and hole depth 10 mm were selected. This diameter allowed for accommodating two or three 1 mm diameter thermocouple holes located at a distance of 1 mm from the test hole.The drilling of deep holes was chosen in order to obtain a reasonable amount of data (due to long cycle times) and to test the effects of deep hole drilling, which often suffers from poor chip evacuation, leading to increased process temperatures. While research has been conducted on the optimal tool geometry in order to reduce cutting forces and process temperatures in bone drilling [e.g. 16–19], in this research the aim was not to minimise these effects, but to produce consistent cutting force signals of high enough magnitude to reliably assess the machinability of different materials. Walter A1276TFL tungsten carbide twist drills with a 40 helix angle, 140 point angle and wide chip flutes were selected for all cutting tests. Previous research —on materials other than bone (substitutes)— demonstrated that this type of drill geometry is well suited for drilling deep holes [20]. The use of tungsten carbide drills was expected to minimise the amount of tool wear, in order to reduce the variation in cutting forces due to a change in point geometry brought about by wear.2.3. Experimental apparatus and procedureThe experimental apparatus for the force measurement consisted of five major components: A Takisawa MAC-V2 machining center, a Kistler 2-component dynamometer (type 9271A) with a custom-made clamping device mounted on top, a Kistler 2-channel charge amplifier (type 5001), analog-to-digital (A/D) converter, and PC with LabVIEW 8.2 data acquisition software. The apparatus for the temperature measurement consisted of a PICO thermocouple data logger connected to the PC, running PicoLog data acquisition software, and 1 mm diameter Omega K-type thermocouples (type HKMQSS-IM100G-300). The entire setup is shown in Figure 1.Before drilling a test hole, in the case of SawBones and bovine bones, three thermocouple holes, with depths of 4, 6 and 8 mm, were drilled, see Figure 2. In the case of porcine bone, due to the limited space available, only two thermocouple holes, with depths of 4 and 8 mm, were drilled. Following the preparation of the holes, the thermocouples were inserted, and once the temperatures had stabilised a single test hole with the given feed and spindle speed was drilled. Tests on radial bone drilling were performed on porcine cortical specimens with cutting parameter combinations of 700/0.12, 1000/0.15 and 1500/0.2 RPM / mm/rev in order to compare the effects of short, medium and long drilling cycle times. The cutting parameter window, see Table 2, was established based on information provided by Bertollo and Walsh [16] and parameters used by other researchers in their experiments [21–24].Figure 1: Setup used to conduct experiments on a machining centre to record cutting forces (using a dynamometer and charge amplifier) and temperatures (using thermocouples) when drilling different types of SawBones as well as bovine and porcine bone samplesFigure 2: Bovine bone sample clamped to the vice attached to the dynamometer to record cutting forces, together with thermocouples inserted into pre-drilled holes around the test holeTable 2: Cutting parameters used in the experiment; ”A”: axial drilling direction, ”R”: radial drilling directionSpindle speed (RPM)70010001500Feed rate (mm/rev)0.12A+RAA0.15AA+RA0.2AAA+RTemperatures were not measured, because in radial drilling the bone’s cortical layer was too thin to accommodate thermocouples at the same depths as in axial deep hole drilling, therefore making the acquisition of comparable data impossible.3. Results and Discussion3.1. Analysis of the drilling processFor each material three test holes were drilled. Following signal processing (smoothing) of the force and torque signals, the averages of these were calculated. The graphs provided in Figure 3 serve the purpose of evaluating the overall drilling process and explaining the underlying material removal mechanism. Figure 3: Cutting force vs. drilling time for a) SawBones 1522-01, b) Sawbones 1522-03, c) Sawbones 1522-03, d) SawBones 3401-06 e) Bovine cortical bone f) Pig cortical bone and g) Temperature rise (T) vs. Drilling time for SawBones 3401-06; spindle speed 1000 RPM, feed rate 0.2 mm/rev; Different scales for the y-axes (for a-f) were used, due to the large differences in magnitude between torque and thrust force. The time scale for (g) was extended in order to capture the peak temperatures at the points of measurements. The fluctuation of both signals can be attributed to a high frequency chipping effect recorded by the dynamometer, which was caused by the extreme porosity of the Saw-Bones 1522-01, 03 and 05, which resulted in the rotating tool to break o fragments of material instead of cutting continuous chips. It can be observed from Figures 3 a) to c) that as the drill propagates through these SawBones types it produces a constant thrust force and a steadily increasing torque. It is assumed that the change in torque is caused by an increase in contact area between borehole wall and drilling tool (via the tool margins) as well as the gradual accumulation of fine debris inside the chip flutes. As the drill propagates deeper into the material, the surface area at which the chips are getting ground between the tool and the borehole wall increases, causing higher mechanical resistance. This phenomenon corresponds to the observations of previous research conducted by Wiggins and Malkin [25], where a linear increase in drilling torque corresponding to an increase in drilling depth had been examined. In the case of SawBones 3401-06, see d), the thrust force reaches a peak right at the start during initial penetration, followed by a steady asymptotic decrease, whereas the torque increases slightly with borehole depth. In the case of cortical bovine and porcine bone, see e) and f), a similar trend can be observed, with the difference that the decrease in thrust force is quicker and then levels off.The assumption that friction plays a major role in the cutting process is strongly supported by the temperature measurements, see Figure 3 g), which shows a decrease in peak temperature with increasing drilling depth (TC1>TC2>TC3). Despite the measurement uncertainty associated with the thermocouples (±2.2°C as stated by the manufacturer) and the fact that the temperatures recorded are relatively close together, this phenomenon was observed in each drilling cycle, for each material. This phenomenon is in contrast to what other researchers have observed when drilling holes into materials other than bone and bone substitutes, i.e. the deeper the drill propagates the higher the peak temperature at the cutting point [26]. There it is argued that the dominant heat source in drilling is the plastic deformation and chip formation (shearing) at the cutting zone and the sliding of the chip up the tool’s rake face, whereas the friction between chips and workpiece is of only minor importance.The decrease in peak temperature, with increasing borehole depth, observed in this research however implies that the friction between tool and workpiece is the dominant heat source. Although a few authors, as for example [27-29], observed a similar trend in temperatures, there appears to be a lack of understanding to fully explain this phenomenon. It is believed by the authors of this paper, that heat was generated due to friction not only between the tool margins rubbing along the borehole wall, but also by the chips that were trapped and reground between these two surfaces. This is supported by the considerable quantities of very fine dust that were extracted from the borehole and embedded inside the pores along the borehole wall. In order to verify these assumptions, additional tests using a modified tool were conducted. In an attempt to reduce friction between the tool’s margins and borehole wall, the diameter of one drill was manually reduced from 3 to 2.5 mm by grinding, excluding the tip, where a 2 mm long segment was left intact (i.e. the original diameter), in order to preserve the cutting edges. This modified tool is shown in Figure 4. The extra clearance between the drill’s margins and the surrounding borehole wall was aimed at to not only eliminate direct contact but also to reduce the amount of chip debris being re-cut (ground) at this interface.Figure 4: Modified tool for investigating frictional effects, showing the original tool diameter at the tip and the reduced diameter over a flute length of 25 mmSawbone 3401-05 was chosen as the test material, because it is sufficiently dense and homogenous to produce high and consistent cutting forces. The cutting parameters selected for this experiment were a spindle speed of 1000 RPM and a feed rate of 0.2 mm/min. Axial thrust force, torque and cutting temperatures were measured using the same setup as shown in Figure 1. Figure 5 illustrates the variation in cutting forces and peak temperatures vs. drilling time.Figure 5: Drilling SawBones 3401-06 with modified tool: a) Thrust force vs. drilling time; b) Temperature rise (ΔT) vs. drilling timeIt is evident that unlike in the case of drilling with the unmodified tool, the peak temperatures increase with drilling depth, i.e. TC3>TC2>TC1. Although the peak temperature at TC1 for the modified tool is lower than in the case of the non-modified tool, as was expected, the temperatures recorded at both TC2 and TC3 were higher, which was not expected. Also, the torque increased noticeably after about 2 seconds into the drilling cycle (phase II), equivalent to a borehole depth of approximately 4 mm. Both the increase in torque and peak temperature (at TC2 and TC3) indicate an increase in friction, despite the fact that the margins had been ground away for most of the flute length. This can be attributed to the clot of compacted chip debris inside the front end of the chip flutes, which was formed during the drilling process and resulted in an increase in friction, causing both the cutting temperature and torque to rise noticeably. It is reasonable to assume that the formation of this clot of debris was caused by a deteriorated chip extraction, due to the gap created between the chip flutes and the borehole wall by the modification of the tool. This allowed some of the chips in the flutes to fall through the gap back into the cutting zone, where they got reground and progressively compacted3.2. Comparison between different bone and bone substitute materialsFigure 6 shows the mean drilling force and torque and peak temperature for different spindle speed and feed rate combinations for the bone and bone substitute materials presented in Section 2.Figure 6: Comparison of a) Thrust force b) Torque c) Temperature rise (ΔT) vs. various spindle speed and feed rate combinations, in different bone and bone substitute materials.It can be observed that the spindle speed has no significant effect on either thrust force or torque. In contrast, an increase in feed rate causes a distinct increase in cutting forces, although this relationship is less pronounced in the case of SawBones 1022-01 to 05. This can be explained by the drill taking a heavier cut and removing more material per revolution when operating at an increased feed rate, whereas with an increased spindle speed the amount of material removed per revolution remains paring porcine to bovine cortical bone, the difference in thrust force is approximately 20%, whereas the difference in drilling SawBones 1022-01, 03 and 05 is around 40% and 33% respectively. When drilling SawBones 3401-06 a thrust force was measured that was only approximately 25-30% and 35-45% of that when drilling porcine and bovine bone, respectively. Considering the drilling torque in Figure 6 b) a very similar trend can be observed, but the differences between cortical SawBones and porcine and bovine bones are only about 20% and 30%, respectively. Attempting to correlate the differences in cutting forces to the tensile strength of the materials presents its challenges, due to the highly complex structure of the genuine bone tissue materials. However, it appears that the differences in both thrust force and torque are correlated to the different densities of the various test materials, see Table 1.In terms of drilling temperatures, there is a clear correlation between average peak temperature and cutting parameters, such that a shorter drilling time, brought about by an increase in feed rate and spindle speed, individually or in combination, results in a reduced peak temperature. This observation agrees with the findings of many other researchers e.g. [17, 23, 30–34], as well as the results reported on in the previous section, where it was suggested that the heat generation is mainly caused by friction. 3.3. Axial versus radial drillingIt has been pointed out in Section 2.3 that, due to the anisotropic nature of cortical bone tissue, the drilling direction (i.e., radial or axial) may have an effect on the resultant cutting forces.Figure 7 illustrates the two different drilling directions and the orientation of the tool’s cutting edges relative to the osteons. It is important to point out that the cutting direction in drilling is perpendicular to the drilling direction, due to cutting edges’ rotating motion around the tool axis. Although this is an approximation, because it ignores the axial motion of the drilling tool, due to the high ratio between cutting speed and feed (in this case 10 m/min vs. 9 mm/min), the axial component added to the cutting direction by the feed can be neglected. In the case of the experiment presented in this paper, the point angle of the tool was 140°, which resulted in the osteons in the case of axial drilling to be always cut at an angle of 70° to their main axis, see Figure 7 a). In contrast, in the case of radial drilling, the cutting angle is dependent on the rotational angle of the tool. This ranges from cutting along the osteons, where the cutting direction is parallel to the osteon axes, see Figure 7 b), to cutting across the osteons, where the cutting direction is 90°, see Figure 7 d). In the first case, this results in a lengthwise splitting of the osteons, while in the second case the cutting is similar to the cutting experienced in axial drilling. As the tool rotates there is a gradual transition between these two cutting scenarios, as illustrated in Figure 7 c).Figure 7: Illustration of cutting directions relative to osteon orientation in a) axial drilling b) radial drilling while cutting parallel to osteon axis c) radial drilling while cutting 45° to osteon axis d) radial drilling while cutting 90° to osteon axis (not to scale).Figure 8 shows the cutting forces in radial and axial approach, acquired using the same cutting parameters.Figure 8: Comparison of a) Thrust force b) torque for axial and radial drilling directionsThe results show a small increase in both feed force and drilling torque, all of which fall within the band of their respective standard deviation. It is suggested that in the case of drilling porcine cortical bone, the similarities in cutting forces between axial and radial drilling directions can be attributed to the point angle of the tool, thereby preventing the tool from cutting in purely axial or radial direction. Instead it is a combination, which makes the change from axial drilling to radial drilling less pronounced.4. ConclusionsFrom the results presented in this paper the following conclusions can be drawn:In the drilling of bone and biomechanical test material, the main heat source does not appear to be the plastic deformation and shearing of the material at the cutting point, but the friction caused by the fine chip debris travelling through the drill flutes and rubbing along the borehole wall. As a consequence, the temperature increase is the largest in that region of the borehole wall that is exposed to this flow of chips for the longest period of time. This suggests that a shorter contact time between tool, debris and workpiece is advantageous for reducing the temperatures within the workpiece.Because friction appears to be the main contributor to temperature generation, a tool design that promotes chip extraction, especially when deep holes need to be drilled, could be considered as a major step towards lowering temperatures.Drilling of cortical animal bone tissue resulted in a significantly higher thrust force and torque compared to both compact and, in particular, porous SawBone test materials, the latter of which should not be intended to mimic bovine or porcine cortical bone tissue. A comparison between the cutting forces recorded in this research and those recorded when cutting genuine human bone tissue by other researchers, at this stage, cannot be made, due to the differences in cutting tools and process parameters.In contrast to conventional (e.g. tensile or compression) mechanical tests or single blade penetration, where various loading directions imply significant differences in reaction forces, a correlation between cutting forces and drilling direction does not appear to exist when drilling cortical bone. This could be attributed to one of the key geometric features of twist drills, the point angle, which results in the osteons not being cut in a purely radial or axial direction, but at an angle, regardless of the drilling direction. In order to quantify the extent to which the point angle affects the cutting forces in axial vs. radial drilling, further investigations are required.FundingThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.Ethical approvalNot required.Conflict of interestThe authors of this article do not have any conflict of interest.References[1]NJR, 12th Annual Report, National Joint Registry for England, Wales, Northern Ireland and the Isle of Man. .uk. 2015.[2]Subrata P, Design of Artificial Human Joints & Organs. In: Mechanical Properties of Biological Materials. Springer; 2014. p23–40.[3]Sedlin ED, Hirsch C. Factors Affecting the Determination of the Physical Properties of Femoral Cortical Bone. Acta Orthopaedica Scandinavica 1966;37:29–48.[4]Patel PS, Shepherd DE, Hukins DW. Compressive properties of commercially available polyurethane foams as mechanical models for osteoporotic human cancellous bone. 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