Int J Adv Manuf Technol (2015) 76:1437–1448 DOI 10.1007/s00170-014-6327-0
ORIGINAL ARTICLE
Optimization of processing parameters of a developed new driller system for orthopedic surgery applications using Taguchi method Kadir Gok & Arif Gok & Yasin Kisioglu
Received: 7 June 2014 / Accepted: 26 August 2014 / Published online: 19 September 2014 # Springer-Verlag London 2014
Abstract In bone drilling process during the surgical operations, heating increases due to high bone/drill contact friction that damages the bones and soft tissues. The overheating is usually recognized as the temperature rise exceeds 47 °C, a critical limit, above which the drilling causes osteonecrosis. In this study, a new driller system is developed to prevent the overheating in orthopedic surgeries. It has a closed-circuit cooling system to reduce the undesired temperature rise during the drilling process. It is also designed and manufactured as a prototype and tested experimentally in vitro by drilling fresh bovine bones using different processing parameters. In the drilling tests, the temperature levels of the bones are measured using thermocouple sensors. Based on the measured results, the driller system provides a valuable temperature reduction around 25 % to prevent necrosis in low spindle speeds (rpm) usually preferred by surgeons. The measured temperatures from the tests of the driller system with a cooling system were compared with the use of regular bone drilling process without cooling. The optimum processing parameters of the new driller system with/without coolant are calculated using the Taguchi method, and the most effective parameter is found as rpm. Keywords Necrosis . Bone drilling . Driller system . Taguchi method K. Gok Kutahya Vocational School of Technical Sciences, Dumlupinar University, Germiyan Campus, 43100 Kütahya, Turkey A. Gok (*) Department of Mechanical Engineering, Amasya University, 05100 Amasya, Turkey e-mail:
[email protected] Y. Kisioglu Department of Biomedical Engineering, Kocaeli University, Umuttepe, 41380 İzmit, Kocaeli, Turkey
1 Introduction Biocompatible plates and fixators are commonly used for positioning and fixing the fractured bones in orthopedic surgeries. In order to fix the fractures, the surgeons practice the bones using a surgical drill to fix plates by screws as seen in Fig. 1. They preliminary select a proper drill considering the screw dimensions which are appropriate for fractured bone plate fixation. Drilling is known as a cutting process of the bones with a multipoint drill bit, and end-cutting tool drills the bones with a high contact friction. In drilling processes, an overheating occurs between the drill and the base material due to high contact friction that causes temperature rise. The overheating caused by undesired temperature rise exceeds a critical value, and then the bone and soft tissues are devitalized. In the failure due to overheating, bones and soft tissues remain anemic that causes osteonecrosis. Several studies are available about the critical value of temperature. Hillery and Shuaib [1] showed that bone undergoes serious damage when the temperature rises above 55 °C in 30 s. Eriksson et al. [2] studied in vivo drilling the cortical bones of a rabbit and obtained the thermal necrosis when the temperature exceeds 47 °C in 60 s. Augustin et al. [3] presented that if the temperature increases above 47 °C, it causes irreversible osteonecrosis during the bone drilling process. The cell structures as well as extracellular matrix of the bones can be really disrupted and changed due to necrosis caused by overheating during the bone drilling process. The overheating causes the weakening strength of the bone structure and reduces the success of the fracture fixations. The heating event is usually dependent on process parameters such as cutting tool geometry, cooling system, feed rate (mm/min), and spindle speed (rpm). In order to prevent the overheating, the temperature needs to be reduced or distributed evenly during drilling. In particular, it is difficult to distribute the heat occurred at the drill/bone interface since the thermal
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Fig. 1 Drilling of bone in femoral fracture surgery [4]
conductivity of the bone material is as 0.38 W/mK [5]. Although the surgeons use water as an external coolant on the drilling region to reduce the temperature, this is not considered as a preferred method due to the risk of infection [1] during bone drilling [6]. Many studies were presented about the bone characteristics, processing conditions, measurement of temperature distributions, cutting tool geometries, cooling systems, and numerical analyses of surgical bone drilling processes [7]. The properties of bone structures for the drilling process were presented in the studies of Eriksson et al., Toews et al., Martinez et al., and Chen et al. [2, 8–10]. Effects of processing parameters on the bone drilling process such as rpm and millimeters per minute drill geometry were studied [3, 8, 11–13]. The processing parameters which are extremely important for the bone drilling process should be selected at an optimum level since the selection of incorrect parameters causes the tissue damage [14–17]. Allotta et al. [18] developed a novel handheld drilling tool for the orthopedic surgery with the capability of early detection of interfaces between layers of different bone tissues and automatic feed stop according to the surgical techniques. Hsu et al. [19] presented a modular mechatronic system for automatic bone drilling in which the power will be off during the drilling to prevent excessive protrusion of the drill bit. Sugita et al. [20] attempted to reduce cutting time and damage by controlling the mechanical load and reducing the influence of overload considering the mechanical properties of the bone tissues. Mitsuishi et al. [21] developed a prototype of a sophisticated 9-axis machine tool for a bone cutting to reduce the machining error, replace the artificial joints, and improve the postoperative performance of the implants. The temperature distributions for surgical drilling process were calculated, and different results were obtained [1–3, 11, 22, 23]. The effects of
surgical drill geometry were taken into account for the drilling process [3, 11, 12, 24]. Additionally, the use of coolant is also very important to reduce the heat in the drilling process. Mitsuishi et al. [25] worked with a technique to reduce the surface temperature of the bones from 45 to 35 °C. Harder et al. [26] evaluated the intra-bone frictional heat produced by drilling using different drill materials and methods of cooling. Augustin et al. [27] presented the increase of bone temperature in drilling process using a new designed two-step internally cooled drills. Brand et al. [28] evaluated the temperature levels during solid material drilling and compared the effects of different cooling systems to control the temperature rise. The surgical bone drilling processes were analyzed using finite element method [29–37]. Gok et al. [38] investigated the effects of optimum machining parameters on the cutting force, Gologlu and Sakarya [39] explored the effects of tool path strategies on surface roughness for milling operations, and Kurt et al. [40] worked on the optimization of drilling parameters for the finish surface process during dry drilling using the Taguchi method. A preventive solution for the tissue damages during the drilling has not been exactly developed until this date although there are available drill designs with cooling systems in the studies applying the coolant externally or internally to the drilling zones. However, the external cooling application is not a desired method for a suitable surgical operation. Our aim for this study was to develop a new surgical driller system to prevent the undesired temperature rise during the bone drilling process. The new driller system was designed and manufactured with an internal cooling system and tested experimentally in vitro to measure the temperatures during the bone drilling process. The measured temperature levels with the cooling system were compared with a standard bone drilling process without cooling. Additionally, the optimum
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Fig. 2 Schematic view of the developed driller system
processing parameters for the developed driller system were calculated to investigate the surgical drilling processes with/without cooling system.
2 Development of a new surgical driller system The developed driller system is designed consisting of three main components, drill bit, drill chuck, and closed-circuit cooling system, as shown schematically in Fig. 2. As seen, the drill and drill chuck is working together as a combined driller tool having a cooling system designed in both drill and drill chuck internally by hidden cooling slots. The drill bit is mounted to the drill chuck that is compatible for the different diameters of the drill bits. In the cooling system, the coolant comes from the reservoir and circulates in the closed-circuit slots and returns to the reservoir. This system provides a valuable temperature reduction and undesired surgical infections by pollution of the coolant applied externally during the bone drilling process. The new driller system applies the cooling continuously to the drill bit and drilling region internally with the designed closedFig. 3 The scanning of original surgical drill using 3D optical scanner
circuit cooling slots to reduce the undesired temperature rise. It is also providing an absence of coolant pollution that causes undesired infections during the drilling process. Similar drill bit having internal cooling slots inside the drill bit designed and tested temperature levels by Brand et al. [28]. But, their drill bit is sending out the coolant on the drilling region which is the similar process as in cooling applied externally. This system is causing undesired cooling pollution. The drill bit of the system was designed as in threedimensional (3D) solid model using SolidWorks, a commercial program. The outer geometry of the drill bit was remodeled by scanning an original surgical drill in 3D and used reverse engineering technique to obtain its 3D computeraided design (CAD) model. A 3D optical scanner having cameras was used to scan the surgical drill made of stainless steel as seen in Fig. 3. The scanning purpose of the surgical drill was to define the drill helix and end angle exactly used in surgical applications. The helix and end angle of the surgical drill were measured as 24° and 118°, respectively, in the remodeling process. The scanned data were taken as a point cloud data in STL format and converted to parasolid format using the Rapidform software. The 3D optical scanner,
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Fig. 4 a Surgical drill manufactured with rapid prototyping method. b Surface grinded and sharpened drill. c Coated surgical drill
scanned original surgical drill bit, and remodeled drill bit are shown in Fig. 3. The drill bit was produced as a prototype drill as seen in Fig. 4 using rapid-prototyping machine, Concept Laser, known as Selective Laser Melting machine uses LaserCUSING method for a prototype product. In this machine, the CL20ES stainless steel powder is used to produce the drill bit by the use of multilayers technique as shown in Fig. 4a. The CL20ES is an austenitic stainless steel powder, and its chemical compositions according to 1.4404, X2 CrNiMo 17 13 2, 316L [41] are given in Table 1. The microstructures of the of the CL20ES powder material display a
Table 1 The chemical composition of CL 20ES (stainless steel powder) material [41]
Component
Indicative value (%)
Fe Cr Ni Mo Mn Si P C S
Balance 16.5–18.5 10–13 2–2.5 0–2 0–1 0–0.045 0–0.030 0–0.030
homogeneous structure which are also provided by the manufacturer to illustrate the dense structure [41]. The produced surgical drill was heat-treated in 550 °C temperature up to 3 h and maintained temperature for 6 h considering the definitions given in the user manual of the CL20ES material. Subsequently, the drill bit was kept for cooling down at the ambient atmospheric conditions [41]. The mechanical properties of the drill bit at 20 °C according to DIN EN 50125 after heat treatment provided by the manufacturer are listed in Table 2. The surface of the drill bit was grinded using a CNC grinding machine to obtain a smooth surface as given in Fig. 4b. As seen in the figure, the cutting edges and end point of the drill bit were sharpened based on the general drilling rules, and then the surgical drill bit was coated with a coating material of TiN to enhance the strength as shown in Fig. 4c. The mechanical and physical properties of the TiN are given in Table 2. The drill chuck of the driller system was designed consisting of six main parts, two shell cases, two end closures, and two washers as seen in Fig. 5. These components were assembled together using eight screws for the end closures, mechanical sealing, and O-ring, two needle roller bearings, and inner and outer bushings. The shell cases, end closures, washers, and bushings were designed and manufactured using both CNC lathe and milling machining processes. In order to produce the drill chuck components, the rules of
Int J Adv Manuf Technol (2015) 76:1437–1448 Table 2 The mechanical and physical properties of CL 20ES [41] and (TiN) [42]
CL 20ES stainless steel powder
Coating material (TiN)
Yield point (N/mm2) Tensile strength Elongation (%) Young’s modulus (N/mm2) Thermal conductivity (W/mK)
470 570 >15 200.103 15
Color Hardness (HV) Thickness (μm) Coating heat (°C) Friction coefficient
Golden 2,000–2,300 2–5 200–500 0.6
Hardness (HRC)
20
Thermal oxidation degree (°C) Surface roughness (μm) Ra
500–600 0.2
manufacturing processes were performed considering the precision of 0.001 tolerances. These components were manufactured from Aluminum 7075-T6 material for lightweight purposes. Different types of sealings and O-rings were used among the components to prevent the leaking of the coolant in the drill chuck. After remodeling the drill bit in 3D, the coolant slots in both drill bit and drill chuck were generated using the SolidWorks software. In order to examine the cooling system in both drill bit and drill chuck working together, a coolant flow simulation was carried out using the SolidWorks software as illustrated in Fig. 5. In this simulation process, the drill was rotated within different speeds in drilling process to observe the fluid behavior in the entire system, and the system worked well.
3 Experimental drilling and temperature measurements The surgical driller system shown schematically in Fig. 2 was developed, and the drilling temperature performance was tested experimentally during the drilling of the bovine bones. An experimental setup as seen in a schematic diagram in Fig. 6 was designed to test the developed new driller system to drill the bovine bones in vitro. The devices and components of the experimental unit were established using a mini-desk milling machine with a power of 750 W as illustrated in Fig. 7.
Fig. 5 Coolant flow simulation in the closed-circuit cooling slots
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As seen from the figure, the new driller system was mounted to the milling machine chuck, and the cooling system was connected to a coolant reservoir. The developed drill chuck connected to coolant hoses is stationary, while the milling machine chuck is rotating the drill bit. The drilling of the proximal tibia of bovine was performed with the aid of orthopedic surgeons. The tibia specimens were used in the experiments that were maintained in a saline solution. The tibia specimens were prepared and kept in an ambient temperature conditions of −10 °C based on the guidelines [43] until the tests. Prior to the tests, the specimens were thawed completely at a room temperature for 24 h and immersed to the saline solution [7]. The temperature rise due to contact friction occurred at the drilling region was measured using PT100 thermocouple temperature sensor. The PT100 sensor was used to measure the temperature of the bovine bone sample as illustrated in Fig. 8. As seen, a hole of 6-mm diameter was prepared by drilling in the bone to place the thermocouple rod with the distance of 0.5 mm to the surgical drill bit to measure the bone temperature levels during the drilling. The distance to place the thermocouple was used about 1 mm by researchers. The length of the drilling on the proximal tibia was prepared about 20 mm. The bone drilling tests were performed by the use of both coolant and without coolant at the room temperature of 26 °C. The coolant was used when the new surgical driller system
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Fig. 6 Schematic circuit view of the bone drilling test unit
was tested in the drilling experiments. The drilling tests were also performed using a regular surgical drill bit without cooling. Water was used as a coolant in the developed driller system during the drilling tests. Water was sent with 0.75-bar (0.075 MPa) pressure into the cooling mechanism. Therefore, the bone temperatures were measured in both drilling types and recorded by a computer in Excel format (see Fig. 7). In order to collect the measured bone temperatures using thermocouple sensor, EUC442 control device was used and the data were transferred to the computer via RS485 USB Converter (see Figs. 6 and 7). Effects of processing parameters on the temperature reduction were also measured during the tibial drilling
Fig. 7 The experimental unit of the bone drilling
process. Different types of processing parameters were used in the bone drilling experiments. The spindle speeds of 400, 800, 1,200, and 1,600 rpm and feed rates of 25, 50, 75, and 100 mm/min were selected as levels of bone drilling test conditions. The orthogonal array of L16 with 16 rows was chosen considering the number of experiments (3 factors with mix levels) as given in Table 3. As seen, a total of 16 levels were used during the tests considering the factors of two different types of drill bit diameters (C1= 8 mm and C2=10 mm). To obtain the optimal drilling performance, the smaller and the better quality characteristics for the temperature were adopted. The sound/noise (S/N) ratio [38] considering the number of experiments, n,
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Fig. 8 The placement of the thermocouple in the bone sample during the drilling tests
and the relevant data, Yi, at the ith can be defined as follows: X S 1 n 2 ¼ −10log Y ð1Þ i¼1 i N n
4 Results and discussions In order to inspect the bone temperatures during the bone drilling process, the temperature levels were recorded by the use of EUC442 control device with/without coolant. Particularly, the bone temperatures were measured mostly exceeding
Table 3 Experimental conditions: levels and associated factors Factors
Levels (rpm)
Factors
Levels (mm/min)
A1 A2 A3 A4 A1 A2 A3 A4
400 800 1,200 1,600 400 800 1,200 1,600
B1 B2 B3 B4 B1 B2 B3 B4
25 50 75 100 25 50 75 100
Levels (tool dia/mm) 8
C1
10
C2
the critical temperature value of 47 °C that increases the risk of undesired osteonecrosis under the drilling conditions without cooling. The material in front of the cutting edge is forced to cut by the drill bit during the drilling operations. The broken chips are produced during the drilling since the structure of bones is so brittle. These chips are moving away from rake face of the drill bit by both shearing and friction. As a result, shearing and friction energies occur between the cutting tool and base material (bone). The shearing energy increases with the increase of feed per revolution which is a function of both millimeters per minute and rpm. The friction energy on the rake face increases with the increase of the rpm. A large portion of the shearing energy and the entire friction energy are converted into heat that increases temperatures of both drill and bone. Besides, the temperature of the drill bit elevates rapidly since the drill bit material possesses significantly higher thermal conductivity than the bone. Thus, the heat is transferred from the drill bit having high temperature to the bone material [7]. The S/N ratios of three factors using Eq. (1) were calculated for cooling effects as given in Figs. 9 and 10. As seen, the effects of each processing parameters at different levels can be observed, and the largest S/N ratio always yields the optimum quality with a minimum variance. Therefore, the largest value of the levels determines the optimum level for each factor. The optimum level can be calculated using with/without coolant at A1 for 400 rpm, B4 for 100 mm/min, and C1 for ∅8 mm in terms of temperatures as shown in Figs. 9 and 10. The effects
Fig. 9 Bone temperature values measured with thermocouple sensor
Spindle Speed [rpm] Tool Diameter [mm] S/N [mm/min]
70 Temperature (°C)
60
65.25
40
-30.96 54.25 40.25 -32.08
30
35.50
50
20
Feed Rate [mm/min] S/N [rpm] S/N [mm] 57.50
50.75
45.25
41.75
42.13 -32.34 -32.36
-33.49
-33.06
-34.52
10
55.50
-34.75
-34.46
-36.09
0 400
800
1200 1600
25
50
Level (rpm)
Temperature (°C)
71.50
-33.23 53.50
66.50
Level (mm)
67.25 63.00
56.75 53.25
-35.62
-34.56
46.00
-35.01
-36.30
55.00
-34.45 -34.72
-36.79
-36.21
-37.78 800 1200 1600
of processing parameters on the temperature rise using coolant were found as shown in Figs. 9 and 10. The curves of S/N and temperatures have been plotted as function of arithmetic average of the measured values depending on orthogonal array for
Spindle speed (rpm) Feed rate (mm/min) Tool diameter (mm) Error, e Total
10
Feed Rate [mm/min] S/N [rpm] S/N [mm]
25
rpm
Source of variance
8
78.50
400
Table 4 ANOVA results for the temperatures with coolant
100
Level (mm/min)
Spindle Speed [rpm] Tool Diameter [mm] S/N [mm/min]
Fig. 10 Bone temperature values measured with the NCTS
90 80 70 60 50 40 30 20 10 0
75
50
75
100
mm/min
-28 -29 -30 -31 -32 -33 -34 -35 -36 -37
S/N Ratio
Int J Adv Manuf Technol (2015) 76:1437–1448
8
-30 -31 -32 -33 -34 -35 -36 -37 -38 -39
S/N Ratio
1444
10
mm
the parameters of spindle speed, feed rate, and tool diameter. The temperature is slightly increased with the increase of rpm and tool diameter. On the other hand, the temperature is decreased with the increase of millimeters per minute.
DOF
SS
Variance
F ratio
p value
PCR (%)
3 3 1 8 15
2,201 567 715 32.50 3,516
733 189.06 715 4.06
180.72 46.56 176.24
0.00 0.00 0.00
62.59 16.12 20.34 0.92 100.00
Int J Adv Manuf Technol (2015) 76:1437–1448 Table 5 ANOVA results for the temperatures without coolant
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Source of variance
DOF degree of freedom, SS sum of squares
Spindle speed (rpm) Feed rate (mm/min) Tool diameter (mm) Error, e Total
DOF
SS
Variance
F ratio
p
PCR (%)
3 3 1 8 15
2,470 769 600 15.50 3,855
823 256 600 1.94
424 132.17 309
0.00 0.00 0.00
64.07 19.95 15.56 0.40 100.00
Table 6 Results of the calculated temperature values with/without cooling With coolant
Without coolant
ηcal (dB)
Tcal (°C)
ηcal (dB)
Tcal (°C)
−28.79
27.54
−31.37
37.15
significant parameters were calculated as rpm, millimeters per minute, and tool diameter for p≤0.00001 during the bone drilling process. Similarly, the rpm, millimeters per minute, and tool diameter without cooling were obtained significant parameters since the p≤0.00001. The most significant parameters within the range of specified drilling conditions were the rpm, millimeters per minute, and tool diameter with/without coolant.
4.1 Statistical analysis of variance 4.2 Determination of optimum processing parameters Statistical analysis of variance (ANOVA) was performed to examine the most effective processing parameter on the temperature rise with/without cooling system as given in Tables 4 and 5. As seen, the degree of freedom (DOF) is the number of calculated statistical values free to vary. The sum of squares (SS) is defined as the variability of the processing parameters. The F ratio determines the same variances of two independent variables. This means that we can assume homogeneity of variance and begin a statistical t test. The F ratio does not mean that the data is statistically significant [39]. The PCR is the effect of the measured value on each parameter in percentage. The p values of ANOVA for all processing parameters are shown at a significance level of 95 % as given in Tables 4 and 5. The parameters are different than the levels so that the measured values are different. Thus, it can be stated that the differences of the measured values are resulted remarkably from the differences of the levels [38]. For the response value of the temperatures seen in Table 4 with coolant, the most
Table 7 The confirmation of the test and calculated results Exp. no.
Tmea (°C)
S/Nmea (dB)
Absolute differences (%) jT cal −T mea j T mea
Drilling with cooling 1 26 2 27
−28.46
Drilling without cooling 1 36 2 35
−31.00
3.92
The S/N ratios of the drilling system were calculated considering with/without cooling that shows the effective parameters which are the largest level of millimeters per minute and the lowest levels of rpm and tool diameter. The optimal parameters during the drilling process were obtained as A1B4C1 with/without cooling in terms of temperature. The predictions of S/N ratios for the optimal drilling conditions are calculated by the following equation [38]: S S ηcal ¼ η¯m þ Max : − η¯m þ Max : − η¯m N1 N2 S − η¯m þ Max : ð2Þ N3 where ηcal is the calculated S/N ratios at optimal drilling conditions and η m is the arithmetic mean of S/N rations of the studied cooling form. The max. S/N1, max. S/N2, and max. S/N3 are the largest S/N values of the spindle speed, feed rate, and tool dia, respectively, as given in Figs. 9 and 10. Using Eq. (1), the estimated temperature, Tcal, values considering calculated S/N ratio, ηcal, can be derived for both with/without
100 Table 8 Confidence interval values Temperature (°C)
4.64
Drilling with cooling Drilling without cooling
4.13 3.21
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cooling conditions as follows [38], and the values are given in Table 6. T cal ¼ 10
−ηcal 20
ð3Þ
4.3 Confirmation of the experiments The confirmations of the experiments have been carried out by repeating the tests considering with/without cooling conditions to prove the obtained best parameters from measurements without errors, and the results of experiments are presented in Table 7. As seen, the values of measured temperatures, Tmea, S/Nmea ratios, and absolute differences are given as in %.
4.4 Confidence interval based on the estimated mean value The estimated mean temperature value is the mean value based on the average values of the results measured from the experiments which is higher or lower than the mean value that results 50 % chance [38]. The confidence interval is calculated to verify the quality characteristics of the experiments. Therefore, the confidence interval value for the measured results is calculated, and the interval value is between the max and min values predicted by the following equation [38]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 CI ¼ F 0:05 ð1; ve Þ⋅V e ⋅ þ neff r
ð4Þ
where F0.05(1,ve) is the F ratio at 95 % confidence against the degree of freedom 1 and error of ve, Ve is the error variance, neff is the effective number of replication (Eq. 5), and r is the number of test trials r=2 (see Eq. (4)) [38]. neff ¼
N 1 þ vT
ð5Þ
where N is the total number of experiments and vT is the total main factor degrees of freedom. The confidence interval values considering with/without cooling conditions calculated using Eqs. (4) and (5) are provided in Table 8. The differences of the S/N ratios between the estimated values calculated using Eqs. (2) and (3) and the results measured from the experiments are shown in Table 7. As seen, these differences are smaller than the confidence interval values of 5 % given in Table 8. Therefore, the best factors considering with/without cooling conditions are confirmed as in safe.
5 Conclusions In this work, a new surgical driller system having an internal cooling mechanism was developed to reduce the undesired temperature rise during the bone drilling process in the surgical applications. The internal cooling mechanism of the driller system was designed as a closed-circuit cooling system and worked successfully for cooling of the drilling region. The components of the driller system were manufactured as a prototype product and tested to drill the bovine bones experimentally in vitro along with different processing parameters. In order to test the system, a specific experimental unit was designed and established successfully to measure the temperature levels of the bones for which the thermocouple sensor was used. Statistical analysis was also performed for the measured bone temperature values to optimize the processing parameters. Based on the generated results, the following conclusions can be made. (a) The new surgical driller system with lower rpm has showed significant bone temperature reduction about 25 % during the bone drilling process. Additionally, the bone temperature level decreases as the feed-rate increases since the duration of the bone drilling is decreasing. In contrast, bone temperature increases with increasing of spindle speed (see Figs. 9 and 10). (b) The developed surgical driller system provides a clean environment for the surgical operations and also eliminates the infection risks due to the pollution of the external cooling. Therefore, the internal cooling system worked well in the designed closed-circuit slots that are extremely important in surgical drilling applications. (c) The most significant processing parameter effects of the bone temperature levels were calculated as the spindle speed (rpm) using both with/without cooling. Based on the optimum results from the experiments, the measured temperatures were obtained lower than those of the calculated ones. The calculations were performed well to determine the confidence interval of the optimal factor settings for with/without cooling.
Acknowledgments This work was financially supported by the Scientific Research Projects Unit of Kocaeli University under the project number of 2012/44. The authors are also applied to Turkish Patent Institute for the patent of drill bit and drill chuck designs with the application numbers of “2012/14,286” and “2012/14,287” in 07 December 2012. Conflict of interest We certify that there is no conflict of interest with any financial organization regarding the materials discussed in the manuscript. Ethical approval Not required for this study.
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