Effect of Using Coolant on the Formation of Microcracks, Burr and Delamination in Bone Drilling Process

Direct approach for bone fracture treatment usually involves restoring the fractured parts to their initial position and immobilizing them with plates, screws and wires. This approach needs a bone surgery drilling to produce hole for screw insertion. But this drilling process causes mechanical damages, i.e microcracks, burr formation and delamination, that can reduce the stability of the fixation. One of the ways to minimize it is by using coolant. Moreover, it is noted that bone has anisotropic microstucture. The object of this study is to understand the effect of coolant on mechanical damages that occur in bone drilling and to understand the effect of microstructure difference on microcracks that occur in the drilled walls holes. Adult bovine bones and adult goat bones were used in this study as the specimens to represent differences in cortical bone microstructure. Five consecutive holes from the distal to the proximal in each specimen were generated using manual hand-drill (spindle speed (n) = 1000 rpm; drill bit (d) = 4 mm diameter) with the use of coolant as variation. The drilling holes then stained and observed using a microscope. As the result, it was found that the use of coolant can significantly reduce the drilling temperature. Microcracks, burr formation and delamination were found to be quite large in the drilling holes without coolant. However, there is no microcrack found in the drilling holes with coolant, there is only a small number of burr formation was found. In addition, it was found that the differences in bone microstructure affect the number and length of microcracks that occur in the wall of the hole. It can be concluded from this study that the application of coolant is very effective to reduce the drilling temperature and enhancing the quality of the hole generated by bone drilling and the higher the density of osteon in cortical bone, the easier the microcrack to initiate and propagate.

Development of a Low-Frequency Vibration-Assisted Drilling Device for Difficult-to-Cut Materials

In the drilling process of difficult-to-cut materials, conventional drilling has resulted in various problems such as high drilling temperature and poor machining quality. Low-frequency vibration-assisted drilling has great potential in overcoming these problems since broken chips are generated. In order to promote the application of low-frequency vibration-assisted drilling device in machining difficult-to-cut materials. In this paper, a low-frequency vibration-assisted drilling device is developed by using a novel ring flexure hinge as the elastic recovery mechanism. First, based on the theory of elastic mechanics and mechanical vibration, the stiffness of the ring flexure hinge is designed theoretically, and the influence of its structural parameters on its deflection is analyzed. And then the correctness of the theoretical design is further verified by static and dynamic simulation and stiffness test. Finally, the vibration performance of the device is tested under no-load condition, and the actual drilling test is conducted to verify the drilling performance. The results show that the device could realize the axial low-frequency vibration with constant frequency-to-rotation ratio and amplitude stepless adjustment and present good working stability under no-load and load conditions. In the actual drilling test of titanium alloy and carbon fiber reinforced plastic (CFRP)/ titanium alloy laminated structure, the device under appropriate processing parameters breaks the titanium alloy chip into small pieces and reduces the drilling temperature by 44% and inhibits the secondary damage of CFRP. It is demonstrated that the device could meet the actual processing requirements. And it also provides guidance for the design of low-frequency vibration-assisted drilling device.

Investigation of Drilling Temperature in Relation to Machining Conditions and Cutting Time

Controlling drilling temperature and thrust forces play a significant role in reducing tool wear and improving machining efficiency. In this work, drilling experiments are set up to measure flank surface temperature via thermocouple sensor wires passed through the coolant holes of 10mm twist drill and brazed to the drill flank surface. The testing setup is an inverted drilling jig where the workpiece (Aluminum 6061-T6 rod) is chucked into the spindle of a vertical machining center. Thrust forces are co-measured using Kistler type 3-component plate dynamometer attached to the table. A design of experiment (DOE) using JMP-SAS/STAT® was adopted for selecting combinations of cutting speed and feed values that cover a wide range. Drilling temperature rise and thrust forces are found to correlate with cutting conditions of feed (f), maximum cutting speed (V), and drilling depth (Dp). Nonlinear regression analysis produced correlating equations of flank temperature rise and thrust forces to conditions and follow a mechanistic power law of the form a1fa2Va3Dpa4 where a1, a2, a3 and a4 are identified via regression fitting.

Study on Deep Hole Drilling Process and Cutting Performance

In deep hole machining, drilling parameters for twist drills have an important impact on tool life and economic efficiency. In order to explore the influence of drilling parameters on twist drills, this paper established a drilling model for twist drill 45 steel, orthogonal experiments were designed. AdvantEdge FEM finite element software was used to simulate the drilling force, torque and drilling temperature. The drilling force, torque and drilling temperature were analyzed by using the finite element simulation value as the orthogonal experimental value. In order to ensure that the simulation results have certain reference value, the drilling experiment was carried out in order to ensure the simulation results have reference value. Finally, the optimal combination of drilling parameters was obtained.

Experimental Study of Drilling Temperature, Geometrical Errors and Thermal Expansion of Drill on Hole Accuracy When Drilling CFRP/Ti Alloy Stacks

The drilling of holes in CFRP/Ti (Carbon Fiber-Reinforced Plastic/Titanium alloy) alloy stacks is one of the frequently used mechanical operations during the manufacturing of fastening assemblies in temporary civil aircraft. A combination of inhomogeneous behavior and poor machinability of CFRP/Ti alloy stacks in one short drilling brought challenges to the manufacturing community. The impact of the drilling temperature and time delay factor under various cutting conditions on hole accuracy when machining CFRP/Ti alloy stacks is poorly studied. In this paper, the drilling temperature, the phenomenon of thermal expansion of the drill tool, and hole accuracy are investigated. An experimental study was carried out using thermocouples, the coordinate measuring machine method, and finite element analysis. The results showed that the time delay factor varied from 5 (s) to 120 (s), influences the thermal-dependent properties of CFRP, and leads to an increase in hole roundness. Additionally, the thermal expansion of the drill significantly contributes to the deviation of the hole diameter in Ti alloy.

Experimental study on drilling temperature and hole quality in drilling of carbon fiber reinforced plastic/titanium stacks

Carbon fiber reinforced plastic/titanium stacks are widely used in aviation field due to the superior properties such as high strength-to-weight and high yield strength. Drilling these stacks in single shot is still challenging because of the disparate properties of two stack constituents. Various hole defects occur during drilling stacks, especially for stacking sequence titanium alloy to carbon fiber reinforced plastics. Cutting parameters of titanium alloy, compared to those of carbon fiber reinforced plastics, have more significant influences on hole quality of stacks. The contribution of this work is to research the influence of cutting parameters of titanium alloy on hole qualities during drilling of stacks including drilling temperature, hole diameter deviation, titanium burrs, and interface damage. A novel assessment approach is developed to character the interface damage area based on the greyscale of digital image. Interface damage factor is proposed to evaluate interface damage quantitatively. Experimental results indicate that maximum drilling temperature reaches the peak value when drilling of the interface region. Hole diameter of titanium layer is always lager than that of the carbon fiber reinforced plastic layer. In the same layer, the deviation of hole diameter increases as spindle speed of drilling titanium layer increases. In addition, the damage areas obtained through damage assessment method show a good correlation with the experimental damage results in shape and distribution. Interface damage increases with the rise of the spindle speed of drilling titanium layer.