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Publication# Cutting Force Modelling for Drilling of Fiber-Reinforced Composites

Résumé

Although used in a very large variety of applications, drilling is one of the most complex and least understood manufacturing processes. Most of the research on drilling was done in the field of metal cutting for mechanical parts since, in this case, high precision and quality are needed. The use of composite materials in engineering applications has increased in recent years, and in many of these applications drilling is one of the most critical stages in the manufacturing process. This is because it is among the last operations in the manufacturing plan of composite parts. Delamination and extensive tool wear are among the problems which drilling of composite materials are currently facing. A major difference between metallic and composite plates is their structure: isotropic for metals and anisotropic for composite materials; meaning that while for metallic materials all the structure will respond in a similar manner under the machining loads, the composite structure will have localized responses from the same loads, leading to defects in the internal structure of the remaining work-piece material (i.e. delamination). Delamination can lead to failure in use and parts with such defects are usually discarded. Delamination is not usually visually detectable and special testing is necessary, affecting the costs of the final parts. Delamination during drilling was found to occur at tool entry (peel-up) or tool exit (push-out) and depends on the loads at inter-laminar level. The work presented in the current thesis focuses in providing reliable information about the thrust and torque distribution along the drill radius (and work-piece thickness) during drilling for varying cutting parameters, drill geometry and work-piece material. Such data should assist in the development of delamination models capable of capturing the influence of the drill geometry and cutting parameters on delamination onset and propagation during both exit and entry of the drill in the work-piece. A cutting force model is proposed to obtain the elementary cutting force distribution along the drill radius which is able to account for changes in axial feed rate and drill geometry. Based on oblique cutting, forces are considered on both rake and relief faces. A generic relationship in the form of a transformation matrix is developed to relate oblique cutting to drilling, valid for any drill geometry. The mathematical description of the drill geometry in the scope of cutting force modeling has been revised. The kinematics of the drilling process is now taken into account for (i) all geometrical parameters of the drill and for (ii) the elementary cutting forces decomposition. Additionally, a new drill type and its geometric features have been described mathematically and the definition of the geometrical parameters has been generalized so that other drills types or variations could be easily implemented into the model. It proved therefore possible to adopt simpler expressions for the empirical force coefficients of the cutting force model. Up to four empirical coefficients are used, which are calculated from experiments for each work-piece material and drill type. Most experimental investigations on drilling fiber reinforced composites analyze only the total thrust and torque generated during drilling or separately the forces caused by the chisel edge and cutting lips by drilling with or without a pilot hole. The later type of analysis suggested that is possible to obtain more detailed information about the distribution of the loads in drilling from the analysis of the forces variation during tool entry into the work-piece. Pursuing this direction, an experimental analysis method is proposed to obtain the axial and tangential elementary cutting force distribution along the tool radius or work-piece thickness. The cutting force distribution obtained experimentally was used to calibrate the cutting force model, rather than the total thrust and torque. The experimentally obtained cutting force distribution can also be used alone for analyzing the drilling process (i.e. the loads distribution among the plies of the composite laminate and how this load is influenced by changes in the drill geometry and the cutting conditions).

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Mihai-Bogdan Lazar, Paul Xirouchakis

In comparison with metals, long-fiber reinforced composites have a layered structure, with different properties throughout their thickness. When drilling such structures, internal defects like delamination occur, caused by the drilling loads and their uneven distribution among the plies. The current experimental analysis is focused towards determining the cutting loads distribution (axial and tangential) along the work-piece thickness and tool radius by analyzing the thrust and torque curves when drilling with 3 different drills carbon-fiber (CFRP) and glass-fiber (GFRP) reinforced composite plates. A wide range of cutting parameters is tested. The highest loads are found at the tool tip in the vicinity of the chisel edge for all cases. It is also found that the maximum load per ply varies mainly with the axial feed rate and tool geometry, while the spindle speed has little or no influence. The analysis is useful for selecting the cutting parameters for delamination free drilling and also for conducting drill geometry optimizations. (C) 2011 Elsevier Ltd. All rights reserved.

2011The milling process has been continuously optimized from many aspects: forces, velocities, stability, quality. Numerous papers have been published that report results in the domain of toolpath optimization with criteria such as raising the quality of the machined part, obtaining a stable machining process and maximizing the material removal rate. However, only recently researchers have started investigating the ecological impact of machining processes. With the constantly increasing prices of electrical energy and all environmental problems caused by the production and waste of the energy, it has become indispensable to look for ways to optimize machining processes from the energy consumption aspect, too. It is in this domain, then, that the work described in this dissertation contributes – to add a new criterion for the sustainable operation of machine tools: reducing the energy consumption during the use phase of the machine tool. The total energy spent in a machining process is the sum of power spent by all machine tool subsystems multiplied by the time that they are working. Therefore, the operating strategies for increasing energy efficiency for the high-speed machining processes must be oriented towards the reduction of the total machining time (productive and unproductive) and reduction of instantaneous consumed power in the machining process or, preferably, both. There are many causes for milling process time and energy inefficiency. The major sources of potential time reductions include: (i) the issues related to the geometry and topology of the toolpath („air-time”, overlapping segments, gouge and uncut regions, orientation of the toolpath), (ii) the issues related to the kinematics of the feed drives (the feed velocity/acceleration profile). The possible domains for power consumption reduction are related to the forces that exist in the machining process (cutting, inertial, gravitational and friction forces). The power invested to overcome the cutting force is necessary for the machining process but its consumption should be optimized through constant cutting engagement and feedrate scheduling strategies. The other forces are responsible for the pure mechanical power loss and they should be minimized. The optimization variables for inertial and gravitational forces include the feed motion profile and moving mass configuration, while friction losses are load and/or velocity dependent. In addition, there are some power losses in the electric components of machine tool drives (electrical power losses), whose dependency on velocity and mechanical load can also be modeled. Despite many research efforts to analyze and model the phenomena causing inefficiency of the machining process, they are usually not taken into account during milling process planning. In fact, rare are the attempts to use this knowledge to optimize the milling process with more than one of the above mentioned criteria. The reason for that lies in the fact that increasing the instantaneous values of tool engagement and feedrate in order to minimize total machining time also increases the consumed power and, indirectly, deteriorates the tool and/or destabilizes the cutting process. The existence of conflicting criteria in the objective functions imposes the use of evolutionary algorithms for multiobjective optimization. The process of experimental validation of such an optimization model requires costly and complex external hardware setup and it is also time consuming. Hence, there is an emerging need for the development of an accurate model, which would be used for the simulation of the milling process and testing the competing optimization strategies. The ultimate goal of contemporary manufacturing science and engineering is the development of a holistic virtual machine tool system. The motivation of this PhD research is to contribute to this objective by developing the “virtual milling process for prismatic parts (produced by 2.5D milling)”. This category of objects is selected, because the majority of CNC milling tasks can be performed using 2.5D milling. A very large number of mechanical parts are prismatic (their geometry consists exclusively of features that represent 2D contours extruded in a perpendicular direction). Parts that are even more complex are usually produced from a blank by a 2.5D roughing and 3D–5D finishing. Therefore, development of an accurate model of a 2.5D milling process represents a significant contribution to manufacturing engineering. Starting with above given problem description, we have defined the following research objectives: 1. Model power and energy consumption of a 2.5D milling machine as a function of the toolpath geometry (curvature and continuity), cutting engagement, real feedrate profiles and spindle rotational speed, taking into account moving mass configuration, all power losses (mechanical and electrical) in feed-axis and spindle drives and electromechanical constraints of servo drives; 2. Design an integrated software environment for simulation of the developed models; 3. Validate the model experimentally; 4. Demonstrate the potential of its use for multi-objective process optimization. Cutting engagement calculation is performed by using two competing methodologies, in order to make a comparison of their performances. The first one, the pixel based method, is based on discrete image processing, while the other, the exact method, is based on Boolean operations and vector geometry. The tests performed in more than 20 feature/toolpath combinations demonstrate that the exact method is more precise whereas the pixel-based method is computationally much faster. The machine tool operator commands a constant feedrate value. During the process, based on his/her subjective judgment, he/she can manually tune it to keep the process in the desired boundaries. One of the objectives of this research is to predict the real feedrate values so that they can be integrated in the part program before the process start. The feasible profiles of feed velocities for each toolpath segment are analyzed by answering the following questions: can the commanded feedrate be achieved and with what sequence of acceleration and deceleration phases, how much the tool has to slow down in corners and how does the feedrate profile degenerate in segments where the commanded feedrate cannot be reached? In order to determine all forces and torques, a machine tool model is developed. The following subsystems are modeled as rigid bodies (without vibrations): feed motor shaft, transmission, lead screw/nut pair, lead screw bearings, guides (sliding and rolling design), table with the workpiece and electrospindle (rotor, stator and bearings). All kinetic variables (linear and angular speeds; cutting, inertial and friction forces and torques) are then calculated in the joints of this multi-body system. The modeling of all speeds, forces and torques that occur in the machine tool system during the machining process, allows for calculation of various indicators of process time and energy efficiency: material removal rate, total machining time, useful power (invested in cutting), power loss due to the friction in the joints, power losses in electrical motors, electrical energy consumption of feed and spindle drives. In order to assess the accuracy and the reliability of the developed model two sets of experiments have been performed. The model validation is performed on several subsystem levels and the simulation results show a very good compliance with the experimental results. The first experimental setup consists of the force dynamometer, to measure cutting force components in linear axis directions, two laser optical position sensors, to measure the real feedrate and the power sensor, to measure the total power consumption of the machine tool electrical motors. The machining process was performed on the 6-axis machining center C.B. Ferrari A152 and the data acquisition platform was developed in LabVIEW 2010. The second experimental setup consists of the torque dynamometer, to measure the cutting torque directly on the spindle, the built-in current sensor that measures the motor current proportional to the electrical power consumed by the spindle and, again, the power sensor, to measure the total power consumption of the machine tool electrical motors. This time, MIKRON HPM 600U, a 5-axis milling machine equipped with the controller iTNC530, was used for the experimentation. The developed model is ready for practical implementation. Several examples have been prepared to demonstrate its capabilities in the domains of milling process simulation, sensitivity analysis and prospective optimization strategies. These examples include the prediction of various kinetostatic variables for given toolpaths, choice of the optimum machining strategy (minimization of machining time, moving mass and energy consumption) and process optimization by feedrate scheduling (reduction of cutting force fluctuation). As a conclusion, the major contribution of this PhD research is the development of a comprehensive rigid-body dynamics model of a machine tool system, aimed for the generation, simulation and optimization of 21⁄2D milling process plans. The model features several novelties compared to existing approaches: (i) it takes into account the machine tool rigid body dynamics and real tool kinematics in toolpath planning, (ii) it predicts all mechanical and electrical power losses in the machine tool system (iii) it allows the modeling of machine tool system electromechanical constraints. The model was experimentally validated on two real machine tools in different workshops and its capabilities for the simulation of different toolpath patterns and optimization of process parameters were successfully demonstrated using several examples.

Mihai-Bogdan Lazar, Paul Xirouchakis

Used in a very large variety of applications, drilling is one of the most complex manufacturing processes. Most of the research on drilling was done in the field of metal cutting since, in this case, high precision and quality are needed. The use of composite materials in engineering applications has increased in recent years, and in many of these applications drilling is one of the most critical stages in the manufacturing process. Delamination and extensive tool wear, affecting the quality and the costs, are among the problems which drilling of composite materials are currently facing. Understanding and predicting the cutting forces occurring during drilling of such materials would allow extending the currently used optimization methods and proposing new tool geometries and tool materials. The current paper introduces a new mechanistic model for predicting the cutting force distribution along the cutting edges of a drill. A simple, generic and effective method is proposed to relate drilling to oblique cutting using a direction cosine transformation matrix valid for any drill geometry. The oblique cutting model used considers forces on both rake and relief faces, and a simple system of empirical coefficients (their number is significantly less than other similar models). The empirical coefficients are calculated assuming the work-piece material is isotropic. The model is validated on experiments carried out on carbon-fiber and glass-fiber reinforced composites using two different drill types (tapered drill reamer and 2-facet twist drill), which are described in more detail in a previous published paper. The mathematical expression of the drill geometry is also reviewed; removing certain assumption, generalizing some definitions and introducing new drill geometry and features.