Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (2008-2018)
Recent turn-milling machine tools are capable of carrying out turning, drilling, boring, milling and grinding operations simultaneously, hence they are widely used in industry to produce complex parts in a single set-up. Turn-milling machines have translational axes with a high speed spindle to hold the cutting tool and a low speed spindle to carry the workpiece. The resulting five-axis turn-milling machines can machine parts with complex curved tool paths. This thesis presents the mechanics and dynamics of turn-milling operations to predict cutting forces, torque, power, vibrations, chatter stability and dimensional surface errors in the virtual environment.First, the kinematics of five-axis turn milling operation is modeled using homogenous transformations. The engagement of rotating-moving tool with the rotating workpiece is identified using a commercial graphics system, and used in predicting the chip thickness distribution. The relative vibrations between the tool and workpiece are modeled, and superposed on the chip thickness in the engagement zone. Unlike in regular turning and milling operations with a single spindle which leads to a single and constant delay, turn milling has two time delays contributed by two rotating spindles and three translational feed drives. The regenerative chip thickness with dual delay is used to predict the cutting forces at tool-workpiece engagement zone, which are transformed to three Cartesian directions of the machine. The resulting coupled differential equations with two delays and time periodic coefficients are solved in the semi-discrete time domain to predict chatter stability, cutting forces, vibrations, torque, power and dimensional surface errors simultaneously.The thesis presents the first comprehensive digital model of turn milling operations in the literature, and can be used to predict the most productive cutting conditions ahead of costly physical trials currently practiced in the industry.
This thesis presents a trajectory generation algorithm, a control strategy, and a geometric errorcompensation methodology for a novel 9-axis micromachining center which combines a 3axis micromill with a 6 degree of freedom magnetically levitated rotary table. The proposedtrajectory generation algorithm resolves redundant degrees of freedom by numerically solvingfor axes positions from desired tool positions and orientations. Differential axes positions arefound while ensuring the stroke limits of the drives are respected and singularities are avoided.The differential solution is numerically integrated to obtain the axes positions with respect todisplacement. The axes commands are then scheduled in time, while respecting the velocity,acceleration, and jerk limits of each of the drives, and traversing the toolpath as fast as possible.The experiments showed trajectories that resolved redundancies, avoided singularities, andrespected all physical limits of the drives.A control strategy which combines the capabilities of the micromill and the rotary table isintroduced. A sliding mode controller with a LuGre friction compensator is designed to controlthe position of the micromill, based on identiﬁed physical parameters. A lead-lag positioncontroller with an integrator and a notch ﬁlter is designed to control the rotary table. Sincethe translational axes of the micromill and rotary table are in parallel, the tracking error of themicromill is sent as a reference command to the rotary table, compensating the tracking errorsof the micromill with the higher bandwidth of the rotary table. In experiments, the dual stagecontrol law improved tracking error over the micromill alone.The geometric errors of the 3-axis micromill is compensated by using the precision motion ofthe 6 degree of freedom rotary table. The geometric errors of the 3-axis micromill are measured with a laser interferometer, ﬁt to quintic polynomials, and incorporated into the forwardkinematic model. The tooltip deviation is found by subtracting the ideal tooltip position fromthe tooltip position affected by geometric errors. Rotary table commands, from all 6 axes, thatcompensate for these deviations are found using a gradient descent algorithm. Experimentsshowed reductions in end effector deviations.
The pipelines used in the offshore extraction of oil and gas are connected by threaded joints. Any geometrical error or vibration marks left on the thread surface during the machining process can lead to stress concentration and fatigue failure of the joint. Such instances in the past have led to massive oil leakage and environmental disasters. Threading is a form cutting operation resulting in wide chips with complex geometries. Multi-point inserts used in mass production can have different custom profiles on each tooth. The chip thickness as well as the effective oblique cutting angles, cutting force coefficients, and direction of local forces vary along the cutting edge. Since the tool moves one thread pitch over each spindle revolution, the vibration marks left by a tooth affect the chip thickness on the following tooth. Threading of oil pipes imposes additional complexities due to the flexural vibrations of thin-walled pipes, which lead to severe chatter instability.This thesis develops a novel and generalized model to formulate, simulate, and optimize general multi-point threading processes. A systematic semi-analytical methodology is first proposed to determine the chip geometry for custom multi-point inserts with arbitrary infeed strategies. A search algorithm is developed to systematically discretize the chip area along the cutting edge considering the chip flow direction and chip compression at the corners. The cutting force coefficients are evaluated locally for each element, and the resultant forces are summed up over the engaged teeth.Multi-mode vibrations of the tool and pipe are projected in the direction of local chip thickness, and the dynamic cutting and process damping forces are calculated locally along the cutting edge. A novel chip regeneration model for multi-point threading is developed, and stability is investigated in frequency domain using Nyquist criterion. The process is simulated by a time-marching numerical method based on semi-discretization. An optimization algorithm is developed to maximize productivity while respecting machine's limits. The proposed models have been verified experimentally through real scale experiments.The algorithms are integrated into a research software which enables the industry to optimize the process ahead of costly trials.
Flexible parts and tools are often found in machining operations, such as boring large cylinders, or turning long and slender shafts. The excessive flexibility of such tool or shaft may cause static deflection, forced vibration, and even chatter vibrations, which result in poor surface finish, tool breakage, and even damage to the machine, and thus become the main constraints in achieving higher productivity. This thesis presents an active damping solution to such problems, by using a novel three degrees of freedom linear magnetic actuator, which can increase the damping and stiffness of flexible structures in machining. The actuator is comprised of four identical magnetic actuating units; the magnetic force output of each actuating unit is linearized with regard to the input current by biasing magnets. Fiber optic sensors are integrated into the actuator to measure the displacements of the structure during machining. The magnetic actuator is used for three purposes: active damping of boring bar, increasing its static stiffness, and monitoring cutting forces based on the control current signals and fiber optic displacement sensor signals. The active damping is achieved by controlling the magnetic force as a function of measured vibrations. Three different types of controllers (loop shaping controller, Derivative-Integral controller, and H∞ controllers) have been developed to actively damp the displacements of a flexible boring bar during machining tests. The actuator can deliver 248 N force up to 850 Hz, and 107 N force up to 2000 Hz which is limited by the current amplifier used in the experimental setup. The cutting force is estimated through a Kalman filter, which was experimentally verified to be effective up to 550 Hz. Both the dynamic stiffness and static stiffness of the boring bar have been increased considerably with the designed magnetic actuator, leading to a significant increase in the chatter-free material removal rates. Although the proposed magnetic actuator is demonstrated for active damping of a slender boring bar in the thesis, the proposed magnetic actuator principle can be applied to suppress vibrations of rotating shafts, long boring bars and flexible structures in machine tools and other machineries.
The final shape of mechanical parts is mainly determined through turning, boring, drilling and milling operations. The prediction of the cutting forces, torque, and power of the machining process, and surface errors and vibration marks left on the parts is required to plan the machining operations and achieve shorter production cycle times while avoiding damage on the part, tool and machine. Past research has focused on developing dedicated mathematical models for each machining operation and tool type. However, the tool geometry and configuration of the machining set-up varies widely depending on the part geometry and application. This thesis presents a generalized mathematical model of machining operations carried out using geometrically defined cutting edges. The mechanics of cutting between the tool edge and the work material are modelled to predict the friction and normal forces on the rake face of a single cutting edge. The combined static and dynamic chip thickness is modelled as a function of tool geometry, the kinematics of machining operation and the relative regenerative vibrations between the tool and workpiece. The cutting forces are transformed to process coordinates by considering the orientation of cutting edge and the kinematics of the machining operation, and are applied on the structural dynamics of the machine tool and workpiece by distribution along the cutting tool–workpiece contact zone.The cutting forces, vibrations, chatter stability and surface errors are simultaneously predicted in a semi-discrete time domain. The geometry and force transformation models are unified in a parametric, mathematical model which covers all cutting operations. The application of the proposed model is demonstrated on turning, drilling and milling operations; multifunctional tools that combine drilling-boring and chamfering in one operation; and two parallel face-milling cutters machining a plate from both sides. The proposed mathematical models are experimentally validated by comparing the measured forces, surface errors, vibrations and chatter stability charts against simulations. The thesis shows the first unified, generalized mathematical modelling of metal cutting operations in the literature. The proposed model is expected to widen the application of science-based machining process simulation, planning and optimization methods in the virtual production of parts.
Machine tool’s productivity and ability to produce a component of the required quality is directly influenced by its dynamic stiffness at the tool center point. Lack of dynamic stiffness may lead to unstable regenerative chatter vibrations which are detrimental to the performance. The chatter vibrations are influenced by the changing structural dynamics of the machine as the tool moves along the tool path, resulting in position-varying machining stability of the system. Evaluation of these varying dynamics at the design stage is a complex process, often involving the use of large order finite element (FE) models. Complexity and computational costs associated with such FE models limit the analyses to one or two design concepts and at only a few discrete positions. To facilitate rapid exploration of several design alternatives and to evaluate and optimize each of their position-dependent dynamic behavior, a generalized bottom-up reduced model substructural synthesis approach is proposed in this thesis. An improved variant of the component mode synthesis method is developed and demonstrated to represent higher order dynamics of each of the machine tool components while reducing the computational cost. Reduced substructures with position-invariant response are synthesized at their contacting interfaces using novel adaptations of constraint formulations to yield position-dependent response. The generalized formulation is used to evaluate the position-dependent behavior of two separate machine tools: one with a serial kinematic configuration, and another with hybrid serial-parallel kinematics. The reduced machine model is verified against full order models and is also validated against measurements by including joint characteristics in the model. The effects of position and feed-direction-dependent compliances on machining stability are investigated by using a novel position and feed-direction-dependent-process-stability performance criterion that evaluates the productivity of machine tools in its entire work volume. Parameters limiting the target productivity levels are identified and modified; and, the complete dynamics are rapidly re-analyzed using the developed models. Optimal design modifications are shown to increase productivity by ~35%. The proposed methods in this thesis enable efficient simulation of structural dynamics, stability assessment as well as interactions of the CNC and cutting process with the machine tool structure in a virtual environment.
Micro-cutting operations are used to manufacture miniature parts in biomedical, optics, electronics, and sensors industry. Compared to chemical manufacturing processes, micro-cutting has the advantage of producing three-dimensional features with a broad range of materials. Tool geometries and cutting conditions need to be properly selected to achieve desired surface finish and avoid premature wear or breakage of the fragile micro-tools. The mechanics and dynamics of micro-cutting have to be modeled in order to predict the process behavior and plan the operations ahead of costly physical trials.The chip thickness is comparable to the tool edge radius in micro-cutting, which brings strong size effect to the prediction of cutting force. A generalized analytical model based on slip-line field theory is proposed to predict the stress distribution and cutting force with round tool edge effect. Plastic deformation of workpiece material is modeled considering strain hardening, strain-rate and temperature effects on the flow stress. A numerical model is developed to simulate chip formation and cutting force using finite element method. The simulation results obtained from the numerical and analytical models are compared against experimental measurements to evaluate their predictive accuracy. The cutting force coefficients are modeled as functions of tool edge radius and uncut chip thickness from a series of slip-line field and finite element simulations. The identified cutting force coefficients are used to simulate micro-milling forces considering the actual tool trajectory, radial tool run-out and the dynamometer dynamics. Micro-milling forces which have sub-Newton amplitude are predicted directly from material constitutive model with experimental proof.A specially devised piezo-actuator mechanism is developed to identify the frequency response function of the micro-mill up to 120 kHz. The process damping coefficient in the ploughing region is identified from the finite element simulations. Dynamic micro-milling force with the velocity dependent process damping mechanism is modeled, and the chatter stability is predicted in frequency domain. Chatter tests are conducted to experimentally validate the dynamic model of micro-milling. The proposed mechanics and dynamic models can be used to simulate micro-cutting operations with various workpiece materials and tool geometries, and provide guidance for micro-cutting planners to select optimum cutting conditions.
The prediction of chatter instability in machining steel and thermal-resistant alloys at low cutting speeds has been difficult due to unknown process damping contributed by the contact mechanism between tool flank and wavy surface finish. This thesis presents modeling and measurement of process damping coefficients, and the prediction of chatter stability limits for turning and milling operations at low cutting speeds. The dynamic cutting forces are separated into regenerative and process damping components. The process damping force is expressed as a product of dynamic cutting force coefficient and the ratio of vibration and cutting velocities. It is demonstrated that the dynamic cutting coefficient itself is strongly affected by flank wear land. In measurement of dynamic cutting forces, the regenerative force is eliminated by keeping the inner and outer waves parallel to each other while the tool is oscillated using a piezo actuator during cutting. Classical chatter stability laws cannot be used in stability prediction for general turning with tools cutting along non-straight cutting edges; where the direction and magnitude of the dynamic forces become dependent on the depth of cut and feed-rate. A new dynamic cutting force model of regeneration of chip area and process damping, which considers tool nose radius, feed–rate, depth of cut, cutting speed and flank wear is presented. The chatter stability is predicted in the frequency domain using Nyquist stability criterion.The process damping is considered in a new dynamic milling model for tools having rotating but asymmetric dynamics. The flexibility of the workpiece is studied in a fixed coordinate system but the flexibility of the tool is studied in a rotating coordinate system. The periodic directional coefficients are averaged, and the stability of the dynamic milling system is determined in the frequency domain using Nyquist stability criterion. The experimentally proven, proposed stability models are able to predict the critical depth of cut at both low and high cutting speeds.
Aerospace, die and mold, and automotive industries machine parts at high cutting speeds to reduce production cycle periods. Machine tools which carry out the cutting operations rely on either precision ball screw or linear motor direct drives to accurately position the workpiece relative to the cutting tool. However, the precise positioning capability of the drives is limited by low servo bandwidth and poor disturbance rejection resulting from structural flexibilities in ball screw drives as well as weak dynamic stiffness/robustness in direct drives. This thesis proposes modeling, parameter identification, control and online parameter estimation techniques which aim at increasing the servo bandwidth and disturbance rejection ability of high speed machine tool feed drives.A hybrid finite element methodology is used to model the structural dynamics of ball screw drives. As part of the model, two stiffness matrices are developed for connecting the finite element representation of the ball screw to the lumped-mass representation of the nut. The developed model is used to analyze the coupled axial-torsional-lateral vibration behavior of a critical structural mode that limits high bandwidth control of ball screw drives. Moreover, a method for accurately identifying the mass, damping and stiffness matrices representing the open-loop dynamics of ball screw drives is developed. The identified matrices are used to design gain-scheduled sliding mode controllers, combined with minimum tracking error filters, to effectively suppress the critical axial-torsional-lateral mode of ball screw drives thereby achieving high bandwidth control and good disturbance rejection. For direct-driven machines, a high bandwidth disturbance adaptive sliding mode controller is designed to improve the dynamic stiffness of the drive, compared to similar controller designs, without increasing the controller’s complexity. Furthermore, the cutting forces applied to the drive are estimated accurately using a disturbance recovery algorithm and used to improve the dynamic stiffness of low-frequency structural modes of direct-driven machine tools.Finally, a method for estimating the changing mass of the workpiece during machining operations with cutting forces that are periodic at spindle frequency is introduced. The techniques presented in this thesis are verified through simulations and/or experiments on single-axis ball screw and linear motor feed drives.
This thesis presents experimentally verified optimal feedrate generation and high performance precision control algorithms developed for 5-axis machine tools. A feedrate scheduling algorithm has been introduced to minimize the cycle time for 5-axis machining of curved tool-paths. The variation of the feed along the tool-path is expressed in a cubic B-spline form as a function of the arc displacement. The velocity, acceleration and jerk limits of the five axis drives are considered in finding the most optimal feed along the tool-path to ensure smooth and linear operation of the servo drives with minimal tracking error. Improvement in the productivity and linear operation of the drives are demonstrated through 5-axis experiments. In an effort to design an accurate contour controller, analytical models are developed to estimate the contour errors during simultaneous 5-axis machining. Two types of contouring errors are defined by considering the normal deviation of tool tip from the reference path, and the normal deviation of the tool axis orientation from the reference orientation trajectory. A novel multi-input-multi-output sliding mode controller is introduced to directly minimize the tool tip and tool orientation errors, i.e. the contouring errors, along the 5-axis tool-paths. The stability of the control scheme is proven analytically, and the effectiveness of this new control strategy has been demonstrated experimentally. An identification technique for identifying the closed loop transfer function of machine tool feed drives has been introduced. The drive system is identified in closed loop, including the feed drive mechanism, motor amplifier, and the control law. A short Numerical Control Program is used for exciting the axis dynamics without interfering with the servo control loop. A generalized drive model is utilized to capture the key dynamics of the drive systems, while guaranteeing the stability of the identified model dynamics by solving a constrained optimization problem. Methods developed in this thesis have been evaluated on a table tilting 5-axis machining center. Their application to other 5-axis machines would require modeling of the kinematic chain and the drive dynamics to be considered in the control law design and trajectory generation.
This thesis presents models and algorithms necessary to simulate the five-axis flank milling of jet-engine impellers in a virtual environment. The impellers are used in the compression stage of the engine and are costly, difficult to machine, and time-consuming to manufacture. To improve the productivity of the flank milling operations, a procedure to predict and optimize the cutting process is proposed. The contributions of the thesis include a novel cutter-workpiece engagement calculation algorithm, a five-axis flank milling cutting mechanics model, two methods of optimizing feed rates for impeller machining tool paths and a new five-axis chatter stability algorithm.A semi-discrete, solid-modeling-based method of obtaining cutter-workpiece engagement (CWE) maps for five-axis flank milling with tapered ball-end mills is developed. It is compared against a benchmark z-buffer CWE calculation method, and is found to generate more accurate maps.A cutting force prediction model for five-axis flank milling is developed. This model is able to incorporate five-axis motion, serrated, variable-pitch, tapered, helical ball-end mills and irregular cutter-workpiece engagement maps. Simulated cutting forces are compared against experimental data collected with a rotating dynamometer. Predicted X and Y forces and cutting torque are found to have a reasonable agreement with the measured values.Two offline methods of optimizing the linear and angular feeds for the five-axis flank milling of impellers are developed. Both offer a systematic means of finding the highest feed possible, while respecting multiple constraints on the process outputs. In the thesis, application of these algorithms is shown to reduce the machining time for an impeller roughing tool path.Finally, a chatter stability algorithm is introduced that can be used to predict the stability of five-axis flank milling operations with general cutter geometry and irregular cutter-workpiece engagement maps. Currently, the new algorithm gives chatter stability predictions suitable for high speed five-axis flank milling. However, for low-speed impeller machining, these predictions are not accurate, due to the process damping that occurs in the physical system. At the time, this effect is difficult to model and is beyond the scope of the thesis.
Master's Student Supervision (2010-2017)
Opening large number of holes takes considerable amount of time during manufacturing and assembly of aircrafts. Traditionally, tools having the same diameter of each hole have been used in drilling which take considerable amount of time for tool change and fixturing. Recently, orbital drilling technology has been introduced to open holes with a single set-up. The combined orbital motion around the hole and helical penetration in axial direction are either given by stationary computer numerically controlled (CNC) machines or hand held, portable heads that are attached to aircraft body with suction pads. Although the tool path and machine were developed, the process mechanics and dynamics have not been modeled to predict cutting forces, torque, power and chatter stability diagrams to identify most productive and safe cutting conditions. This thesis presents mathematical model to simulate the mechanics and dynamics of orbital drilling process. The mechanics of the process are modeled by identifying the chip thickness distribution along the peripheral and bottom cutting edges of the helical end mills used in orbital drilling, The pitch length of the path, tool and hole diameters, spindle speed, feed and material properties are used in the model which is experimentally proven by comparing predicted and measured cutting forces. The flexibilities of the orbital drilling head and tool are incorporated to the mechanics model to predict the dynamics of the system. It is shown that the additional delay contributed by orbital motion of the tool can be neglected, and the regenerative delay is dominated by the spindle speed. However, structural dynamic modes of the system need to be oriented along the tangential feed direction since it varies continuously along the orbital path. The chatter stability of the system has been developed in both frequency and semi-discrete time domains. The experimentally verified stability model considers spindle speed, tool and hole geometries, structural dynamics, material properties and orbital drilling pitch length. The proposed orbital drilling model allows optimal selection of spindle speed, feed, orbital speed, pitch length and tool diameter for a given work material without overloading the machine and chatter while achieving highest possible material removal rates.
Additive manufacturing (AM) technologies are used in three-dimensional (3D) printing of parts by depositing the material layer-by-layer on the computer numerical controlled machine tools. While laser and electron beam guns are used to melt, and deposit the metals, thermoplastic materials are heated and deposited by the extruders. When the material deposition is not synchronized with the tangential velocity of the machine, an excess material is accumulated at sharp curvatures where the machine slows down. This thesis presents a novel algorithm for the synchronized deposition of thermo-plastic materials with the tangential path velocity of the machine under constant temperature. The temperature of the thermoplastic filament needs to be kept at a constant temperature (i.e. 220 Celsius) in a heater chamber. The transfer function of the temperature and current input to the heater is modelled as a first order system whose parameters are time varying as a function of material’s extrusion rate. An adaptive pole placement controller is designed to maintain the temperature of the material by manipulating the current supply to the heater as the extrusion rate vary. The tool path is first smoothed by a fifth order B-spline. The tangential path velocity is also smoothed by a third order spline while respecting heater’s power limit as well as the jerk, acceleration and velocity limits of two drives which are used in printing the material layer by layer. The extrusion rate is controlled proportionally to the tangential path velocity while keeping the temperature of the deposited thermo-plastic material at the desired temperature by adaptively controlling current supply to the heater. The experimentally proven algorithm leads to more uniform material deposition at sharp curvatures and resulting improved dimensional accuracy of printed parts. The proposed methodology can be extended to laser and electron beam based metal printing applications.
The current trend in industry is to achieve intelligent, Computer Numerical Controlled (CNC) machine tools which can monitor its performance and take corrective actions automatically during machining operations. Cutting forces are the most accepted indicators of the tool condition, load on the machine and part, and abnormalities in the machining operations. The objective of this thesis is to predict the cutting forces from the current drawn by each drive during five axis machining operations. The cutting forces generated at the tool–workpiece contact zone are transmitted to the three translational and two rotary drive motors through ball screws and gear boxes. The torque received by individual motors is transformed as disturbance current by the motor amplifiers. The cutting force transmitted to each feed drive acts as a disturbance to the closed loop servo controller, which reacts by supplying torque command in addition to the torque required to overcome the friction and inertial motions. The accurate prediction of cutting forces from the motor current measurements requires the separation of the effects of cutting and inertial motion forces from the total motor current values. The transfer function between the applied force at the tool tip and motor current is identified at each drive. The effects of structural modes are canceled through extended Kalman Filter designed for each drive. Both Coulomb and Viscous Friction forces have been identified, and their effects are also removed from the state measurements of all drives. The cutting forces at the tool tip are predicted by applying extended Kalman Filter on motor current signals, and transmitting them to the tool tip through forward kinematic model of the machine, the contributions are proven using machining tests conducted on a five axis machining center.
This thesis presents a six degree of freedom magnetically levitated rotary tablethat has one unlimited rotation axis. The actuation force is achieved by the Lorentzforce. The underside of the actuator has an axial Halbach array mounted circumferentially near the outside edge. Force is produced in the stator coils which are made using a printed circuit board. The purpose of this table is for a micro-machining rotary table application. Beneﬁts of this table are its compact, lightweight, no friction and high precision characteristics.Control of the table in six degrees of freedom is achieved by dividing the stator into quarters. Each quarter is driven by a linear three phase current ampliﬁer.For each quarter two forces can be generated, one in the levitation direction andone in the tangential direction. This creates eight independently controlled forcesallowing for full six degrees of freedom control. Position feedback for the stageis achieved by using four capacitive displacement measurement probes and fouroptical encoders around a circular optical grating. Performance of the table hasbeen tested and the results show that the closed loop bandwidth for all axes is between ∼ 250Hz and ∼ 550Hz. Regulation error in the X,Y and Z axes is less than55nm while the A,B and C axes are better than 1.2µrad (0.248 arc seconds). Forcecapacity has been tested up-to 70N with a theoretical limit of 140N.
The growing demand for high speed machining in aerospace, automotive and die and mold industry has directed the interest of research community towards prediction and reduction of machining cycle time. In this thesis, a cycle time prediction scheme is proposed for milling operations based on identified CNC machine dynamics in exact-stop and continuous mode. Various system identification techniques are utilized to identify the implemented trajectory generation and corner smoothing technique and feed drive dynamics of the CNC system. An analytical approach for predicting cycle time based on the identified CNC system dynamics and given part program is presented. It is shown that the cycle time of NC machining process is predominantly affected by trajectory generation and corner smoothing techniques implemented on CNC systems. The closed-loop feed drive dynamics does not have much influence on the cycle time, since the tracking delay is insignificant in position control servos. The proposed algorithm is validated in experiments and experimental results has shown that the cycle time prediction error remains within 5% for various 2-axis, 3-axis and 5-axis toolpaths. In the later half of the thesis, a new decoupled approach for five-axis corner smoothing is presented to reduce the cycle time of milling operations. Toolpath position and orientation are smoothed by inserting quintic and normalized septic micro-splines, respectively between the adjacent linear toolpath segments. Optimal control points are calculated for position and orientation splines to achieve C³ continuity at the junctions between the splines and the linear segments while respecting user-defined corner position tolerance and orientation tolerance limits. Synchronization of position and orientation splines is carried out. After geometrical modification of the toolpath, feedrate planning is performed using C³ continuous cubic acceleration feedrate profile to preserve jerk continuity in toolpath motion. The proposed C³ continuous toolpath motion is compared against the unsmooth and C² continuous motion in experiments and simulations to show improvements in cycle time, tracking accuracy and smoothness throughout the toolpath.
Feed drives of High Speed Machine (HSM) tools deliver fast motions for rapid positioning of tool or work-piece. The inertial forces generated by acceleration and deceleration of large machine tool components excite structural modes of the machine tools and cause residual vibrations. Unless avoided, the vibrations lead to poor surface finish and instability of the drive's control loop. In this thesis, structural flexibilities are represented by linear and torsional springs and dampers to develop a mathematical model of the feed drive dynamics. The model includes the contribution of structural vibrations in measuring table position by a linear encoder. An identification algorithm is introduced to facilitate the estimation of rigid body and structural dynamics in frequency domain. The identified mathematical model is used to mimic the real machine in simulations with the purpose of analyzing the interaction between structural dynamics and a high bandwidth adaptive sliding mode controller. Meanwhile, efficiency of finite element modeling approaches in predicting this interaction prior to the physical production is investigated by replacing the machine dynamics by a FEM based model. The mathematical model is used to design a Kalman Filter which estimates the table's acceleration by taking double digital derivative of the encoder signal. The table's acceleration is used to modify the control loop to minimize the effect of undesired structural vibrations. It is shown that the vibrations can be actively damped, and the bandwidth of the drive can be increased. The increase in the servo loop bandwidth provides smoother motion and improves the tracking performance significantly.
The present Computer Numerical Controlled (CNC) machine tools can provide internal states of the machine (such as speed, feed, current, power, torque, and axis tracking errors) to external computers, which in turn can manipulate spindle speeds and feeds through Ethernet communication tools. This thesis presents on-line detection and avoidance of chatter vibrations, on-line prediction of cutting torque and its adaptive control during milling operations. Chatter is detected by monitoring the frequency spectrum of sound signals during machining operations. The forced vibrations that occur at spindle and tooth passing frequencies are removed through a comb filter. The chatter frequency and its magnitude are predicted. The spindle speed is automatically changed to enter the process into the nearest stability pocket if it lies within the first five stability lobes. If the process cannot be stabilized due to missing lobes at low speeds, the spindle speed is harmonically varied without violating the power limit of the spindle drive. The algorithm is implemented on a five axis Mori Seiki NMV5000 Machining Center with a FANUC 30i controller. The communication with an external PC is handled through Ethernet and FOCAS command library of Fanuc.The cutting torque is also predicted by monitoring the current of a three phase induction motor in real time. The cutting torque is estimated through Extended Kalman Filter from the steady state model of the motor after removing the friction component. The estimated torque is used to keep the cutting torque on the machine at desired and safe levels by manipulating the feed rate with adaptive pole placement controller.The thesis shows that it is possible to add process monitoring and control functions to the machine without having to add costly and impractical sensors on the machine, leading to safer and more productive machining operations.
Peripheral milling of thin walled aerospace components takes considerable amount of machining time as blank blocks are cut down to thin webs under excessive structural oscillations during the process. Unstable chatter vibrations and stable forced vibrations cause poor surface finish on the machined part. Predicting the process mechanics in advance eliminates the time consuming trial and error approach in reducing the vibrations which are within the tolerance limits of the part. This thesis presents the mathematical modeling of the peripheral milling of the thin walls with helical end mills. The cutting forces, vibrations and dimensional form errors left on the finish surface are predicted under stable but forced vibration conditions. The chatter stability diagram of the operation is predicted by using both frequency and semi-discrete time domain models. The relative vibrations between the flexible part and slender end mill are consi-dered. The tool and the workpiece are discretized along the contact axis to include effect of varying cutting forces and structural dynamics. The differential milling forces are evaluated from the static chip loads contributed by the rigid body motion of the milling operation, and dynamic chip loads caused by the relative vibrations between the flexible tool and flexible thin part. The different cylindrical end mill geometries with regular and non-uniform pitch and helix angles, and low speed process damping effects are included in the dynamic force model. The dynamic properties of the flexible structures are represented by expe-rimentally evaluated modal model in order to reduce the number of linear, periodic, delayed differential equations solved in frequency and time domain computations. The periodic, delayed differential equations are solved by the semi discrete time domain method to predict the amplitude of vibrations and forces. The equations of motion are simplified to constant coefficient type ordinary differential equations, and surface location errors are calculated by frequency domain solver. Chatter stability lobes are calculated using semi discrete time domain and fre-quency domain methods. Chatter stability solvers are validated by conducting chatter tests for roughing and finishing stages of thin walled aluminum part at high cutting speeds, and low speed machining of rigid steel block.
Feed drives are used in positioning of machine tools. The drives are actuated either by linear or rotary servo motors. The ball screw drives are driven by rotary motors; hence they have flexibility and added friction due to nut interface. Direct drives are driven by linear motors which have more mechanical stiffness, but less disturbance rejection due to missing load reduction mechanism. This thesis presents the modelling and control of drives with rigid and flexible structures.A single degree of freedom flexible oscillator is mounted on a high speed, rigid feed drive table for experimental illustration of system identification and the active control method proposed in the thesis. The rigid feed drive dynamics include the mechanical component of the rigid body mass and viscous damping, and the electrical component of the power amplifier and motor. The flexible component is modelled by springs, mass and damping elements. Both rigid and flexible dynamics of the system are identified experimentally through unbiased least square, sine sweep and impact model tests. The vibration of the single degree of freedom system is actively damped by an acceleration feedback inserted in the velocity loop. A Kalman filter is used to minimize the drift and noise on the acceleration measurements. The position loop is closed with a proportional controller.It is experimentally demonstrated that the vibrations of the flexible structure can be well damped. However, the acceleration feedback used at the resonance frequency greatly minimizes the bandwidth close to the vibration frequency. Further methods need to be used to expand the bandwidth beyond the natural frequency of the flexible structure by coping with the anti-resonant effect of the acceleration feedback.
The manufacturing planning of parts is currently based on experience and physical test trials. The parts are modeled, and Numerically Controlled (NC) tool paths are generated in Computer Aided Manufacturing (CAM) environment. The NC programs are physically tested, and if the process faults are found, the NC program is re-generated in the CAM environment. The objective of this thesis is to develop Virtual Turning System that predicts the part machining process ahead of costly physical trials.Tool–workpiece engagement geometry is calculated along the tool path by a proposed polycurve method. The part geometry is imported as a stereolithography (STL) model from the CAM system, and the cross section around the turning axis is reconstructed. The tool and part cross sections are modeled by polycurves, which are constructed by series of arcs and lines. The tool–part geometries are intersected using boolean operations to obtain the engagement conditions.The turning process is modeled by predicting the chip area and equivalent chord angle. The process forces are modeled proportional to the material dependent cutting force coefficients, depth of cut and equivalent chord length that depends on the nose radius and approach angle of the tool. The chatter stability of the process is examined using Nyquist criterion at each tool–workpiece engagement station along the path.The virtual turning simulation simulates the forces and detects the chatter stability, and adjusts the feeds at each tool-part engagement station. The physical turning of parts with arbitrary geometry can be simulated, and cutting conditions that leads to most optimal machining operation is automatically determined without violating the limits of the machine tool and part.
This thesis presents a continuous tool motion trajectory generation algorithm for high speed free form surface machining. A NURBS toolpath generation algorithm is presented to fit the discrete motion commands generated from free-form CAD-models. By using a NURBS representation of the machine part, the toolpath is interpolated continuously to direct the synchronized motion of the 5-axis CNC machine. The higher continuity of the motion trajectory allowed for tighter machining tolerances and reduced feedrate fluctuations and the undesired acceleration harmonics in the overall feed motion and in each of the motor motions. An optimal and feasible feedrate profile have been used to continuously maneuver the cutting tool with the interpolated reference tool position and tool orientation commands such that the kinematic constraints of the drives are not violated. Commonly used least squares curve fitting of discrete data points forces the curve to weave through the data points and results in a fluctuating toolpath. By making use of the defined basis function distributions of the NURBS control points, a higher smoothness fit has been achieved through a minimization on the chord error and the third derivative of the curve. The feasibility of this toolpath generation algorithm has been extended using the double spline representation to represent both the tool position and the tool orientation with minimal fitting error. The real time interpolation of the fitted NURBS toolpath has also been implemented using the multi-segment Feed Correction Polynomial. This method provides an adaptive mapping between the nonlinear relationship of the NURBS curve parameter and the curve displacement to allow for a consistent feedrate in the cutting motion. Additionally, the kinematic compatibility conditions are considered based on the inverse kinematics of the 5-axis CNC machine. The proposed algorithm ensures that an overall efficient feed constraint is placed such that none of the individual drives are overdriven. The results from experiments and simulations are presented to demonstrate the effectiveness of the developed trajectory generation algorithms.
The complexity and variation of parts are continuously increasing due to technologically oriented consumers. The objective of present manufacturing industry is to increase the quality while decreasing the machining costs. This thesis presents a smart machining strategy which allows the automated prediction of chatter-free cutting conditions using sensors integrated to Computer Numerical Controlled (CNC) machine tools. The prediction of vibration free spindle speeds and depth of cuts require to have material's cutting force coefficient and frequency response function (FRF) of the machine at its tool tip. The cutting force coefficients are estimated from the cutting force measurements collected through dynamometers in laboratory environment. The thesis presents an alternative identification of tangential cutting force coefficient from average spindle power signals which are readily available on machine tools. When tangential, radial and axial cutting force coefficients are needed, the forces need to be collected by piezoelectric sensors embedded to mechanical structures. The structural dynamics of sensor housings distort the force measurements at high spindle speeds. A Kalman filter is designed to compensate the structural modes of the sensor assembly when the spindle speed and its harmonics resonate one of the modes the measuring system. The FRF of the system is measured by a computer controlled impact modal test unit which is integrated to CNC. The impact head is instrumented with a piezo force sensor, and the vibrations are measured with a capacitive displacement sensor. The spring loaded impact head is released by a DC solenoid controlled by the computer. The impact force and resulting tool vibrations are recorded in real time, and the FRF is estimated automatically. The measured FRF and cutting force coefficient estimated from the spindle power are later used to predict the chatter free depth of cuts and spindle speeds. The machine integrated, smart machining system allows the operator to automatically select the chatter-free cutting conditions, leading to improved quality and productivity.
Rotating shafts are used in the power train components of aircraft and automotive engines. The shafts are turned on lathes. Engine cylinders and bearing housings are finish machined using boring bars with single or multiple inserts. The cutting forces excite the structural dynamics of the turned shafts or boring bars during machining, leading to a poor surface finish and possible damage to the machined parts. This thesis presents mathematical models of single and multiple point turning/boring operations with the aim of predicting their outcome ahead of costly physical trials on the shop floor. Turning and boring operations are conducted at low angular speeds where the system dynamics is dominated by the process damping mechanism. The dynamic forces are modeled proportional to the static and regenerative chip thickness, tool geometry, and velocities of the vibration. The process damping coefficients, which are dependent on the material, tool geometry, cutting speed and vibrations, are identified from chatter tests conducted at the critical speeds and depths. The structural dynamics of the long boring bars are modeled using the Timoshenko Beam elements in Finite Element model which allows parametric placement of the boundary conditions, such as the bearing supports. The dynamics of the interaction between the cutting process and the structure are modeled. The stability of the operations is solved in frequency domain, analytically when the velocity and vibration dependent process damping is neglected. When the process damping is included, but the periodicity of the dynamic forces is neglected, the stability of the process is solved using the Nyquist criterion. When the periodicity and process damping are considered, the dynamic system is represented by a set of differential equations with periodic, time delayed forces. The stability of such systems, which are found in the line boring of crank and cam shaft housings, is solved in the time domain using an analytical but semi-discrete method. The thesis presents a complete set of solutions in predicting the static and dynamic forces, as well as the critical depths of cuts and speeds to avoid chatter vibrations in single point, multi-point and line boring operations.