A.G. Mamalis

Macroscopic and microscopic phenomena of electro-discharge machined steel surfaces: An experimental investigation

Abstract Electro-discharge machining is a thermal process with a complex metal removal mechanism, involving the formation of a plasma channel between the tool and workpiece electrodes, melting and evaporation action and shock waves, resulting in phase changes, tensile residual stresses, cracking and metallurgical transformation. These properties determine the operational behaviour of machined parts and are included in the term “surface integrity”. Experimental results pertaining to the physico-chemical changes occurring during electro-discharge machining of steel (structural, medium-carbon and alloyed steels) surfaces are reported in this paper. Surface morphology and finish are assessed and correlated with overall process parameters and metal removal rates. Metallurgical transformations and new structures, surface damage in the form of cracking and “white layer” formation, microhardness variations and residual stress patterns are quantitatively and qualitatively examined and discussed. The results confirm the inherent complexity of the process, as well as the assumptions that must be made in order to achieve acceptable predictions of the mechanical and physico-chemical characteristics of the machined surfaces.

Residual Stress Distribution and Structural Phenomena of High-Strength Steel Surfaces Due to EDM and Ball-Drop Forming

Summary Experimental results pertaining to the surface integrity of high-strength steel plates, namely dual phase, microalloyed and high-alloyed chromium steels are reported. The macroscopic and microscopic phenomena, i.e. metallurgical structure of the surface and sub-surface layers. residual stress state. microhardness and plastic deformation, which occur in such steel surfaces electro-discharge machined. due to the intense thermal nature of the process involved. and in mechanically ball-drop formed steel surfaces at low speed impact, as well as their dependence on the main processing parameters is quantitatively and qualitatively examined and discussed. Superposition of thermal and mechanical processing due to EDM and subsequent ball-drop forming is also examined.

Automated guided vehicle system design for FMS applications

Abstract The issues of designing and installing a system of Automated Guided Vehicles (AGVs) in a Flexible Manufacturing System (FMS) are examined in this work. The development, advantages and future trends of AGVs are briefly reviewed. Then the basic features of an AGV in an FMS environment, as well as a computerized procedure for the optimum vehicle selection, are discussed. Guide path layout considerations are made and docking/pallet transfer methods are suggested. A routing algorithm is presented to facilitate both routing and control of the vehicles, and the most commonly used methods for zone blocking, loading/unloading and traffic control and communications are outlined. Operation rules are reviewed and an example of an AGVS in an FMS is presented to demonstrate the potential of simulation in both design and evaluation of such systems.

Determination of due dates in job shop scheduling by simulated annealing

The scheduling of n jobs to m machines in a job shop is considered. A predefined due date, a release time and the minimization of the maximal jobs lateness is the objective assigned to each job. A search space consisting of triads (job, operation, machine) is formulated, and an iterrative improvement approach, the simulated annealing method, is then used to obtain feasible and global optimal solution. The simulated annealing method is applied to two alternative energy functions to model the maximum lateness. For calculation of the first energy function at each step, complete schedules are created and the lateness of each job is derived by abstracting the jobs completion time from the corresponding due date. The second energy function is calculated on the basis of partial estimates often used by pairwise interchange techniques. The convergence of the algorithm in relation to the initial temperature, temperature iterrations and temperature cycles has been verified in various case studies. Specific characteristics of the scheduling, such as its dimensionality and the deviation of the total processing time from the due dates, were considered. Common characteristics derived were subsequently used for the definition of an efficient annealing schedule.

Design of a system for computer-aided engineering of manufacturing facilities

Abstract This paper first defines the activities involved in manufacturing facility design and reviews relevant computer tools; these include CAD used to create static models, and simulators also tackling logistical behaviour information. A feasibility study of hard-merging CAD and simulation is briefly described. Next, the architecture of software flexibly combining CAD, simulation and intelligence is presented, based on the object-oriented Fusion method. The system includes an object library with rules defining behaviour, knowledge based pertinent to various ‘jobs’ within the activities of facility design, a reasoning engine that draws conclusions during and after execution of these jobs, and a graphical library of facility components potentially displayed through a real-time link to a CAD system. The novelty of the system pertains to flexible encapsulation of object behaviour and of intelligent design decisions, as well as to the capability of approximating a virtual environment through accurate graphics models.

On-line scheduling in metal removal processing using variable routings and control strategies

Abstract In this paper, a simulation program for the analysis of operations in a manufacturing system is presented. The simulation program is integrated using a control algorithm and a relational data model implemented on the ORACLE RDBMS into a scheduling control system. The control algorithm considers the on-line use of alternative machines tools and/or alternative control strategies in order to optimize a given criterion, which in the present case corresponds to the total delay of jobs in the shop. A series of variables, such as the average machine utilization, were chosen to be the output. A sequential optimization approach, where the dynamically calculated average delay per job is assumed to represent the system state up to the current decision stage, is used for the control algorithm. An application of the scheduling control system to the manufacturing of a piston pump is also presented. The on-line utilization of variable routings and strategies was investigated, demonstrating the overall performance of the algorithm, namely a reduction of the total processing time up to 30%.

Ball-drop forming of coated steel sheets

Abstract Experimental results are presented for the ball-drop forming process, pertaining to the influence of some variables in this low-speed impact process on the curvature acquired by initially-flat relatively-thin strips of low carbon and hot-dipped galvanized steels. The effects of the ball size, the mass of the balls dropped, the drop height, and the geometry and coating of the target material on the bulge profile and the surface topography of the strip are discussed. Attention is directed to the formation of the plastic zones of the dynamically deformed workpiece in relation to the curvatures produced. For the forming conditions examined, a double curvature with associated deformation zones was obtained; for large ball-sizes and drop-heights, concave shapes of the strip were formed; uncoated steel specimens showed higher bulgings than hot-dipped galvanized ones; geometrical discontinuities influence bulging performance; surface roughness is mainly determined by the crater dimensions and increases with increasing ball-size and drop-height.

On the plane plastic flow of rectangular billets forged with diamond-shaped closed dies

Abstract Slip-line field (s.l.f.) analysis and force-plane diagrams, simply drawn from force-equilibrium considerations, are used to predict forging loads during intermediate, i.e. post-incipient, deformation stages in the plane-strain closed-die forging of rectangular billets with diamond-shaped dies. The theoretically calculated loads are compared with experimentally predicted values by forging billets of the model material Plasticine: agreement is found to be fair only.

On the Yoshida Buckling Test of High Strength and Coated Low Carbon Steels

Summary The Yoshida Buckling Test, i.e. the uniaxial stretching of a square thin steel plate, was performed on a range of high strength and low carbon coated and uncoated steels to investigate their behaviour regarding buckling during the loading and unloading, deflection and failure problems encountered in actual sheet metal forming operations. The effects of loading and gripping conditions and the material properties on the initiation, growth and removal of the buckle and the straining occurred are discussed. A finite element analysis was performed to predict the stress distribution over the deformed steel plate.

Force Polygons for the Plane Strain Forging of Rectangular Billets with Diamond-Shaped Dies

Summary Force polygons, simply drawn from force equilibrium considerations, were employed to predict forging load during the incipient and succeeding stages of deformation when forging rectangular Plasticine billets with diamond-shaped closed dies of different semi-angles. The incipient stages of deformation are analysed by using slip-line field theory and the subsequent stages of plastic deformation by the sliding block method. Theoretically predicted loads were compared with experimentally obtained ones and found to be in good agreement.