Ph.D. in mechanical engineering, teacher-researcher in systems engineering and numerical analysis, and head of projects at ESME Sudria Lyon, Andrea Bareggi received the Joseph Whitworth Prize 2015 in June, a prize for excellence in academic research awarded by the Manufacturing Industries Division Board of the Institution of Mechanical Engineers (IMechE). He describes for ESME Sudria the motivation behind the research that earned him this distinction, and which is at the heart of his scientific article co-written by Garret O’Donnell of the Department of Mechanical & Manufacturing Engineering at Trinity College Dublin: “Thermal and mechanical effects of high-speed impinging jet in orthogonal machining operations: Experimental, finite elements, and analytical investigations” (download the publication here).
Andrea Bareggi (right) with Clive Hickman, Chief Executive of The Manufacturing Technology Center, during the Joseph Whitworth Prize award ceremony held at the prestigious Marble Hall in London
“In the context of Industry 4.0, or the introduction of digital technology into industrial production, the machining of metallic materials must be revitalized in order to regain the importance it had acquired during the 19th and 20th centuries. Although mass production uses plastic materials, a majority of transportation and civil engineering production is based on machining technologies.
One of the major problems of machining metallic materials, in particular of materials that are difficult to machine such as titanium alloys like Ti6Al4v and Inconel (which are used extensively in the aerospace industry), is the removal of heat during machining and the lubrication to reduce the forces during the process (see below).
Fig. 1 – The traditional use of machining fluid to lubricate and eliminate heat in the machining zone
If the heat is not removed immediately, the cutting tool (or insert) can break from thermal shock and imperfections can arise on the surface of the machined piece due to the strong temperature gradient. The traditional way to remove heat is to add mineral oil directly between the tool and the piece to be machined. This solution worries ecologists, however, for mineral oil is extremely harmful to the environment. Even more worrisome is the effect that cutting oil has on human health, as it is toxic to the skin and especially dangerous if inhaled. Minimum Quantity Lubrication (MQL) is one of the most widespread solutions allowing for less mineral oil use. Today, MQL is the prevailing solution for materials that are difficult to machine. Nevertheless, MQL increases the risk of inhalation by the operator, for the oil particles are vaporized in a high-pressure and high-speed air flow. A variety of solutions are currently under development, but the trend is for dry machining (machining without mineral oil) for traditional machining, and near dry machining for materials that are difficult to machine.
The research I conducted these past few years at Trinity College Dublin and ESME Sudria were based on dry machining. Since heat must be removed during machining, I proposed the use of a well-aimed high-pressure air jet to cool the machining zone. To carry out this industrial research project, I drew upon experimental techniques to measure the temperature in the cutting zone by placing a thermocouple in the insert (a particularly difficult procedure, considering the reduced dimensions and hardness of the insert, which is made up of a tungsten carbide substrate and several protective layers of titanium nitride and titanium carbide). The cutting forces were analyzed by a dynamometer placed on the lathe (see image below). This data was used to develop an orthogonal cutting model – the simplest type of cut one can perform – to determine the machining parameters (speed and pace). The idea is to maximize the amount of material machined (the main productivity index is the Material Removal Rate, MMR), all the while minimizing the cutting temperature and forces by using the air jet.
Fig. 2 – Experimental assembly with the thermocouple and dynamometer
Finite Element Modeling (FEM) is a calculation technique that arose in the 1960s. Over time, it has become the integration method for differentials which deal with dynamic phenomena. The formulation used to describe the problem of machining is called Arbitrary Lagrangian-Eulerian. Combined with adaptive remeshing, the finite element approach allows us to factor in machining parameters and to visualize the distribution of the effective stress, strain, and strain rate inside the machined material (see image below).
Fig. 3 – FEM allows for the measurement of machining parameters which otherwise are extremely difficult to obtain
The new feature of this research work is the discovery of a mechanical effect of the air jet on the chip forming the face of the insert. Depending on the direction of the air jet, this mechanical effect can increase or reduce the energy required for machining low-carbon steels by around 8%. The important thing to consider is that this effect is independent of the temperature, as demonstrated by the finite element analysis: in the image below, the position of the air jet is seen from a qualitative point of view and the heat transfer coefficients are evaluated at a pressure of 7 bar.
Fig. 4 – The two positions of the air jet during the tests and the mechanical and thermal modeling by finite elements
The tests and the model showed that the position which is perpendicular to the insert face (overhead position) is the more efficient position in terms of the energy required for machining, in contrast to the traditional position of methods such as Minimum Quantity Lubrication. This finding opens new avenues in the search for techniques that can reduce the energy needed for machining. Further research work in this area will lead to greater knowledge of the velocity field of the fluid around the machining zone, with more efficient drain utilization by way of an air jet. In addition, an analysis of air compression costs is necessary for any commercial developments of this new technique. These developments are currently underway at ESME Sudria Lyon’s Centre de Modélisation et Calcul Numérique.”