OPTIMISATION OF SURFACE FINISH USING

GENETIC ALGORITHM AND EXPERIMENTAL VALIDATION

FOR END-MILLING OPERATIONS

ABSTRACT

Optimization of cutting parameters is valuable in terms of providing high precision and efficient machining. Optimization of machining parameters for milling is an important step to obtain better surface finish. The machining parameters are:-•Cutting Speed•Feed per tooth•Depth of cut

OBJECTIVE FUNCTION

The purpose of this study is to investigate the optimal cutting parameters for minimizing machining time and maximizing surface finish of the milling operation using genetic algorithm while maintaining material removing rate and stability of the process and experimental validation is need to be done to estimate surface roughness using experimental data.

INTRODUCTION

Machining is the most important of the manufacturing processes. Machining can be defined as the process of removing material from a work piece in the form of chips. The term metal cutting is used when the material is metallic. Most machining has very low setup cost compared to forming, molding, and casting processes. However, machining is much more expensive for high volumes. Machining is necessary where tight tolerances on dimensions and finishes are required.

Importance of Machining

END-MILLINGEnd-milling process is used to machine and produce a flat surface or a pair of flat surfaces. When the operation is performed at the end of a work piece, as shown in fig, a single flat surface is produced. The surfaces produced may be horizontal, vertical or inclined with respect to the top of the machine table.

APPLICATIONS1. Profile milling2. Tracer milling3. Face milling4. Plunging

GENETIC ALGORITHM

A genetic algorithm (GA) is a search technique used in computing to find exact or approximate solutions to optimization and search problems. Genetic algorithms are categorized as global search heuristics. Genetic algorithms are a particular class of evolutionary algorithms (EA) that use techniques inspired by evolutionary biology such as inheritance, mutation, selection, and crossover.

Mathematical modellingMilling is a metal cutting process in which the cutting tool intermittently enters and leaves the work piece, unlike turning, in which the tool is always in contact. In the milling process, material is removed from a work piece by a rotating cutting tool. While the tool rotates, it translates in the feed direction at a certain speed. A milling process can be modeled as cutting simultaneously with a number of single-point cutting tools. A basic dynamic model of 2-DOF end milling with a flexible tool is illustrated in Fig. This model is dynamic in the sense that the effect of tool and work piece vibrations on the instantaneous tooth feed is considered, and the part is assumed to be rigid. The chip thickness during machining periodically varies due to the vibration of the machine structure and cutting tool. The cutter is assumed to have z number of teeth and zero helix angle. Since stability is determined by the average effect during one revolution, the helix angle is ignored. If the tool is flexible, its motion in both the x and y directions affects the chip load and cutting forces, leading to chatter instability under certain conditions. The process parameters are the spindle speed N, the feed per tooth (chip load) if, the radial depth-of-cut br and the axial depth of cut b. In development of a dynamic force model for end milling, the objective of this study is to combine the concept of the equivalent orthogonal cutting conditions with the concept of a regenerative force model to predict the cutting force.

DETERMINATION OF CUTTING FORCES

The tooth of a milling cutter can be viewed as a single point tool referring to a rotating coordinate system; hence, the cutting force in milling can be calculated from a predictive force model as presented in Fig. 1 from cutting conditions, the property of the work material, the tool geometry and the corresponding instantaneous dynamic re-generative undeformed chip thickness. Consider the case of up milling with an assumed circular tool path. The cutter is subjected to normal and tangential cutting force components which may be projected into x and y coordinate directions using rotational positioning of cutting edge φ.

1. The tangential force Ft is proportional to the material dependent specific force Ks, the feed per tooth f, the chip width b, and the rotational position of cutting edge ϕ.

2. The normal radial force is assumed proportional to tangential force Fn

where kt is the cutting force coefficient. Resolving Ft and FN into cutting forces in the x and y direction and summing for all the teeth engaged will result in

Substitution of Eqs. 1 and 2 into Eqs. 3 and 4 yields x and y cutting forces, each with three distinct terms: depth of cut b, sine and cosine of φ. Equations 5 and 6 can be rewritten as

Where z is the number of teeth on the cutter. The x and they cutting force components given in Eqs. 5 and 6 may be evaluated analytically for the optimized cutting parameters and compared with measured values.