Asit Baran Puri

An analysis and optimisation of the geometrical inaccuracy due to wire lag phenomenon in WEDM

Abstract An extensive study of the wire lag phenomenon in Wire-cut Electrical Discharge Machining (WEDM) has been carried out and the trend of variation of the geometrical inaccuracy caused due to wire lag with various machine control parameters has been established in this paper. In an extremely complicated machining process like Wire-cut EDM, which is governed by as many as ten control factors, it is very difficult to select the best parametric combination for a particular situation arising out of customer requirements. In the present research study, all the machine control parameters are considered simultaneously for the machining operation which comprised a rough cut followed by a trim cut. The objective of the study has been to carry out an experimental investigation based on the Taguchi method involving thirteen control factors with three levels for an orthogonal array L27 (313). The main influencing factors are determined for given machining criteria, such as: average cutting speed, surface finish characteristic and geometrical inaccuracy caused due to wire lag. Also, the optimum parametric settings for different machining situations have been found out and reported in the paper.

Modelling and analysis of the wire-tool vibration in wire-cut EDM

Abstract Although wire-cut electrical discharge machining (WEDM) has revolutionised the present day metal cutting, die making and press tool manufacturing industries, further improvements are still required to meet the increasing demand of product precision and accuracy in those sectors of manufacturing. The machining performance in this regard is greatly influenced by the wire-tool vibration occurring during machining. This paper briefs an account of the vibrational behaviour of the wire and presents an analytical approach for the solution of the wire-tool vibration equation considering multiple spark discharges to investigate into the characteristic effects of wire vibration in WEDM. It includes the output of the model depicting the influence of the pulse discharge frequencies under various wire tensions on the maximum amplitude of wire vibration. Also, the trend of variation of the maximum amplitude of the wire vibration with the ratio of ‘the height of a job’ to ‘the span of the wire between the guides’ has been studied and reported in the paper.

Optimization of surface roughness in ball-end milling using teaching-learning-based optimization and response surface methodology

Surface roughness is one of the most important requirements of the finished products in machining process. The determination of optimal cutting parameters is very important to minimize the surface roughness of a product. This article describes the development process of a surface roughness model in high-speed ball-end milling using response surface methodology based on design of experiment. Composite desirability function and teaching-learning-based optimization algorithm have been used for determining optimal cutting process parameters. The experiments have been planned and conducted using rotatable central composite design under dry condition. Mathematical model for surface roughness has been developed in terms of cutting speed, feed per tooth, axial depth of cut and radial depth of cut as the cutting process parameters. Analysis of variance has been performed for analysing the effect of cutting parameters on surface roughness. A second-order full quadratic model is used for mathematical modelling. The analysis of the results shows that the developed model is adequate enough and good to be accepted. Analysis of variance for the individual terms revealed that surface roughness is mostly affected by the cutting speed with a percentage contribution of 47.18% followed by axial depth of cut by 10.83%. The optimum values of cutting process parameters obtained through teaching-learning-based optimization are feed per tooth (fz) = 0.06 mm, axial depth of cut (Ap) = 0.74 mm, cutting speed (Vc) = 145.8 m/min, and radial depth of cut (Ae) = 0.38 mm. The optimum value of surface roughness at the optimum parametric setting is 1.11 µm and has been validated by confirmation experiments.

Determining cutting force coefficients from instantaneous cutting forces in ball end milling

The precise prediction of cutting forces helps in improving the machining performances. This involves the modelling of cutting force components in tangential, radial and axial directions and determination of respective specific cutting force coefficients. In the present work, an improved method of identification of specific cutting force coefficient is proposed for ball end milling cutter using semi-mechanistic force model. The cutter is discretised into a finite number of axial discs along the axis of the cutter. Using the geometry of ball end milling cutter, true uncut chip thickness is modelled based on the trochoidal trajectory of a cutting edge element. Specific cutting force coefficients have been determined through inverse method. Also, a fourth order polynomial curve fitting method has been employed to establish a mathematical relationship between the said coefficients and axial depth of cuts. Several experiments have been carried out at different feed rate and axial depth of cut to determine the specific cutting force coefficients based on the proposed identification method. Validation results show good agreement between predicted and experimental results. Compared to conventional identification model, the specific force coefficient identification process discussed in the present paper is fast, convenient and accurate.

Analysis of cutting force coefficients in high-speed ball end milling at varying rotational speeds

ABSTRACT In high-speed ball end milling, cutting forces influence machinability, dimensional accuracy, tool failure, tool deflection, machine tool chatter, vibration, etc. Thus, an accurate prediction of cutting forces before actual machining is essential for a good insight into the process to produce good quality machined parts. In this article, an attempt has been made to determine specific cutting force coefficients in ball end milling based on a linear mechanistic model at a higher range of rotational speeds. The force coefficients have been determined based on average cutting force. Cutting force in one revolution of the cutter was recorded to avoid the cutter run-out condition (radial). Milling experiments have been conducted on aluminum alloy of grade Al2014-T6 at different spindle speeds and feeds. Thus, the dependence of specific cutting force coefficients on cutting speeds has been studied and analyzed. It is found that specific cutting force coefficients change with change in rotational speed while keeping other cutting parameters unchanged. Hence, simulated cutting forces at higher range of rotational speed might have considerable errors if specific cutting force coefficients evaluated at lower rotational speed are used. The specific cutting force coefficients obtained analytically have been validated through experiments.

Analysis of Surface Texture of High Aspect Ratio Blind Micro Holes on Titanium Alloy (Ti-6Al-4V) in Micro Electrical Discharge Drilling

Efficiency of any machining process depends on the effectiveness of final outcomes. Surface integrity plays an important role in functional performance of a part or component. Traditionally, surface roughness is considered to be the principal parameter to assess the surface integrity of a machined surface. In this paper, the influences of machining parameters like gap voltage, capacitance and depth of hole on the surface finish parameters like Ra and Sa of micro holes have been studied in micro electrical discharge drilling. The high aspect ratio blind micro holes were drilled on titanium alloy (Ti-6Al-4V) with cylindrical tungsten tool electrodes. The experimentation was carried out adopting a full factorial design (33). The simultaneous effects of machining parameters on responses were analysed using response surface methodology. Multiple linear regression models were developed for responses to obtain the correlation between machining parameters and machining outputs. Multi-objective optimization has been performed with the aid of the desirability function approach.

Advancements in Micro Wire-cut Electrical Discharge Machining

A WEDM process may be called as a Micro WEDM (MWEDM) process when it is used for manufacturing micro parts. The rest underlying theory is same as traditional WEDM. A micro part is machined with good dimensional accuracy and surface finish when the kerf (slot) width becomes considerably smaller compared to that obtainable in conventional WEDM. Truly speaking, the cutting tool in WEDM/MWEDM is not the wire, but it is the pulse (electrical discharge). A minimum kerf is ensured in Micro WEDM, when (i) pulse sizes are made extremely small, (ii) extremely thin wire is used (ϕ = 20–100 μm), and (iii) process inaccuracies along with the discharge gap are minimized. It is required for Micro WEDM to maintain the pulse energy in the order of 10−5–10−7 J. Process inaccuracies are minimised by minimising the amplitude of wire vibration and wire lag. Hence, for an MWEDM machine, the following subsystems are modified as compared to a conventional WEDM setup: (a) the machine tool configuration is designed mainly to eliminate stray capacitance or leakage of charges from the gap, (b) the pulse generator produces discharges with pulse energy preferably in the order of 10−6–10−7 J, (c) a pulse discrimination system is installed to avoid arc or other abnormal discharges, (d) a closed-loop controlled proper wire transportation system is needed, (e) the wire diameter should be 20–80 μm and preferably of tungsten, (f) a suitable oil dielectric is used and lastly, (g) a precise closed-loop controlled servo mechanism is used for gap and work-table feed control to operate at steps at submicron level.

Chatter and dynamic cutting force prediction in high-speed ball end milling

ABSTRACT Machine tool chatter is a serious problem which deteriorates surface quality of machined parts and increases tool wear, noise, and even causes tool failure. In the present paper, machine tool chatter has been studied and a stability lobe diagram (SLD) has been developed for a two degrees of freedom system to identify stable and unstable zones using zeroth order approximation method. A dynamic cutting force model has been modeled in tangential and radial directions using regenerative uncut chip thickness. Uncut chip thickness has been modeled using trochoidal path traced by the cutting edge of the tool. Dynamic cutting force coefficients have been determined based on the average force method. Several experiments have been performed at different feed rates and axial depths of cut to determine the dynamic cutting force coefficients and have been used for predicting SLD. Several other experiments have been performed to validate the feasibility and effectiveness of the developed SLD. It is found that the proposed method is quite efficient in predicting the SLD. The cutting forces in stable and unstable cutting zone are in well agreement with the experimental cutting forces.

Analysis of surface texture of bulk metallic glass in micro electrical discharge drilling

The influences of machining parameters has a like gap voltage, capacitance and spindle speed on machining performances for surface finish Ra and Sa of high aspect ratio through micro holes drilled in zirconium-based bulk metallic glass Vitreloy 1 or Vit 1 with cylindrical tungsten tool electrodes in micro electrical discharge drilling were studied. The experimentation was carried out adopting Taguchis L27 (313) orthogonal array. The simultaneous effects of machining parameters on machining performance were studied by applying response surface methodology. Adopting analysis of variance technique, it was observed that the machining parameters selected have great significant effects on responses. Second order regression models were developed for responses and found excellent for acceptance. Predicted optimal designed value for responses was computed using Taguchis algorithm. Validity of the models was checked statistically and found acceptable. The proposed model could be considered as valuable tools for process planning for cost effective machining of Vit 1. Tool electrode material tungsten was deposited on the hole surface as observed from energy dispersive X-ray spectroscopy study.