S. Chandrashekhar

An investigation into the influence of helix angle on the torque-thrust coupling effect in twist drills

This paper presents an experimental investigation into the torque-thrust coupling effect in twist drills. A set of cross-coupling stiffness coefficients (Kft and Ktf) are defined. The influence of helix angle on torsional and the cross-coupling stiffnesses is studied. The coupling interaction (as measured by Kft and Ktf) is strongly influenced by the helix angle and has a distinct maxima around 28°. Also, the magnitudes of the coefficients increase parabolically with drill diameter. The pure torsional stiffness also has a maxima around the same helix angle. Under axial restraint, the torsional stiffness increases by 10 to 25% owing to the coupling effect. The larger the torque-thrust interaction, the larger is this increase in torsional stiffness. The findings of this investigation is proposed as a probable basis for the traditional popularity of a 28 to 30° helix angle for drills.

On the design and development of a new BTA tool to increase productivity and workpiece accuracy in deep hole machining

This paper presents the design and method of development of a new three-pad BTA tool. The advantages of this cutting tool over a conventional two-pad BTA tool are brought out using a probabilistic approach in both the static and dynamic case. Experiments were conducted with the new three-pad tool, and it confirmed the theoretical prediction that the three-pad tool is more stable than the conventional BTA tool. Also, the cause of grooves observed on the workpiece produced by the two-pad tool has been explained. Further, it is observed that the third pad stiffens the tool thereby improving the chip breaking effect which improves the quality of the holes produced at high feeds. Thus this new development increases the productivity and improves workpiece accuracy in deep hole machining.

An experimental investigation for the stochastic modelling of the resultant force system in BTA deep hole machining

Abstract This paper presents the measurement and a statistical analysis of the resultant force system, consisting of an axial force and torque, in BTA deep hole machining. The measurements were performed using a specially designed two-component piezoelectric dynamometer and adopting the rotating cutting tool-stationary workpiece procedure. The dynamometer was calibrated for static and dynamic outputs and techniques were employed for increasing the measuring accuracy and reducing the cross-interference by obtaining the elements of the system transfer function. Experiments were carried out to measure the mean values and the dynamic fluctuations of the axial force and torque. The recorded data was processed and analysed to establish all major statistical properties of the axial force and torque. Results show that the dynamic fluctuations of the axial force and torque in BTA deep hole machining can be represented by a stationary wideband process with a gaussian density distribution function. Such a mathematic...

An experimental investigation into the comparison of the performance characteristics of TiN and ZrN coatings on split point drills using the static and stochastic models of the force system as a signature

This paper presents a comparison of the performance characteristics of TiN (Titanium nitride) and ZrN (Zirconium nitride) coatings on split point drills. The objective of this work was to choose the better coating for machining tough materials like INCONEL. This leads to an increased productivity of drilling holes in certain components of environmental control systems and fuel control systems (in aerospace industry) made of tough materials like INCONEL. The comparison of the performance characteristics was based on the measurement of the mean values and dynamic fluctuations of the cutting force and the number of holes drilled under the same optimum machining conditions. The measurements were carried out using two specially designed piezoelectric dynamometers. The dynamometer was calibrated from static and dynamic outputs and techniques were employed for increasing the measuring accuracy and reducing the cross interference by obtaining the elements of the tranfer function. Power spectrum plots of the drift force, axial force and torque were obtained so that these plots may be used as a signature. Results show that the ZrN coating is better than the TiN coating because (a) the mean values of the axialforce, drift force and torque are smaller, thus improving the roundness, (b) the dynamic flutuations of the forces and torque about the mean are smaller thus improving the surface quality of the holes produced, (c) the number of holes before the failure of the drill is about three times more for a ZrN coated split point drill as compared to a TiN coated drill.

A STOCHASTIC CHARACTERIZATION OF THE MACHINE TOOL WORKPIECE SYSTEM IN BTA DEEP HOLE MACHINING Part II: Response Analysis and Evaluation of the Tool Tip Motion

ABSTRACT In this paper, the solutions to the stochastic differential equations, which mathematically represent the machine tool work-piece system in BTA deep hole machining are presented. The solutions to the parametric stochastic differential equations have been obtained using the well known averaging technique. The non-parametric inhomogeneous equations have been solved using the Fokker-Planck equation. Based on these solutions, the true motion of the tool tip has been described using the maximum, average and minimum deviation curves. These curves predict that helical grooves will be formed on the workpiece and such helical grooves were observed on the workpieces. Also, the maximum, average and minimum values of deviation of the tool tip which is a measure of the roundness error are established. Based on these results an upper bound for the roundness error as a function of depth of hole is derived. The measurement of roundness of the specimens reveals that the experimental values be in the zone predicte...

An experimental investigation into the influence of ZrN and TiN coatings on drills in drilling Hasteloy

SUMMARY This paper presents a comparison of the performance characteristics of titanium nitride (TiN) and zirconium nitride (ZrN) coatings on drills. The objective of this work was two-fold: to choose the best coating for machining tough materials tike Hasteloy; and to determine a method of predicting the wear of drills. A longer life of drills leads to increased productivity of drilling holes in certain components of environmental-control systems and fuel-control assemblies (in the aerospace industry) made of tough materials like Hasteloy. The comparison of the two coatings and tool wear was based on the measurement of static and dynamic forces and torques, power spectral density and acoustic-emission plots. The measurements were carried out using piezoelectric dynamometers and an acoustic-emission device. Upon analysis of the data, it was found that drills coated with TiN performed better than those coated with ZrN. It was also observed that the root-mean-square value of the acoustic-emission density an...

ON THE STOCHASTIC DESCRIPTION OF THE TOOL TIP MOTION AND ITS INFLUENCE ON THE SURFACE TEXTURE IN BTA DEEP HOLE MACHINING

Abstract This paper presents the solutions to the stochastic differential equations which mathematically represent the machine tool workpiece system in BTA deep hole machining. The stochastic differential equations are derived assuming an appropriate model for the tool-workpiece interaction, and the machine tool work-piece system. The solutions to the non-parametric inhomogeneous differential equations are obtained using the Fokker-Planck equations. Based on these solutions, the true motion of the tool tip has been described using the maximum, average, and minimum error curves. Also, using the statistical description of the true motion, equations are derived which mathematically describe the CLA and RMS value of the surface produced. Based on these results, it is seen that the CLA and RMS value of the given surface vary 1n a given band depending upon certain machining paraj meters. The measurement of the surface texture of the specimens shows that the experimentally measured CLA values agree reasonably we...

On the prediction of the roundness error in BTA deep hole machining based on the stochastic description of the cutting tool motion

This paper presents a methodology of predicting the maximum possible out-of-roundness of the hole produced, in BTA deep hole machining, as a function of certain machining parameters. Based on the solutions of the stochastic differential equations representing the machine tool-workpiece system in BTA deep hole machining, and the true cutting tool motion, an index describing the upper bound of the roundness error is defined. A parametric analysis of the out-of-roundness index is carried out. This analysis points out that at a low length-to-diameter ratio of the cutting tool, the axial force is the predominant factor causing the tool-tip deviation from the ideal motion, and that the radial and tangential forces are the major causes for the tool tip deviation at high length-to-diameter ratio of the cutting tool. The experimental measurement of out-of-roundness of the specimens under different machining conditions shows that the roundness error obtained lies within the zone described by the theoretical prediction.