Non Conventional Machine

Module 9 Non-conventional machining Version 2 ME, IIT Kharagpur Lesson 36 Ultrasonic Machining (USM) Version 2 ME, IIT Kharagpur Instructional Objectives i. ii. iii. iv. Describe the basic mechanism of material removal in USM Identify the process parameters of USM Identify the machining characteristics of USM Analyse the effect of process parameters on material removal rate (MRR) v. Develop mathematical model relating MRR with USM parameters vi. Draw variation in MRR with different process parameters vii. Identify major components of USM equipment viii. State the working principle of USM equipment ix.Draw schematically the USM equipment x. List three applications of USM xi. List three limitations of USM 1. Introduction Ultrasonic machining is a non-traditional machining process. USM is grouped under the mechanical group NTM processes. Fig. 9. 2. 1 briefly depicts the USM process. Force, F Slurry of abrasive and water Horn Vibration frequency f ~ 19 – 25 kHz Amplitude, a ~ 10 à ¢â‚¬â€œ 50 ? m Tool Work Fig. 9. 2. 1 The USM process In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 ? over the workpiece. Generally the tool is pressed downward with a feed force, F. Between the tool and workpiece, the machining zone is flooded with hard abrasive particles generally in the form of a water based slurry. As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material is brittle, due to crack initiation, propagation and brittle fracture of the Version 2 ME, IIT Kharagpur aterial. Hence, USM is mainly used for machining brittle materials {which are poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-discharge machining (ECM and ED)}. 2. Mechanisms of Mater ial Removal in USM and its modelling As has been mentioned earlier, USM is generally used for machining brittle work material. Material removal primarily occurs due to the indentation of the hard abrasive grits on the brittle work material. As the tool vibrates, it leads to indentation of the abrasive grits.During indentation, due to Hertzian contact stresses, cracks would develop just below the contact site, then as indentation progresses the cracks would propagate due to increase in stress and ultimately lead to brittle fracture of the work material under each individual interaction site between the abrasive grits and the workpiece. The tool material should be such that indentation by the abrasive grits does not lead to brittle failure. Thus the tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys.Other than this brittle failure of the work material due to indentation some material removal may occur due to free flowing impact of the abrasives against the work material and related solid-solid impact erosion, but it is estimated to be rather insignificant. Thus, in the current model, material removal would be assumed to take place only due to impact of abrasives between tool and workpiece, followed by indentation and brittle fracture of the workpiece. The model does consider the deformation of the tool.In the current model, all the abrasives are considered to be identical in shape and size. An abrasive particle is considered to be spherical but with local spherical bulges as shown in Fig. 9. 2. 2. The abrasive particles are characterised by the average grit diameter, dg. It is further assumed that the local spherical bulges have a uniform diameter, db and which is related to the grit diameter by db = ? dg2. Thus an abrasive is characterised by ? and dg. db db db db dg Fig. 9. 2. 2 Schematic representation of abrasive grit Version 2 ME, IIT KharagpurDuring indentation by the abrasive grit onto the w orkpiece and the tool, the local spherical bulges contact the surfaces and the indentation process is characterised by db rather than by dg. Fig. 9. 2. 3 shows the interaction between the abrasive grit and the workpiece and tool. Tool db abrasive grit db Work A B db 2x C D ?w Hemispherical material removed due to brittle Fig. 9. 2. 3 Interaction between grit and workpiece and tool As the indentation proceeds, the contact zone between the abrasive grit and workpiece is established and the same grows.The contact zone is circular in nature and is characterised by its diameter ‘2x’. At full indentation, the indentation depth in the work material is characterised by ? w. Due to the indentation, as the work material is brittle, brittle fracture takes place leading to hemi-spherical fracture of diameter ‘2x’ under the contact zone. Therefore material removal per abrasive grit is given as 2 ? w = ? x 3 3 Now from Fig. 9. 2. 3 AB 2 = AC 2 + BC 2 ? db ? ?d ? ? ? = ? b ? ? w ? + x2 ? 2 ? ? 2 ? 2 x = db? w neglecting ? w2 as ? w