NON-CONVENTIONAL MACHINES

1 Introduction

In conventional machining, there must be direct contact between the tool and the work material. The non-conventional machines employ the modern and latest technology in processing. In these machining methods, no direct contact between the machine tools and the workpiece.  The tools used are infrared beam, laser beam, electric arc, plasma cutting, and electric beam.

The non-conventional machining methods are generally used when traditional methods are not technically or economically feasible. Some examples for this are:

• Machining of tough and very hard materials
• Machining of complex shapes
• High accuracy and surface finish requirements
• Low cutting forces or clamping forces requirements

2 Types of Non-Conventional Machines

The different types of non-conventional machines are listed below.

1 Electrical discharge machining (EDM)

2 Electron beam machining (EBM)

3 Plasma arc machining, (PAM)

4 Laser beam machining (LBM)

5 Ion-Beam machining (IBM)

6 Ultrasonic machining (USM)

7 Abrasive jet machining (AJM)

8 Water jet cutting (WJC)

9 Electrochemical machining (ECM)

10 Electrochemical grinding (ECG) and

11 Chemical machining (CHM)

The non-conventional machining methods can be classified according to the source of energy used for removing metals from the workpiece: Mechanical, Thermal, Chemical, and Electrochemical.

Mechanical: The material is removed from the workpiece by the high-velocity stream of abrasives or fluids or both. Mechanical energy is used in USM, AJM, and WJC non-conventional methods.

Thermal: The thermal energy is applied to a very small portion of the work surface, causing that portion to be removed by fusion or vaporization of the material. In this process, electrical energy is converted into thermal energy. The thermal energy is used in EDM, EBM, PAM, LBM, and IBM non-conventional methods.

Electrochemical and chemical process:  The metals are susceptible to chemical attack by certain acids or other etchants. In chemical machining, chemicals selectively remove material from portions of the workpiece, while other portions of the surface are protected by a mask. The chemical energy is used in ECM, ECG, and CHM nonconventional methods.

In this blog, the applications and limitations of all non-conventional machines and the working of the widely used non-conventional machines are discussed.

3. Working of Non-Conventional Machines

i Abrasive Jet Machining

       In this process, a small layer of material is removed from the workpiece using abrasive particles of aluminium oxide, silicon carbide, or glass powder with particle size 10 to 50 microns. The larger size particles used for rough machining and smaller size for finish work.

In AJM the compressed air or high-pressure gas is supplied to the mixing chamber at a pressure of 2 to 8 kg/cm2. In mixing chamfer abrasive particles from the feeder and high-pressure gas are mixed. A vibrator is used to maintain the required amplitude in the mixing chamber to control the flow of abrasive. These abrasives gas mixer finally pass through the nozzle at very high speed (150 to 300 m/min). Since nozzles are subjected to very high abrasion wear, they are made of hard materials such as tungsten carbide or ceramic. The high-speed abrasive particles strike the workpiece surface, where the material is to be removed. Due to the repeated impact, a small bit of material is separated from that surface. The metal removal rate in AJM depends on the nozzle diameter, the composition of the abrasive gas mixture, the hardness of abrasives, velocity of jet, distance of workpiece from the jet, and the workpiece material. The main parts of the AJM are shown below.

ii Electrical discharge machining (EDM)

Electrical discharge machining (EDM) is also known as spark-erosion or electro-erosion machining. The main parts of the electrical discharge machine are shown below.

In this process, an interrupted repetitive electric spark discharged from the tool (cathode) to the workpiece (anode) through a spark gap (0.005 to 0.05 mm). Suitable dielectric slurry, which is a non-conductor of electricity, is forced through a spark gap at a pressure of 2kg/cm². When the proper voltage (50 to 450v) is applied, the electrons emitted from the cathode (tool) and the gap is ionized. The collection of more electrons in the gap results in the resistance drops, causing an electric spark to jump between the workpiece and the tool. Each electric spark causes a focused stream of electrons to move with very high velocity from the cathode towards the anode. The collision of spark with the work results in the generation of compression shock waves on high spots of workpiece closest to the tool. This consequently develops a local rise in temperature of 10,000°C is sufficient to melt a part of the workpiece metal. The spark creates the electric and magnetic force produces a tensile force, which tears off the materials from the high spots of the workpiece.

iii Chemical Machining (CMM)

              In chemical machining, the material is removed from the workpiece through the controlled chemical attack with acid or alkalies. The area of the workpiece from where the material is not to be removed is protected by etchant resistance material. The workplace to be machined is cleaned from the dust and oil, by dipping in a solution of mild alkaline. After cleaning, the workpiece is dried and later coated with the etchant resistant material (called maskant). The coating is applied by brushing, dipping, or spraying (up to 0.2 mm thickness). When this has dried, the desired shape to be processed on the workpiece is cut on the maskant with a scribing knife.   The workpiece is later dipped completely in a tank of chemicals that dissolve away the exposed metal. Accuracy in the range of 0.01 mm can be achieved on relatively shallow depths of cut with a surface finish of about 5 microns. The chemical machining process is shown below.

4 Limitations

• Very slow process
• High machining cost
• Not suitable for every type of material
• Skilled operator required
• Mostly use for producing prototype parts.

5 Applications                     

All non-conventional machining methods are not suitable for machining of all types of materials. The methods that can be suitable for the machining of materials are given below.

The applications of non-conventional machining methods are given below

6 Conclusions

The non-traditional machining methods are effectively used in machining of hard materials like ceramics and composites or machining under very tight tolerances. Therefore for understanding the non-conventional machining methods, the classification, applications, limitations and the working of the widely used non-conventional machining methods are discussed in this blog.