Non-Conventional Methods of Machining | Industries | Engineering

In this article we will discuss about the non-conventional methods of machining used in industries. The methods are: 1. Elastic Emission Machining (EEM) 2. Electro-Chemical Grinding (ECG) 3. Hot Machining 4. Electron Beam Machining (EBM).

Method # 1. Elastic Emission Machining (EEM):

In elastic emission machining process, machining is carried out by a spinning polyuera-thane ball in a suspended liquid medium of fine abrasives (0.1 to 0.01 µm in size). When ball approaches the workpiece surface, collision with abrasive takes place within an area of 1—2 mm diameter and amount of material removal depends on the machining time.

Since the collision of fine particles causes elastic failures of the interatomic bonds at the surface, EEM produces a highly finished surface without a work-affected layer. Further since the machining unit (minimum controllable amount of material that can be removed) in EEM is 0.01 µm or less, it can be called atomic unit material processing.

The energy impinges on an area much smaller than the stress fields created by grinding, producing elastic failures at the atomic scale without crystal dislocations or cracks. Surface produced has very little surface alterations.

Processes similar to EEM include hydro-dynamic polishing, and hydroplane polishing.

Hydrodynamic Polishing:

It is similar to EEM. The hydrodynamic pressure lifts up the workpiece to avoid contact with the tool, while the liquid-suspended fine particles collide with the workpiece surface at near-horizontal angles, achieving a high degree of anisotropy (Anisotropy is the property of the finished surface to progress in only one direction).

Method # 2. Electro-Chemical Grinding (ECG):

Based on electro-chemical machining process and refinement to it, the process of electrolytic grinding has been developed in which metal is removed both by electro-chemical decomposition (about 90%) and abrasion of the metal (about 10%). Thus the wheel wear is very less. In this process a metal disc or electrode wheel is revolved in close proximity to the workpiece.

The wheel employed is made of fine diamond particles in metal matrix. The particles are slightly projecting out from the surface and come in contact with work surface with very little pressure. The work is connected to positive terminal of battery and wheel to negative and thus current flows between the work and wheel. The wheel and its spindle are insulated from the rest of the machine.

The short-circuit between the wheel and work is prevented due to point contact made by the fine diamond points. During grinding process, a continuous stream of non-corrosive salt solution is passed through work and tool and it acts both as electrolyte and coolant.

The electrolyte is entrapped in the small cavities between the projecting diamond forming electrolytic cells. When these cells come in contact with the work the current flows from the wheel to work, it leads to chemical decomposition of work. Cutting fluid, which acts as electrolyte is a cheap-corrosive alkaline solution.

This process is best suited for very precision grinding of hard metals like tungsten carbide tool tips as the grinding pressure is very less and the temperature is very low due to which the defects like grinding cracks, tempering of work, transformation of layers and dimension control difficulties are eliminated.

Accuracy of the order of 0.01 mm can be achieved by proper selection of wheel grit size and abrasive particles. Surface finish of 0.05 to 1 µm CLA is possible. Metal removal rate is very low (of the order of 15 mm³ per sec) and power consumption is high.

Electro-Chemical Turning:

The machine has motion of lathe and metal removal tool is a cathode which is separated from the rotating work surface (anode) by a film of electrolyte. A suitably shaped tool can produce a desired form on a hard metal in a very short time.

Method # 3. Hot Machining:

Hot machining is employed for machining high strength, high hardness and high temperature resistant materials which are difficult to machine at room temperature. Machining of hard metals at elevated temperatures is applied mainly to turning and milling operations.

Since the shear strength of metal decreases at elevated temperature as compared to that at room temperature, the magnitude of cutting forces on the tool is lower. Further as the chip formation by plastic deformation in the shear plane ahead of tool becomes easier at elevated temperature and the cutting forces involved are less, therefore, power requirements are low.

But as the property of tool material at elevated temperature is also changed due to its being in contact with high temperature material, therefore, tool life is also affected. It is found that tool life is maximum for certain temperature of workpiece (for particular work material and tool material) at which total metal removal rate per tool grinding will be maximum, irrespective of the speed.

It is observed that for mild steel and carbide, the maximum material is removed at 260°C for all the speeds.

Thus the factors which mainly determine how readily a workpiece material may be machined are:

i. Tool life,

ii. Tool forces,

iii. Power consumption,

iv. Metal removal rate,

v. Accuracy,

vi. Finish obtainable.

It may be generally summarised that for a given tool life, the metal removal rate may be increased upto 400 per cent at elevated temperature and the tool life is maximum for a particular temperature.

However, strict dimension control has to be observed as dimensions at elevated temperature will be different and the requirements are corresponding to room temperature.

Methods of heating the workpiece vary according to the type of operation, workpiece material and the possibilities of thermal damage. It is desirable that only a small zone which has to be cut by the tool be heated and preheating of the whole piece should be avoided.

The various methods of heating the workpiece are:

(i) Simple heating,

(ii) Flame heating,

(iii) Induction heating,

(iv) Arc heating.

The best way out of all these is induction heating method in which maximum cleanliness can be maintained; is very easy to control and convenient also. However, it is costly method, but most efficient one as it heats only a small zone ahead of the tool.

Method # 4. Electron Beam Machining (EBM):

In this process, the material is removed with the help of a high velocity (travelling at half the speed of light, i.e. 160,000 km/sec.) focussed stream of electrons which are focussed magnetically upon a very small area. These heat and raise the temperature locally above melting point, and the process thus does not depend on heating of the material to the point of evaporation.

The sputtering coefficient, i.e. number of atoms or molecules removed for each ingoing ion increases with the atomic weight of the bombarding ion, and with increase of incidence, the highest yield being obtainable by the grazing incidence. It finds uses for deposition of films in electronics industry.

In typical apparatus, the material to be deposited is made the anode in a low-pressure argon or other rare gas atmosphere. Gaseous ions bombard the cathode, sputtering its material on the substrate. It can also be used for etching circuit patterns on integrated circuit substrates.

This method using d.c. power source is simpler and less expensive but limited for etching conductive materials only. For dielectrics, more costly radio-frequency equipment is required. Its advantages over chemical etching are that it provides better resolution, since undercutting is eliminated and there is no need to rely on powerful etchants that can propagate along cracks and possibly degrade photo resist mark.

Because of small metal removal rate, tolerances of the order of 0.005 µm can be attained. However etch rates are easily controlled. It is a relatively expensive process; etch rate are slow, and thermal radiation damage may occur in some materials.