Introduction Mechanical Energy – Machining Process
In mechanical energy methods, the material is removed by mechanical erosion of the workpiece material.
- Abrasive Jet Machining (AJM)
- Water Jet Machining (WJM)
- Ultrasonic Machining (USM)
ABRASIVE JET MACHINING (AJM) – Mechanical Energy
In abrasive jet machining process, a high speed stream of abrasive particles mixed with high pressure air or gas are injected through a nozzle on the workpiece to be machined. It is shown in the image.
CONSTRUCTION AND WORKING OF AJM – Machanical
It consists of mixing chamber, nozzle, pressure gauge, hopper, filter, compressor, vibrating device, regulator, Mechanical Energy, efc.
- The gases generally used in this process are nitrogen, carbon dioxide or compressed air.
- The various abrasive particles used in this process are aluminium oxide, silicon carbide, glass powder, dolomite and specially prepared sodium bicarbonate.
- Aluminium oxide is a general purpose abrasive and it is used in sizes of 10, 25 and 50 micron. Silicon carbide (SIC) is used for faster cutting on extremely hard materials. It is used in sizes of 25 and 50 microns. Dolomite of 200 grit size is found suitable for light cleaning and etching. Glass powder of diameter 0.30 to 0.60 mm are used for light polishing and deburring.
- As the nozzle is subjected to a great degree of abrasion wear, it is made up of hard materials such as tungsten carbide, synthetic sapphire (ceramic), etc., to reduce the wear rate.
- Nozzles made of tungsten carbide have an average life of 12 to 20 hours, whereas synthetic sapphire nozzle have an average life of 300 hours. Nozzle tip clearance from work is kept at a distance of 0.25 to 0.75 mm.
- The abrasive powder feed rate is controlled by the amplitude of the vibration of mixing chamber. A pressure regulator controls the gas or air flow and pressure.
Working : Mechanical Energy
- Dry air or gas (N2 or CO2) is entered into the compressor through a filter where the pressure of air or gas is increased.
- The pressure of the air varies from 2 kg / cm- to 8 kg / cm2.
- Compressed air or high pressure gas is supplied to the mixing chamber through a pipe line. This pipe line carries a pressure gauge and a regulator to control the air or gas flow and its pressure.
- The fine abrasive particles are collected in the hopper and fed into the mixing chamber. A regulator is incorporated in the line to control the flow of abrasive particles.
- The mixture of pressurised air and abrasive particles from mixing chamber flows into the nozzle the at a considerable speed.
- Nozzle is used to increase the speed of the abrasive and it is increased particles upto 300 m/s.
- This high speed stream of abrasive particles from the nozzle, impact the workpiece to be machined. Due to small chips of material repeated impacts, get loosened and a fresh surface is exposed.
- A vibrator is fixed at the bottom of the mixing chamber. When it vibrates, the amplitude of the vibrations controls the flow of abrasive particles.
- This process is widely used for machining hard and brittle materials, non-metallic materials (germanium, glass, ceramics and mica) of thin sections. This process is capable of performing drilling, cutting, deburring, etching and cleaning the surfaces.
- Abrasive Jet Machining (AJM) blasting process differs from sand process. AJM is basically meant for metal removal with the use of small abrasive blasting particles, whereas the sand involve process is a surface cleaning process which does not any metal cutting.
METAL REMOVAL RATE PROCESS PARAMETERS – MECHANICAL ENERGY
The metal removal rate depends upon the following parameters.
- Mass flow rate.
- Abrasive grain size.
- Gas pressure.
- Velocity of abrasive particles.
- Mixing ratio.
- Nozzle tip clearance.
Mass Flow Rate – Machining Process
At particular pressure, the material removal rate increases with the abrasive flow rate. But, after reaching an optimum value, the material removal rate decreases with further increase in abrasive flow rate. This is due to the fact that the mass flow rate of gas or air decreases with the increase of abrasive flow rate.
Abrasive Grain Size – Mechanical Energy
The various abrasive particles used in AJM process are aluminium oxide (A12O3), silicon carbide (SiC), glass powder, dolomite and specially prepared sodium bicarbonate. Aluminium oxide is a general purpose abrasive and is used in sizes of 10, 25 and 50 microns. Silicon carbide is used for faster cutting on extremely hard materials. It is used in sizes of 25 and 50 microns.
Dolomite of 200 grit size is found suitable for light cleaning and etching. Glass powder of 0.30 to 0.60 mm are used for light polishing and deburring. In general, larger sizes are used for rapid removal rate while smaller sizes are used for good surface finish and precision.
Nozzle Tip Clearance or Stand-off Distance
The distance between the nozzle tip and the workpiece has great influence on the diameter of cut, its shape, size and also on the rate of material removal. The material removal rate first increases with the increase of tip clearance from workpiece upto a certain limit after that it remains unchanged for a certain tip clearance and then decreases gradually as shown in image.
ADVANTAGES OF AJM
- This process is suitable for cutting all materials. Even diamond can be machined by using diamond as abrasive.
- There is no heat generation during this process. So, thermal damage to the workpiece is avoided.
- Very thin and brittle materials can be cut without any risk of breaking.
- There is no direct contact between the tool and workpiece.
- Low initial investment.
- Good surface finish.
- It can be used to cut intricate hole shapes in hard and brittle materials.
DISADVANTAGES OF AJM
- Material removal rate is slow.
- Soft material cannot be machined.
- Machining accuracy is poor.
- Nozzle wear rate is high.
- The abrasive powder used in this process cannot be reused because of decrease of cutting capacity and clogging if the nozzle due to contamination.
- There is always a danger of abrasive particles getting embedded in the workpiece. So, cleaning is essential after the operation.
- It requires some kind of dust collection system.
APPLICATION OF AJM
This process is widely used for,
- Machining of hard and brittle materials like quartz, ceramics, glass, sapphire, etc.
- Fine drilling and microwelding.
- Machining of semi-conductors.
- Machining of intricate profiles on hard and brittle materials.
- Cleaning and polishing of plastics, nylon and teflon components.
WATER JET MACHINING – MECHANICAL ENERGY
INTRODUCTION – Machining Process
Water Jet Machining (WJM) process is an extension of abrasive jet machining process. In this process, high pressure and high velocity stream of water is used to cut the relatively soft and non-metallic materials like paper boards, wood, plastics, rubber, fibre glass, leather, etc.
When the high velocity of water jet comes out of the nozzle and strikes the material, its kinetic energy is converted into pressure energy including high stresses in the work material. When this induced stress exceeds the ultimate shear stress of the material, small chips of the material get loosened and fresh surface is exposed.