ADVANCED MACHINING PROCESS

 What is Machining?

Machining is the process used to remove material, typically metal, to create parts for machines, tools, transportation, and more. Machine shops and machinists use equipment like lathes, mills, and drill presses to turn material into useful tools using precise cuts.

Fig no 1 Machining (source www.theengineerspost.com)

What is Advanced Machining?

Advanced machining processes are material-removing processes different from conventional machining processes, in which a well-guided wedge-shaped tool removes the material in the form of chips by producing contact stresses. There are a variety of ways in which material is removed using these processes. One method is producing stresses in the workpiece by different means but not with a well-guided wedge-shaped tool. There are several processes in this category, e.g., ultrasonic machining, water jet machining, abrasive jet machining, laser beam machining, and chemical machining.

  Fig no 2 Advanced Machining (source mechkatta.com)

Machining through Robots

Robot machining can be a game-changer when there is a requirement for more complex shapes in the manufacturing process. Sometimes, it gets very tough to achieve the desired geometry using traditional machines. On the other hand, in some applications it is not possible to use milling or turning as the part which has to be machined is too big to fit into the traditional machines.

After looking at the above situations, the best solution is to use industrial robots with more than 5 axes. Depending upon the application, a lot of rigidity may be required. While using the robot, one can omit the problem of complexity but will face the problem of accuracy. You want the robot to be as accurate as possible and the factors including backlash and robot warmup have to be regularly monitored and reoriented for best results. To make it more precise one can use vision system, it will help the robot to get to the right spot at the right time.

Visionary systems

For achieving best results in machining, the machine should be rigid and that is the key part. Using Robots for work poses some challenges, as the force applied is huge and it creates some kick back. This can result in compromise of accuracy. Hence, to omit those errors robots has to be repositioned exactly.

To achieve this, external vision systems have been developed, which will reposition the robot at the right spot when problems occur. The path of the robot for the operation is pre-programmed and the vision system helps the robot to remain to be on the desired path for better accuracy and precision.

Aerospace machining using robot milling

On an aircraft, rivets are used to fix the cabin to the aircraft frame. Earlier, this was performed manually using drills by the workers.  These days composites like titanium are used in the manufacturing of aircraft and it is nearly impossible to drill the hole manually. Manufacturers are now using advanced robots for performing the process with ease. Also, they are using a vision system to make it more precise.

Chemical Machining

Chemical Machining is the clean removal of metal from pre described areas without altering the integrity or properties of the metal by means of a photochemical process. This process is primarily used in creating small thin metal parts of complex design with no burns or stresses to the parts. 

Working Principle

• In this process, material is removed from the workpiece through a controlled etching or chemical attack of the workpiece material.

• Material can be material from selected area of workpiece or from the entire surface of the workpiece, according to requirement.

• If selective machining is desired, the areas of the workpiece which are not be machined are covered with a resistant material, called a resist or maskant.

• The workpiece to be machined is first cleaned in trichlorethylene vapour or in a solution of mild alkaline at 85 to 90⁰C, followed by washed in a clean water. This removes dust and oil from the workpiece.

• After cleaning, the workpiece is dried and coated with the maskant material.

• The workpiece is then immersed in a chemical reagent. So, chemical reaction takes place and the metal is removed from the workpiece.

Fig no 3 Chemical Machining (source mechcontent.com)

Application

Chemical machining process is applied in great number of usages where the depth of metal removal is critical to a few microns and the tolerances are close.

The major application of chemical machining is in the manufacture of burr free components.

Advantages 

• Burr-free components are produced.

• Most difficult to machine materials can be processed.

• High surface finish is obtained.

• Any metal can be machined.

Disadvantages

• Since the process is slow, metal removal rate is low.

• Manufacturing cost is high.

• Workpiece thickness, that can be machined, is limited.

• Large floor area is needed.

 Laser Beam Machining

A laser beam machining is a non-conventional machining method in which the operation is performed by laser light. The laser light has maximum temperature strikes on the workpiece, due to high temp the workpiece gets melts. The process used thermal energy to remove material from a metallic surface.

 Working Principle

In this process, the Laser Beam is called monochromatic light, which is made to focus on the workpiece to be machined by a lens to give extremely high energy density to melt and vaporize any material.

The Laser Crystal (Ruby) is in the form of a cylinder as shown in the above figure or Diagram with flat reflecting ends which are placed in a flash lamp coil of about 1000W.

The Flash is simulated with the high-intensity white light from Xenon. The Crystal gets excited and emits the laser beam which is focused on the workpiece by using the lens.

The beam produced is extremely narrow and can be focused to a pinpoint area with a power density of 1000 kW/cm². Which produces high heat and the portion of the metal is melted and vapourises.

 

Fig no 4 Laser Beam Machining (source eggkatta.com)

Application

• The laser beam machining process is used for making very small holes.

• Mass macro machining production.

• LBM is used in surgery.

• Selective heat treating of materials.

Advantage

• Any material can be machined including non-metal.

• The production rate is high.

• There is no direct contact between the tool and the work.

• There is no tool wear.

• No mechanical force on the work.

• The heat-affected zone is very small.

 Disadvantage

• The overall efficiency of Laser machining is very low.

• It is limited to thin sheets.

• The life of the flash lamp is short.

• It is not possible to remove a large number of metals.

• The machined holes are not round and straight.

Ultrasonic Machining 

Ultrasonic machining is an abrasive process which can create any material into hard and brittle form with the help of its vibrating tool and the indirect passage of abrasive particles towards the work piece. It is a low material removal rate machining process.

 

Working Principle

The tool present in the machine for cutting the materials is made from a soft material as compared to the work piece. The tool is usually made from materials such as soft steels and nickel. When the tool vibrates, the abrasive slurry (liquid) is added which contains abrasive grains and particles. The abrasive slurry is added till the work pieces interacts with the grains. Due to the particles of liquid added, the work brittleness of the work piece abrades the surface meanwhile the tool deforms gradually.

Fig no 5 Ultrasonic Machining (source Wikipedia)

Applications

•  Machining very precise and intricate shaped articles.

•  Drilling the round holes of any shape.

•  Grinding the brittle materials.

•  Profiling the holes.

•  Engraving. Trepaning and coining.

•  Threading.

•  Slicing and broaching hard materials.

Advantages

• Machined all sorts of hard materials

• Produces fine finished and structured results

• Produces less heat

• Various hole cut shapes due to vibratory motion of the tool  

Disadvantages

• Requires a higher degree of integrity and skills

• No certified record of radiography

• Unnecessary large grain sizes causes defects

Electro Discharge Machining

Electrical Discharge machining is the process of metal removal from the work surface due to an erosion of metal caused by electric spark discharge between the two electrodes tool (cathode) and the work (Anode).

Working Principle

It consists of an electric power supply, the dielectric medium, the tool, workpiece, and servo control.

The workpiece is connected to the positive terminal and the tool is connected to a negative terminal of the DC power supply.

An air gap of 0.005 to 0.05 mm is maintained between the tool and the work.

The die electric fluid which is non-conductor of electricity is forced under pressure through the gap.

When a DC power is supplied, the fluid in the gap gets ionized and produces a spark between the tool and workpiece, causing a local rise in temperature at about 1000 degrees Celsius, when melts the metal in a small area of the workpiece and vaporizes.

The DC supply generates a pulse between 40 to 3000 V and the frequency of spark at the rate of 10000 sparks per second can be achieved.

The electric and magnetic fields on heated metal cause a compressive force which removes the metal from the work surface.

Fig no 6 Electro Discharge Machining(source EngineeringClicks)

 

Applications 

• Drilling for micro holes in the nozzle.

• This is used in thread cutting.

• Used in wire cutting.

• Rotary form cutting.

• Helical profile milling.

• Curved hole drilling.

Advantages 

• It can be used for any hard material and even in the heat-treated condition.

• Any complicated shapes made on the tool can be reproduced.

• High accuracy of about 0.005 mm can be achieved.

• Good surface finish can be achieved economically up to 0.2 microns.

• Machining time is less than the conventional machining process.

Disadvantages 

• Excessive tool wear.

• High power consumption.

• The sharp corner cannot be reproduced.

• High heat developing causing the change in metallurgical properties of materials.

• The workpiece must be an electrical conductor.

• Surface cracking may take place in some materials.

• Redressing of a tool is required for deep holes.

• Over-cut is formed.

• Difficult finding expert machinists.