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ROOHUL AMIN

REG NO: A1EWT-216005

SUBJECT: “PRODUCTION AUTOMATION”

PROGRAMM: B.TECH (Mech) 4 YEAR (WEEKEND)

SEMESTER: SUMMER 2016

PIMSAT KARACHI

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Machining

Definition: -

Machining is one of various processes in which a piece of raw material is cut into a desired final shape and size by a controlled material-removal process.

Explanation: -

Machining is a part of the manufacture of many metal products, but it can also be used on materials such as wood, plastic, ceramic, and composites. A person who specializes in machining is called a machinist. A room, building, or company where machining is done is called a machine shop. Machining can be a business, a hobby, or both. Much of modern day machining is carried out by computer numerical control (CNC), in which computers are used to control the movement and operation of the mills, lathes, and other cutting machines

Machining operations

There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture.

The three principal machining processes are classified as turning, drilling and milling. Other operations falling into miscellaneous categories include shaping, planing, boring, broaching and sawing.

1. Turning operations: - Turning operations are operations that rotate the workpiece as the primary method of moving metal against the cutting tool. Lathes are the principal machine tool used in turning. In turning, a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a cylindrical shape. The primary motion is provided by rotating the workpiece, and the feed motion is achieved by moving the cutting tool slowly in a direction parallel to the axis of rotation of the workpiece.

Assignment no.03“Types of machining operations

used for manufacturing metal and plastic parts”

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2. Milling operations: - Milling operations are operations in which the cutting tool rotates to bring cutting edges to bear against the workpiece. Milling machines are the principal machine tool used in milling. In milling, a rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool's axis of rotation. The speed motion is provided by the rotating milling cutter. The two basic forms of milling are:

a) Peripheral millingb) Face milling.

3. Drilling operations: - Drilling operations are operations in which holes are produced or refined by bringing a rotating cutter with cutting edges at the lower extremity into contact with the workpiece. Drilling is used to create a round hole. It is accomplished by a rotating tool that typically has two or four helical cutting edges. The tool is fed in a direction parallel to its axis of rotation into the workpiece to form the round hole. Drilling operations are done primarily in drill presses but sometimes on lathes or mills. In boring, a tool with a single bent pointed tip is advanced into a roughly made hole in a spinning workpiece to slightly enlarge the hole and improve its accuracy. It is a fine finishing operation used in the final stages of product manufacture.

4. Miscellaneous operations: - Miscellaneous operations are operations that strictly speaking may not be machining operations in that they may not be swarf producing operations but these operations are performed at a typical machine tool. Burnishing is an example of a miscellaneous operation. Burnishing produces no swarf but can be performed at a lathe, mill, or drill press.Reaming is one of the sizing operations that remove a small amount of metal from a hole that already drilled. Other conventional machining operations include shaping, planing, broaching and sawing. Also, grinding and similar abrasive operations are often included within the category of machining.

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Stages in metal cutting

Machining operations usually divide into two categories, distinguished by purpose and cutting conditions:

Roughing cuts Finishing cuts

Roughing cuts are used to remove large amount of material from the starting work part as rapidly as possible, i.e. with a large Material Removal Rate (MRR), in order to produce a shape close to the desired form, but leaving some material on the piece for a subsequent finishing operation. Finishing cuts are used to complete the part and achieve the final dimension, tolerances, and surface finish. In production machining jobs, one or more roughing cuts are usually performed on the work, followed by one or two finishing cuts. Roughing operations are done at high feeds and depths

While finishing operations are carried out at low feeds and depths.

“Cutting speeds are lower in roughing than in finishing.”

A cutting fluid is often applied to the machining operation to cool and lubricate the cutting tool. Determining whether a cutting fluid should be used and if so choosing the proper cutting fluid, is usually included within the scope of cutting condition. Today other forms of metal cutting are becoming increasingly popular. An example of this is water jet cutting. Water jet cutting involves pressurized water in excess of 620 MPa (90 000 psi) and is able to cut metal and have a finished product. This process is called cold cutting, which eliminates the damage caused by a heat affected zone, as opposed to laser and plasma cutting.

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Plastics Machining

Plastic parts are produced by various methods with the primary types being:

1. Injection molding2. Thermoforming3. Blow molding

After manufacturing, plastic parts may also be machined to final specifications, and assembled to other plastic components. Plastics Machining processes are sometimes used as secondary operations on molded and formed parts, but are most commonly used in the fabrication of flat plastic sheet, or rod and bar stock into finished products.

Differences between machining plastics and metals

There are several key differences between machining plastics and metals:

a. Plastics exhibit greater thermal expansion, up to 10 times more than that of metals

b. Plastics dissipate heat at a slower rate than metals c. Plastics have more elasticity than metals d. Plastics have lower melting temperatures than metals.

These differences require changes in both cutting tool speeds and feed rates. Cutting tool materials also affect the machining of plastics. High speed steel cutting tools can be used in short run production, but tungsten carbide or diamond tipped tooling are required for higher production rates.

The range of plastic materials responds to machining differently and is typically divided into three main categories:

a. Soft plastics: -such as polyethylene and polypropylene, which will produce long curly chips when machined

b. Hard plastics:- including Acrylonitrile Butadiene Styrene, or 'ABS' and polycarbonate, which result in short crystal like shards when machined

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c. Reinforced plastics: - which contain abrasive fibers requiring very sharp tooling of specific geometries for machining Because of heat buildup both in part and in the tool bit, coolants are often used to control temperatures.

Coolant mediums can include: -

1. Clean compressed air2. Mist sprays3. Water soluble oils4. Light cutting

Careful selection of a coolant is needed to avoid any adverse reactions with the plastic being machined.

Machining Processes: - The primary types of processes used to machine plastics include:

1. Sawing2. Milling3. Routing4. Drilling 5. Turning 6. Water jet Cutting 7. Laser Cutting

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Polishing

The most common plastic polishing methods are vapor polishing, flame polishing, and mechanical buffing. Each process is best suited for particular configurations and materials.

Types of polishing1) Flame polishing uses a hot flame to flow a surface. Operator skill is critical

with this method. When done properly, flame polishing produces the clearest finish, especially on acrylics. Flat external surfaces are the most applicable configuration for this method.

2) Vapor polishing utilizes a chemical vapor which attacks the surface of the plastic, causing it to flow. The best plastics for this process are polycarbonate, Ultem, and poly sulfonic. Acrylics do respond to this method, though not quite as well. The most appropriate acrylic polishing technique depends on the configuration of the component. Vapor polishing is not for an inexperienced person as special equipment is needed in order to avoid vapor contact with the operator. However, when done properly, it can provide absolute optical quality finishes. This method can be used for both internal and external features, including threads and drill holes.

3) Mechanical polishing is the most common, easiest to do, and is useful on any plastic. This method tends to leave very fine scratches on the surface and while it will usually look nice, it is not an acceptable polishing method for optical work. In addition, buffing would degrade the surfaces of the two other methods above.

Annealing of Plastics Annealing, or stress relieving, of plastics is an important step in the

plastic machining process. While not all components require annealing, it can provide important advantages for some materials and part configurations.

Many manufacturers believe that their own internal annealing process is sufficient. However, in practice, batch processing can yield inconsistent results.

In addition, post-machining annealing can improve the properties of some components, especially those in the harder materials. These materials are sensitive to stress cracking, accumulated from the rod stock or precision machining. To achieve the lowest possible stress and the most stable component, consider stress relieving after machining. This operation is a must post plastic polishing. Plastic components polished without annealing will likely crack in operation.