May 24

Definition of Jig

Jig is a device that firmly holds and locates work piece and also guides cutting tool during machining.

Elements of Jig


Jig body

This is also called base. It supports work piece by locators and clamps. The base is fitted with four feet which rest on the machine table.

Jig feet

Jig has four feet at the bottom. Jig rests on machine table only with these feet. This increases accuracy. Jig feet are welded or cast with jig body. Sometimes, feet are press-fitted. They are usually hardened and ground to flat surface. In some cases jig feet is not present.

Drill bush or latch

Drill bush guides tools like drills and reamers during machining. Bushes are fixed in jig plates or bush plates. These bushes are made in hardened steel. When the bush wears out, it can be replaced with a new one. There will be one bush for each hole to be drilled. The size of bush depends on the size of the hole to be drilled.

Bush plate or Jig plate

Bushes are fitted to the bush plate. Bush plate may be a rigid one or may be of hinged type. Hinged type plate is called leaf or latch. This can be swung open to load as well as to un-load work piece. It is swung again for closing.


Locators help the work piece, to rest in proper position, with reference to tools. Therefore holes are drilled accurately in required position. Locators are fitted in these holes. Depending upon the work piece shape, various locators are used. Usually locators are detachable type. They are fixed to the jig body or frame. Worn-out locators can be replaced by new ones.

Clamps or knob

Clamps are used for holding the work piece rigidly in its position; they also keep work piece firmly in contact with locating pins. Clamps should be thick enough so that they do not bend, while clamping,. Generally, surface of clamps are case hardened.

Fool proof element

This element prevents loading of work piece in the wrong position. That is, this element makes sure that work piece is correctly loaded and positioned. Fouling pegs cross pieces or pins are used as fool proof element.


Advantages of Jigs

· Reduce manufacturing time-Marking out; measuring and setting the work piece are not required. This saves time.

· Increase dimensional accuracy: Work piece is located automatically; Tool is properly guided in machining.

· Identical parts are produced easily: Parts of same size and shape are interchangeable. Therefore assembly operations can be done without any difficulty.

· Increase rate of production:

Many work pieces are machined in single setup.

Many tools cut work piece at the same time.

· Work handling time is reduced-This is due to quick setting and locating of work piece.

· Metal removal rate is increased.

Work piece is rigidly clamped by jigs. Therefore speed, feed and depth of cut can be increased for machining. That is, more metal is removed in lesser time.

· Operator fatigue is reduced.

Work handling operations such as marking, centering, clamping are very much reduced. Hence operator’s tiredness reduced.

· Semi-skilled operator is adequate, because setting of tool and work are simpler. Salary to semi-skilled labour is lesser than skilled labour.

· Quality control cost is less.

· Overall cost of production is reduced.

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May 24

Jet propulsion

Jet propulsion is based on newton’s second law and third law of motion. Newton’s second law states that the rate of change of momentum in any direction is proportional to the force acting in that direction. Newton’s third law of motion states that for every action there is an equal and opposite reaction. The types of jet engines are explain below.

Turbo-propeller engines

This is also called as turbo-prop engine. It differs from the turbo-jet in that, it uses a propeller to increase the mass flow of air. In this type, the turbine drives the compressor and propeller.


Air from atmosphere is taken into the compressor through diffuser. In the diffuser, the air is slowed down and compressed. The compressed air from the compressor enters the combustion chamber where the fuel is sprayed. Fuel-air mixture is burnt and thus heat is supplied at constant pressure. The combustion products are then expanded partially in the turbine. The combustion products are then expanded partially in the turbine. The exhaust gases from the turbine are then expanded in the exit nozzle. In the nozzle pressure energy is converted into kinetic energy. Thus the gases emerge from the unit with very high velocity. This increased velocity produces a reaction or thrust in the opposite direction. The total thrust produced in the sum of the thrusts produced by the nozzle and the propeller. This total thrust propels the air craft.

The angular velocity of the shaft is very high. The propeller cannot run at a higher angular velocity. Therefore, a reduction gear box is provided before the power is transmitted to the propeller. In some cases, two turbines are used to drive the compressor and propeller separately.

Advantages of turbo-prop engines

· Low specific weight and less frontal area.

· Simple in construction.

· Lesser vibration and noise. Easy maintenance.

· High power for take-off and climb of the propeller.

· High propulsive efficiency at speeds below 800 km\hr.

Ram-jet engine

A ram-jet is the simplest of all propulsive devices. Compressor and turbine are not required for ram-jet units. It consists of three main components namely diffuser system, combustion chamber and exhaust nozzle.


The diffuser consists of two parts namely supersonic diffuser and subsonic diffuser. The atmospheric air enters the unit with supersonic velocity and slowed down to the sonic velocity in the supersonic diffuser. This produces shock waves and increase in pressure. Then the air passes through the subsonic diffuser and its velocity further reduces to subsonic value. Due to this, the pressure of air increases to ignition pressure. Thus, the compression of air to the ignition pressure takes place only by ram effect in the diffuser. Then the high pressure air enters the combustion chamber. Fuel is injected in the combustion chamber and the air-fuel mixture is burnt. The hot products of combustion are then expanded in the exit nozzle. In the nozzle pressure energy is converted into kinetic energy. This increased velocity of exhaust gases produces an opposite thrust. The aircraft is propelled by this thrust. The ram-jet operates at very high speed.


· No moving parts.

· Light wright when compared with turbo-jet engine.

· More thrust than a turbo-jet

· Unlimited flight speed.

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