SPOT WELDING

Write the definition of welding

Spot welding is a pressure resistance welding in which overlapping sheets are joined by local fusion at one or more spots. Heat is produced by electrodes placed above and below the workpiece.

Write the principle of resistance welding.i.e. heat is generated because of resistance to the flow of elect current through the workpiece.

PROCEDURE:
1. Job should be cleaned from grease, dirt, paint, scale, rust, oxides etc.
2. The tip of electrodes must be cleaned by emery paper regularly.
3. To avoid the electrodes getting overheated, water is kept running for cooling.
4. Proper welding current is to be set on current selector switch.
5. Proper time has to be set on timer.
6. Electrodes are brought against the overlapping surfaces of workpiece and small pressure is applied for just physical contact.
7. Welding current is switched on for proper time.
8. As current passes, a small spot area is heated and weld takes place. TEMP is about 815 to 930 degree centigrade.
9. Now current is cut off and extra force/ pressure is applied to electrodes' tips. Hold until the metal cools down and gains strength.
10. After that electrode pressure is released to remove the spot welded piece.

Draw figure and write advantages & disadvantages.
Write application/ uses of spot welding.

Instructions

Dear students,
I have put almost everything on this blog for yr preparation for examination.Moreover I would like to mention that:
-Gas welding is on other blog " mp coursematerial".
-There may be some spelling mistakes, because I could not do it till date.

FORMING

This is a primary shaping operation i.e. attaining the required form of a job by pressing, forging, rollong, spinning, extrusion, piercing, blanking, punching, drawing, squeezing, bending, coining, sizing etc,
Kindling refer sheet metal processes in MP COURSE MATERIAL on blog or site.
MECHANICAL WORKING OF METALS:
It is shaping of metals(plastic deformation) by means of external force.
Mechanical working may be either hot or cold working.
Hot Working: The working of metals above the recrystalline temperature but below the burning point.Some metals such as lead and tin have low recrystalline range so can be worked at room temp, but most of the metals have high recrystalline temp.
Write advantages and disadvantages.
Various hot workinh processes are: Hot rolling, hot forging, hot spinning, hot extrusion, hot drawing or cupping hot piercing etc.
Kindly go through these hot working processes-explained on site.
Cold Working: The working of metal below the recrystalline temp and deformation is achieved by slip of planes.Force required is more as compared to hot working.
NB: During cold working strength and hardness of metal increases, but stresses are set, which is annealed,if to be further worked to remove the hardness and residual stresses.
Write some advantages and disadvantages.
Cold working processes are- Sheet metal work, shearing, drawing squeezing, bending cold heading, rotary swaging, coining, sizing, spinning, extrusion, cold drawing, bending, streach forming, hobbing, embossing, staking, ironing, peening,etc.

Dictated material

Dear students,
Kindly go through the material, which I have dicdated in my lectures as follows:
-Types of production systems.
-Classification of Manufacturing Processes.
-MP coursematerial on blog and site too,e.g.industrial safety, first aid and sheet metal etc.
-Chart from input of BF to output as Pig Iron and then CI, WI and STEEL in furnaces
(Cupola, Puddling and Bessemer converter, EAF & open hearth).
-Carbon % in PI, CI, WI and Steels (MS, MCS, HCS, HSS and 18:4:1)
-Moulding Sand(classification and preparation of moulding sand)
-CORE and preparation of core sand.
-Riser, Runner and Gating system
-Differance between Welding, Braze welding, Brazing and Soldering.

Example for answering a welding question.

TIG WELDING:
-write the definition of welding.
-TIG welding or Gas Tungsten Arc Welding(GTAW) is a non-pressure arc welding.
-Write principle of arc welding.
-Draw figure.
Welding heat is produced from an electric arc established between the job and bare non-consumable tungsten electrode.A shielded gas(argon, heliumn nitrogen etc) is used as flux.Filler metal if required is fed separately.
ARGON: It requires a lower arc voltage, easier arc starting and provides a smooth arc action.It is economical and heaviest.
HELIUM: Higher arc voltage, hence is used where higher heat input is required.
Sometimes active carbon dioxide is used more economical and requires higher current.Used normally with only DC with +ve electrode.
Argon and helium are used for both AC and DC power sources.
Welding operation is done by striking an arc between the workpiece and tungsten electrode mounted on a special holder.Inert gas is used as flux.The arc is allowed to impinge on the job and a weld pool is created.The welding torch and filler metal are generally kept inclined at angles 70-80 degree and 10-20 degree respectively with the flat workpiece.
-Write advantages and disadvantages.
-Write applications
-Finally precautions.

Welding contd.

Types of electrodes: bare, coated, consumable and non-consumable.
Consumable electrodes: Molen metal from the electrodes and base metal are mixed under the arc provides necessary joint.Electrode is serving the purpose of filler rod as wellas input for heat.
Non consumable electrodes: not consumed during welding.Separate filler rod is used if necessary to fill the gap.These are of carbon & graphite for DC welding.
Tungsten electrodes are used for DC & AC welding and have better control over the heat input because sources are separate.Used in carbon arc & TIG welding.
Bare electrodes are without coating of flux and flux is used separately as non-consumable electrodes.The striking of arc is difficult in such electrodes especially with AC supply.
Coated electrodes are coated with flux and called STICK electrodes.The problem is moisture picked-up by yhe flux and dissociates into oxygen and hydrogen and being absorbed by the liquid metaland released during solidification causing porosity and so advised to keep the electrodes in a dry place.

FLUX:During welding, when metal is heated in air, the oxygen from air combines the metal and forms oxides,which results poor quality, low strength weld and even some cases welding becomes impossible. To protect the weld pool from oxygen of ambient air,flux is used and does not allow the air to come in contact with fused metal.
Thus fluxes may be divided into the following categories.
Gas Forming: Components are organic matters such as starch,wood, pulp etc, which form gas layer, thus isolating the welding zone from the ambient air.
Slag Forming: The slag forming components are chinaclay, felspar, manganese and titanium ores etc and enhance slag formation.Slag covering the molten metal to come in contact with air.

RESISTANCE WELDING

This is a process of welding in which heat is obtained from resistance of the work to the flow of current and then applying pressure. No filler metal or flux is added.
PRINCIPLE OF RESISTANCE WELDING:
In this welding,a low voltage(typically 1 volt) and very high current(typically 15000 A)is passed to the point for a very short time(typically 0.25 sec). This high ampere current heats the joint.The following are the types of resistance welding.
-Spot welding
-Seam welding
-Projection welding
-Resistance butt welding
-Flash butt welding
-Percussion welding
Electrodes for resistance welding: Since electrodes carry large amount of current/ pressure and also help to remove the heat from weld zone. Thus to prevent overheating, the electrodes must have higher electrical conductivity and hardness, hence copper alloys are preferred.
Copper-Cadmium for welding Al and Mg.
Copper-Chromium for welding MS and low alloy steels.

Go through the advantages/ disadvantages and uses for every type of welding.

ARC WELDING

Arc welding is non pressure fusion welding and with or without filler metal depending upon thickness of base plate.
PRINCIPLE OF ARC WELDING:
Arc is generated between anode +ve pole of DC and -ve pole of DC cathode. When these two poles are brought together and separated for a small distance(1.5 to 3 mm)and current flows through a path of ionized particles, called plasma and electric arc is formed.Heat is generated as the ions strike the cathode.Various arc weldings processes are:
-Carbon arc welding
-Flux shielded metal arc welding
-TIG(GTAW) welding
-MIG(GMAW) welding
-Submerged arc welding
-Electroslag welding
-Electrogas welding
-Plasma arc welding
Dear students first four processes are to be studied with making figures(sketch) available on my site.
As explained in the class that for every welding answer first write definition of welding and then type of welding and principle. Make figure and explain.

WELDING

Kindly refer welding contd in my blog of mp course material upto types of flames.

Welding is permanent fastening and process of joining of two metals with pressure or without pressure, by using or without using filler metal by fusion. Fusion takes place by means of heat.Heat is obtained/produced from electric arc,electric resistance,chemical reaction,friction or radiant energy.
Important Terms
Autogenous Welding- Similar metals without adding filler metal.
Homogeneous Welding-Similar metals with adding of filler metal.
Heterogeneous Welding-Dissimilar metals using filler metal.
Base Metal-
Bead or weld Bead-
Weld Pass-
Backing-
Root-
Tack Weld-
Toe of Weld-
Weld Metal-

Classification of Welding:
Make Welding Trees as classification, which is explained in the class and the trees are available on my website.

Difference between Welding, Braze Welding, Brazing and Soldering has been discussed in the class and dictated also.Memorise it thoroughly as instructed.

PATTERN ALLOWANCES

After drawing figures of patterns and writing few lines about patterns.I would like to add about pattern allowances as laid under.

1.Shrikage or contraction allowance
2.Machining or finish allowances
3.Draft or Taper allowance
4.Distortion or camber allowance
5.Shake or rapping allowance
6.Mould wall movement allowance
As explained in the class write few lines with figures.

I have already dictated the following in the class.
Classification of sand.
Preparation of moulding sand.
Preparation of core sand.
Riser,Runner and Gating.
Engineering Materials-Metal, Non-metal, Ferrous & Non-ferrous metals etc.