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To study the oxy-acetylene gas welding and cutting.
THEORY:
Oxy-Acetylene Welding. In this process, heat is obtained from the combustion of acetylene with oxygen. Pressure and/or filler metal may or may not be used. This process produces the hottest flame and is currently the most widely used fuel for gas welding.
Production of Oxygen.
Oxygen is obtained commercially either by the liquid air process or by the electrolytic process.
(2) In the electrolytic process, water is broken down into hydrogen and oxygen by the passage of an electric current. The oxygen collects at the positive terminal and the hydrogen at the negative terminal. Each gas is collected and compressed into cylinders for use.
Production of Acetylene Gas:
Acetylene gas is obtained by dropping lumps of calcium carbide in water. The gas bubbles through the water, and any precipitate is slaked lime.
EQUIPMENT:
Oxygen Cylinder
A typical oxygen cylinder is shown. It is made of steel and has a capacity of 220 cu ft at a pressure of 2000 psi (13,790 kPa) and a temperature of 70°F (21°C). Attached equipment provided by the oxygen supplier consists of an outlet valve, a removable metal cap for the protection of the valve, and a low melting point safety fuse plug and disk. The cylinder is fabricated from a single plate of high grade steel so that it will have no seams and is heat treated to achieve maximum strength. Because of their high pressure, oxygen cylinders undergo extensive testing prior to their release for work, and must be periodically tested thereafter.
Acetylene Cylinder
Acetylene cylinders are equipped with safety plugs which
have a small hole through the center. This hole is filled with a metal
alloy which melts at approximately 212°F (100°C), or releases at
500 psi (3448 kPa). When a cylinder is overheated, the plug will melt and
permit the acetylene to escape before dangerous pressures can be developed.
The plug hole is too small to permit a flame to burn back into the cylinder
if escaping acetylene is ignited. 
The regulators for oxygen, acetylene, and liquid petroleum fuel gases are of different construction. They must be used only for the gas for which they were designed.Most regulators in use are either the single stage or the two stage type. Check valves must be installed between the torch hoses and the regulator to prevent flashback through the regulator.
The equal pressure torch is designed to operate with equal pressures for the oxygen and acetylene. The pressure ranges from 1 to 15 psi (6.895 to 103.4 kPa). This torch has certain advantages over the low pressure type. It can be more readily adjusted, and since equal pressures are used for each gas, the torch is less susceptible to flashbacks.
HOSE
The hoses used to make the connection between are made especially for this purpose.
(1) Hoses are built to withstand high internal the regulators and the torch pressures.
(2) They are strong, nonporous, light, and flexible to permit easy manipulation of the torch.
A further increase in the oxygen supply will produce an
oxidising flame in which there is more oxygen than is required for complete
combustion. The inner cone will become shorter and sharper, the flame will
turn a deeper purple colour and emit a characteristic slight "hiss", while
the molten metal will be less fluid and tranquil during welding and excessive
sparking will occur. An oxidising flame is only used for special applications,
and should never be used for welding.
PROCEDURE:
When setting up welding and cutting equipment, it is important that all operations be performed systematically in order to avoid mistakes and possible trouble.
a) Cylinders.
b) Pressure Regulators.
(2) Connect the acetylene regulator to the acetylene regulator and the oxygen regulator to the oxygen cylinder. Use either a regulator wrench or a close fitting wrench and tighten the connecting nuts sufficiently to prevent leakage.
(5) Release the regulator screws to avoid damage to the
regulators and gages. Open the cylinder valves slowly. Read the high pressure
gages to check the cylinder gas pressure. Blow out the oxygen hose by turning
the regulator screw in and then release the regulator screw. Flashback
suppressors must be attached to the torch whenever possible.
Connect the red acetylene hose to the torch needle valve which is stamped "AC or flashback suppressor". Connect the green oxygen hose to the torch needle valve which is stamped "OX or flashback suppressor". Test all hose connections for leaks at the regulators and torch valves by turning both regulators’ screws in with the torch needle valves closed. Use a soap and water solution to test for leaks at all connections. Tighten or replace connections where leaks are found. Release the regulator screws after testing and drain both hose lines by opening the torch needle valves. Slip the tip nut over the tip, and press the tip into the mixing head. Tighten by hand and adjust the tip to the proper angle. Secure this adjustment by tightening with the tip nut wrench.
d. Adjustment of Working Pressure.
Adjust the acetylene working pressure by opening the acetylene needle valve on the torch and turning the regulator screw to the right. Then adjust the acetylene regulator to the required pressure for the tip size to be used. Close the needle valve. Adjust the oxygen working pressure in the same manner.
(2) Open the torch oxygen valve to drain the oxygen regulator and hose. When gas stops flowing and the gauges read zero, close the valve.
(3) When the above operations are performed properly, both high and low pressure gauges on the acetylene and oxygen regulators will register zero.
c. Release the tension on both regulator screws by turning the screws to the left until they rotate freely.
d. Coil the hoses without kinking them and suspend them
on a suitable holder or hanger. Avoid upsetting the cylinders to which
they are attached.
b) Adjust the preheating flame to neutral.
d) If the cut has been started properly, a shower of sparks will fall from the opposite side of the work. Move the torch at a speed which will allow the cut to continue penetrating the work. A good cut will be clean and narrow.
e) When cutting billets, round bars, or heavy sections, time and gas are saved if a burr is raised with a chisel at the point where the cut is to start. This small portion will heat quickly and cutting will start immediately. A welding rod can be used to start a cut on heavy sections. When used, it is called a starting rod.
PRECAUTIONS:
3) Purge both acetylene and oxygen lines (hoses) prior to igniting torch. Failure to do this can cause serious injury to personnel and damage to the equipment.4) oxygen and gas should be stored separately. Always chain store cylinders. Acetylene cylinders may explode unless stored upright.
6) Leaks - ensure no gas is escaping from any part of equipment.
Personal Protection
2) Eyes - protect them with a helmet and visor of a grade
designed for the type of welding; wear eye protection during slag removal
or chipping and grinding.
THEORY:
TYPES OF JOINT:
There are five basic types of joints for bringing two members together for welding. These joint types or designs are also used by other skilled trades. The five basic types of joints are described below and shown in figure.
(1) B, Butt joint - parts in approximately the same plane.
(2) C, Corner joint - parts at approximately right angles and at the edge of both parts.
(3) E, Edge joint - an edge of two or more parallel parts.
(4) L, Lap joint - between overlapping parts.
(5) T, T joint - parts at approximately right angles,
not at the edge of one part
b. Groove Weld: In this position, the axis of the weld lies in an approximately horizontal plane and the face of the weld lies in an approximately vertical plane.
c. Horizontal Fixed Weld: In this pipe welding position, the axis of the pipe is approximately horizontal, and the pipe is not rotated during welding.
d. Horizontal Rolled Weld: In this pipe welding position,
welding is performed in the flat position by rotating the pipe.
b. In vertical position pipe welding, the axis of the
pipe is vertical, and the welding is performed in the horizontal position.
(2) Minimum spatter adjacent to the weld.
(3) A stable welding arc.
(4) Penetration control.
(5) A strong, tough coating.
(6) Easier slag removal.
(7) Improved deposition rate.
DIAGRAMS
ARC WELDING MACHINES
Most of the alternating current arc welding machines in use are of the single operator, static transformer type. For manual operation in industrial applications, machines having 200, 300, and 400 amphere ratings are the sizes in general use. Machines with 150 ampere ratings are sometimes used in light industrial, garage and job shop welding.
The transformers are generally equipped with arc stabilizing
capacitors. Current control is provided in several ways. One such method
is by means of an adjustable reactor in the output circuit of the transformer.
In other types, internal reactions of the transformer are adjustable. A
handwheel, usually installed on the front or the top of the machine, makes
continuous adjustment of the output current, without steps, possible.
EXPERIMENT # 3
To study the spot welding.
THEORY:
Resistance welding is a process used to join metallic parts with electric
current. There are several forms of resistance welding, including spot
welding, seam selding, projection welding, and butt welding.
In all forms of resistance welding, the parts are locally heated until a molten pool forms. The parts are then allowed to cool, and the pool freezes to form a weld nugget. On a typical machine, the operator has control over the current setting, electrode force and weld time.
To create heat, copper electrodes pass an electric current through the work pieces. The heat generated depends on the electrical resistance and thermal conductivity of the metal, and the time that the current is applied. The heat generated is expressed by the equation
E = I2·R·t
where E is the heat energy, I is the current, R is the electrical resistance and t is the time that the current is applied.
Copper is used for electrodes because it has a low resistance and high thermal conductivity compared to most metals. This ensures that the heat is generated in the work pieces instead of the electrodes. When the electrodes get too hot, heat marks on the surface of the work pieces can form. The electrodes also become susceptible to "mushrooming". Electrode mushrooming reduces their usable lifetime. To prevent these problems, the electrodes are cooled with water. The water flows inside a cavity in the electrodes, removing excess heat.
The electrodes are held under a controlled force during welding. The amount of force affects the resistance across the interfaces between the work pieces and the electrodes. The force is adjusted to immediately create heat at the interface between the work pieces. The force also refines the grain structure of the weld. If the force is too low expulsion, or weld splash, can occur.
The heat needed to produce a molten pool depends on the thermal conductivity and melting point of the metal being welded. A material with a high thermal conductivity will quickly conduct heat away from the weld pool, increasing the total heat needed to melt the pool
Different types of Resistance Welding
Spot welding: Most common type of resistance weld. Electrode
and material are held stationary while heat and pressure are applied.
Projection welding: With the use of small projection the electrical
current is applied at very specific locations. This also allows the use
of flat electrodes, which are most easily dressed to an original condition.
Seam welding: Similar to spot welding, but the material moves between two rotating weld wheels (electrodes) with continuous current being applied.
Roll Spot Welding: The material moves a short distance (~2 mm) and stops while the machine welds. Then moves again and is welded.
Butt welding: Typically used to join the ends of two pieces of wire. The ends of the wire are clamped and brought together under pressure, then current is applied to form the weld.
SPOT WELDING:
A spot weld is made by applying pressure to two or more pieces of overlapping
metal, then passing an electrical current through a localized contact area,
for a very specific time period, to heat the area till a weld nugget forms.
Force is continued to be applied after the weld time until the weld has
solidified and forged.
To minimize heat dissipation the weld is formed very quickly. This requires very large electrical transformers.
Using schedules to setup machines:
P C T
P=Pressure
C=Current
T=Time
PCT are the factors used to construct a weld schedule. To use the schedule you look up the size of the thinnest piece of metal being welded. Looking across, you will see values for the correct amount of force, the proper weld time value, and the amount of amperage necessary to weld. Also, you will typically see values for electrode diameter, electrode size (expressed as an RW or MT size), and weld strengths you should expect to realize.
The relationship between these factors can be expressed as:
H = I2 x R x T x K
H = Heat
I = Current
R = Resistance
T = Time
K = Heat losses through conduction and radiation
With this formula we can see that welding heat is proportional to the square of the current. If the current is doubled, the heat is quadrupled. Welding heat is also proportional to the time. Thus if the current is increased, the time can be decreased. Pressure also has a direct relationship to the heat being generated. Increasing pressure lowers the resistance, and hence lowers the amount of heat generated. Alternatively, lowering the pressure, increases the resistance, and thus increases the amount of heat being generated.
Normally a short weld time is preferred to minimize heat losses trough radiation. (You want to heat the area being welded, not that area plus another larger peripheral area that is not being welded.) When you have a large area being heated without being welded, you are actually "spinning your wheels". There is no advantage to heating a large peripheral area. If you generate enough heat, you actually make the weld weaker by crystallizing the metal.
Components
The three main components of the welding machine are the Control, Transformer,
and Secondary Conductor.
Control
The purpose of the weld control is to accurately time the functions
involved in resistance welding. The various functions that are timed are
Squeeze, Weld, Hold, and Off.
Squeeze: allows the weld heads to come together and generate the appropriate amount of pressure prior to welding.
Weld: is the amount of time that the transformer is actually applying electricity to the weld area.
Hold: is the amount of time that pressure is applied to the weld after the transformer has been turned off. This allows the weld to forge and cool under pressure.
Off: is used when the machine is in repeat mode. It specifies an amount of time in between weld initiations.
The control actually turns the different valves and switches on and off to run the machine. The control will include a small valve transformer, which supplies 115V to the solenoid valve that controls the pneumatic cylinder. There is also a switch to run the transformer. This switch may be a set of "ignitron tubes", or a set of thyristors, more commonly called SCR's (Silicon Controlled Rectifiers).
Controls can get very complicated. They may be able to run multiple valves and multiple transformers from one control. Many models are capable of interfacing with PLC's and PC's.
Transformers
The principle of the transformer is based on Electromagnetic Induction
and was discovered in 1831. This is not new technology! The transformer
consists of a primary winding, secondary winding, and a magnetic core,
onto which the two windings are placed. Without getting into the physics
involved, as a voltage and current are applied to the primary winding,
a subsequent voltage and current is induced in the secondary winding. Typically,
we will take a primary voltage of 230V or 460V at up to ~800 amps, apply
it to a primary winding and induce a secondary voltage of 2 to 15 volts
with currents approaching 100,000 amps. Three phase applications can generate
significantly higher amperages.
Secondary Conductor
This includes everything that is attached to the secondary winding
of the transformer. The weld arms, holders, and tips are all part of the
secondary circuit. We will talk about this later as a maintenance item.
PROCEDURE AND OPERATION OF EQUIPMENT:
Cooling systems should be inspected for flow periodically. An adequate
flow would be approximately 3 to 5 gallons per minute for average size
machines being run at a normal duty cycle (large machines or high duty
cycles can necessitate higher flow rates). If you observe a flow rate significantly
below this, maintenance is required. Because many water channels are very
small, a little mineral scale can have a significant impact. Also, water
cooled cables tend to fray and release small pieces of wire into the cooling
system. These can accumulate into a "hair ball" forming a blockage. Operators
have the best opportunity to spot these problems before they cause a machine
shutdown. Excess heat should always be reported and investigated.
When properly set up resistance welding machines should produce no flash. Weld flash is a fire hazard and requires that any flammable substances be properly protected. Weld flash is also an abrasive. Moving parts should be protected from exposure to weld flash whenever possible. Also, since weld flash is made of metal (steel) it can be conductive. If allowed to accumulate across platen and fixture insulation, flash will cause a slow burning, gradually extending back to inaccessible areas. Eventually this will begin to interfere with the welding process, and a major repair and cleaning are called for. Transformers should also be protected from weld flash. Flash can get in the winding causing a short in the windings.
Electrodes require regular maintenance. The size and shape of the electrode largely determines the quality of the weld. Therefore, it is of primary importance that the weld face remains constant. Because of this, tips should be frequently changed or dressed. Several dressing devices are available. They might be air operated cutters or manual files. Electrodes sometime will "pickup" the material being welded. When this happens it should be removed as soon as possible. However, when welding coated materials this pickup continually occurs and it is not reasonable to clean the electrode after each weld. Care should be exercised when handling electrodes that the tip tapers are not damaged. Damaged tapers can cause poor electrical contact and water leakage. A conductive copper lube should be used on tip tapers to ease in removal and ensure a good electrical connection.
Particular attention should be given to the entire secondary circuit,
especially the joints. These joints should be examined for excess heat.
Periodically they should be disassembled and cleaned. A copper lube should
be used on these joints also to eliminate any oxidation. Oxidation causes
a resistive buildup, which if left unchecked, will lead to increased resistance
and poor weld quality. Operators should pay attention to the operation
of the transformer, in particular to any odd or different sounds that the
transformer might make. In particular, you are listening for "half cycling".
Under normal conditions the transformer makes a smooth hum when welding.
When half cycling, the transformer makes a loud barking or grunting noise.
This is especially hard on the transformer and if left unchecked, can cause
the transformer to fail.