PCC WEL 102

ACETYLENE

• Acetylene is a gas formed by the mixture of calcium carbide and water.– Acetylene is a colorless gas, with a very distinctive odor– It becomes very unstable if it is compressed to more than

15 psi.– The cylinder is packed with a porous material called

Acetone. It is a chemical liquid that dissolves acetylene under great pressure in a cylinder.

• The cylinder is equipped with a safety fuse plugs located at the bottom of the cylinder

– An acetylene cylinder should only be opened at the most 1 – 1 ½ turns.

Oxy-Acetylene Torch Cutting• Only Ferrous metals can be cut with a Oxy-Acetylene torch

– Cutting is allowed because of oxidation (high speed rusting process)

– The oxygen does the work

• The cutting torch consists of two oxygen needle valves and oxygen pressure lever. There is only one acetylene valve.– Torch tips vary in size and use: The small holes on the outside of

the tip are preheat holes, and the large one in the center is the cutting hole.

• Oxy-Acetylene Torch Formula– Fe + O = FeO + Heat

Adjusting the Flame• Neutral flame

– Equal one to one ratio of oxygen & acetylene– One of the hottest flames known– 5589 F (5600 degrees F)

• Oxidizing flame– More oxygen is present in the flame– Resembles a neutral flame, but has a shorter and a more

of a pointed inner cone with a almost a purple color rather than white

• Carburizing flame (Carbonizing)– Excess of acetylene– 3 flame zones– Weakens metal by putting carbon into it (annealing?)

• Neutral flame

• Oxidizing

• CarburizingReturn Carb: Return

Neutral:

• Oxygen & Acetylene Regulator Pressure– Our shop for using the cutting torch (not gas welding)

• Acetylene 3-4psi

• Oxygen 20 psi

• Oxygen pressure may vary depending on the thickness of the metal to be cut.

½” plate & less

• Lighting the torch1. Turn on the acetylene valve (1/2 turn)

2. Use striker to ignite flame

3. If you are using a torch with two oxygen needle valves, open the first one at least 3 – 4 turns. Then use the second needle valve to adjust for a neutral flame. Then depress the oxygen lever and additional adjustment may be necessary with the secondary valve.

Cutting Demo

Welding Machines/ElectricityShielded Metal-Arc Welding

SMAW

Weld 1: Ch. 5

Weld 2: Ch. 11

• Welding Current– When an electrical current moves through a wire heat is

generated by the resistance. The greater the flow the greater the resistance.

– Welding current is provided by an AC, DC, or AC/DC welding machine.

– Arc-electricity jumping from an air gap between electrode and the base metal.

• Arc temp- 6000ºF to 10,000ºF

– The primary current to a welding machine is 220 or 440 volts.

• Primary current – line off the pole to breaker box (welder)

• Secondary current – line to an electrical appliance (wall outlet)

– Actual voltage for a welding current is 18 to 36 volts

Electrical Terms

• Electricity- the flow of electrons through a conductor

• Electron – the negatively charged part of an atom

• Polarity – the direction of the flow of electrons

Two ways to make electricity

• Chemically –– Example: A car battery

• Alternator vs. Generator – what is the difference?

• Mechanically – – Example: A motor generator – hydro-electric power –

Wind-mill farm – etc.

• Conductor – A substance capable of readily transmitting electricity or heat– Good conductors – Gold, Copper, Iron, Mercury, Nickel, Platinum,

Silver, & Aluminum.Conductors & Insulators

• Conductivity – The ability of a conductor to carry a currentInsulators/Electrical Diagram

• Resistance – the opposition of the material in a conductor to the passage of an electrical current causing electrical energy to be transformed into heat.

• Alternating Current (AC) – An electrical current having alternating positive and negative values.– First half of cycle the current flows in one direction, and for the next half it flows in the

opposite direction.

– The current reverses its direction of flow 120 times per second, it requires 1/60 of a second to complete a cycle or hertz.

– The frequency is indicated as 25, 40, 50, and 60 cycles per second.

• In the U.S. AC is established at 60 cycles per second.

AC Website Tutorial

• Direct Current DC – Electrical current which flows in one direction only

Direct Current Straight Polarity – DCSP “or”

Direct Current Electrode Negative –DCEN

Direct Current Reverse Polarity - DCRP “or”

Direct Current Electrode Positive - DCEP

• Polarity is important because it has a direct relationship with the amount of heat generated within the base metal. – Polarity does not affect AC welding situations– DCRP will generate more heat within the base metal

• Reverse Polarity is also used for making some welds in the vertical & horizontal positions.• One way to know if you have the right polarity setting is the sharp crackling sound of the arc.• An incorrect polarity setting will cause the arc to give off a hissing sound & will make it

difficult to control the welding bead. Example: E-6010 has to run on reverse polarity– DCSP will concentrate more heat through the electrode and a smaller area within the

base metal.

• POLARITY RULE – Hook the stinger to what they ask for!!!!– EXAMPLE: DCEN-stinger(electrode) to the negative terminal– DCEP-stinger(electrode) to the positive terminal

• Electrical Circuit – A complete path of electricity through a conductor; starting from the negative terminal and ending at the positive terminal.– Open circuit – there is not a complete path of electricity to follow. (exp = circuit

when no welding is being performed and machine is running.) • This voltage varies from 50 – 100 volts

– Closed circuit – there is a complete path of electricity to follow. (exp= circuit when welding is being done.)

• Working voltage – voltage drops between 18 and 36 volts

• Ampere – amount or rate of current that flows in a circuit.– Amps are measured with a ammeter

• Volt – electrical pressure within a circuit– Similar to pressure that makes water flow in pipes, the pump provides the

pressure. In an electrical circuit the power source provides the force that pushes the current through the line; Voltage does not flow only the current flows.

– Voltage is measured with a voltmeter

• Static electricity – electricity at rest or electricity that is not moving.

• Dynamic electricity – electricity in motion in an electrical current.

• Constant Potential – A generation of stable voltage regardless of the amperage output produced by the welding power supply.– Important in MIG welding

• Voltage Drop – Voltage will drop as the distance from the welding power supply increases.– If welding leads are too long, the voltage will drop affecting the weld.

Types of Welding Machines• Transformer

– Produces AC current

– Least expensive to purchase

– Lightest & smallest

– Old fashion name was “BUZZBOX”

– Current may be changed by plugging leads into sockets or by rotating a hand wheel

– No moving parts

• Rectifier or commonly called “Inverter”– Basically a transformer containing an electrical device which changes AC into DC

currents– They produce both AC or DC currents with the ability to produce straight or reverse

polarity.– Very efficient – Basically quiet operation– HI & LOW range setting– Not very mobile– Few moving parts– Our shop uses these for the arc booths.

36

volts

Beginning Weld Ending Weld

Steep slopping volt-amp curve

• Constant Potential (MIG)– Maintains approximately the same voltage regardless of the amount of current drawn.

Unlike a constant current machine (stick welding) where there is a steep slope or drop-off of amps

– If the wire (or electrode) is fed at a faster rate the current increases, if it is slowed down it decreases.

– Easier to train an operator– Achieve good welds– Not very mobile– Less splatter– Continuous welds– Shielding Gas– Moving Parts

36

volts

Beginning Weld Ending Weld

• Motor-Generator– Welder powered by a gasoline motor

– Provides DCSP or DCRP & AC

– Expensive

– Many moving parts

– Loud

– Very mobile

– Alternative power source

• Selecting a Welder

• “Duty Cycle” – number out of 10 minutes a welder can stay at a given output. (example 60%)

– Primary use and type of welding– Advantages of DC constant current type arc welders

• DCRP or DCEP– This produces deeper penetrating welds than DCSP

– Can be used in all positions other than flat or down hand

– Electrodes to weld nickel, aluminum, & copper use DCRP

• DCSP or DCEN– Recommended for EXX2X electrodes that have high metal deposition

rates

– Can be used for welding in other positions other than flat

– Disadvantages of DC constant current• More expensive than AC machine with the same current output

and duty cycle• Magnetic Arc Blow

– Advantages of AC constant current• Moderate penetration• Large-diameter electrodes can be used with high ac currents to

obtain greater filler metal deposition rates and faster welding speeds.

• Less expensive than DC machines of same quality, current output, and duty cycle.

• No magnetic arc blow• Low operating and maintenance cost• High electrical efficiency & noiseless operation

– Disadvantages• Not all SMAW (Shielded Metal Arc Welding) electrodes can

be used with AC current

• Welders with both DC/AC current capabilities are naturally better, but more expensive.

• Magnetic Arc Blow – condition that causes the arc to wander while welding in corners on heavy gauge metal or if using large electrodes.– DC current flows in one direction the metal becomes magnetized.

– The arc is deflected, resulting in excessive spatter.

– Arc blow breaks the continuity of the deposited metal, (the bead), making it necessary to refill the crater

– The process of refilling the crater often slows down welding, but also leaves weak spots in the weld.

1. The magnetic field around the electrode is deflected at the end of the joint (A & B)

2. The concentration of the magnetic flux at the ends of the joint forces the arc toward the center of the base metal.

3. The arc is seen to blow away from the area directly under the electrode.

4. Area C is alright

1. The arc is blown towards the right. ( A )

2. The arc travels straight down (C ).

3. The arc and filler metal are blown toward the workpiece lead end of the weld (B)

• Preventative measure to stop arc blow– Place ground connections as far from the weld joint as possible

– If forward arc blow is a problem, connect the work piece lead ground near the end of the weld joint.

– Reduce welding current

– Weld towards a heavy tack-weld

– Change to an AC welding machine

– Use back step method• Makes a number of short welds starting away from the beginning of the

joint, each section is welded back toward the previous section.

• Never turn the dial up or down while a person is in the arc booth welding!!!

Proper Welding Equipment• Helmet with shade 10 lens

– Always make sure there is an plastic cover lens on the outside of the expensive shade 10 lens.

• Welding Gloves• Welding jacket• Chipping hammer/wire brush• Proper leads and connections