NAME 345 - Lecture 15

Metal Cutting

Md. Habibur RahmanLecturer

Department of Naval Architecture & Marine EngineeringBangladesh University of Engineering & Technology

Dhaka-1000, Bangladesh

⚫Plasma

⚫Oxy Fuel

What is Plasma?

The Fourth State of Matter

One common description of plasma is to describe it as the fourth

state of matter. We normally think of the three states of matter as

solid, liquid and gas. For a common element, water, these three

states are ice, water and steam. The difference between these

states relates to their energy levels. When we add energy in the

form of heat to ice, the ice melts and forms water. When we add

more energy, the water vaporizes into hydrogen and oxygen, in

the form of steam. By adding more energy to steam these gases

become ionized. This ionization process causes the gas to

become electrically conductive. This electrically conductive,

ionized gas is called a plasma.

4th State of Matter

Plasma Plasma consists of charged particles that conduct the electrons

across the gap. Both the glow of a neon tube and the bright fluorescent light bulb are

examples of low-temperature plasmas.

Plasma results when a gas is heated to a high enough temperature to convert into positive and negative ions, neutral atoms, and negative electrons. The temperature of an unrestricted arc is about 11,000°F

The temperature created when the arc is concentrated to from a plasma is about 23,000°F.

Plasma Arc Cutting (PAC)is a cutting process that uses an arc and a high-velocity, ionized gas coming through a nozzle to cut all metals that can not be cut with flame

Such as:

Stainless steel

Aluminum

Copper

How PAC works Plasma cutters work by sending an electric arc through a gas that is passing

through a constricted opening. The gas can be shop air, nitrogen, argon, oxygen. etc.

This elevates the temperature of the gas to the point that it enters a 4th state of matter. Scientists call this additional state plasma. As the metal being cut is part of the

circuit, the electrical conductivity of the plasma causes the arc to transfer to the work.

The restricted opening (nozzle) the gas passes through causes it to squeeze by at a high speed. This high speed gas cuts through the molten metal.

The gas is also directed around the perimeter of the cutting area to shield the cut.

How a Plasma Cutter works A complete plasma cutter consists of a

power supply,

a ground clamp,

and a hand torch.

The main function of the power supply is to convert the AC line voltage into a user-adjustable regulated (continuous) DC current.

The hand torch contains a trigger for controlling the cutting, and a nozzle through which the compressed air blows. An electrode is also mounted inside the hand torch, behind the nozzle.

PAC System

Advantages of PAC Can cut all metals

Fast cutting speeds

Little distortion from heat

No hazardous gases

Oxygen based system do not leave nitride deposits

Nitrogen based systems are for aluminum, stainless steel and nickel

Disadvantages

Plasma Torch is expensive

Not portable (needs electricity)

Metal fumes created can be a health hazard

Puts tremendous heat into the metal around the cut

Hardens materials next to the cut

PAC How it cuts

The plasma gas (compressed air) is forced through a nozzle and heated by the arc which reaches to 5,0000°F

How to Use

Clean metal

Hook up ground cable

Draw cutting pattern on metal

Hold torch 1/16-1/8” from metal (torch stand will help maintain this)

Hold torch at a 70 to 90° angle

Pull button when arc begins pull torch across metal

Setting the PAC TorchIf torch amps is set too high travel speed needs to be increased

Thickness Current Travel Speed

1/16” 35 amps 175 in/min

1/8” 40 amps 90 in/min

1/4” 40 amps 40 in/min

1/4” 80 amps 100 in/min

3/8” 40 amps 18 in/min

3/8” 80 amps 55 in/min

1/2” 40 amps 20 in/min

1/2” 80 amps 35 in/min

Cutting Speed

Too Slow- Molten metal will collect on bottom of work piece and need to be ground off (this is called Dross)

Too Fast- Metal will not be cut all the way through

Oxy-Fuel Cutting

Metal to be cut should be clean, marked with a punch or soapstone, and placed in a suitable position for cutting.

DO NOT cut over a concrete floor.

Use a container or special cutting table device to catch the molten metal.

Oxyfuel cutting is limited to ferrous metals.

Steels with a high tungsten or chromium content or stainless steel cannot be cut with oxyfuel.

Cast iron is more difficult to cut than steel.

An excess fuel flame is used

More oxygen pressure is needed

Oxy-Fuel Cutting

Definition:

Burning is the rapid oxidation of a material

Does Metal Burn? Virtually all materials will burn if they are first heated to their

ignition temperature.

Steel ignition temp is when it is cherry red, 1300-1400° C

Oxygen burns preheated metal and blows it away from the work piece.

Oxy-Fuel Cutting Process

Clean metal

Draw line

Make jig to keep cut straight

Hold tip 1/16-1/8” above metal

Preheat metal to cherry red color

Open oxygen valve and push torch through metal (Angle tip slightly toward direction of cut)

Flame typesCarburizing Neutral Oxidizing

Cutting torchA cutting torch head is used to cut materials. It is similar to a welding torch, but can be identified by the oxygen blow out trigger or lever. With fuel and oxygen tube, the cutting torch has an additional tube for high-pressure cutting oxygen.

The metal is first heated by the flame until it is cherry red. Once this temperature is attained, oxygen is supplied to the heated parts by pressing the "oxygen-blast trigger". This oxygen reacts with the metal, forming iron oxide and producing heat. It is this heat that continues the cutting process. The cutting torch only heats the metal to start the process; further heat is provided by the burning metal.

The melting point of the iron oxide is around half that of the metal; as the metal burns, it immediately turns to liquid iron oxide and flows away from the cutting zone. However, some of the iron oxide remains on the work piece, forming a hard "slag" which can be removed by gentle tapping and/or grinding.

3690 Cutting Tip

3690-P Cutting Tip

Cutting Tips

CUTTING TORCHES

Torch Body

❖Most welding torches are designed so the body of the torch can accept either welding tips

or a cutting attachment. This type of torch is called a combination torch.

❖The high-pressure cutting oxygen is controlled by a lever on the torch handle

Cutting Torch Tips

❖Although the orifice arrangements and tip material are much the same among the

manufacturers, the part of the tip that fits into the torch head often differs in design.

❖If the joints leak, the preheat gases could mix with the cutting oxygen or escape to the

atmosphere, resulting in poor cuts or the possibility of flashbacks.

❖You must keep the tip orifices and passages clean and free of burrs and slag. If the tips

become dirty, they should be put aside for restoration.

OXYGAS CUTTING OPERATIONS - EQUIPMENT SETUP

Make sure the following steps are taken before any attempt is made to light the

torch:

❖Secure the cylinders so they cannot be accidently knocked over. After securing the

cylinders, remove the protective caps.

❖Standing to one side, crack each cylinder valve slightly and then immediately close

the valve again. This blows any dirt or other foreign matter out of the cylinder valve

nozzle.

❖Connect the fuel-gas regulator to the fuel-gas cylinder and the oxygen regulator

to the oxygen cylinder.

❖Back off the regulator screws to prevent damage to the regulators and gauges

and open the cylinder valves slowly. Open the fuel-gas valve only one-half turn and

the oxygen valve all the way.

❖Connect the RED hose to the fuel-gas regulator and the GREEN hose to the

oxygen regulator. Notice the left-hand threads on the fuel-gas connection.

❖To blow out the oxygen hose, turn the regulator screw in (clockwise) and adjust

the pressure between 2 and 5 psig. After the hose has been purged, turn the screw

back out again (counterclockwise) to shutoff the oxygen. Do the same for the fuel-

gas hose.

❖Connect the RED (fuel-gas) hose to the connection gland with the needle valve

marked “FUEL.” and the GREEN (oxygen) hose with the needle valve marked “OXY.”

❖With the torch valves closed, turn both regulator screws clockwise to test

the hose connections for leaks

❖Select the correct cutting tip and install it in the cutting torch head.

❖Adjust the working pressures. The fuel-gas pressure is adjusted by opening

the torch needle valve and turning the fuel-gas regulator screw clockwise.

Adjust the regulator to the working pressure needed for the particular tip

size, and then close the torch needle valve.

❖In lighting the torch and adjusting the flame, always follow the

manufacturer’s directions for the particular model of torch being used.

❖In general, the procedure used for lighting a torch is to first open the torch

oxygen needle valve a small amount and the torch fuel-gas needle valve

slightly more, depending upon the type of torch. The mixture of oxygen and

fuel gas coming from the torch tip is then lighted by means of a spark igniter

or stationary pilot flame.

Tips frequently become spattered with metal or other materials that may cause inferior work.

To clean tips:

Rub emery cloth, steel wool, or the file on a tip cleaner to clean and square the tip point.

Insert the correct size cleaner into each orifice. Push in and pull out; do not twist or bend the cleaner.

Open oxygen slightly during this procedure to blow out foreign particles.

Care should be taken not to damage the threads and seats of the tips.

Care of Oxyfuel Tips

TIP MAINTENANCE❖In cutting operations, the stream of cutting oxygen sometimes blows slag and molten metal

into the tip orifices which partially clogs them.

❖When this happens, you should clean the orifices thoroughly before you use the tip again.

❖You should follow the recommendations of the torch manufacturer as to the size of drill or

tip cleaner to use for cleaning the orifices.

❖If you do not have a tip cleaner or drill, you may use a piece of soft copper wire.

❖Do not use twist drills, nails, or welding rods for cleaning tips because these items are likely

to enlarge and distort the orifices.

One Piece

Tip Cleaner

Two Piece

JUDGING CUTTING QUALITY

In general, the quality of an oxygas cut is judged by four characteristics:

✓The shape and length of the draglines

✓The smoothness of the sides

✓The sharpness of the top edges

✓The amount of slag adhering to the metal

DRAG LINES

Drag lines are line markings that show on the surface of the cut. Poor drag are long and

irregular or curved excessively. When the draglines are short and almost vertical, the sides

smooth, and the top edges sharp, you can be assured that the slag conditions are satisfactory.

SIDE SMOOTHNESS

A satisfactory oxygas cut shows smooth sides. A grooved, fluted, or ragged cut surface is a

sign of poor quaility.

TOP EDGE SHARPNESS

The top edges resulting from an oxygas cut should be sharp and square (fig. 4-31, view D).

Rounded top edges, such as those shown in view E of figure 4-31, are not satisfactory. The

melting of the top edges may result from incorrect preheating procedures or from moving the

torch too slowly.

SLAG CONDITIONS

An oxygas cut is not satisfactory when slag adheres so tightly to the metal that it is difficult to

remove.

Correct CutNotice that the top edge is square and that the drag lines show a slight curve.

Oxygen Pressure Too LowProduces a cut with rough surface, makes it difficult to hold cut and results in slow speed with too much lag.

Speed Too FastProduces a rough cut with pronounced drag line very similar to too low oxygen pressure.

Oxygen Pressure Too HighProduces a rough surface, melts down top edge and wastes oxygen.

Too Much Acetylene in PreheatsReduces cutting speed approximately 25%. Forms carbon deposits on cut surface and wastes acetylene.

Correct CutHere the factors of tip size, pressures and speed are correct…results in square top edge and uniform narrow kerf.

Preheats Too HeavyTop edge is melted down, kerf irregular and excess gas is consumed by the preheat flame.

Preheats Too SmallWastes time as maximum speed cannot be obtained. Low cutting speed results in gouges at bottom edge of cut.

Oversize TipProduces a kerf which is too wide, causing not only a waste of plate but also a waste of both gases.

Undersize TipInefficient cut because piece will not drop when end is reached as slag has not cleared the kerf and cutting time will be excessive.

Quality Cut Extremely Fast

Extremely SlowIrregular Pressure

AWS Cut Quality

Advantages

Can cut complex shape

Can cut carbon and low carbon steel

Inexpensive process

Can have multiple cutting torch

Disadvantages

It is slower than other cutting systems

Cut accuracy is not as good as plasma, water jet ,laser

Creates heat affected zone

Safety Backflash

Turn Oxygen off first

Watch where you cut

Don’t blow molten metal toward another person

Don’t melt the hoses

Don’t drop metal on hoses

Don’t let hoses lay on hot metal