Chapter 7 Flame Cutting 2012 Delmar Cengage Learning

Chapter 7 Flame Cutting © 2012 Delmar, Cengage Learning

Objectives • Describe the oxyfuel gas cutting process and list the commonly used fuel gases • Discuss which metals can be cut using the oxyfuel gas cutting process • Describe the eye protection that must be used for flame cutting • Tell how to determine the correct size and type of cutting tip for a specific job • Demonstrate how to properly set up a cutting torch © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) • Demonstrate how to properly clean a cutting tip • Demonstrate how to safely light the torch • Tell what actions can be taken to ensure a smooth cut when using a hand torch • Describe how to accurately lay out a line to be cut • Demonstrate how to set the working gas pressure on the regulator • Explain the chemical process that takes place during the burning away of the metal when using an oxyfuel gas cutting torch © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) • Explain what the kerf surface can reveal about what was correct or incorrect with the preheat flame, cutting speed, and oxygen pressure • Demonstrate how to make a machine cut and then evaluate the results • Describe soft slag and hard slag and what causes them • Describe the various types of guides that can be used to guide the torch for a more accurate cut © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) • Demonstrate how to make a flat, straight cut in thin plate, thick plate, and sheet metal • Demonstrate how to make a flame-cut hole • Describe the two major methods of controlling distortion of the metal during the heating or cutting process • Demonstrate how to make a straight line cut in the vertical and overhead positions • Discuss the various factors that can become a problem when cutting © 2012 Delmar, Cengage Learning

Objectives (cont'd. ) • Demonstrate how to cut out internal and external shapes • Demonstrate how to make a square cut on pipe in the horizontal rolled position, the horizontal fixed position, and the vertical position © 2012 Delmar, Cengage Learning

Introduction • Oxyfuel gas cutting (OFC) – Group of oxygen cutting processes • Raises temperature of metal to kindling temperature • Most commonly preformed with oxyacetylene cutting – Most people use the oxyfuel cutting torch – Manual, mechanized, and automatic OFC processes are used in the industry © 2012 Delmar, Cengage Learning

TABLE 7 -1 Fuel Gases Used for Flame Cutting. © 2012 Delmar, Cengage Learning

Metals Cut by the Oxyfuel Process • Include: – Iron base alloys – High alloy and high alloy carbon steels • Preheated before cutting • Nonferrous metals – Seldom cut with OFC because of postcut cleanup • A few reactive nonferrous metals can be cut © 2012 Delmar, Cengage Learning

Eye Protection for Flame Cutting • Recommended filter plates – Identified by shade number related to type of cutting • Use suitable eye protection for flame cutting – Goggle should have vents near the lenses – Cover lenses or plates should be provided – All lens glass should be ground properly – Filter lenses should be clearly identified © 2012 Delmar, Cengage Learning

TABLE 7 -2 A General Guide for the Selection of Eye and Face Protection Equipment © 2012 Delmar, Cengage Learning

Cutting Torches • Oxyacetylene hand torch – Most common – May be a part of a combination welding and cutting torch set or a cutting torch only • Combination welding-cutting torches offer more flexibility • Dedicated cutting torches are usually longer and have greater gas flow © 2012 Delmar, Cengage Learning

Cutting Torches (cont'd. ) • Oxygen is mixed with the fuel gas – Forms a high-temperature preheating flame – Must be completely mixed before they leave the tip • Methods used to mix the gases – Mixing chamber – Injector chamber © 2012 Delmar, Cengage Learning

Cutting Torches (cont'd. ) • Equal-pressure torches – Use a mixing chamber • Injector torches – Work with equal gas pressures and low fuel-gas pressures • Venturi creates a low-pressure area • Location of cutting lever – May vary from one torch to another • Machine cutting torch – Operates similar to hand cutting torch © 2012 Delmar, Cengage Learning

Cutting Tips • Most are made of copper alloy – Some are chrome • Prevents spatter from sticking to the tip • Tip designs – Change for different uses and gases • Tip diameter – Determines thickness of metal that can be cut • Acetylene – Use only with tips designed for acetylene © 2012 Delmar, Cengage Learning

FIGURE 7 -10 Comparison of standard and high-speed cutting tips. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Oxyfuel Cutting, Setup, and Operation • Setting up a cutting torch – Exactly like setting up oxyfuel welding equipment • Except for gas pressure adjustment • The chapter covers: – Safety – Pressure regulator setup and operation – Welding and cutting torch design and service – Reverse flow and flashback valves – Hoses and fittings – Types of flames and leak detection © 2012 Delmar, Cengage Learning

Hand Cutting • Key points – It is important to be steady • Be comfortable • Be free to move the torch – Practice cutting movement before lighting torch – A heartbeat will cause slight ripples in the cut – Brace the torch with the non-dominant hand © 2012 Delmar, Cengage Learning

Hand Cutting (cont'd. ) • Inner cones of the flame should be 1/8 -3/8 inch from surface plate – Called coupling distance • Start a cut on the edge of a plate – Hold torch at a right angle to the surface – Point torch to start cut at the very edge • Never use a cutting torch to cut open a used can, drum, tank, or other sealed containers © 2012 Delmar, Cengage Learning

Hand Cutting (cont'd. ) • Start a cut away from the edge of the plate – Hold inner cones as close as possible to metal • Starts and stops – Easier if one side of the metal is scrap • Drag – Distance from bottom of cut to top • Proper alignment of preheat holes – Speeds up and improves the cut © 2012 Delmar, Cengage Learning

A B FIGURE 7 -23 For longer cuts, the torch can be moved by sliding your gloved hand along the plate parallel to the cut: (A) start and (B) finish. Always check for free and easy movement before lighting the torch. Larry Jeffus View Welding Video Click here to view closed captioning © 2012 Delmar, Cengage Learning

Layout • Laying out a line to be cut – Can be done with a piece of soapstone or chalk line • Soapstone should be sharpened properly • Chalk lines make a long, straight lines on metal and are best used on large jobs – Use a scribe or punch for a more accurate line • Punched line is easier to see when cutting © 2012 Delmar, Cengage Learning

Selecting the Correct Tip and Setting the Pressure • Each manufacturer uses its own numbering system to designate the tip size – Equivalent drill size of each tip cleaner • Listed on the back • Use the tip cleaner you are familiar with – Try it on various torch tips – Remember which tip cleaner was used on a tip © 2012 Delmar, Cengage Learning

The Chemistry of a Cut • High-pressure stream of pure oxygen is directed on the metal preheated to its kindling point – Kindling point: lowest temperature at which a material will burn • Works on any metal that rapidly oxidizes – Iron, low carbon steel, magnesium, titanium, and zinc • Some metals release harmful oxides when they are cut – May need extra ventilation and a respirator © 2012 Delmar, Cengage Learning

The Chemistry of a Cut (cont'd. ) • Burning away metal – Chemical reaction with iron (Fe) and oxygen (O) • Oxygen forms an iron oxide: Fe 3 O 4 • Light gray in color – Heat helps carry the cut along • May overheat small strips of metal cut from a larger piece © 2012 Delmar, Cengage Learning

Figure 7 -37 As a hole is cut, the center may be overheated. Larry Jeffus. View Welding Video Click here to view closed captioning © 2012 Delmar, Cengage Learning

The Physics of a Cut • Record of the cut – Preserved on both sides of the kerf – Shows what was correct or incorrect • Preheat flame • Cutting speed • Oxygen pressure © 2012 Delmar, Cengage Learning

Preheat • Size and number of preheat holes in a tip – Effect both edges of the metal • Excessive preheat flame melts or rounds top edge • Excessive heat leaves slag along the bottom edge • Too small a flame causes cutting stream to wander © 2012 Delmar, Cengage Learning

Speed • Cutting speed should produce drag lines – Should have a slight slant backward • Hold the tip at a 90 degree angle to plate • Oxygen may not cut through the metal if cutting speed is too fast – Too slow a cutting speed results in the cutting stream wandering • Causes gouges in side of the cut © 2012 Delmar, Cengage Learning

Pressure • Pressure is correct – Cut sides are flat And smooth • Pressure too high – Cutting stream expands • Pressure too low – Cut may not go completely through the metal © 2012 Delmar, Cengage Learning

Slag • Two types – Soft slag • Very porous, brittle, and easily removed from a cut – Hard slag • Attached solidly to the bottom edge of a cut • Higher the unoxidized iron content – More difficult the slag is to remove • Found on bad cuts © 2012 Delmar, Cengage Learning

FIGURE 7 -43 A good cut showing a steady stream of sparks flying out from the bottom of the cut. Larry Jeffus View Welding Video Click here to view closed captioning © 2012 Delmar, Cengage Learning

Plate Cutting • Low carbon steel plate – Cuts quickly and accurately • Thicker the plate – More difficult the cut • Heavy cuts – Equipment and operator technique must be near perfection • Properly cut plate – Can be assembled and welded with little postcut cleanup © 2012 Delmar, Cengage Learning

Cutting Table • Pieces should be supported – Prevent torch flame from cutting into the table • Special cutting tables expose only a small metal area to the torch flame – Some use parallel steel bars • Others use cast iron pyramids • Flame and oxygen stream – Run between support bars or over edge of the table © 2012 Delmar, Cengage Learning

Torch Guides • Guide or support allows for better control – Produces more even cutting – Allow height and angle of torch head to remain constant – Speed must be controlled by welder – Various types of guides can be used FIGURE 7 -47 Using angle irons to aid in making cuts. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning

Distortion • Metal bends when heated during cutting – Major problem when cutting plate • Major ways of controlling distortion – Make two parallel cuts on the same plate at the same speed and time – Start the cut a short distance from the edge and skip short tabs every two to three feet © 2012 Delmar, Cengage Learning

Cutting Applications • Common cutting problems – Changing positions – Sparks – Hot surfaces – Tip cleaning – Blowback © 2012 Delmar, Cengage Learning

Pipe Cutting • Two ways to cut pipe freehand – Small diameter pipe • Hold torch straight up • Move from center to each side – Large diameter pipe • Point torch tip toward center of pipe • When cutting pipe, hot sparks can come out of the end of the pipe nearest you – Use proper protection and procedures © 2012 Delmar, Cengage Learning

Summary • OFC cutting torch – One of the most commonly used tools in welding – Can produce almost machine-cut quality • When misused can produce difficult problems • Equipment setup and torch tip cleaning – Essential elements to produce quality cuts – Take your time – Do not rush through a cut too quickly © 2012 Delmar, Cengage Learning