CHAPTER XI

OXY-ACETYLENE CUTTING

(=142=) By heating a bar of wrought iron or steel to a welding heat and holding it in a stream of compressed air, or a strong blast, it will at once begin to melt and sizzle, emitting an incandescent and scintillating light. This light is dangerous to observe at close range without colored glasses. The burning of the metal can be maintained for hours, without any other source of heat except that caused by the combustion of the iron. The oxy-acetylene cutting process is based upon this principle, in that a neutral flame is applied in order to heat the part being cut to the desired temperature. Once the melting-point is reached, pure oxygen under pressure is applied to maintain oxidation and force out the burned portion.

(=143=) The apparatus used for cutting does not differ to any great extent from that of the welding class, except that a different torch is employed. There are combination regulators and torches manufactured, but a combination tool is always regarded by most authorities as a loss in efficiency, either on one side or another. While a low-pressure welding regulator may be used on the oxygen line for cutting, yet its use upon large work, where the pressure is high and the regulator must pass a great deal of gas very freely without freezing up, this low-pressure regulator will be a serious handicap and cause much trouble, if used.

[Illustration: FIG. 80.--The Cutting Torch Eats its Way through Steel of any Size with Remarkable Ease, Leaving a Clean-cut Edge. This View Shows a Cutting Torch in Operation at the Ordnance Welding School, U. S. Army.]

(=144=) An ideal arrangement on the oxygen line for cutting is to have a double or “twin” regulator attached to the oxygen drum, one side of which will do for welding and the other, being high-pressure type, will produce a constant flow of high-pressure gas, suitable for the cutting jet. Then when cutting is done a three-hose torch should be employed. One of its oxygen connections which governs the neutral flame can be connected to the low-pressure regulator, while the oxygen jet should be controlled by the high-pressure regulator, the third connection will furnish the acetylene gas for the preheating flame. However, in place of this three-hose arrangement, most cutting is accomplished by means of a two-hose apparatus, wherein only one hose is used to convey the oxygen from a single regulator to the torch. On such apparatus much trouble is usually experienced in cutting old metals where a great deal of scale is present or in a close place where the torch is apt to get hot.

(=145=) Many times part of the scale or metal will pop up against the tip and cause the oxygen jet to flicker. This slight variation may cause an excessive pressure of oxygen to be introduced into the preheating flame momentarily, by backing up the oxygen in the cutting jet. This lean mixture of gas will generally flash back instantaneously and will deposit a layer of carbon on the inside of the tip, which causes much annoyance to the operator. This condition is to be found where there is but one oxygen line. In the two-hose arrangement this is entirely overcome, due to the independence of the pressure on each line.

(=146=) The high-pressure regulator differs from the low-pressure regulator in these respects: The diaphragm, see Fig. 16, is much smaller in diameter, which makes it less sensitive, and of course much stronger. The diaphragm springs are usually much heavier; the nozzle contains a larger opening for passing gas freely without freezing; and to take care of the increased pressure on the line, usually a higher pressure working gauge is added to the regulator. Such a regulator is capable of passing much more gas than the low-pressure type, but as far as being as sensitive and maintaining a constant, absolute flow of gas, its design will not permit it to do so. In cutting, these requisites are not necessary. In welding, however, the delicate adjustment of the flame demands a very sensitive regulator and usually the larger the diameter of the diaphragm the more sensitive the adjustment.

[Illustration: FIG. 81.--End Views of Cutting Tips, Showing Possible Arrangements of Preheating Flames in Regard to Oxygen Jet. The Black Circles Represent the Preheating Flames, which Vary in Number and Arrangement According to the Nature of the Work, the Possible Limit being a Continuous Circle, as Shown. The White Circles Illustrates the Oxygen Jet, which, too, Varies in Size According to the Work.]

(=147=) The cutting torch differs from the welding torch in many respects. The tip itself, when looking at its end, may resemble any one of the views shown in Fig. 81. In the welding torch, but one hole is to be found in the tip; in the cutting tips, two or more holes are to be found. In all cases the center hole passes pure oxygen, whereas in the surrounding holes, both oxygen and acetylene mix and when lighted give a neutral flame. This will hereafter be called the preheating flame. The gases issuing from these openings are controlled by three valves, one of which may have a trigger or lever arrangement for quick action, and it will control the center jet of oxygen which really does the cutting. This is under much higher pressure than the preheating flame. The other two valves will control the oxygen and acetylene gases used for the preheating flame. In lighting such a torch, the acetylene is turned on in the same manner as has been taught when welding, until it just leaves the end of the tip. Then the oxygen valve is opened, which controls the preheating flame, and enough is permitted to pass to produce a neutral flame. As soon as this has been accomplished, the third valve should be quickly opened and held so a moment, to see if the neutral flame has been changed. Generally this operation will deprive the neutral flame of some of its oxygen, and a feather flame, showing too much acetylene and not enough oxygen gas, can be noticed. This will necessitate turning on slightly more oxygen at the torch valve. The third valve is then shut off and the torch is ready to start cutting.

[Illustration:

(_Courtesy of the General Welding & Equipment Co._)

FIG. 82.--Cutting a Heavy Shaft.]

[Illustration: FIG. 83.--Position to Hold Torch in when Cutting Metal.]

(=148=) On small cutting jobs, about as much acetylene pressure is used on the line as there would be if it were a welding job. The oxygen pressure, however, is generally much greater, and a pressure anywhere from ten to two hundred pounds should be used, depending upon the thickness of the metal and the conditions which must be met. In extreme cases where very heavy cuts are to be made, a much higher pressure than has been mentioned should be used, but the limitations given will cover a wide range of work. To start a cut it is necessary to bring the preheating flame in contact with one edge of the metal to be cut and play it there until the metal is red hot. As soon as this condition is reached the torch is held steady--the neutral flame just touching the metal; then the third valve controlling the cutting jet of oxygen is opened. This oxygen, under high pressure, quickly acts upon the hot metal and severs it instantaneously, melting and oxidizing the metal so that it will not flow together, in one and the same operation. As soon as this occurs the torch should be advanced as rapidly as possible in the direction the metal is to be cut. The more rapid the advancement and the steadier the torch is held the cleaner the cut will be; and incidentally, less gas consumed in the execution of the job. In cutting, as in welding, it is always well to give the torch a chance, and when the operator sees much molten metal splashing directly back on the torch, he should change the angle slightly to avoid his apparatus becoming overheated. It has been found that if the cutting torch is held at the angle shown in Fig. 83, the most satisfactory results can be expected.

(=149=) At the present time only such metals as steel and wrought iron can be successfully cut. When it comes to cast iron no method has yet been discovered to cut it with any degree of success by the oxy-acetylene flame, on account of the high melting-point of the oxide and various other matters. The day is looked forward to, however, when after sufficient time and study has been devoted to this subject, that cast iron can be as successfully cut as any other metal, by introducing another gas or agent to destroy some of the reactions which retard its application at the present time.

(=150=) The use of the cutting torch in preparing steel work, for welding of large size, plays an important part, in quickly and efficiently “V-ing” out and getting it ready for use. Care should be taken, after its use, to see that the heavy oxide which it leaves is largely destroyed, before any more metal is added.

[Illustration: FIG. 84.--Method of Cutting with Two Welding Torches. Torch _A_ is Adjusted so that a Neutral Flame will do the Preheating, while a Fork in the Oxygen Line Supplies Oxygen only to Torch _B_, and it does the Cutting.]

(=151=) Frequently the welder has a call for a cutting torch, where none is available, yet an extra welding torch or two may be on hand. If this is the case, two welding torches may be fastened together in such a manner that a temporary job of cutting may be handled. The arrangement shown in Fig. 84 illustrates this point. If no extra welding torch is available, a carbon burning torch or any piece of copper tubing which has a valve in one end, suitable for taking a hose connection, and the other end free to have a welding tip brazed on, can be used in the same manner. The welding torch will give the neutral flame and the extra line of oxygen will do the cutting. It is well to remember that oxygen, no matter under what pressure, cannot be expected to act upon cold metal. A red heat is absolutely necessary. There are various short cuts, it is true, in obtaining this heat, and where a large shaft is to be cut, the operator would not think of playing his torch upon such a piece of metal until it was red hot in the locality in which he wished to start his cut. This would consume too much time and gas. Generally a hammer and cold chisel are brought into play and a slight curl on the metal is obtained as shown in Fig. 85. The moment this becomes red hot, the oxygen jet may be turned on, and the cut commenced. As soon as started, the operator is able to “carry-on” at will.

[Illustration: FIG. 85.--When no Edge is Available to Start the Cut on Large Work, Much Time may be Saved by Making a Curl with a Cold Chisel, as Shown.]

(=152=) An armored hose is generally used on the oxygen line for cutting, as well as on the acetylene line, as there is much more pressure used in cutting than in welding. This type of hose wears much longer and does not kink to the extent that the unprotected hose does. The armor protects both lines from being burned by the melted metal, which is very apt to come in contact with the rubber, were it not protected in some manner.

(=153=) The question often arises in welding circles, as to why, since the cutting torch contains a series of neutral flames, it would not be just as well to use such a method in welding, as no doubt more heat could be obtained and a greater surface handled. The answer to such a question would be, that the opportunity for oxidation is so great that successful welding could not be expected, although if this were the last means at a welder’s disposal, he would certainly be justified in making a weld in this manner. He should be very careful, however, to see that his extra oxygen supply is completely shut off and that there is no possible chance for that gas leaking into the weld.

(=154=) To plunge a flame, such as is used in the cutting torch, under water and see it continue to burn while submerged, looks quite marvelous to the average layman. Yet in cutting piling along water fronts this is continually being done. Not only does the torch stay lighted, but it retains much of its efficiency as a cutting tool, and some instances have been recorded where cutting has been accomplished at a depth of thirty feet under the sea. It is true that the water conducts a large part of the heat away very rapidly, but to facilitate such operations, an air line is brought down which ejects air under the torch and clears the water away to some extent, but this is not necessary. In order to explain this phenomenon in a very simple way, it will be stated that nothing will burn unless oxygen is present, and the more oxygen used, up to a certain point, the more rapidly will the burning take place. When submerging the cutting torch, it is presumed that the flame obtains what added oxygen is necessary from the cutting jet and this together with the velocity of the flame and its hydrogen enveloping flame permits the neutral flame to continue burning.