Oxy-Acetylene Welding and Cutting Part 5

A flux is used to float off physical impurities from the molten metal; to furnish a protecting coating around the weld; to a.s.sist in the removal of any objectionable oxide of the metals being handled; to lower the temperature at which the materials flow; to make a cleaner weld and to produce a better quality of metal in the finished work.

The flux must be of such composition that it will accomplish the desired result without introducing new difficulties. They may be prepared by the operator in many cases or may be secured from the makers of welding apparatus, the same remarks applying to their quality as were made regarding the welding rods, that is, only the best should be considered.

The flux used for cast iron should have a softening effect and should prevent burning of the metal. In many cases it is possible and even preferable to weld cast iron without the use of a flux, and in any event the smaller the quant.i.ty used the better the result should be. Flux should not be added just before the completion of the work because the heat will not have time to drive the added elements out of the metal or to incorporate them with the metal properly.

Aluminum should never be welded without using a flux because of the oxide formed. This oxide, called alumina, does not melt until a heat of 5,000 Fahrenheit is reached, four times the heat needed to melt the aluminum itself. It is necessary that this oxide be broken down or dissolved so that the aluminum may have a chance to flow together. Copper is another metal that requires a flux because of its rapid oxidation under heat.

While the flux is often thrown or sprinkled along the break while welding, much better results will be obtained by dipping the hot end of the welding rod into the flux whenever the work needs it. Sufficient powder will stick on the end of the rod for all purposes, and with some fluxes too much will adhere. Care should always be used to avoid the application of excessive flux, as this is usually worse than using too little.

SUPPLIES AND FIXTURES

_Goggles._--The oxy-acetylene torch should not be used without the protection to the eyes afforded by goggles. These not only relieve unnecessary strain, but make it much easier to watch the exact progress of the work with the molten metal. The difficulty of protecting the sight while welding is even greater than when cutting metal with the torch.

Acetylene gives a light which is nearest to sunlight of any artificial illuminant. But for the fact that this gas light gives a little more green and less blue in its composition, it would be the same in quality and practically the same in intensity. This light from the gas is almost absent during welding, being lost with the addition of the extra oxygen needed to produce the welding heat. The light that is dangerous comes from the molten metal which flows under the torch at a bright white heat.

Goggles for protection against this light and the heat that goes with it may be secured in various tints, the darker gla.s.s being for welding and the lighter for cutting. Those having frames in which the metal parts do not touch the flesh directly are most desirable because of the high temperature reached by these parts.

_Gloves._--While not as necessary as are the goggles, gloves are a convenience in many cases. Those in which leather touches the hands directly are really of little value as the heat that protection is desired against makes the leather so hot that nothing is gained in comfort. Gloves are made with asbestos cloth, which are not open to this objection in so great a degree.

[Ill.u.s.tration: Figure 9.--Frame for Welding Stand]

_Tables and Stands._--Tables for holding work while being welded (Figure 9) are usually made from lengths of angle steel welded together.

The top should be rectangular, about two feet wide and two and one-half feet long. The legs should support the working surface at a height of thirty-two to thirty-six inches from the floor. Metal lattice work may be fastened or laid in the top framework and used to support a layer of firebrick bound together with a mixture of one-third cement and two-thirds fireclay. The piece being welded is braced and supported on this table with pieces of firebrick so that it will remain stationary during the operation.

Holders for supporting the tanks of gas may be made or purchased in forms that rest directly on the floor or that are mounted on wheels. These holders are quite useful where the floor or ground is very uneven.

_Hose._--All permanent lines from tanks and generators to the torches are made with piping rigidly supported, but the short distance from the end of the pipe line to the torch itself is completed with a flexible hose so that the operator may be free in his movements while welding. An accident through which the gases mix in the hose and are ignited will burst this part of the equipment, with more or less painful results to the person handling it. For that reason it is well to use hose with great enough strength to withstand excessive pressure.

A poor grade of hose will also break down inside and clog the flow of gas, both through itself and through the parts of the torch. To avoid outside damage and cuts this hose is sometimes encased with coiled sheet metal.

Hose may be secured with a bursting strength of more than 1,000 pounds to the square inch. Many operators prefer to distinguish between the oxygen and acetylene lines by their color and to allow this, red is used for the oxygen and black for acetylene.

_Other Materials._--Sheet asbestos and asbestos fibre in flakes are used to cover parts of the work while preparing them for welding and during the operation itself. The flakes and small pieces that become detached from the large sheets are thrown into a bin where the completed small work is placed to allow slow and even cooling while protected by the asbestos.

Asbestos fibre and also ordinary fireclay are often used to make a backing or mould into a form that may be placed behind aluminum and some other metals that flow at a low heat and which are accordingly difficult to handle under ordinary methods. This forms a solid mould into which the metal is practically cast as melted by the torch so that the desired shape is secured without danger of the walls of metal breaking through and flowing away.

Carbon blocks and rods are made in various shapes and sizes so that they may be used to fill threaded holes and other places that it is desired to protect during welding. These may be secured in rods of various diameters up to one inch and in blocks of several different dimensions.

CHAPTER III

ACETYLENE GENERATORS

Acetylene generators used for producing the gas from the action of water on calcium carbide are divided into three princ.i.p.al cla.s.ses according to the pressure under which they operate.

Low pressure generators are designed to operate at one pound or less per square inch. Medium pressure systems deliver the gas at not to exceed fifteen pounds to the square inch while high pressure types furnish gas above fifteen pounds per square inch. High pressure systems are almost unknown in this country, the medium pressure type being often referred to as "high pressure."

Another important distinction is formed by the method of bringing the carbide and water together. The majority of those now in use operate by dropping small quant.i.ties of carbide into a large volume of water, allowing the generated gas to bubble up through the water before being collected above the surface. This type is known as the "carbide to water" generator.

A less used type brings a measured and small quant.i.ty of water to a comparatively large body of the carbide, the gas being formed and collected from the chamber in which the action takes place. This is called the "water to carbide" type. Another way of expressing the difference in feed is that of designating the two types as "carbide feed" for the former and "water feed" for the latter.

A further division of the carbide to water machines is made by mentioning the exact method of feeding the carbide. One type, called "gravity feed"

operates by allowing the carbide to escape and fall by the action of its own weight, or gravity; the other type, called "forced feed," includes a separate mechanism driven by power. This mechanism feeds definite amounts of the carbide to the water as required by the demands on the generator.

The action of either feed is controlled by the withdrawal of gas from the generator, the aim being to supply sufficient carbide to maintain a nearly constant supply.

_Generator Requirements._--The qualities of a good generator are outlined as follows: [Footnote: See Pond's "Calcium Carbide and Acetylene."]

It must allow no possibility of the existence of an explosive mixture in any of its parts at any time. It is not enough to argue that a mixture, even if it exists, cannot be exploded unless kindled. It is necessary to demand that a dangerous mixture can at no time be formed, even if the machine is tampered with by an ignorant person. The perfect machine must be so constructed that it shall be impossible at any time, under any circ.u.mstances, to blow it up.

It must insure cool generation. Since this is a relative term, all machines being heated somewhat during the generation of gas, this amounts to saying that a machine must heat but little. A pound of carbide decomposed by water develops the same amount of heat under all circ.u.mstances, but that heat can be allowed to increase locally to a high point, or it can be equalized by water so that no part of the material becomes heated enough to do damage.

It must be well constructed. A good generator does not need, perhaps, to be "built like a watch," but it should be solid, substantial and of good material. It should be built for service, to last and not simply to sell; anything short of this is to be avoided as unsafe and unreliable.

It must be simple. The more complicated the machine the sooner it will get out of order. Understand your generator. Know what is inside of it and beware of an apparatus, however attractive its exterior, whose interior is filled with pipes and tubes, valves and diaphragms whose functions you do not perfectly understand.

It should be capable of being cleaned and recharged and of receiving all other necessary attention without loss of gas, both for economy's sake, and more particularly to avoid danger of fire.

It should require little attention. All machines have to be emptied and recharged periodically; but the more this process is simplified and the more quickly this can be accomplished, the better.

It should be provided with a suitable indicator to designate how low the charge is in order that the refilling may be done in good season.

It should completely use up the carbide, generating the maximum amount of gas.

_Overheating._--A large amount of heat is liberated when acetylene gas is formed from the union of calcium carbide and water. Overheating during this process, that is to say, an intense local heat rather than a large amount of heat well distributed, brings about the phenomenon of polymerization, converting the gas, or part of it, into oily matters, which can do nothing but harm. This tarry ma.s.s coming through the small openings in the torches causes them to become partly closed and alters the proportions of the gases to the detriment of the welding flame. The only remedy for this trouble is to avoid its cause and secure cool generation.

Overheating can be detected by the appearance of the sludge remaining after the gas has been made. Discoloration, yellow or brown, shows that there has been trouble in this direction and the resultant effects at the torches may be looked for. The abundance of water in the carbide to water machines effects this cooling naturally and is a characteristic of well designed machines of this cla.s.s. It has been found best and has practically become a fundamental rule of generation that a gallon of water must be provided for each pound of carbide placed in the generator. With this ratio and a generator large enough for the number of torches to be supplied, little trouble need be looked for with overheating.

_Water to Carbide Generators._--It is, of course, much easier to obtain a measured and regular flow of water than to obtain such a flow of any solid substance, especially when the solid substance is in the form of lumps, as is carbide This fact led to the use of a great many water-feed generators for all cla.s.ses of work, and this type is still in common use for the small portable machines, such, for instance, as those used on motor cars for the lamps. The water-feed machine is not, however, favored for welding plants, as is the carbide feed, in spite of the greater difficulties attending the handling of the solid material.

A water-feed generator is made up of the gas producing part and a holder for the acetylene after it is made. The carbide is held in a tray formed of a number of small compartments so that the charge in each compartment is nearly equal to that in each of the others. The water is allowed to flow into one of these compartments in a volume sufficient to produce the desired amount of gas and the carbide is completely used from this one division. The water then floods the first compartment and finally overflows into the next one, where the same process is repeated. After using the carbide in this division, it is flooded in turn and the water pa.s.sing on to those next in order, uses the entire charge of the whole tray.

These generators are charged with the larger sizes of carbide and are easily taken care of. The residue is removed in the tray and emptied, making the generator ready for a fresh supply of carbide.

_Carbide to Water Generators._--This type also is made up of two princ.i.p.al parts, the generating chamber and a gas holder, the holder being part of the generating chamber or a separate device. The generator (Figure 10) contains a hopper to receive the charge of carbide and is fitted with the feeding mechanism to drop the proper amount of carbide into the water as required by the demands of the torches. The charge of carbide is of one of the smaller sizes, usually "nut" or "quarter."

_Feed Mechanisms._--The device for dropping the carbide into the water is the only part of the machine that is at all complicated. This complication is brought about by the necessity of controlling the ma.s.s of carbide so that it can never be discharged into the water at an excessive rate, feeding it at a regular rate and in definite amounts, feeding it positively whenever required and shutting off the feed just as positively when the supply of gas in the holder is enough for the immediate needs.

[Ill.u.s.tration: Figure 10.--Carbide to Water Generator. A. Feed motor weight; B. Carbide feed motor; C. Carbide hopper; D. Water for gas generation; E. Agitator for loosening residuum; F. Water seal in gas bell; G. Filter; H. Hydraulic Valve; J. Motor control levers.]

The charge of carbide is unavoidably acted upon by the water vapor in the generator and will in time become more or less pasty and sticky. This is more noticeable if the generator stands idle for a considerable length of time This condition imposes another duty on the feeding mechanism; that is, the necessity of self-cleaning so that the carbide, no matter in what condition, cannot prevent the positive action of this part of the device, especially so that it cannot prevent the supply from being stopped at the proper time.

The gas holder is usually made in the bell form so that the upper portion rises and falls with the addition to or withdrawal from the supply of gas in the holder. The rise and fall of this bell is often used to control the feed mechanism because this movement indicates positively whether enough gas has been made or that more is required. As the bell lowers it sets the feed mechanism in motion, and when the gas pa.s.sing into the holder has raised the bell a sufficient distance, the movement causes the feed mechanism to stop the fall of carbide into the water. In practice, the movement of this part of the holder is held within very narrow limits.

_Gas Holders._--No matter how close the adjustment of the feeding device, there will always be a slight amount of gas made after the fall of carbide is stopped, this being caused by the evolution of gas from the carbide with which water is already in contact. This action is called "after generation" and the gas holder in any type of generator must provide sufficient capacity to accommodate this excess gas. As a general rule the water to carbide generator requires a larger gas holder than the carbide to water type because of the greater amount of carbide being acted upon by the water at any one time, also because the surface of carbide presented to the moist air within the generating chamber is greater with this type.