The use of oxygen for metal cutting was introduced in this country by the British Oxygen Co., Ltd., in 1905. Since that date the process has steadily developed until it is now probably the largest and most important application of oxygen in industry. The process is based on the well-known fact that a jet of oxygen directed upon a previously heated spot of metal ignites it, with the result that the metal acting as its own fuel burns away rapidly in the form of iron oxides. It is found necessary in practice to maintain an independent heating jet as well as the oxygen cutting jet in order to make the cutting operation continuous. The additional heat thus furnished renders the oxides fluid, so that they can readiy be blown away through the cut by the separate jet of oxygen.

For hand cutters, employed in destructional work, acetylene is almost invariably used as the combustible gas, for the oxy-acetylene flame has the highest calorific value, and quickly heats up the surface of the metal, even when it is badly corroded, as is often the case where scrap is being dealt with. Other combustible gases, such as hydrogen and coal gas, are frequently employed instead of acetylene on constructional work, notably in connection with oxygen cutting machines, where, on account of its low heating value, coal gas is very successfully employed.

We do not propose to deal with that branch of metal cutting here, as this is concerned with oxy-acetylene hand cutters.

Wrought Iron And Steel

The cutting of wrought iron, mild steel and cast steel, in the form of plate, bar, tube, or structural sections, is quite a simple matter and should not present any difficulty. By using the rivet cutting nozzle, rivet heads may be completely cut away without risk of injury to the surface of the plate. Tube cutting nozzles are used for cutting out boiler tubes and similar work where the cut has to be made from the inside of the tube. The nozzle operates in tubes down to 11/4in. bore and the cut may be made from lin. to 3in. from the end of tube, the roller guide being adjustable. Particulars of nozzle size, cutting speeds, pressure of oxygen, etc., are given in cutting tables. The pressures may be taken as correct for hand cutting, on the thickness of plate given, although the speeds obtained will vary according to the condition of the plate and the skill and experience of the operator.

It is a mistake to use too high a pressure when cutting; the rate of cutting is not increased to any appreciable extent, and oxygen is wasted unnecessarily.

If the plates to be cut are heavily corroded or thickly painted, it is advisable to clean the plates as much as possible before starting to cut. This is necessary from the point of view of safety to the operator, because the paint gives off injurious fumes liable to gas the operator, and the oxide surface may split and fly when hot, thus possibly causing bad burns should it alight on the operator ; the oxide is also liable to get into the cutting nozzle, and cause a back fire.

When cutting galvanised plates the operator should wear a gasman's respirator (see page 93).

When working for any length of time on plate more than three inches thick, it is an advantage to use a battery of cylinders, and a regulator warmer. The battery of cylinders obviates the necessity of continually stopping the work in progress in order to change cylinders, and the warmer serves the dual purpose of preventing the regulator freezing, and of increasing the rate of cutting.

A spade type of guide is usually supplied with hand cutters, and is quite satisfactory for all destruc-tional work, but if the line of cut is required to be as clean and regular as possible, a roller type of guide is to be preferred, a straight or curved line of cut being maintained by means of a suitable steel guide laid across the plate.

For very heavy cutting on steel above 12in. thick, the C.I. cutting pipe, will be found a useful tool; it is designed to work at a minimum pressure of 70 lbs. per square inch, and is capable of maintaining a large supply of oxygen and acetylene suitable for thick material.