Cutting

For cutting, the set-up is a little different. A cutting torch has a 60 or 90-degree angled head with orifices placed around a central jet. The outer jets are for preheat flames of oxygen and acetylene. The central jet carries only oxygen for cutting. The use of a number of preheating flames, rather than a single flame makes it possible to change the direction of the cut as desired without changing the position of the nozzle or the angle which the torch makes with the direction of the cut, as well as giving a better preheat balance. Manufacturers have developed custom tips for Mapp, propane, and polypropylene gases to optimize the flames from these alternate fuel gases.

The flame is not intended to melt the metal, but to bring it to its ignition temperature. The torch's trigger blows extra oxygen at higher pressures down the torch's third tube out of the central jet into the workpiece, causing the metal to burn and blowing the resulting molten oxide through to the other side. The ideal kerf is a narrow gap with a sharp edge on either side of the workpiece; overheating the workpiece and thus melting through it causes a rounded edge.

Cutting is initiated by heating the edge or leading face (as in cutting shapes such as round rod) of the steel to the ignition temperature (approximately bright cherry red heat) using the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet. The oxygen chemically combines with the iron in the ferrous material to instantly oxidize the iron into molten iron oxide, producing the cut. Initiating a cut in the middle of a workpiece is known as piercing. It is worth noting several things at this point:

The oxygen flowrate is critical — too little will make a slow ragged cut; too much will waste oxygen and produce a wide concave cut. Oxygen Tances and other custom made torches do not have a separate pressure control for the cutting oxygen, so the cutting oxygen pressure must be controlled using the oxygen regulator. The oxygen cutting pressure should match the cutting tip oxygen orifice.

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Уфимский государственный Кафедра общеобразовательных

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Consumable electrode methods

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. An electric current is used to strike an arc between the base material and a consumable electrode rod or 'stick'. The electrode rod is made of a material that is compatible with the base material being welded and is covered with a flux that protects the weld area from oxidation and contamination by producing C02 gas during the welding process. The electrode core itself acts as filler material, making separate filler unnecessary. The process is very versatile, requiring little operator training and inexpensive equipment. However, weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though specialty electrodes have made possible the welding of cast iron, nickel, aluminium, copper and other metals. The versatility of the method makes it popular in a number of applications including repair work and construction.

Gas metal arc welding (GMAW) is a semi-automatic or automatic welding process that uses a continuous wire feed as an electrode and an inert or semi-inert shielding gas to protect the weld from contamination. When using an inert gas as shield it is known as Metal Inert Gas (MIG) welding. A constant voltage, direct current power source is most commonly used with GMAW, but constant current .systems as well as alternating current can be used. GMAW welding speeds are relatively high due to the automatically fed continuous electrode, but is less versatile because it requires more equipment than the simpler SMAW process.

Заведующий кафедрой

общеобразовательных дисциплин

____________ В.П. Павлов

Уфимский государственный Кафедра общеобразовательных

авиационный технический дисциплин

университет

ЭКЗАМЕНАЦИОННЫЙ БИЛЕТ № 3

Факультет: АТС (СП) Вопрос: 1