Cutting with the electric arc can be done very rapidly and economically and is done a great deal in foundries, scrap yards, Fig. 87, and other places. The slot cut is not so narrow as that cut by a gas flame, which is discussed later, but the metal is good after fusing out and may be used again. This is not always the case with metals cut by gas processes because there is a chemical action between the gases and the metals, whereas the arc is composed of the vapors of the metals worked upon and, aside from a slight oxidation when cutting iron or steel, there is no injurious change in the metals cut. Some of the more volatile elements may be reduced by the high temperature but the mass being cut will remain unchanged.
Fig. 87. Cutting Scrap with a Cirbon Electrode Courtesy of C & C Electric and Manufacturing Company.
Cutting is done with the graphite electrode and requires from 100 amperes on sheet metal up to several hundred on heavy castings and forgings. The maximum current which it is practical or necessary to use should never exceed 1000 amperes and the usual cutting operations take from 400 to 600 amperes. Direct current at 70 volts is used, the same as for welding, and almost any source of supply will give satisfactory results if proper means are provided for controlling the current. However, apparatus which has been specially developed for the service is much better and more reliable than makeshift devices and, as experience shows that the cost of cutting is just as important as the cost of welding, dead resistances should not be used.
The rate of cutting has been found to be very close to one square inch of cross section per minute for each 100 amperes used in the arc. This rate will be increased slightly for sheet-metal work but applies very closely for heavy sections. On the basis given, a section 4 inch by 6 inch (24 square inches) can be cut in 4 minutes with 600 amperes, and experience shows this to be true. A steel plate 1 inch thick and 1 foot wide (12 square inches) can be cut in 2 minutes with 400 to 450 amperes; copper, aluminum, and other metals can be cut at about the same rates as steel sheet. When cutting it is necessary to make the slot wide enough to allow the arc to reach to the bottom of the cut instead of jumping to the sides and the piece should be placed so that the molten metal can run out of the slot. Work should begin at the top of the piece.