Undoubtedly, one of the best means of cutting steel readily available in the well equipped workshop is the oxy‑acetylene or oxy‑LPG process. Clean, accurate cuts, often requiring no further finishing treatments, are quickly obtained. When this equipment is not available a rough but handy substitute can be found in arc cutting, which relies not on burning of the metal as with the oxy cutting technique, but the simple melting and forcing away by the high heat intensity, forceful arc. Special cutting electrodes naturally give the best results; when not available, best results are obtained with those structural electrodes which combine a naturally forceful arc with good current carrying capacity. Dipping of these electrodes in water for a few minutes prior to arcing will increase both of these characteristics.

Because of the high arc voltage the actual current being used with special cutting electrodes will be different from that shown on the machine. Most manufacturers nominate machine amps but the best setting will vary with each machine. Select a current close to the maximum without detrimental over‑heating of the electrode.

When not using special cutting electrodes, the region of the maximum current for the next largest size to the size selected e.g. for a 3.2mm electrode choose a hot4mm size current. Direct the electrode into the work in the direction of cutting, working from the outside edge. Use an up and down sawing motion, the up arc length being increased to increase heating, the down arc length being decreased to force the molten metal out of the groove.

Because it is purely a melting action, stainless steels and cast iron, non‑ferrous metals, etc, can also be cut, the thickness of cuts being mainly determined by the amperage capacity of the plant in use. In better class work a finish grind is desirable.

The same principle may be used for the piercing of holes for bolts etc., particularly in hardened materials not easily drilled, such as spring leaves. In piercing, draw a long arc over the spot to be pierced, using a small circular motion until the metal surface is molten. Then the electrode tip is forced down through the metal. Size and shape of the hole may then be trimmed by the cutting action.

CARBON ARC

For the farm workshop that does not run to an oxy‑acetylene welding, cutting and heating plant, the carbon arc torch can prove a handy tool.

This comprises two copper coated carbon electrodes held at an angle to each other with an arc being drawn (4‑6mm) between them. The flame produced is held over the work as the source of heat, being moved closer or further away to control the heat input. The arc is independent of the work and it may be used as a source of localised intense heat for heating for bending etc. brazing and soldering, fusion welding or hard surfacing.

The twin carbon torch is best used with AC welders one electrode being connected to the electrode terminal, the other to the work terminal. Typical amperage setting are 40 amps for 6mm carbons ('/4"), 55 for 8mm (Shs'") and 70 for 9.5mm (3/a'"). If used on DC sets then a size larger carbon should be selected for connection to the positive pole than the negative carbon.

A single carbon may be used on DC in a standard jaw type holder with the work being connected to positive pole and the electrode to the negative pole. An arc can be struck and maintained as a localised heat source.

A single carbon can also be used as a source of resistance heating for soft soldering a joint. Here either AC or DC can be employed and no arc is established. The carbon is shaped to a point and drafted along the previous fluxed joint with the solder being added to the joining with the other hand. No head‑shield is required as no arc is drawn, the point of contact being shielded by the gloved hand from the eyes when making or breaking the circuit.

SOME WORKSHOP HINTS ON JOINT PREPARATION

Clean joints are best

Spend a little extra time on joint preparation. it is worth it. A very rusty weld area can only result in poor welds. Light rust may be brushed away but heavy scale can be cleaned by moving over the surface with the oxy torch or a grinder.

Grease and oil impregnated parts will burn clean under the oxy flame or can be cleaned up with solvent first. (Watch out for flammable solvents retained in hollow sections). This is particularly important with open textured cast irons when years of service in oil ensure porous welds if absorbed lubricants are not removed first.

Don't expect super‑penetration

Relying on a normal electrode to deeply penetrate a tight joint even

if you do raise the amps‑ is not enough. Joints should be opened to receive the filler metal. Even if butt welds can't be oxy cut to ideal joint preparation, grind or arc cut a bevel if this is indicated by the material thickness. Where such refinements are not available or practical, gap to allow greater penetration and if necessary use a permanent (steel) or temporary( copper) backing bar.

It is particularly important to get full penetration on most joints if the reverse side cannot be welded.

The lower the penetration properties of the electrode, the more the preparation must be opened up to accept the weld metal. In cast iron, grind or chip the cracked section to a shallow U preparation rather than a vee.