Saturday, August 7, 2010

First steps with a MiG

Always practice on scrap metal and do not attempt any welding to the bodywork of your car until you are capable of producing consistently good results.
Safety is the most important consideration. If your workshop does not already possess one, then buy a fire extinguisher. Never weld in the vicinity of a petrol tank nor any other container which holds or has held combustible fluids, especially if the container is now empty or near-empty (an empty petrol tank contains more explosive fumes than a full one). Remember that paint, underseal and certain other materials to be found in a car (such as some sound-deadening material) can be flammable, and that many can be ignited by heat moving along a panel which is being welded. Keep a fire extinguisher handy for blowing out small welding fires.
Always use a proper welding mask. If you view the electric arc with the naked eye then you will later suffer an immensely painful phenomenon called arc eye. Arc eye is painful enough to drive most sufferers to seek hospital attention. The radiation given off by the MiG is not just harmful to eyes, but to skin as well, so always ensure that you are well protected.
Always wear protective clothing, especially strong leather gloves, and a hat (to prevent your hair from catching fire as the sparks shower) is a good idea. It is as well to wear old, thick items of clothing and stout leather shoes (red-hot weld splatter will burn through flimsy shoes and your sock — and when it reaches your foot it hurts like hell), as you will inevitably burn holes in most of them.
Never take liberties with the electric current, which is of a low voltage but quite powerful enough to kill you. Ensure that you weld only in dry conditions, and keep trailing leads off damp floors.
In MiG welding, an electric current passes down the MiG wire, melting both the end of the wire and the metal underneath it. so that the two fuse together. If the surface of the metal has any contaminants on it, including paint, rust or oil, then this will mix with the molten metal and weaken the joint. When welding typically thin car body steel, the steel panels become molten right through, and paint or other contaminants on the underside can be drawn up into the molten metal, again weakening the joint.
When first attempting to weld, try to run a bead onto a flat sheet of 18g or preferably 20g steel rather than attempting a joint between two pieces. Begin by cleaning the metal top and bottom thoroughly of all
rust, paint and grease. Trim the wire protruding from the MiG nozzle to around 10 mm. Place the earth clamp on the steel, put on all protective clothing then switch on the machine. Place the wire against the steel, pull the face visor in front of your eyes then press the trigger and begin to push or drag the gun along the surface of the steel, keeping the gun at an angle of around 70 degrees from the horizontal. Do not allow the mask to get too close to the weld, because sparks will quickly ruin it. Wrap-around face masks, particularly those which attach to the user's head, are recommended, because the alternative lollipop-type flat masks can allow in extraneous light from the top and sides which contract the pupils and make the viewed image of the welding process appear very dim.
When you first attempt to weld it will appear thateverything happens at once — sometimes too quickly foryou to establish gun movement before burning through begins. The solution is to keep on practising and adjusting the settings on the MiG to suit the steel you are welding until you master the art. The author is not possessed of particularly steady hands, and he has never found achieving good welds with the MiG an easy matter. The greatest problem is that of running the weld away from the intended join. He overcomes this problem to a great extent by resting the side of the MiG pistol grip against a solid object such as a length of scrap box section steel which is arranged so that it is in line with the intended join. Many people use proper head-mounted welding visors rather than the 'lollipop' type of mask typically supplied with cheaper welders, and this allows them to use their 'spare' (and heavily gloved) hand to help guide the MiG. Basically, the visor is tilted upwards so that the wearer can place the pistol grip onto the metal and support it using both hands (do not allow your hand too close to the 'business' end), then a flick of the head moves the visor downwards over the eyes, and welding can begin. Do not blame the author if you crick your neck trying this, though! The alternative is to wear MIG-proof goggles, but the author does not personally like these because radiation given off during a MiG welding session will tan your skin.
MiG 'plug' welding is an easy method of producing neat and strong joints. This simulates a spot weld, and is achieved by drilling holes in the uppermost of two panels which are to be joined, then clamping the panels tightly together and filling the holes with weld. The weld fuses to the bottom panel and to the side of the hole in the top panel. After surplus weld has been ground down, the results can be very neat and strong. However, do not use plug welds if you envisage ever having to remove the panel thus welded because, unlike spot welds, plug welds can turn out to be irregularly shaped and they cannot simply be drilled or cut out like spot welds. If you do elect to plug weld, the welds should be as frequent as the original spot welds which they are replacing.
Please check with your local vehicle testing authorities that plug welds are still acceptable for roadworthiness test purposes (MOT in the UK) before rebuilding your car using this technique. Although plug welds are perfectly acceptable at the time of writing, legislation does change and the author and publishers cannot be held accountable for future laws!
There are various types ofjoint which you will have to deal with. The butt joint is, as the name suggests, a join between two sheets of metal which butt against each other. A small gap should be left in between the two so that the weld can properly penetrate the joint, and the ideal tool for achieving this is the 'Inter-Grip'. This small device (sold in packs of five) can hold flat or curved panels tightly together for butt welding equally well. They are available from Frost Auto Restoration
Techniques (see address in rear of book). The author always tacks the two pieces of metal before continuously welding them, because if you start continuous welding at one end of the join and weld the whole lot in one go, distortion is very likely to occur.
Other joints include right angles (which can be difficult) and stepped joints (detailed in the following paragraph). Practice all types of joint because they will all be needed during a typical restoration.
A joddler (variously referred to as a 'joggler"jodder' and, more properly, as an edge setter) is a great aid. This tool places a step into the edge of a panel to allow it to overlap yet remain at the same level as the panel to which it is to be joined. The better joddlers incorporate a % in. punch, for punching holes in steel through which you can produce neat plug welds.
Two types of commercially manufactured joddler are commonly available. The less expensive is the scissors type, which can incorporate a plug weld hole punch.
The more expensive alternative works rather like a can opener and utilises two stepped wheels which are
pressed either side of the steel and then turned using a 'A in. ratchet drive as a winder. The author uses the scissors type, but found that the effort needed to step an edge into steel of greater thickness than 20g was too
high. He made up a cheap alternative using a large mole wrench, with two stepped blocks welded into the jaws (see photograph and illustration). The adjustable mole wrench allows pressure to be progressively built up as two or more passes are made over thick steel with the tool.
The joddled joint has a great advantage over the butt joint. Because the two halves of a joddled joint can be pulled tightly together and because the stepped edge of the joddled panel is parallel with the rest of the panel,
the two panels naturally tend to lie flat when they have been welded together. With the butt joint, it is easy to inadvertently weld the panels up so that they are not quite in line with each other. This becomes important when one of the two panels being joined is under any sort of stress. One instance which springs readily to mind is when a lower side (quarter) repair panel is being welded into position. The cutting process which removed the unwanted metal can easily have distorted the remaining metal. A joddled repair panel pulls this back into correct alignment when the panels are temporarily clamped with pop rivets or self tapping screws prior to welding.
If you have access to a spot welder, then you can utilise a useful alternative to joddled joints. By spot welding a strip of steel behind one edge so that it overlaps the other, you have a nice, flush joint to weld.
The alternative to doing the welding yourself is to bring in a skilled welder as and when required. There are many self-employed and mobile MiG and gas welders who may be hired by the hour, and they can usually be found listed in any commercial telephone directory.
When hiring a skilled welder it is as well to prepare as much work per visit as possible, otherwise the travelling expenses could eclipse the actual welding charges! For most DIY restorers who will only ever restore the one car, hiring a skilled welder is probably a better solution than learning to weld, because you will get better results and be able to drive your car safe in the knowledge that the welds will not spring open the first time you drive over a pot-hole!

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