MIG WELDING SAFETY

Setting Up a MIG Welder

Whether you have purchased a new or second hand welder, it will have to be set up before use. New welders will have an instruction manual, but secondhand units may come without documentation. Nowadays many manuals can be found free on the Internet, however the following will hopefully prove useful if a manual is not available. Whilst different models will vary in design, the basic layout should be similar to that illustrated in Figure 1. The following instructions are based on setting up the typical MIG welder supplied for home use. The gas content of the bottle will vary depending on the type of metal to be welded.        75% Argon 25% CO ² for thin mild steel.        100% Argon for Aluminium & Stainless steel.        100% CO² for Mild steel. 100% CO² produces more heat and deeper weld penetration, but results in oxide formation in the weld. It is also unsuitable when welding thin steel. Use the 75% Argon 25% CO ² for steel panels with the appropriate 6mm wire.

Ensure the welder is NOT connected to the power supply. Attach the gas bottle to the welder with the steel band provided. The small disposable cylinders will usually have a protective plastic cap over the threaded top of the bottle. Remove the cap and screw the regulator on clockwise taking care not to cross the threads. Make sure the regulator is turned to zero and connect the gas tube between the regulator and the welder.

Remove the side panel of the welder to reveal the spool holder and wire feed rollers. Usually MIG wire is supplied in 0.6mm or 0.8mm wire size. Mild steel can be welded with either size but stainless and aluminium are usually welded with 0.8mm wire. The correct type of wire must be used depending on the composition of the material to be welded. Having selected the correct type of wire unscrew the knob or nut on the spool shaft and fit the spool of wire. Always use clean wire. Old oxidised wire will rapidly wear the liner. Replace the spool retainer on the shaft and do not overtighten. It acts as a gentle brake and prevents the spool from over running, but it must allow the wire to run freely.

Slacken the tension roller adjuster (turn anti-clockwise) to release the pressure on the wire feed roller, then lift the tension roller clear of the feed roller (see Figure 1). The feed roller usually has two grooves in it. One groove is for 0.6mm wire and the other for 0.8mm wire. The feed roller can be reversed so that the appropriate groove is aligned with the guide tube. Check to make sure the groove matches the wire size you are using. If the welder has been used previously, remove any wire left in the wire liner. Use sharp wire cutters to cleanly clip the end of the new wire and straighten the first 15cm, then carefully thread the wire into the guide tube. Ensure the wire is sitting properly in the groove of the wire feed roller, then lower the pressure roller and gently tighten the tension roller adjuster. It must not be too tight or it will damage the feed motor. If it is too loose, the wire will not feed properly. Remove the gas shroud and unscrew the contact tip. Replace the side panel and connect the welder to the power supply, then press the on button. Press the welding torch trigger until the wire is extruded at the end of the torch. Switch off the power and replace the contact tip and gas shroud. Always check the size of the tip is correct for the wire being used.

Adjusting Wire Speed, Power and Gas Flow

Ensure the ground clamp is not near the welding torch, then switch the welder on and extrude approximately 10 - 15mm of wire from the end of the torch. This distance is known as the "stick out" and is the distance of the contact tip to the work. This is the distance to achieve optimum welding conditions. If you have a manual, look up the table advising the settings for various types and thickness of material. If not, the following advice will hopefully help. The controls on the front view of the welder in (Figure 1) may differ from yours. The power knob for example may be replaced with a selection of switches, but the three main conditions governing the use of the machine are the same. They are Gas regulation, Power and Wire feed speed. More expensive professional machines often combine and synchronise power and feed speeds into one control knob. Before adjusting the settings on the welder, obtain a piece of scrap metal to practise on (make sure it is similar to the metal you intend to weld). Attach the ground clamp firmly onto the scrap metal. Make sure you are wearing the correct safety clothing and helmet, then adjust the following controls.

The GAS REGULATOR setting is important bcause too low a setting will cause the weld to suffer from porosity. Welding gas is expensive, so keeping the flow as low as possible without compromising the weld is important. The regulator will require adjusting for different power settings. As a starting point 2/3 litres/minute should be adequate.

The POWER SWITCH setting has the greatest influence over the weld. Low power results in a shallow scruffy weld, whilst high power will penetrate more deeply and wider. Assuming you have no users manual, adjusting the power setting half way should be a reasonable starting point when experimenting on scrap metal. Obviously thinner metal requires less power. Holes will be blown through thin metal if the power control is set too high.

The WIRE FEED SPEED also effects the weld. If wire speed is set too low, penetration will be too shallow. Setting the wire speed too fast will result in penetration being too deep for thin metal. Practising with a thin piece of steel, start with the speed setting half way. The weld will probably sputter and crackle. Slowly decrease the speed setting until the crackling changes to a pleasant buzz, like cooking bacon. This is the correct speed. If the power setting is changed so will the speed setting need to be adjusted.

Tuning the welder to get the best settings is an important process. Use a piece of scrap metal to practise welding whilst trying various tuning settings. The user's manual should have a chart showing typical settings for various types of metals. If you do not possess a manual, try to find one on the Internet.

Welding Thin Steel

Before attempting to weld for the first time, practise on scrap metal. Use similar metal to that which you intend to work, and ensure the wire stick out on the gun is between 5 - 10mm. Some MIG machines have a table recommending power and wire speeds for various thickness metals. This table is often in the MIG owners manual. If no information is available, try position 2 on the power control and 4 or 5 on the wire feed. Car renovation usually involves welding thin steel. 18 gauge steel is 1.2mm thick and care has to be taken when welding it. High power settings and fast wire feed will produce high temperatures during welding. Thin steel will warp if exposed to high temperature so power and wire feed must be kept relatively low, but high enough for good weld penetration. Attach the ground clamp to the scrap metal. Make sure you are wearing the recommended MIG welding helmet and safety clothing. Hold the gun at a 45⁰ angle with the nozzle tip 5 - 10mm from the work-piece and squeeze the trigger. Move the gun smoothly. If the wire speed is high enough you should hear a coarse crackling sound with a bit of spatter. Start to turn the wire speed down until the sound turns to a smooth buzz like that of frying bacon. If you find you have blown holes in the steel, the power and wire speed are too high or you are moving the gun too slow. Scruffy welds with a high profile and little penetration can be a sign of low power, too much stick out or other errors (see Figure 3). When you can weld short lengths that penetrate through the steel without producing a hole, the conditions are right. There are many good books and web sites with good advice on welding, but only practise will make you competent. If you try continuous long welds on thin steel without stopping, it will warp. This can be overcome by tack welding.

Tack welding is a technique used to help prevent thin steel from warping. Small brief welds are made at approximately 40mm spacing. Figure 2 illustrates the technique being applied to a butt weld and may give the impression that the welds are executed in sequence, however it is wise to tack at wide spacings and come back to fill in the spacings between. This gives the metal around the weld, time to cool down. If you own a compressor, play low pressure air around the welds before continuing. When enough tack welds have been made, you have a choice of welding short lengths between them or stitch welding. Stitch welding consists of joining many tackwelds together. This takes patience and time, but has the advantage of preventing warped panels. When the panels have been welded together, the welds can be ground flush. Always check for pin holes after grinding and weld them up.

Types of Welding Joints

The type of welding joints used will depend on various factors, including material thickness, shape, strength and whether the weld has to be flush etc. Figure 4 shows a typical selection along with their common names. External panels on a car will require a flush finish on the outside, with minimum distortion. In the case of wheel arches it is also desirable to achieve a flush joint on the inside, because water and dirt thrown up from the road will tend to gather on ledges and crevices, which can lead to corrosion. Figure 5 shows the repair job required on my rear wings.

Stage 1 requires assessing the degree of corrosion and making a template of the rusty steel to be removed.

Stage 2 involves making a steel replica from the template and scribing a line on the wing where the weld join is required. The rear wing has a swaged edge which will most probably have been made in the factory with swaging rollers. If you own swaging rollers you will be able to produce a swage deep enough to include the wired edge. Producing a wired edge is explained in Removing and Making a New Rear Panel. Unfortunately it is not likely for a DIY restorer to own rollers and the alternative is to use joddlers. Joddlers are shown in Figure 4. They are simple to use and inexpensive. The edge of the steel is placed in the jaws and the handles are squeezed. By slowly working round the perimeter of the metal, a swaged edge is produced. Unfortunately the swage is not deep enough to allow the edge to be rolled around the wire whilst retaining the same swage depth as the original wing profile. The solution is to weld the wire around the inside of the swaged edge. To replicate the curvature of the original wing, a contour gauge can be used.

Stage 3 requires using a butt weld to get a nice smooth finish on both inner and outer sides of the wing. When you are happy that the new steel is the correct size and shape, it is necessary to cut away the corroded steel. Use a hacksaw to cut through the wired edge first, then use the cutting tool of choice to cut along the scribed line on the wing. If you are using Monodex cutters, be careful to cut on the corroded side of the scribed line. Clamp the new piece of steel in place and continue to shape and file it until it fits the old wing perfectly. Tack weld the new piece in place then stitch weld it. I must admit, I used short welds between close tacks and got slight distortion on the curved part of the wing, but this was my first ever non practise weld. In the photograph of stage 2 there are a couple of polished dollies in the background. I used these to correct the distortion after grinding the welds. When everything looked almost perfect I finished the job with a little body solder which can be seen in the photograph in stage 3. This left the wing looking distortion free and after painting looking perfect.

Body soldering is an alternative to using polyester filler. The advantage of body solder is that it becomes an integral part of the body and if done properly, will not crack or shrink. It takes paint primer really well and I personally find it feathers better than polyester filler. If you mess up filing it down, it can be re-heated and worked again. Body solder kits are available which have all the necessary paddles, lead solder, flux, tallow and brushes etc. The area to be filled must be free from rust and meticulously cleaned then brush coated with solder paste which acts as a flux to tin the metal. The paste is then gently heated until it appears to go brown. Using a clean rag the paste is wiped over until it turns bright silver. Body solder is usually supplied in sticks composed of 70% lead and 30% tin and it melts quite easily. By placing the ends of the sticks on the tinned area and heating with a butane torch, melt blobs of solder on the area to be filled. A hard wood paddle is then used to spread the melted solder, which spreads like butter until it is covers the area to be filled. Whilst it is still warm the area should be washed with cold water to remove traces of the solder paint. A flexible body file is then used to shape the body solder to the original body contours. Care should be taken when finally sanding the finish. A good mask should be worn to avoid breathing the fine lead dust.

Other panels on a Morgan will require different types of weld. The bulkhead on old Morgans usually rusts extensively where it is overlapped by the inner front wing. On wet days water flowing down the front wing is guided into this area. Nowadays the bulkhead is manufactured from stainless steel, but in the past the inner wing and bulkhead were made of mild steel and joined together before painting the car. Welding new steel into the bulkhead involves joddled, lap, fillet and corner welds.