WELDING METHODS & EQUIPMENT

Part Two - Arc Welders

Arc welders are the simplest form of electric welder and are suitable for welding steel and cast iron. Note though, that successfully welding cast iron, such as exhaust manifolds etc, is highly specialised and technically difficult so is beyond the scope of most home users. Despite their reputation for being hard to master and difficult to use on thin sheet they are very effective, reliable and have the lowest operating costs.

They work on a simple principle that will be evident to anyone who has seen a tree after it has been struck by lightning; it is the fact that electric arcs concentrate enormous amounts of heat into a very small area.

In Arc welding, the electric arc is deliberately struck (by tapping or scratching the rod on the metal to be welded) between a special rod and the metal surface to be welded. The heat produced melts the rod and fuses it with the metal below. This can be illustrated as follows

Both Edges of the metal are heated by the ARC (A) until they melt and flow together forming one piece (B). Simultaneously more molten metal and flux is added from the rod (C) which fills the crater and covers the top of the weld with slag (D)

Picture copyright SIP (Industrial Products) Ltd

There is one problem with all this heat and molten metal coming together, and that is its reaction with Oxygen and other gases in the atmosphere. If these come into contact with the molten weld pool the resultant weld will be very porous and erratic. To combat this, the special rod used in Arc welding has a coating all around it, called flux. The flux melts along with the rod but floats onto the top of the weld pool, protecting the molten metal beneath from the problematic atmospheric gasses. Once the weld cools the melted flux, or slag as it is now known, can be chipped off to reveal a clean, strong weld beneath. One major advantage of the flux coating is that Arc welding can be performed out of doors, when even a light breeze can prevent the use of other welders.

However, poor and inexperienced welders often find the slag can become trapped in the weld pool itself, seriously weakening the joint. Whatsmore, electric arc welders have limited controls and so rely much more on the welding skill of the operator. This, coupled with the fact that the weld pool and slag coating retain rather than dissipate heat, means that often as not, holes are blown through the sheet being welded. The thinner is the sheet, the greater is the chance of blow through.

The special welding rods however, are available in differing diameters and so for thinner sheet, smaller diameter rods help since they require less power to melt. Incidentally, never buy your welding rods in packets of 10 or so from motor accessory shops. Bought in small quantities like this they are horrendously expensive per rod and 10 won’t last 5 minutes!! Instead, visit a local hire or tool shop (or mail order them from us!) and buy 5kg packets at a time. 5kg packets cost anything from £12 – £20 depending on rod diameter (smaller diameters cost more). Don’t buy or use rods with a damaged flux coating. Treat them gently and store them in a warm dry place and they will last indefinitely.

Because they require greater skill when welding thinner steel, Arc Welders really come into their own when being used to fabricate sturdy structures from materials that are no less than 3mm thick. For ‘heavy’ fabrication work of this type and in the hands of a skilled operator, arc welders are both ideal and cost effective. Sadly, most automotive work however, falls outside the ideal conditions for an Arc welder and so for this type of work, other types of welders must be found.

Having said all this, if an Arc Welder is what you need, here are some handy tips on selecting a suitable machine.

Given that Arc Welders are best suited to heavier fabrication work, and that this requires a higher output current, you should be considering a welder with an output of at least 140 Amps. Be careful though, all this power generates heat in the welder’s transformer so if you are going to be using it for prolonged periods make sure it comes with a cooling fan (often called "Turbo" models). Related to this is the so called Duty Cycle which in simple terms is the percentage of time the welder can actually be used in any given period without it automatically switching off to cool down. For instance, a duty cycle of 40% at 120 Amps means that you could weld for up to 40 minutes in any 100 minute period at a 120 Amp output setting. In practise you won’t actually be welding continuously for 40 minutes (!!) but rather as you stop and start to complete different weld runs, the time the welder is ‘resting’ should average 60 percent of the time to prevent it switching off to cool. Copper wound transformers tend to produce less waste heat and so have better duty cycles than aluminium equivalents so these are a bonus.

Better welders have variable transformers that can be varied to suit the diameter of welding rod being used but should come ready fitted with a gauge to give an approximate idea of the size of rod corresponding to the output setting. In practise the exact setting will be determined by the operator so a few practise welds are in order when changing rod sizes and output settings.

As we just mentioned, the welder must be fitted with a thermal overload device to switch it off automatically if it gets too hot. Check too that it has a strong steel case and is balanced when lifted by its carrying handle. Very heavy duty welders should be wheel mounted otherwise they are a back injury waiting to happen. Even for smaller ‘hand portable’ welders a small trolley is a wise investment to save lifting injuries.

Better quality machines have thick, very flexible cables with a sturdy earth clamp and welding rod holder. The quality of these is a sure indicator of the quality of the overall machine since these are the first items to get ‘skimped’ in low cost manufacture. Another good sign are large, stud type connectors on the front of the machine to which the welding cables are fastened by big knurled knobs. These allow easy replacement of the welding cables and again are omitted on cheaper welders, the cables being fed through the case directly into the machine.

All in all, because of their simplicity and lack of moving parts, Arc Welders seldom go wrong and last for many, many years. Entry level machines start at around £60 but a budget of £100 -£200 (depending on output power) will get you an excellent machine with all the features and benefits described, far less cost than any other kind of electric welder so "you pays your money and you takes your choice" as it were. However, don’t forget what we said earlier about the difficulties of using them for thin metal, automotive work. For this sort of thing, MIG welders are a better option and we will explore these in more detail next month.

These and Other Tools are AVAILABLE NOW! from the Difflock 4x4 Multi Store

A good quality fan cooled Arc Welder with infinitely variable output, heavy-duty studs for attaching welding cables and a sturdy case and carrying handle. Shame about the low quality hand held welding helmet included in the box!

For further information about any aspect of welding, Please email: info@difflock.com

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