And just as automakers turn out sedans, pickups, sports coupes and SUV's, there are several "makes" of welding machines, each serving a different purpose. The most common are called MIG, TIG, Stick and Oxyacetylene welders. There are also more expensive but versatile multi-process machines, as well as engine-driven (fuel-powered) welders for work off the electrical grid. If you're unfamiliar with the different welding processes, check out Skills to Learn before proceeding here.
As a new or aspiring welder, your prospects for employment will increase if you understand the features of many different types of equipment. Being able to decide which model works best for a particular assignment, and which filler rod, wire or stick electrode best meets code requirements will help qualify you to work as a supervisor, project assistant, weld technician or purchaser at your company. This article starts with the basics of how to choose a machine, then shows you how to read spec sheets included in the product sales literature. You'll also get to test your knowledge by evaluating the pros and cons of two sample machines on the market.
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Most welding is done on carbon steel, in the form of either pipe or sheet metal. Carbon steel (which is ordinary steel) can handle a lot of heat. Unlike the other metals listed below, this one is very forgiving when too much heat gets applied by a novice welder. Nearly all welding processes accommodate carbon steel. And you don't need a lot of features on the machine to produce a good-looking weld.
Stainless steel is much more finicky in how it deals with heat. Composed of steel, chromium and nickel, this alloy steel is used for food/beverage vessels and many other products, largely because of its anti-corrosion properties. It's typically welded using MIG or TIG machines and requires less current than carbon steel. You can also find stainless steel stick electrodes. This allows you to use a stick welding machine to get the job done. Of course, the base metal must be thick enough to stand the heat.
Aluminum is on another planet entirely. As a non-ferrous metal, aluminum conducts heat so well that you constantly need more of it to keep your puddle molten. At the same time, the work piece distorts easily if it gets too hot. Consequently, aluminum frequently requires more complex equipment to get the job done. You can use a MIG machine (especially one with a pulse welding feature), but many wirefeed mechanisms have trouble feeding the aluminum filler wire, so a special spool feeder must also be purchased. A good TIG welding machine is designed to weld aluminum. An AC power option is standard. An inverter, square wave, balance control and pulse feature are also helpful for welding aluminum. Naturally, these features add to the product cost.
Although it's not the preferred choice, a stick welding machine can also weld aluminum. Like stainless steel, the base metal must be thick enough to stand the heat.
Titanium (used on custom bicycles and airplanes), chromoly (used on motorcycles and automobiles), and other alloy steels and exotic metals have their own thermal sensitivity issues that welders must take into account. Because these metals are so expensive, you don't want to be making mistakes when you weld on them. Hence, they generally require a sophisticated TIG machine, along with plenty of set-up and fit-up, and a seasoned veteran at the controls.
The thicker the metal, the more current needed to weld a joint with good penetration. Since the cost of a welding machine is based in part on how much juice it generates, you'll have to determine in advance the maximum thickness of base metals and fixtures you're going to be working on in your shop.
Thick structural steel and pipe thicker than a half inch requires the use of a heavy duty MIG welding machine or a Stick welder. According to Miller Electric, you need one amp of power for every one-hundredth (i.e. .001) inch of mild steel thickness. For example, a 1/8" (.125 in) sheet of mild steel requires approximately 125 amps. Stainless steel needs about 10% less juice than carbon steel, while aluminum needs about 25% more. Current settings are also tied to the diameter of welding rods, as explained in this Miller tutorial for setting machine parameters.
Conversely, working with very thin metal requires different settings on a more sensitive welding machine. Now the objective is to provide just enough heat to get the job done. Sometimes a low current induces an unstable arc, and that's a welder's nightmare. Besides that, if too much heat enters the base metal, the area surrounding the weld weakens or melts. So many of the features described in the paragraph above about aluminum will also apply when welding on extremely thin stock for any metal type.
If you're careful, you can use an oxyacetylene kit to weld thin ferrous material, but make sure the torch can accommodate a tiny three-ought (i.e. 000) sized welding nozzle. This form of welding is discussed further in the "Cutting and Grinding" section.
As mentioned earlier, knowing where you will be welding most of the time figures into what sort of equipment you should purchase. There are a couple of things to think about:
Power supply: If you're plugging the machine into the wall (i.e. the power grid), your choices are as follows:
Offgrid Scenario: If you're welding outdoors and don't have access to the power grid, you'll need an engine-driven welder, or welder-generator to complete the assignment. Farmers and welders who work in the field typically buy this type of machine. Depending on the model, the generators run on gasoline, diesel or liquid propane (not all three), and may accommodate a stick welding torch, a Tig torch, or a MIG/Flux-core wirefeed unit and gun. The low end of this product niche starts at about $2000 and is used only for stick welding. When reviewing product sales literature, look for the symbols CC (constant current) and CV (constant voltage). CV machines are costlier, but are the right choice if you're plugging a MIG/flux cored welder into it. You'll also need to know your power requirements (i.e. maximum watts) in order to choose the right sized generator. Beware, the state of California only permits the use of generators meeting low carbon emission standards, otherwise known as CARB-compliant.
Windy conditions: If you expect to be welding in unsheltered areas where a breeze is possible, this may negatively affect your welds. The CO2/argon gas used in MIG to shield the weld's molten puddle until it solidifes will be ineffective, resulting in oxidation and porosity. Thus, you'll want to be able to switch the MIG machine over to flux-cored mode (or use a straight flux-cored welder). Alternatively, a stick welder will work in a breezy environment (but not too breezy) . Both of these welding processes contain solid deoxidizers within the wire or rod. They vaporize directly over the puddle during welding, leaving a protective layer of slag behind.
Here are a few key things to look at:
Duty Cycle: This spec tells you how much uninterrupted welding a machine can knock in ten-minutes. Traditionally, duty cycle is defined as the number of minutes out of a 10-minute period a welder can weld at the highest current the machine offers. After reaching the limit, the machine must be allowed to cool down.
Duty cycle is given as a percentage. So you have to do the math in your head, multiplying each percentage point by ten to get the number of minutes you can expect to weld per ten-minute interval. If you exceed the duty cycle, the machine heats up and the circuits inside may fry.
For example, a very inexpensive machine with a maximum current of 70 amps may have a 10 percent duty cycle. This means you can weld for 1 minute out of every 10 without the equipment overheating or burning out.
In general, light industrial/hobbyist welding machines have a duty cycle of 20%, medium duty 40-60%, and heavy duty 60-80%. But nowadays manufacturers are playing with the formula. In order to boast a higher duty cycle, they base the percentage on a lower amperage setting. So with a machine that provides a maximum 140 amps with a 10% duty cycle, you might see a rating of 30% at 115 amps instead.
On the up side, you can use the same tactic to get around a low duty cycle for a machine that otherwise fits all your needs. Just buy the model with a higher maxiumum current than you expect to use. That way, you effectively increase your duty cycle.
Open-Circuit Voltage: This is the voltage emanating from an arc welding torch or gun when current is not flowing. On the one hand, it's sort of dangerous to have a live circuit sitting around on the work bench with the potential to cause a serious injury. (That's why OSHA limits OCV on equipment.) On the other hand, OCV affects the way a torch electrode will perform when striking an arc, so the extra punch you get at start-up is important in some types of welding.
In particular, E6010 and E7018 rods in stick welding require a reasonably high OCV. That enables a crisper arc start as the welder scratches the rod against the metal to begin the weld. A frequent problem for students is the inability to strike an arc, so a low OCV on a small welding machine may only aggravate the situation. So you should take note of this figure in the specs. An OCV of about 80 volts is considered normal in a stick welder. In a MIG welder, it can drop to about 35, but it's not a big deal, since in MIG welding the arc usually starts without any fuss when you pull the trigger.
Thermal Overload Protection: Either a machine has it or doesn't. And you should only buy a machine that has it. This feature automatically cuts output power to your torch or gun if the circuit inside starts overheating. The fan or other cooling mechanism will continue running to help disperse the heat (assuming you leave the machine turned on). In some specs, this feature is clearly stated. But with other products you have to check the equipment manual or ask a sales rep. (See the samples below for more.)
A variety of gases (CO2, argon, oxygen, etc.) are used for different welding processes. In the case of MIG, the type of gas you need depends on the process, the base metal, welding position and environmental conditions. With oxy-fuel welding, you simply need oxygen and a fuel gas. And a TIG machine typically uses argon but on occasion may require helium. Besides reading the equipment literature carefully, you'll want to consider a few other things:
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Hobart Stickmate LX 235AC
Read the product description and spec sheet (PDF) This stick welding machine has a great online price of $299 (plus shipping), with an impressive current range of up to 235 amps and an OCV of 80 volts max. It's recommended for steel and stainless steel, can handle stick electrodes up to 5/16" diameter and a base metal thickness up to 3/8". It also requires a 230V input power source (make sure you have one available, if you're using a home shop). So far, this all looks pretty good.
Now dig a little deeper and you'll discover an irksome deficiency. On the stick electrode chart provide in the specs, one of the most common rods in the business, E6010, can't be used with this machine. E6010 rods are common in pipe welding and are part of the certification test. They require a DC current and this machine is AC only. (E6011 rods are the AC equivalent of E6010, so you can still obtain the same type of weld.)
Besides the AC-only drawback, at 225 amps the machine has only a 20% duty cycle. However, the duty cycle graph shows that if you go down to 100 amps, that figure jumps to 100%. There's no mention of thermal overload protection, just a note saying the fan stays on all the time. (See next example for more on this subject.) Finally, the online seller charges a huge shipping fee of $125+. (Amazon offers the tinier Hobart LX 205 for $299 with free shipping. This one can be plugged into any 115Vsocket. You can buy the LX 235 AC for about $375 on Amazon.)
Too bad about this machine being AC only. If you checked the other listings on the same website, there's an AC/DC model available. The addition of a rectifier piles over $200 onto the price, but at least now the AC current can be converted to DC. Stick welding in AC can be precarious with the polarity changing 60 times per second. (That means the current stops flowing 120 times per minute.) Probably a good idea to try your hand at AC welding in the overhead position before committing to an AC-only stick welder.
Lincoln Electric Power MIG 180-C
Read the product description and spec sheet (PDF). This MIG/Flux Core welding machine, selling for $729 (with a rebate offer) has a range of 30-180 amps in DC only, and requires input power from a 230V source. It can weld steel, stainless steel and aluminum up to 3/16" thick, or with flux cored wire up to 1/2" thick. The duty cycle rating is 30 percent.
Unfortunately, the cycle is listed for 130 amps, not the 180-amp max. There's no mention of thermal overload protection, either. It could be that all Lincoln machines have this protection, so they don't bother saying so in the literature. When in doubt, it's easy enough to email the sales department with any questions. Note: We did check with a Lincoln sales rep, who referred us to Page D-2 of the product manual. Thermal overload protection is standard on all their machines.
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Guide to Buying Your First Welder:
To Rent or Own Welding Equipment?
Choosing an Engine-Driven Welder
Understanding AC TIG Balance Control
Intro to Electricity in Welding Machines
The Role of Inverters
Wiring at home to install a 220 v circuit
Installing 220 / 240 volt Electrical Circuits
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