Cutting metal with a gas torch is accomplished by pressing a lever on the torch that shoots a stream of pure O2 into preheated metal. This triggers a chemical reaction, causing the metal in its path to rust away instantly. For that reason, oxy cutting only works on steel and other ferrous metals. You'll need an alternate cutting process for aluminum, titanium, copper, and other non-ferrous metals.
By all accounts, the oxy-acetylene "kit" remains the cheapest, all-purpose welding-cutting combination on the market today. Acetylene gas mixed with oxygen produces nearly 7000 degrees farenheit. The high heat is necessary to generate the chemical reaction described above. Other fuels, like propane and Mapp gas, may also be used in the oxy-fuel cutting.
In addition, with a special tip called a rosebud the oxy torch allows you to heat metal for forming or annealing purposes. A rosebud is routinely used to loosen tight bolts and nuts. Some welding applications that require pre-heating can likewise be accomplished with the oxy torch.
Heating up metal with a torch
An oxy-acetylene kit should include a torch handle, welding tip (hopefully more than one size), a cutting tip and a rosebud (heating) tip. Unfortunately, many inexpensive kits skip the rosebud (and even the welding tip in some cases), so pay close attention when you're shopping around. The torch handle should be outfitted with flashback arrestors and check valves. These prevent flames from traveling up through the hoses, and the two gases from mixing inside the hoses. Refillable gas cylinders that fuel the torch should come with protective caps, as shown in the photo below right. (Since lots of safety protocols apply to pressurized tanks, be sure to review the literature on this subject before setting up a welding shop.)
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A good kit also includes high-quality regulators for both the oxygen and acetylene (or alternative fuel) tank, as well as a gas hose for each (red and green). Most kits come with tip cleaners, a flint striker and set of goggles. You'll have to supply your own gloves and a suitable work area for welding, which typically means purchasing fire clay bricks, either from a ceramics shop or company that sells fireplace and other refractory supplies.
The diagram above illustrates the basic set-up of an oxy-acetylene cutting operation.
Plasma Cutting works on all metals, ferrous and non-ferrous, thin or thick. It's particularly effective for gouging holes into aluminum or stainless steel, leaving behind no trace of carbon contamination. However, it's less effective on dirty metal and aluminum oxide build-up.
The process utilizes an inert gas (nitrogen, argon, or oxygen) that streams through a narrow tube insde a nozzle. An electrical arc strikes the gas, ionizing it, and that in turn generates plasma, which physicists like to refer to as the "fourth state of matter". (If you're wondering what plasma looks like, think of the Sun. )The concentrated heat produced by this chemistry is sufficient to blow through hard metal like it's a piece of paper.
In the air carbon-arc process, a stream of compressed air is aimed at the point of contact with the base metal. There's a valve in the holder for turning the compressed air on and off. Unlike a welding machine, this equipment includes an air compressor and hose. The size of the compressor ranges between one and ten horsepower, depending on the electrode sizes, amperage and duty cycle at which the machine operates.
You can use this process to cut metal, gouge out defective metal or bad welds, or to bevel a groove for welding. (You'll probably have to do some grinding and other clean-up afterward.) The cuts made are generally not as precise and pretty as plasma cutting, but the cost is far cheaper. Using compressed air also saves you the hassle and cost of filling acetylene and oxygen tanks. In addition, since the area affected is small and the process quick, the surrounding base metal does not reach a high enough temperature to potentially alter its mechanical properties.
Air carbon arc cutting can be either a manually operated venture (as shown in the photo above) or hooked up to a travel carriage for long, even cuts. The manual process can be used in all positions, but overhead cutting is difficult.
You can cut aluminum, copper, cast iron, magnesium, and stainless steels. Holders are available in several sizes depending on the duty cycle of the work performed, the welding current, and size of carbon electrode used. For extra heavy duty work, water-cooled holders are needed.
For more info on CAC, read the overview posted at WeldGuru.com
Nowadays, the most widely used angle grinder handles 4 1/2 inch diameter grinding discs. Larger grinders use discs up to 9 inches and are ideal for beveling thick-walled pipe. The other variable to consider when shopping for a grinder is the motor's amperage. More amps means more power and the ability to grind faster. A 10-amp angle grinder is standard in industry, while students and hobbyists may buy a 6 to 7.5 amp grinder.
Angle grinders accommodate a variety of abrasive discs, which are divided into five categories:
grinding discs - These eliminate material, like excess weld metal, weld spatter, burrs, and the rough edges of oxy-cut material.
cutting discs - These work like a mini-circular saw to cut through metal.
flap discs - These polish and smooth metal in advance of a paint job, powder coating or bend test.
wire brush discs - These are used to remove weld spatter and slag from welds without altering the surface of the base or weld metal.
sanding discs - This requires a special spindle adapter to handle flat, rounded sheets of sandpaper.
Smaller types of grinders and deburring tools are powered by an air compressor rather than an electrical outlet. Since they don't contain a motor or battery, these tools are designed to fit in tight spaces, like corners and interiors.
You'll also find a stationary (aka bench) grinder set up in most shops. This equipment is used to sharpen tools, smooth out bevels cut in metal stock, or for quick rust-removal of small plates before welding. Home improvement stores generally sell affordable models for consumers, so be sure to consider one if you're setting up a shop.
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A tungsten grinder (left) and a bench grinder.
For TIG welding, a special tungsten grinder is also a necessity. Both portable and stationary models are available to grind this exceptionally hard metal down to a sharp point. (And both are expensive.) During welding, the tungsten tip will occasionally come in contact with the molten puddle, losing its point. That's why the grinder must always be closeby. In the field, the portable version is plugged into an outlet on the welder- generator. See the Guides and Video page for a good look at this on YouTube.
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Alternate Fuel FAQ
Inside a Plasma Cutter
Basics of Field Welding Repair on Construction Equipment
Portable Welding and Cutting Equipment for Maintenance/Repair
The Great Debate: Plasma Cutter or Oxyfuel Torch?
Tech Tips For Better Plasma Cutting
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