There are several ways to cut steel and most other metals. Steel was traditionally cut with a gas torch, or oxyacetylene torch, and although this method is still widely used, more sophisticated and affordable methods are now available. Nevertheless, the oxyacetylene torch and its associated bottles and gauges will find many uses around a metal boat building project, although it’s not a necessity. The gas torch is quick, efficient, and low in operating costs. With this equipment you’ll need a light- to medium-duty kit with a 90-degree angle, and specialized cutting tips.
The basic oxy kit consists of the cutting torch, tips of various sizes, a set of gas regulators, a flint lighter, goggles, a special wrench, couplings for oxygen and acetylene tanks, and two lengths of hose to lead from the tanks to the torch. This package could cost less than $400 (£230) if you’re able to pick up used equipment at a favourable price. A small cart to hold the bottles would be useful; you can either buy one or make one yourself. The cylinders are usually leased from the gas supplier and you’ll only need to pay for the refills.
The oxyacetylene torch cuts metal through a rapid oxidization process in two continuous steps. While the torch heats a small area of metal to a cherry-red colour (about 1,500 to 1,600°F), a small stream of pressurized oxygen is directed from a central tip within the torch against the hot metal. The stream of oxygen causes the metal to “burn” rapidly and the metal separates as the torch is moved along the line of the desired cut. There are many different cutting tips and they can be used to influence the size, speed, and accuracy of the cut. A special plate-cutting, drag-step tip can cut steel plate from 1/8 to 1/4 inch (3 to 6 mm) thick with precision at the rate of about 2 feet (61 cm) per minute. The resulting cut using this tip will be between 1/16 and 3/16 inch (1.5 and 4.5 mm) wide. Using this equipment is something of an art form, and considerable experience is required to achieve the type of fine cutting that is required when plating your hull.
The main drawback is that torch cuts are rough around the edges and usually need some cleaning up before they are suitable for welding to other parts. You should avoid the disgusting habit of some low-cost builders who plate the hulls oversize and simply torch off the overlaps at the chine. The oxyacetylene torch can also be used for some specialized welding operations, but for boatbuilding it’s better to use the other equipment discussed below, such as arc (stick), metal inert gas (MIG) or tungsten inert gas (TIG) welders (see Chapter 5). Reserve the oxy equipment for cutting where precision is not required.
ANGLE GRINDER
You should buy the best-quality angle grinder you can find; it will get a lot of use. This is a much-used tool in boat construction and can perform a variety of jobs, ranging from cutting lengths of flat and round bar to smoothing out the edges of torch-cut plate. You can use this tool to bevel thicker plates by grinding off the excess metal before welding. Fitted with wire brushes, it can be used to clean off rust, mill scale, and weld splatter. Another use is to smooth off the welds on the hull topsides and superstructure. (Take note, though, that we, along with most classification societies, do not recommend grinding welds below the waterline.)
When you fit your angle grinder with a special wheel, it can be used to cut the slots in the frames to accept the stringers. Another use is to make many small cuts in metal bar. This feature is useful for making frames, snaping the ends of stringers, and similar tasks. Don’t use your grinder for heavy-duty cutting; the grinding/cutting wheels don’t last very long. For instance, when you use it to cut 1/8-inch (3 mm) plate, you can expect to get only about 12 to 15 feet (3.5 to 4.5 m) from a single blade. When you use it for cutting, consider your angle grinder to be more of a convenience tool than a fully fledged cutting device.
Buy a 7- to 9-inch (178 to 228 mm) heavy-duty, commercial-grade angle grinder. Make sure it has heavy-duty switches and a high-efficiency cooling fan. Don’t order this item by mail. You need to hold the grinder, see how it balances in your hands, and feel the weight. You’ll be holding this tool for many hours, so make sure it feels right if you want long and trouble-free service.
NIBBLER
A nibbler will cut thinner plate, but it’s an expensive tool, especially considering its limited use on most boatbuilding projects. You could rent one if you really find it useful. This tool is like a pair of electric scissors, and it slices through thin metal by taking small nibbles, hence the name. The nibble is an up-and-down punching action and makes a cut about 1/4 inch (6 mm) wide. When it’s used by an experienced operator, this tool can produce a smooth cut with clean edges. The cutting rate will vary, depending on the thickness of the metal, but on 1/8-inch (3 mm) steel it can cut about 2 to 4 feet (60 to 120 cm) per minute.
PLASMA CUTTER
A plasma cutter is ideal for cutting plate and other steel and metal sections, so rent or buy the best you can afford. This device cuts without distortion and can be used to trim plates in position. In the hands of an experienced operator, the plasma cutter produces a clean, sharp cut without any sign of distortion. The cutting action is very fast, and steel plate up to 5/16 inch (8 mm) in thickness presents no problems for this device. It is not suitable for aluminum, however. The narrow cut of 1/8-inch (3 mm) makes for neat and efficient cutting. Be forewarned, however, that the plasma cutter uses a fair amount of electricity, and the cutting tips do not have a long life. This tool is especially suited to cutting plate, and it finds angle and other shapes something of a problem, so alternative cutting methods should be used for those sections.
