Volumes of production

Compared with the alternatives for cutting thick metals, thermal cutting is an economical process for small-batch production.

Unit price vs. capital investment

Unless a cutting template is introduced, the process does not require tooling. In an automated process, the information for the shape can be provided by CAD files. Both of these factors mean that costs are kept down.

Speed

The speed is greatly affected by the type of material used and its thickness. The process may be carried out manually, or it can be highly automated, with multitorch, computer-operated systems. Speeds can reach up to 10 feet per minute.

Surface

Cutting can be controlled to produce different grades of surface depending on the cost-versus-edge quality—that is, longer cutting times equal better edge finish. The finish of the edge is also determined by the material, but generally plasma-arc cutting (see p.33) will give the best finish.

Types/complexity of shape

The process is best suited to heavy-gauge materials. Metals of below 3/8 inch may distort as a result of the intense heat, as might narrow sections. As in all sheet-cutting operations, nesting one shape within another (as you would when cutting cookies from dough) to optimize space in between shapes produces an economical use of the material. The cut is generally at 90 degrees to the plate. Other angles can also be achieved, although this is not as easy to set up for oxyacetylene cutting as it is for plasma cutting.

Scale

Using handheld cutting tools there is no maximum size, while in the automated process part sizes are restricted to the size of the machinery.

Tolerances

Depend on the thickness of the material but, as a rule of thumb, they vary between ±1/17 inch for ¼–13/8-inch-thick materials.

Relevant materials

Limited to ferrous metals and titanium.

Typical products

Heavy construction, including shipbuilding and machine components.

Similar methods

Electron-beam machining (EBM) (p.24), plasma-arc cutting (p.33), laser cutting (p.46), and water-jet cutting (p.42).

Sustainability issues

Oxyacetylene cutting is extremely energy intensive because of the phenomenal temperatures required to maintain the heat of the flame, along with the slow speed at which the torch needs to move, which increases cycle times. In addition, several harmful chemicals are produced from both the fuel and the work piece.

Further information

www.aws.org

www.twi.org.uk

www.iiw-iis.org

Sheet: Sheet-Metal Forming

Product	Acme Thunderer whistle
Designer	Joseph Hudson
Materials	nickel-coated brass(image shows the brass before plating)
Manufacturer	Acme Whistles
Country	UK
Date	1884

The Acme Thunderer is shown here in its pre-assembled state, and before it is nickel-plated. This shows how many formed components go into making the final product.

Making objects from sheet metal is one of the earliest methods of human production. The Egyptians, for instance, made soft precious metals, such as gold, into sheets, from which they cut sometimes highly intricate forms.

One of the most refined applications for sheet-metal forming can be found in the production of the common whistle. Falling under the generic heading of solid-state forming, the production of whistles is an industrial craft, a multistage process that is based on the conversion of a sheet material into a three-dimensional object by cutting, press forming (see metal cutting, p.59), and, finally, plating sheets of brass. However, this overly simplified description masks the fact that this is a precise method of industrial production that requires extremely high levels of tolerance to produce a whistle with the perfect pitch.

The basic geometry of a whistle’s body consists of three parts: an underside, mouthpiece, and top and side. The pieces of brass are stamped out to form the flat nets and are then pressed into shape using a male and female jig. These components are soldered together, polished, and plated in nickel. The final simple step: a cork pea is pushed into the mouthpiece.

In any wind instrument, the sound is the result of air flowing at different rates over a very sharp edge, producing two vibrating columns of air. After 135 years, Acme Whistles, in Birmingham in the UK, has tailored this highly precise process into an art form, producing a reject rate of just 3 percent for their whistles. Considering the potential for the slightest, sometimes invisible, imperfection to produce the wrong sound, this really is a feat of crafted industrial manufacturing.

–  The beauty of this process is that it allows the creation of a complex form with a highly precise component.

–  Reasonable tooling costs.

–  Limited to sheet materials.

–  The product may have to go through a number of stages.