SMEs that fabricate metal components must consider the internal structure of the metal being processed (see furniturelink's metal properties). Steel, for example, has three atomic structures depending on the temperature of the alloy. These structures dramatically influence heat-dependent fabricating technologies such as welding and casting.
The grain structure of various metals also plays an important role in the behaviour of metals during cold-forming operations such as tube/sheet bending, spinning and extrusion.
Chris Lefteri's book Metals: Materials for Inspirational Design (see right) is an excellent resource for designers wanting to harness the potential of metals. The author lists each metal by type and gives examples of products with the designer's name. The book introduces several recent innovations, including foamed, shredded and inflatable metals.
Another designer friendly source with brief descriptions of current manufacturing technologies is Materials and Design: (third edition) by Michael Ashby and Kara Johnson (see right).
Listed below are common metal processing technologies used by furniture SMEs. For each process furniturelink has provided a "best-pick link" (¶) and an example of furniture using the process, plus the designer's name and production date. A full list of Internet research links by category is given in the sidebar (see below right).
Tubing (round, square, etc.) and solid rod can be bent to shape over hardened steel dies to produce a wide range of furniture parts.
Three types of machines are used in tube bending: manual - human-powered forming suitable for tubes up to one inch diameter; semi-automatic - hydraulic- or electric-powered equipment, some with advanced features; CNC - computer-controlled equipment that accurately performs 3 axis' bends.
David Roland (1979)
Most SMEs will subcontract casting of furniture parts to specialist companies using one of these casting technologies: die - requires production runs of 2,500+; sand - accommodates small runs or large; resin/investment - allows medium-size runs; plaster - often used for prototyping.
Patrick Messier (2004)
Cutting sheet mechanically for furniture components is done with punch presses (for cut-outs) or shears (trimming to size).
More intricate cutting patterns can be obtained with laser, plasma and water-jet-cutting equipment, although the high cost of this technology requires most SMEs to subcontract this work.
Beatrice Mahud (2004)
Common forming techniques applicable to furniture SMEs include: spinning - a circular blank is formed over a rotating mandrill to produce symmetrically shaped cones, domes, etc.; rolling - thin sheet is fed through offset rollers to produce cylinders, cable trays, etc.; pressing - male and female dies and hydraulic pressure are combined to produce small parts from thinner gauge metals; braking - sheet is clamped/levered or pressed to produce straight-line bends at the required angle.
Robin Bush (1963)
With the exception of water-based painting, most SMEs will subcontract one of these finishing methods: anodizing - an electrolytic process that coats the surface of aluminum components with a hard, dyed or clear film; electroplating - can be performed on a wide range of metal substrates (and plastics) using a variety of surfacing/plating metals. Steel furniture parts are commonly plated with chromium and/or nickel. This process can involve toxic chemicals and should only be used where surface durability is essential; solvent-based painting - for environmental reasons is not to be encouraged; water-based painting -the obvious in-house choice for the furniture SME. On spraying, these paints evaporate to form a tough pigmented or clear film. They are more expensive and require additional air flow when compared to solvent finishes. On the plus side, an explosion proof spray booth or solvent-rated breathing mask is not required; powder coating - probably the most common metal finishing technology - entails "spraying" the part with electrostatically charged pigmented plastic powder, which is then subjected to heat in a convection or ultra violet radiation tunnel "oven"; producing a smooth fused coating.
Lisa Norinder (2002)
The average SME can accomplish most welding methods in-house, with the exception of the more "high tech" computer-controlled welding technologies.
Common welding methods include: brazing/soldering - a low capital-cost option where a gas/oxygen torch melts a low temperature alloy into the joint. The softer material is easy to shape with abrasives to provide a smoothly contoured joint; arc - an electric arc is passed from a flux-coated welding rod to the parent metal/joint - mostly used with steel; MIG - in metal inert gas welding the electrode that makes the arc is a continuous feed wire, and the flux is a stream of inert gas - ideal for fillet welds in a wide range of parent metals; TIG - in tungsten inert gas welding the electrode is a non-consumable tungsten electrode shielded by inert gas - ideal for thin sections and accurate fit joints; spot welding - a localised electric current is applied via clamped electrodes to either side of the two metal sheets to be joined - faster than rivets or screws.
René Herbst (1928)
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