Rugby School Thailand Design & Technology Senior School

3.9 Fabrication in wood and metals

Learning Outcomes

By the end of this unit you should have developed a knowledge and understanding of:

the different ways that materials can be shaped into products
how products can be shaped by the fabrication/addition methods
why fabrication processes are used rather than wasting or redistribution processes
shaping and forming processes that involve fabrication/addition
how the different processes produce varying amount of waste and their effect on the environment

Fabrication in wood and metal

Fabrication processes involve joining like or unlike materials. The opposite of wasting, this is essentially an additive process.

Wood and metal are often fabricated while plastic is more commonly formed or moulded

***Fabrication can be broken down into two main types; temporary and permanent which will be referenced by a T, P or both in the following pages.

P - Permanent methods will often fuse the materials together either by melting materials together or by introducing a joining material such as adhesives.

T - Temporary methods use either some kind of joint or fixing which can be dismantled. Bear in mind that even a temporary fixing can be made permanent by the addition of adhesive or some other method of fusing, but a permanent joining method will always be permanent.

Wood Joints (T&P)

1.These are probably the most commonly used method of fabricating wood.

2. They are typically permanent if nails or glue are also used but otherwise, if they fit together, they can come apart.

3. There are many types of wood joints and you can even combine different types such as putting dowels into a mortice and tenon joint to add further reinforcement.

The joining of wood goes back before records began and while many are still produced using traditional hand methods, the increasing use of power tools and specialist machinery mean that they can be cut more accurately and quickly than even a craftsman can produce, but a hand joined piece of furniture is still desirable especially in bespoke products.

Wood joints generally fall into one of two categories: Frame joints and box joints.

Bespoke: made for a particular customer or user.

Wood glues (P)

Wood is a porous material, as a result it can be joined with resin, polymer and water based adhesives.

sometimes wood joints and glue are combined.

The most commonly used glue is PVA which requires wood to be clamped together while the glue sets usually within a few hours.

Cascamite is a resin-based powder which is mixed with water into a ‘milk-like’ consistency and is ideal for laminating and veneering as the thinner consistency makes it easier to spread on large surfaces.

Gorilla Glue is a modern and extremely strong adhesive which requires both surfaces to be wet before being clamped together. Over the next few hours the glue expands like a foam to provide a very strong joint.

Fabrication in metal

Metal is a versatile material that can be worked hot or cold making it suitable for nearly all types of production, and the fabrication of metal is a common process. There are no commonly used metal joints and metal is either fused/welded together or joined using fixings such as screws, rivets and bolts.

Crimping (P)

is a method used in thin sheet metal to join two sheets together.

The edge of one sheet is folded over, then the edge of the other sheet is placed in the fold before rollers press the fold down (known as crimping).

It provides a tight fit and can be further reinforced by welding if necessary.

Next we will look at Soldering, Brazing and Welding which are similar processes:

Soldering

Heating of the work pieces is not required.

brazing

Work pieces are heated but below their melting point.

welding

Work pieces need to be heated up to their melting point.

Soldering (P)

Two or more metal items are joined together by melting and flowing solder (an alloy of Tin and Lead) into the joint, the solder has a lower melting point (180 C) than the adjoining metal.

Soldering differs from welding in that it does not involve melting the work pieces
It is particularly suitable for intricate work such as soldering components onto a circuit board.

Silver Soldering (P)

is used to join precious and semi-precious metals such as Gold, Silver, Brass, and Copper.

The solder is usually referred to as easy, medium, or hard. This refers to its melting temperature, not the strength of the joint. Extra-easy solder contains 56% Silver and has a melting point of 1,145 °F (618 °C).

Extra-hard solder has 80% Silver and melts at 1,370 °F (740 °C).

If multiple joints are needed, then the jeweler will start with hard or extra-hard solder and switch to lower-temperature solders for later joints.

With Silver soldering, small pieces of solder wire are placed onto the metal prior to heating. A flux is used to keep the metal and solder clean and to prevent the solder from moving before it melts. Silver solder is absorbed by the surrounding metal, resulting in a joint that is actually stronger than the metal being joined. The metal being joined must be perfectly flush, as silver solder cannot normally be used as a filler and any gaps will remain.

Before we continue...

... we must understand Flux

Flux is a mixture of various minerals, chemicals, and alloying materials that primarily protect a molten weld metal from contamination by oxygen, nitrogen other contaminants in the atmosphere. You apply it during the welding process. It also prevents the solder from moving before it solidifies.

It can be a white paste applied to the material surface before welding or brazing or it can be embeded in the core of welding rod like in the image.

Brazing (P)

Brazing is a metal-joining process where a filler metal is heated above its' melting point and distributed between two or more close-fitting parts. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux.

It is used for more decorative work as the soft (Brass) brazing rod is easier to file to shape.

Welding (P)

Welding is a fabrication or sculptural process that joins materials, usually metals, by melting the work pieces and adding a filler material to form a pool of molten material (the weld pool). That cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld.

This is in contrast with soldering and brazing, which involve introducing a lower-melting-point material between the work pieces to form a bond between them, without melting the actual work pieces.

There are several types of welding commonly used depending on the job and the material.

We are going to look at Gas welding, Arc welding, MIG welding, TIG welding and SPOT welding.

Gas welding (P)

sometimes referred to as Oxy-Acetylene Welding because it uses Oxygen and Acetylene gases uses a filler metal material like brazing but with a much higher temperature.

The metal to be joined needs to be heated until white hot at which point the welding rod is introduced to the joint.

It can be a slower process but can result in a neater finish and requires inexpensive equipment which does not need an electrical supply.

Arc Welding (P)

sometimes referred to as electric or stick welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point.

The welding region is usually protected by some type of shielding gas.

Arc welding processes were first developed in the late part of the 19th century, and arc welding became commercially important in shipbuilding during the Second World War.

Today it remains an important process for the fabrication of steel structures and vehicles.

MIG Welding (P)

MIG (Metal Inert Gas) welding is a type of welding process that uses an electric arc to generate heat to fuse metal together. The process uses a wire that is continuously fed through a welding gun and onto the workpiece. An inert gas, such as argon or helium, is used to shield the weld from contaminants in the air.

MIG welding is a popular choice for many types of welding projects because

it is relatively easy to learn and use,
it can be used to weld a variety of metals, including steel, aluminum, and stainless steel.
It is also a fast process, making it efficient for many types of production work.

TIG Welding (P)

Also known as Tungsten Inert Gas welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas such as Argon, and a filler metal is normally used.

TIG is most commonly used to weld thin sections of Stainless Steel and non-ferrous metals such as Aluminium, Magnesium, and Copper alloys.

The process grants the operator greater control over the weld than other types of welding, allowing for stronger, higher quality welds.

TIG is a skilled process that is difficult to master, and it is significantly slower than most other welding techniques.

Spot Welding (P)

is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current.

The process uses two shaped copper alloy electrodes to concentrate welding current into a small ‘spot’ and to simultaneously clamp the metal sheet together.

Forcing a large current through the spot will melt the metal and form the weld. A lot of energy can be delivered to the spot in milliseconds and avoids excessive heating of the remainder of the sheet.

Creates a unique 'spot' appearance

Metal Glues

Metal is a non-porous material therefore, it is much harder to join using adhesives.

There is one commonly used metal adhesive called Epoxy Resin.

In its commercial form this adhesive can be stronger than a welded joint!

A domestic version is commonly sold in hardware stores and is known by the brand name Araldite. It is a two part glue consisting of resin and a hardener. Neither part is of use without the other and they are mixed in equal parts.

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