4.3 Joining

In addition to manufacturing an individual component using a single casting, forming or cutting process, we could assemble it from a number of simpler shapes joined together. There are several reasons for employing joining in a manufacturing operation. A product can be too big to make in one piece. The need to transport the product from the place of manufacture to its destination may limit the processes that can be used. It is often simpler to transport the product in parts, and assemble these parts at the relevant location. The building of a house or super-tanker are obvious examples. Joining can also be useful if a product has a complex shape, or of there is a need to combine different materials together. All these factors make it advantageous to join together previously shaped components in order to fabricate a complete and useful product.

But we need to draw a distinction between joining parts together and assembling components into a finished product. Although there is some overlap, I am going to concentrate here on the former. Assembly itself is a subject worthy of study in its own right but beyond the scope of this course.

In general terms, there are three basic methods of joining material together:

• Mechanical joining , using fasteners where the elastic and/or frictional properties of a material are exploited to hold two components together physically (rivets, nuts and bolts, screws and so on).
• Gluing , where a layer of another material is introduced between two surfaces and later solidifies to form a solid joint.
• Welding , where the aim is to create a joint between two surfaces which is similar to, or even indistinguishable from, the bulk material.

Although you can join things with simple glues or even adhesive tape, here we will be concerned with methods of joining solid components in such a way that the joint will remain intact throughout its service life. The designer's aim is to select a joining process and a joint geometry such that the joint itself is not the weak link in the chain. Of course, joining techniques are also available that allow the joint to be taken apart if needed, at some time in the product's life. These are a mainstay of product assembly.