Case Study - 13A plug

Plug pins: selecting the process

The plug pins are made out of brass, but how are they manufactured? The selection of a suitable process involves several stages; here we will follow the approach taken in the tutorial. So let's start with material compatibility<.

Material compatibility

The first step is to consider which processes are suitable for use with brass. Clearly the metal shaping processes are of most interest and the main ones are tabulated opposite. It would also be possible to use machining to make the pins, but even at this stage we can reject this as being too expensive as there are plenty of other options. One of the advantages of metals is that they can be processed in many ways. Unfortunately this does not help us much in choosing a suitable process! The next stage is to think about size and shape.

+ : routine ? : difficult X : unsuitable		Brass
Metal Shaping	Sand casting	+
	Die casting	+
	Lost wax casting	+
	Powder metal forming	+
	Forging	+
	Sheet forming	+
	Rolling	+
	Metal extrusion	+
Machining	Milling	+
	Grinding	+
	Drilling	+
	Cutting	+
Joining	Fasteners	+
	Solder / braze	+
	Welding	+
	Adhesives	+

Size and shape

The shape of a component is often key to selecting a suitable process. With metal parts, it is useful to think of the basic shape, as further steps such as drilling can be used later to produce specific "features" such as holes. The basic shape may be:

Folded or drawn sheet, such as a can or microwave casing.

2D - components with the same cross-section all the way through - which may be short things like plug pins, or long things like window frames.

3D - with a complex geometry such as an engine block. In addition, factors such as symmetry and concave curves will affect which processes can be used successfully. We have a small component which is essentially '2D'. So by referring to further process infomation (available elsewhere on this WWW site) we can see that rolling and sheet forming are not suitable. This sort of information will also help us to decide whether, as well as being able to be make the component, we can make it to a high enough standard.

Finish and quality

The next stage is to think about dimensional tolerances, surface finish, quality etc. These requirements for the plug pin are not very demanding - so we can do little to reduce our list of processes further.

This is about as much as we can do to reject processes for simple technical reasons, all that remains is to think about the processing cost.

Economics

We've already ruled out machining on cost grounds. Can any of the other processes be ruled out because of cost? We're going to want to make millions of pins, so we need a process with an appropriate economic batch size. Taking this into account and using the process data referred to earlier, we're left with die casting, forging and extrusion.

Normally at this stage, there is little more that can be done at a simple level to choose between these 3 processes. Here, however, we will make one choice by thinking about how processing and materials properties are linked.

Processing and properties

The mechanical properties (e.g. strength) of a component depend on how it is made as well as what material is used. Die casting will not give as good a strength as forging or extrusion - especially because of the sharp corners - so we will reject it here leaving only extrusion and forging.

Although both forging and extrusion are near-net-shape processes, they both require further steps to finish the pin (making holes, threads etc.). These post-processing steps will add to the cost, but will not be sufficiently large to affect our choice.