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Introducing engineering
Introducing engineering

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4.2.3 Classifying shapes

Apart from being important in the physical performance of a component, its geometry – shape and size – is also a vital factor in choosing a manufacturing process. Some manufacturing methods are better suited to particular shapes and sizes than others. The shape of a product is usually the best place to start when deciding which processes are feasible. So we need a way to describe shape, although it does not need to be very sophisticated.

Figure 59 is a simple relationship diagram for the shape categories I am going to use.

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Figure 59 A diagram showing how the different categories of shape are related
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Figure 59 is a schematic diagram consisting of seven rectangles arranged in four columns. The rectangles are connected by lines to form a tree structure that is designed to be read from left to right. Each rectangle represents a category of shape. In the first column there is a single rectangle labelled 'shapes'. In the second column there are two rectangles labelled '2D continuous' and '3D'. In the third column there are two rectangles labelled 'sheet' and 'bulk'. In the fourth column there are two rectangles labelled 'solid' and 'hollow'. From the rectangle 'shapes' a line goes to the right and splits into two lines. One line goes to the rectangle '2D continuous' and the other line goes to the rectangle '3D'. No line leaves the rectangle '2D continuous'. From the rectangle '3D' a line goes to the right and splits into two lines. One line goes to the rectangle 'sheet' and the other line goes to the rectangle 'bulk'. No line leaves the rectangle 'sheet'. From the rectangle 'bulk' a line goes to the right and splits into two lines. One line goes to the rectangle 'solid' and the other line goes to the rectangle 'hollow'.

Figure 59 A diagram showing how the different categories of shape are related

2D (continuous)

If the profile of an object does not change along its length – like a pipe, electrical cable or aluminium cooking foil – then it can be classified as having a simple (continuous) shape. For convenience I am going to call these 2D (shorthand for two dimensional) shapes. Many 2D products are used as the raw material for processes that make them into three-dimensional (3D) shapes. PVC window frames for example are made from continuous extrudate (the product of the process of extrusion, Figure 60) which is cut into suitable lengths and then joined together by fusion welding. Nails are made from short lengths of wire in a couple of simple cutting and forming steps.

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Figure 60 A range of extrusion profiles
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Figure 60 comprises 11 lengths of material arranged such that the end sections are visible. The profiles of the section appear to be the same along their whole length. The sections are too complex to describe here but they do share certain characteristics. They look like they could be made from sheet but closer examination reveals that this would not be possible. The profiles are too complex and intricate. The material thicknesses are not all constant for each profile. Most of the profiles are completely hollow, nearly hollow or have parts that could be described as hollow.

Figure 60 A range of extrusion profiles

3D

Most objects have profiles that vary in all three axes. Many processes are suitable for the production of 3D shapes, so we need some further breakdown of this high-level classification. For various reasons it is best to split 3D shapes into two types, which I shall call sheet and bulk shapes.

Sheet products have an almost constant section thickness, which is small compared with their other dimensions, but without any major cavities. Buckets and car body panels (before assembly) are examples of 3D-sheet products (Figure 61). Although their overall shapes can be very complicated, you could imagine the same shape being made out of a sheet of material that is folded or draped, like origami paper shapes.

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Figure 61 3D-sheet products: (a) plastic bucket; (b) car body panel
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Figure 61 comprises two photographs labelled 'a' and 'b'. Photograph 'a' shows a plastic bucket complete with a plastic handle. The bucket has a traditional shape with a wider top tapering to a smaller base. There is a rim around the top of the bucket. The plastic handle has a semi-circular shape and its ends are connected to integral supports moulded just below the rim of the bucket. Photograph 'b' shows a metallic object. The view is such that it is difficult to discern its shape. It is definitely not flat and looks like a deep tray except that it has a relatively large lip around its edge and the middle surface is not flat either. The middle surface looks contoured and there are additional ridges and indentations.

Figure 61 3D-sheet products: (a) plastic bucket; (b) car body panel
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Figure 62 3D-bulk-solid product (used with permission of Brunswick Corporation)
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Figure 62 is a photograph of a three-bladed ship's propeller. The blades have semi-elliptical shapes and are twisted. The hub of the propeller is cylindrical and looks to be hollow but it is not evident whether the hollowness goes all the way through the hub.

Figure 62 3D-bulk-solid product (used with permission of Brunswick Corporation)

Many 3D products fall into the category of bulk shapes, and have complex forms, often with little symmetry. If they have no significant cavities in them we can call them solid (Figure 62) but if they do have cavities, they will be classed as hollow. The cavities in hollow objects can be quite simple but they can also be more complex, involving re-entrant angles (Figure 63). A re-entrant angle in a cavity means that at least one part of the cavity is larger than the opening into it. So if you cast such an object, you usually have to destroy the mould, or at least the part of it that creates these cavities. Making 3D-bulk-hollow shapes with re-entrant angles using a forming process is even more difficult.

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Figure 63 Aluminium alloy cylinder block showing complex internal cavities that make this a 3D-bulk-hollow shape
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Figure 63 is a three-quarters perspective photograph of a rectangular shaped object. Using the width as a basis for measurement, it looks to be about twice as deep as it is wide and four times as long as it is wide. The outer surface has contours, indentations and protuberances. At one end there is an additional structure on one side making it twice as wide as the rest of the object. The top surface of the object is smooth and flat. There is a line of 6 relatively large vertical holes along the length of the object. On either side of these holes there is a single vertical cavity running the length of the object that curves around each of the central holes. There is a major vertical cavity traversing the width of the object at the wider end. A number of other vertical holes are evident in the top surface of the object.

Figure 63 Aluminium alloy cylinder block showing complex internal cavities that make this a 3D-bulk-hollow shape

Shape is important because it provides a 'coarse filter' for choosing processes. It allows you, on the one hand, to rule out many unsuitable processes for manufacture of a given component and focus your attention on just those processes that could be used for the shape you are working with. On the other hand, it can show you immediately that you may need or want to take a more imaginative approach by, for instance, dividing one complex shape into several simpler ones that can be joined together later. Conversely, there may be potential to combine simple shapes into more complex ones to minimise the number of processing steps.

Activity 34 (self-assessment)

Consider the following list of components and objects and classify each according to the shape classification given in Figure 59:

  • a.plastic tray used to hold the confectionery within a box of chocolates
  • b.garden hose pipe
  • c.open-ended spanner
  • d.plastic (PET) lemonade bottle
  • e.rail (from a railway track)
  • f.flower pot.

Answer

  • a.3D sheet
  • b.2D continuous
  • c.3D bulk solid
  • d.3D bulk hollow
  • e.2D continuous
  • f.3D bulk hollow.

We'll next look at two different categories of process: that of joining and, since the early 2000s, a new technology that has the potential to disrupt fundamentally our entire approach to making things. This is 'additive manufacturing' which you might recognise under the title '3D printing'. It is not so much a process as a radically different way of going about creating individual objects.