Shaping processes: attributes and origins 421

18.4 Shaping processes: attributes and origins 421

Table 18.2 Typical levels of finish required in different applications, and suitable processes

Roughness ( µm)

Process R ⫽ 0.01

Typical application

Lapping R ⫽ 0.1

Mirrors

Precision grind or lap R ⫽ 0.2–0.5

High-quality bearings

Precision grinding R ⫽ 0.5–2

Cylinders, pistons, cams, bearings

Precision machining R ⫽ 2–10

Gears, ordinary machine parts

Light-loaded bearings, non-critical

Non-bearing surfaces

Unfinished castings

RMS roughness, R

Maximum roughness

Mean surface

Microns Millimeters

Figure 18.10

A section through a surface, showing its irregular surface (artistically exaggerated in the vertical direction). The irregularity is measured by the RMS roughness, R.

rolls produce sheets. Shaped rolls make more complex profiles—rail track is one of these. Extrusion is a particularly versatile continuous process, since complex prismatic profiles that include internal channels and longitudinal fea- tures such as ribs and stiffeners can be manufactured in one step.

Tolerance and roughness

We think of the precision and surface finish of a component as aspects of its quality. They are measured by the tolerance, T, and the surface roughness, R. When the dimensions of a component are specified the surface quality is speci- fied as well, though not necessarily over the entire surface. Surface quality is crit- ical in contacting surfaces such as the faces of flanges that must mate to form a seal or sliders running in grooves. It is also important for resistance to fatigue crack initiation and for aesthetic reasons. The tolerance T on a dimension y is specified as y ⫽ 100⫾0.1 mm, or as y ⫽ 50 ⫹0.01 ⫺0.001 mm, indicating that there is more freedom to oversize than to undersize. Surface roughness, R, is specified as an upper limit, e.g. R ⬍ 100 µm. The typical surface finish required in various products is shown in Table 18.2. The table also indicates typical processes that can achieve these levels of finish.

Surface roughness is a measure of the irregularities of the surface (Figure 18.10). It is defined as the root-mean-square (RMS) amplitude of the surface profile:

yxx 2 () d (18.1)

422 Chapter 18 Heat, beat, stick and polish: manufacturing processes

Sand casting Die casting Investment Casting Low-pressure casting

Metal shaping

Sheet forming Powder methods Electro-machining

Spark amic

plug Cer shaping

Conventional machining

Injection molding

Blow molding Compression molding Rotational molding

CD

P olymer shaping

Thermo-forming

Polymer casting

case

Resin-transfer molding Filament winding Lay-up methods

Composite

shaping

Vacuum bag

Tolerance (mm)

Precision machining

rod metal ceramic polymer composite

Figure 18.11 The process—tolerance bar chart for shaping processes and some finishing

processes, enabling process chains to be selected. The chart is color coded by material and annotated with the target values for the case studies.

It used to be measured by dragging a light, sharp stylus over the surface in the x-direction while recording the vertical profile y(x), like playing a gramophone record. Optical profilometry, which is faster and more accurate, has now replaced the stylus method. It scans a laser over the surface using interferome- try to map surface irregularity. The tolerance T is obviously greater than 2R; indeed, since R is the root-mean-square roughness, the peak roughness, and hence absolute lower limit for tolerance, is more like 5R. Real processes give tolerances that range from 10R to 1000R.

Figures 18.11 and 18.12 show the characteristic ranges of tolerance and roughness of which processes are capable, retaining the color coding for mate- rial family. Data for finishing processes are added below the shaping processes. Sand casting gives rough surfaces; casting into metal dies gives a smoother one. No shaping processes for metals, however, do better than T ⫽ 0.1 mm and R ⫽ 0.5 µm. Machining, capable of high dimensional accuracy and surface finish, is commonly used after casting or deformation processing to bring the tolerance or finish up to the desired level, creating a process chain. Metals and ceramics can be surface-ground and lapped to a high precision and