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Cutting can be done in one of three ways, by guillotine, blanking or routing. The guillotine or shear (Fig. 7.4) operated either by foot pedal or power, makes long, straight cuts (4–10 ft dependent on machine size).

Blanking is a punch and die process. Any irregular shape can be cut out as a hole in the thick (comparatively) steel plate to make the die. A match-ing piece of similar material (the punch) made so that it will just pass through the hole, is mounted above the die in a press. When a piece of thin sheet is placed between punch and die the punch is pressed down and cuts one piece of matching shaped sheet. The process is very versatile and can reproduce accurate components, at a very high speed if necessary, from subminiature size to dimensions limited only by the power of the press.

Turning

Parting

Facing

Fig. 7.2 Lathe.

Horizontal mill

Vertical mill

Fig. 7.3 Milling machines.

Fig. 7.4 Guillotine.

The tools (that is, the punches and dies) are expensive but the labour cost of the finished component is low, therefore this method is economical for long production runs with the runs needing to be longer as the compo-nent size (and therefore tool size) increases. Presses vary enormously in appearance and even in their exact function. A simple hand-operated machine, called a fly press, is shown in Fig. 7.5, but hydraulic power presses of 500 tonne capacity are quite common equipment.

Routing also reproduces irregular sheet-metal shapes. In this method, a special milling cutter run at very high speed (18 000–24 000 rpm) cuts through a pack of five to ten sheets of material. The slot, which may be about 0.25 in. wide, can be of any length and follow any path guided by a template or model of the shape required. The tools are much cheaper than blanking tools and there is virtually no upper size limit, but the accuracy of profile is rather dependent on the skill of the operator and the labour cost per part is high. The name ‘routing’ is sometimes given to the milling process of making shaped recesses in plate material.

Fig. 7.5 Fly press.

Forming sheet metal is probably the major activity of aircraft produc-tion. Straight bending of flanges or single-curvature skin panels, such as those on the parallel central part of a fuselage or the surface of a con-ventional wing, is very easy. Forming parts with double curvature, such as the skin panels at the nose of an aircraft or the lips around the flanged lightening holes (see Fig. 9.2), is considerably more difficult, but the processes are well understood in the industry and seldom cause major problems. There are some further comments on the design aspects of forming in Chapter 9. Bending or folding of straight flanges is carried out in a folding machine (Fig. 7.6) or a press brake.

(Fig. 7.7). The latter machine can also be set up for blanking or piercing (i.e. blanking small holes). Double curvature forming may be carried out in a press with punch and die tools similar to those described above, but arranged with sufficient clearance so that the sheet is not cut between the two parts of the tool but is formed to the shape of the punch. Again such tools are very expen-sive and only justified by a long production run or the absence of an alter-native method.

A similar but cheaper, and in some ways more adaptable, version is the rubber press method. The punch is still required but is mounted on the bottom plate of the press and instead of a die the top plate carries a rec-tangular block of rubber contained inside a strong box or fence with the

Machine bed Folder bar

Pivot Beam

Fig. 7.6 Folding machine.

underside open.As the top and bottom press plates are squeezed together, the rubber moulds itself and wraps the sheet metal around the punch.

Punches (or form blocks) can be made of materials which are more easily worked than the steels used for matched punch and die sets, and the rubber (or sometimes now a plastic called polyurethane) is long lasting.

Purpose-built rubber presses are very large and may work at several thou-sands of tonnes load.

The process of forming sheet metal depends to some extent on the property of material called work hardening (see Section 6.2.2) and luckily aluminium, the major material of aircraft structures, has good work-hardening characteristics. To illustrate this point, imagine a component made of sheet aluminium and shaped like a deep dish or saucer. The manufacturing tools required will be a punch and die which respectively fit the inside and the outside of the finished component. Also required is a clamping ring, or pressure plate, to hold the unpressed metal blank in position over the die ready to receive the punch. As the punch is pressed down it starts to stretch the sheet into the hollow of the die, because the clamping ring (intentionally) prevents material being drawn in from the sides. Now as the material is being stretched it must get thinner and wherever it is thinnest we would expect it to be weakest and therefore to stretch more, so that it progressively becomes thinner in one small area.

However, because of work hardening this is not the case. The initial small stretch ‘works’ the material, improving the strength of the sheet locally, transferring the deformation to a new point and so by progressive action ensures that stretching takes place evenly over the whole area of the dish.

Fig. 7.7 Brake press.

7.3 Jointing