Conventional Machining

Conventional machining is done using seven basic techniques, which are:

• Drilling/reaming
• Turning
• Milling
• Sawing
• Shaping/planing/slotting
• Broaching
• Abrasive machining (i.e. grinding)

These techniques have been well established for many years. Most of the advances have been confined to tooling improvements, which have permitted higher material removal rates. The early high-carbon steel tools have been superseded by high-speed steels (tungsten/cobalt alloy steels), cemented carbides and ceramics.

The Machining Centres have also been developed, which are capable of automatic tool changes and of doing difficult types of machining without the need for transferring work to a different machine and re-setting up. In this way, a much more versatile machine tool has evolved. However, the biggest single machining advance in modern times (especially with regard to aircraft manufacture) has been the introduction of Numerically Controlled (NC) machines. In particular, NC milling has revolutionised airframe manufacture.

NC machines are machines in which motion is controlled by a series of numbers, either via punched tape or magnetic tape. Instructions on the tape are based on the Binary System (or a variant), which is common to most electronic computing devices. The primary advantage of NC machining is the ability to accurately control the spindle, the tool or the workpiece movements in three directions (x, y and z axes) independently or simultaneously. NC machines are capable of producing compound shapes and contours and are especially suited to the task of generating integral spars, ribs and stiffeners in slabs or forgings.

NC machines usually incorporate a feed-back system, which ‘tells’ the control unit how much actual movement is made. Analysis is then done and final compensation eliminates any error (i.e. the motion ceases when the input and feed-back signals agree). Electrical control of the machine servo-motors can control movements as small as 0.0005 mm.

CNC machines (i.e. Computer Numerically Control) differ from NC machines only in that the electronic control unit on the CNC machine is more sophisticated in that it is adaptable to a wide variety of software and can accommodate a diverse range of programs. Although the capital cost of NC/CNC machines is high, the following advantages make such machines technically desirable and economically viable, where super-light, complex, high-tech, manufacture is concerned:

• Complex shapes with integral features are possible
• The number of jigs and fixtures is reduced
• A reduction in manufacturing time
• Adaptable to short runs
• Greater accuracy and consistency
• Program can be changed to accommodate modifications