Manufacturing Processes - MFRG 315 - 5. Special or non - Traditional Processes

5.1 Introduction

Until the middle of the last century production of many (metal) parts started as a casting, which then underwent some forming operations (eg: rolling, drawing, forging) followed by some machining (eg: turning, milling) to provide accurate dimensions where needed. These processes have been extensively developed and for a great many materials and products offer very efficient manufacturing routes.

However as greater demands are placed on components, materials which are stronger, less dense or possessing better strength at elevated temperatures are increasingly needed. In some cases the traditional processes are not efficient (very short tool lives) or unable to produce the required features. In such cases there are now a range of 'new' processes which are able to form the feature, however in many cases these processes are very much less efficient than the traditional processes and they are no more accurate which partly explains why these processes have not been taken up and used where they are not essential.

An example of a process giving access to new materials is powder metallurgy. Prior to the development of this process metal alloys could only be produced that could be melted without separation or significant melting losses. This limitation was somewhat restrictive for casting tool steels. By starting with powdered metals, any composition can be achieved by mixing, followed by compaction and sintering. Hence many new alloys have been developed since powder metallurgy became available.

5.2 Key Characteristics

Electro-chemical machining (ECM), Electro-discharge machining (EDM), laser and electon beam machining are all capable of producing holes (including non-round holes) with very large length to diameter ratios, in very hard materials including super alloys and ceramics.

ECM and EDM can be used to produce 3 dimensional cavities in hard steels. However both processes also require elaborate filtering and separation systems (to remove metallic sludge from the working fluid) and disposal of the sludge, which as it may contain a range of metals, is likely to be costly.

Ultrasonic machining (USM) and water jet cutting (WJC) are both purely mechanical processes.
USM uses a tool vibrated at 20 - 100kHz which is separated from the workpiece by a slurry containing abrasive. This can cut into hard and / or brittle materials.
WJM involves forcing water at very high pressure through a small nozzle so that it emerges at supersonic speed. This process is ideal for cutting soft materials such as paper, cloth and plastics. Adding abrasive grains to the water, abrasive jet machining (AJM) improves the performance when cutting harder materials such as metals and particularly fibre reinforced plastics.

These processes can best be studied by reading the relevant chapters in one or two of the references, paying particular attention to:

• Electro-chemical machining
• Electro-discharge machining
• Laser and electron beam machining
• Water jet cutting
• Ultrasonic machining