Wasting Techniques

The processes covered on the previous 12 sheets have dealt with shaping and forming techniques which relied on starting with raw materials either in the form of a billet of steel or granules of thermoplastic and turning them into a finished component. With a few exceptions, these processes involve complex machinery and are only economical for large quantity production.

The techniques that you will be more familiar with involve starting out with standard sections such as 18mm MDF, 3mm Acrylic, 20mm diameter mild steel, then cutting and machining them. These are known as wasting techniques and are clearly a disadvantage in that, as the name suggests, waste is generated and the techniques tend to be time consuming. Wasting falls into four categories: mechanical, electrical, chemical and thermal.

Mechanical
This is the process you will be most familiar with. It involves the use of either wedge- shaped tools (the teeth on a saw or a file, a lathe cutting tool or drill bit) or abrasive particles bonded to form a solid (disc sander, grinding wheel) or flowing in a ÔfluidÕ (sand-blasting, ultrasonic machining).

Electrical
The most common method of electrical machining is arc-discharge machining (or spark erosion). It works as a result of the eroding effect of sparks generated at voltages from 20-500 V between an electrode and the workpiece (which therefore must be a metal). The workpiece must be contained in a semi-conducting liquid such as paraffin. It is used to machine hardened tool steels and to create a grainy texture on injection moulded products.

Chemical
If you have constructed a PCB, you will be familiar with this method. It depends on masking those areas which are required and exposing the remainder to chemical (etches) which attack the material.

Thermal
Thermal techniques remove material by melting or vaporising it. Oxyacetylene cutting uses gas to melt through steel plate from 6 - 150 mm thick. Lasers can be used on both ferrous non-ferrous materials such as textiles. There is less distortion compared to oxyacetylene cutting but it is limited to thicknesses of about 20 mm.
Hot wire cutting is used to cut materials such as polystyrene. A low voltage is passed through a wire causing it to heat up to the melting point.


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