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.
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.
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,
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.
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 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.