Stock removal techniques

Common processes for removing material from stock in order to produce a component:

The lathe

A machine tool that rotates the workpiece material so that when abrasive or cutting tools are applied to the workpiece, it can be shaped to produce an object that has rotational symmetry about the axis of rotation.

Examples of objects that can be produced on a lathe include, table legs, crankshafts or camshafts.

The lathe holds the stock material in a chuck and rotates the stock relative to a stationary tool. Translating the saddle and cross-slide that support the tool post in an axial and/or radial direction generates an interaction between the stock material and the tool that removes material as swarf.

The lathe is generally suited to making cylindrical components. While the component can be parted off and reversed in the chuck to machine the other end, it is unwise to assign tight tolerances to the concentricity of holes bored from each end. Similarly the thickness of a flange may vary if it is machined in two set-ups.

 

Three-axis milling machine

Utilises a rotating tool or cutter to remove material from the component.

Various configurations of three-axis milling machine exist, but typically the spindle that carries the cutter is moved to provide the vertical axis, while the component is attached to a moving bed that provides the two horizontal axes of motion in mutually orthogonal directions. The component can be held in a vice, a chuck or by clamping it to the bed of the machine. The milling machine is suited to making a wide range of components but has limited access to the component at any given time during the process, although this can be improved by using a rotary table or inclinable bed to support the component.

 

Five-axis computer numerical control machine tool

In the case of a five-axis milling machine two orthogonal rotary axes are added to a three-axis milling machine and under computer numerical control these machines are capable of cutting complex three-dimensional shapes such as centrifugal compressor disks or propellers. Five-axis milling machines are capable of machining five faces of a component in one set-up. The component may be held in a vice, a chuck or clamped to a faceplate, or the face of the rotary table depending on the configuration of the particular machine tool.

 

Wire cutting

Wire cutting utilises an electro-chemical process to allow a thin conducting wire to cut its way though a conductive material while the component is mounted on a two-axis stage. The process is ideal for flat two-dimensional components with intricate outlines.

There is a requirement for a pilot hole to feed the wire through at the start. More sophisticated versions of the machine offer a limited two-axis tilt facility in the head that allows the machining of tapers and cones. The process is capable of high accuracy machining and of producing thin sections with little danger of part failure under machining loads and deformations.

 

Electro discharge machining (EDM)

EDM employs a similar principle to wire cutting, but uses a graphite or copper electrode instead of a wire. The electrode or die is slowly lowered into the material and excess material is electro-chemically eroded. It is usual to use a roughing die followed by a finishing die as the die itself is consumed – all be it at a slower rate than the component. This technique is very useful for manufacturing moulds and tooling for injecting moulding systems.

For example, look at the detail in an Airfix™ plastic model kit.

 

Volume production processes

Casting

Die-casting techniques are suitable for medium to large production runs and allow the

manufacture of components at near finished size, which minimises the machining required.