Advanced workpiece materials

Tool selection

As mentioned, there are many materials that fall under group N materials, this therefore makes tool selection difficult.

 

In general, many group N workpieces require high quality surface finish and tight geometric tolerances. Low cutting resistance tools are usually selected, but compatibility between workpiece and tool materials should be taken into consideration to ensure effective machining.

Aluminium / aluminium alloys

Although pure aluminium and aluminium alloys are bracketed together as workpiece materials, there are a several differences.

 

The clearest difference is the hard particle content in the structure. Aluminium is a very light, soft and highly malleable metal and can be machined to any shape. Pure aluminium has little strength, so it is important to increase strength by adding various alloy elements such as silicon (Si).

 

Si is very effective in increasing the strength and hardness of aluminium. However, Si itself forms hard particles within the structure. Which means that adding a large amount of Si increases the hard particle content, which can results in poor machinability. Other alloy elements added to aluminium include Cu, Mg and Zn.

 

When machining pure aluminium with relatively few hard particles, chip control and poor surface finish caused by welding can be problematical. This is because pure aluminium is very soft and highly malleable, causing continuous type chips and making the surface finish vulnerable to damage.

Magnesium / magnesium alloys

Magnesium is lighter than aluminium (atomic weight Mg=24.31). It has some degree of strength and is used occasionally for aircraft structural parts.

 

Magnesium easily reacts with oxygen and water and this reaction can cause ignition of the chips after machining as they react with the oxygen and moisture in air. To prevent the chips from reacting with air, it is necessary to ensure that plenty of water-soluble oil coolant is used. However, this reaction produces hydrogen gas, so full air ventilation during the machining process is necessary.

Copper / copper alloys

Copper is a metal with very high electrical conductivity* properties and is often used for the manufacturing of parts for the electronics industry, such as printed circuit boards (PCB) for computers.

 

Compared to aluminium, copper is very heavy, but it remains soft and highly malleable. This tends to cause welding during machining leaving poor surface finishes. Care should be taken on applications where the material overhangs to prevent bending due to the materials, high malleability.

Graphite

Graphite is very fragile. Fine particles from broken graphite can cause abrasive wear on tools.

It is also a combustible material and care should be taken about the heat generated during cutting.

Wood

Compared to metal, wood is very soft and has little rigidity. Therefore, if it bends or if vibration occurs during machining the surface finish can deteriorate.

Even if it is not machined at high speeds, wood burns easily and can be charred by the effects of cutting heat. Additionally, wood is not suited to wet cutting and care should be taken during machining.

GRP, FRP

GRP stands for Glass fibre reinforced plastic. FRP stands for Fibre reinforced plastic.

GRP and FRP contain reinforced fibre and the machined surfaces tend to peel.

Polyvinylchloride (PVC)

PVC melts easily due to the heat generated during machining, causing welding on the cutting edge.

It is also soft and often used for pipe material but it has little rigidity so care should be taken with these applications.