We’ve discussed CNC automation control systems and the G-Code that provides the instruction for these machines to follow in another article. This time we’re going to take a closer look at CNC milling and its use by industrial automation companies in Malaysia.
CNC precision engineering down to the micron.
In the past milling machines were operated by hand to perform fabrication tasks. This changed when CNC automation became available. Computer-controlled milling machines, fully automated and extremely precise, are known as machining centres.
A computer controlled CNC milling machine uses a rotary cutting tool to shave material, usually metal, off a from workpiece that is fixed to a table or platform. Makino CNC milling machines that industrial automation companies in Malaysia use are able to fabricate parts as small as a few microns to pieces as heavy as 6985 kg.
Types of machining centre configurations and basic operation.
There are two types of machining centre configurations: 1. Vertical Machining Centres (VMC) and 2. Horizontal machining Centres (HMC). The former is preferred over horizontal machining centres when work is done on only a single workface while the latter is preferred for work on large, bulky workpieces.
The machine arm that holds the rotary cutting tool is controlled by highly accurate electric servo motors and move on three main axes; X, Y and Z (X = side to side, Y = up and down, Z = height). A rotary platform adds an additional B-axis, which are able to rotate upwards/downwards and sideways so that other faces of the workpiece can be worked on. This minimizes the need for a human operator.
Fabrication process of Vertical Machining Centres.
CNC milling machines can be broadly categorized by the types of cutting heads they use, in which there are two types:
Face milling is used to cut flat surfaces into workpieces, or to carve flat-bottomed cavities. This cutting process typically begins at the end corners of a milling cutter and the teeth of the heads mainly facing downwards. For softer metals with lower melting points lubricants, such as WD40 to prevent the chips from melding onto the cutting heads.
Also known as side milling, this mode of cutting the action happens mainly along the circumference of the cutter (cutting sideways into the workpiece). Peripheral milling heads differ from face milling in that the the former has cutters that have additional teeth on one or both sides and are generally used for slotting and straddle milling.
Unlike drilling, in which the tool moves along its axis, that is, up and down – a cutter moves perpendicular to its axis, either in circular motions or sideways. By making passes over the material repeatedly, taking off bits of a workpiece each time. Because of the friction and massive heat generated during cutting it is common for the parts involved to be cooled by water or other lubricants.
The process of shaving off material slowly off is called shear deformation, where the material of a workpiece is ‘pushed off’ in small amounts each time. CNC milling machines use cutters with many small, but hard and temperature-resistant ‘teeth’ (tungsten carbide, high-speed steel heads, etc.) that spin at high-speed and rely on making many small cuts each time to ‘wear’ the material off the workpiece.
Milled surfaces have a tendency to leave circular, revolution marks where the cutters pass. Depending on the flatness of the cutter head and the angle at which the cutting was done, the degree of roughness of the marks can vary. To fix this a slow run is usually performed, and in the most precise examples only microscopic imperfections are left on the workpiece surfaces caused by irregularities of the cutting head itself.