Saturday, November 26, 2022

How does the CNC milling machine work?

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The introduction of computerized numeric control (CNC) has exponentially expanded the applications of industrial machines through programmable automation of production and the achievement of movements impossible to perform manually, such as circles, diagonal lines and other more complicated figures that make it possible to manufacture parts with highly complex profiles. This also translates into the optimization of many essential variables of any manufacturing process: productivity, precision, safety, speed, repeatability, flexibility and waste reduction.

The multiplicity of milling machines that exist today has comfortably expanded towards the proliferation of their CNC-equipped peers. In fact, there are also special kits to transform old milling machines into a CNC milling machine.

Basically, CNC milling machines are very similar to conventional milling machines and have the same moving parts, that is, the desk, the Cutting head, the spindle and the side and cross shift carriages. However, they do not have levers or cranks to activate these moving parts, but rather a screen inserted into a panel full of controls and a Metallic box where the electrical and electronic components that regulate the operation of motors destined to carry out the same work that the levers and cranks of the old machines did are housed. Among these components is the CNC, which is a computer mainly responsible for the movements of the milling machine through the corresponding software. The combination of electronics and drive motors or servomotors is capable of achieving all possible milling operations.

To understand the movement control exerted by the CNC, we are going to briefly review how a conventional milling machine works.

CNC milling machine

The figure schematizes a typical milling machine. In this type of machines, the cranks activate the moving parts manually so that the cutting tool (milling cutter) moves linearly in at least three axes, which are called main axes:

X axis: horizontal and parallel to the workpiece clamping surface. It is associated with the movement in the longitudinal horizontal plane of the milling table.

Axis y: forms a trihedron of direct direction with the X and Z axes. It is associated with the movement in the transverse horizontal plane of the milling table.

Z axis: where the cutter is mounted, it is the one that has the cutting power and can adopt different positions depending on the possibilities of the head. It is associated with the vertical displacement of the machine head.

If the milling machine has a fixed table, these three movements are executed by the spindle.

Now, it is clear that the milling of more complex parts will require a greater number of axes whose trajectory is not only linear, but also rotary. It is at this point that the concept of CNC comes into play, giving rise to a multiplicity of complementary axes independently controlled and determined by the movement of rotary tables me steerable heads. This gives rise to a variety of machine models that make it possible to machine the part through different planes and approach angles.

In the following figure we see an example of a CNC milling machine with its basic components and main (X, Y, Z) and complementary (B, W) axes.

Components of a CNC milling machine

Components of a CNC milling machine

1 – Column
2 – Workpiece
3 – Milling table, with movement in the X and Y axes
4 – Strawberry
5 – Cutting head including spindle motor
6 – CNC control panel
7 – Coolant hoses
X, Y, Z – Main displacement axes
B – Complementary rotary movement axis of the cutting head
W – Complementary axis of longitudinal displacement of the cutting head

The primary function of the CNC is to control the movements of the table, the transverse and longitudinal slides and/or the spindle along their respective axes by means of numerical data. However, this is not all, because the control of these movements to achieve the desired final result requires the perfect adjustment and the correct synchronization between different devices and systems that are part of every CNC process. These include the main and complementary axes, the transmission system, the workpiece clamping systems and the tool changers, each of which presents its modalities and variables that must also be properly stipulated.

This rigorous control is carried out by software that is supplied with the milling machine and that is based on one of the CNC numerical programming languages, such as ISO, HEIDENHAIN, Fagor, Fanuc, SINUMERIK and Siemens. This software contains numbers, letters and other symbols – for example, G and M codes– that are encoded in an appropriate format to define a program of instructions capable of performing a specific task. The g-codes are functions of machine movement (rapid movements, advances, radial advances, pauses, cycles), while the M codes are miscellaneous functions that are required for part machining, but are not machine motion (spindle start and stop, tool change, coolant, program stop, etc.). From this it follows that to operate and program this type of machine basic knowledge in machining operations in conventional equipment, basic knowledge of mathematics, technical drawing and handling of measuring instruments is required.

Currently the use of programs CAD (computer aided design) and CAM (computer-aided manufacturing) is an almost obligatory complement to any CNC machine, so, generally, the manufacture of a piece involves the combination of three types of software:

  1. CAD: performs the design of the part.
  2. CAM: calculates the displacements of the axes for machining the piece and adds the feedrates, turning speeds and different cutting tools.
  3. Control software (included with the machine): receives the CAM instructions and executes the movement commands of the moving parts of the milling machine according to said instructions.

The following video illustrates the manufacture of a part using CAD/CAM:

CNC milling machines are specially adapted for milling profiles, cavities, surface contours and die-cutting operations, where two or three axes of the milling table must be controlled simultaneously. Although, depending on the complexity of the machine and the programming carried out, CNC milling machines can work automatically, an operator is usually needed to change milling cutters, as well as to mount and dismount workpieces.

Among the industries that routinely use CNC milling machines are the automotive industry (design of engine blocks, molds and various components), aerospace (aircraft turbines) and electronics (making molds and prototypes), in addition to those dedicated to the manufacture machinery, instruments and electrical components.

Let’s see in this video some types of CNC milling machines, as well as examples of operation.

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