What is a CNC machine tool?

(1) From machine tools to CNC machine tools, machines are no longer brainless

The machine tool is the "mother machine" of other machines.

The steel produced by steel mills is not a variety of strange shapes that we see in life, but materials with more regular shapes such as plates, pipes, ingots, etc. These materials need to be processed into parts of various shapes by using machine tools for cutting; There are also some parts with higher precision requirements and finer surface roughness requirements, which must be cut out or ground out with a fine and complicated process on the machine tool.

CNC stands for Computer Digital Control. CNC is an automated manufacturing technology that allows fast, efficient, repetitive cutting, drilling, machining, and material manufacturing through computer-controlled equipment. A CNC machine consists of a miniature or microcomputer that acts as the controller of the machine. All cutting processes are performed by a complex array of machining tools controlled by instructions entered into a computer.

Once the specific part has been designed and the machining instructions have been written, the CNC machine is programmed with those instructions. The raw material is fed into the machine, the setup is complete, and then the computer does everything else: rotate the part, cut through the layers, drill the hole at the desired depth, and even automatically change the cutting head. The same parts can be machined over and over again with little change.

The basis of every CNC project is a 2D or 3D CAD drawing, which is converted into computer code for the CNC machine to execute.

The design of CNC machines is very precise. They are programmed to perform tasks of specified dimensions, similar to robots. Some common CNC machine types include:

CNC milling machine

CNC milling machines are usually very large and have built-in tool changers, automatic feed mechanisms for loading materials, and various electrical sensors for safe monitoring of machining.

CNC tools CNC lathes

The lathe rotates the workpiece on the spindle, while the fixed cutting tool is close to the workpiece to cut the chips. Because of this geometry, lathes are ideal for parts that are symmetrical around certain axes, which can get stuck in the spindle. The action of cutting a workpiece on a lathe is called "turning".

CNC engraving machine

CNC milling machines are very common mechanical parts. These types of CNC controllers typically have no other human-machine interface other than through a computer.

Planers are often used to produce larger sized parts from wood, plastic or metal. The most common router is a 3-axis setup (X, Y, and Z). The 3-axis setup will allow contour cutting, bagging and 3D embossing.

Figure: Parts with complex shapes, high manufacturing accuracy and surface roughness requirements: gas turbine impeller

Like all machines, the original machine tool includes a power device, transmission device and executive device, relying on the input power of the motor rotation, through the transmission device with the workpiece to be processed or the tool for relative motion, as for where to cut the knife, how much, how fast to cut and so on, it is directly controlled by people in the processing process.

Figure: Three core devices of traditional equipment

Because the speed of the motor used by the traditional machine tool is basically constant at work, in order to achieve different cutting speeds, the traditional machine tool has designed an extremely complex transmission system. Machines of this complexity are rare in modern design.

The transmission structure of traditional machine tool (x5032 vertical milling machine) is very complicated

With the development of servo motor (servo motor is a motor that can accurately control the position and speed of the motor within a certain range) technology and its application in CNC machine tools, direct control of the speed of the motor has become convenient, fast and efficient, and basically stepless transmission, the structure of the transmission system is greatly simplified, and there are even many links of the motor directly connected to the actuator. And omitted the drivetrain.

This "direct drive" mode is now a major trend in the field of mechanical design.

(2) What can numerical control system do? Process information and control dynamics

Numerical Controller System is the brain of numerical control machine tools.

General CNC machine tool composition

For general CNC machine tools, often include man-machine control interface, CNC system, servo drive device, machine tool, detection device, etc., the operator with the help of some computer-aided manufacturing software, the various operations required by the processing process (such as spindle speed changes and other steps as well as the shape and size of the workpiece) are expressed by part program code. And through the human and control interface input to the CNC machine tool, then by the CNC system to process and calculate the information, and according to the requirements of the parts program control servo motor, to achieve the relative movement of the tool and the workpiece, to complete the processing of parts.

The machining process of CNC machine tools

Numerical control system to complete a lot of information storage and processing work, and the information processing results in the form of control signals to the subsequent servo motor, the effect of these control signals rely on two core technologies: one is the interpolation of curves and surfaces, one is the machine tool multi-axis motion control.

(3) The shape of the part is too "free"? It's a matter of interpolation

If the motion trajectory can be expressed analytically, the entire motion can be decomposed into a composite motion of several coordinates of independent motion, and the motor can be directly controlled.

However, the shape of many parts in the manufacturing process can be said to be very "Free", neither round nor square, and even do not know what shape, such as automobiles, ships, aircraft, molds, art and other products often encounter Curves and surfaces that cannot be described analytically, which are called Free Form Curves or free surfaces.

 Parts containing free-form surfaces

To cut out these "free" shapes, the relative motion between the tool and the workpiece is correspondingly very complex. Specifically in the operation, it is necessary to control the workpiece table and the tool to move according to the designed position-time curve, and control the two to reach the specified position at the specified time with the specified attitude.

Machine tools can be well between the workpiece and the tool to complete the linear segment, arc or other analytical spline curve relative movement, and this complex "free" movement and how to complete? The answer depends on interpolation.

CNC machine tool for complex surface processing

The so-called interpolation is the process of determining the motion path of the tool on the CNC machine tool according to a certain method. According to the given speed and trajectory, add some new intermediate points between the known points of the trajectory, and control the workpiece table and the tool through these intermediate points, and then the whole movement can be completed.

Between these intermediate points, they are connected by line segments, arcs or splines. It is equivalent to using several small line segments and arcs to approximate the required curves and surfaces, which is the essence of interpolation.

The popular interpolation algorithms include point-by-point comparison method, digital increment method, etc. The interpolation using Nurbs spline curve is favored by high-end CNC machine tools because of its high efficiency and good precision.

(4) The posture of the knife is not correct and can not be processed? Five-coordinate linkage can be done in minutes

Machining complex surfaces not only needs to be machined in theory, but also needs to consider the relative position relationship between the tool and the surface being machined.

On the one hand, if the posture of the tool is not suitable, it will lead to the low quality of the surface of the machining; On the other hand, the tool will also interfere with the structure of the machined parts, and there is no way to process without adjusting the relative posture of the tool. This requires giving more freedom of movement to CNC machine tools to make them more dexterous.