Machine tool refers to a machine used to manufacture other machines. It is also known as a working machine or machine tool and is traditionally called a machine tool.
Machine tools are generally divided into metal cutting machine tools, forging machine tools, woodworking machine tools and others.
In modern mechanical manufacturing, there are many ways to process mechanical parts. In addition to cutting, there is also casting, forging, welding, stamping, extrusion and more. However, parts with high precision and fine surface roughness generally need to be machined using a cutting method on a machine tool.
Machine tools play an important role in building national economic modernization.
Types of machine tools
There are many varieties and specifications of CNC machines, and classification methods may vary. However, they can generally be classified according to function and structure using the following four principles.
Classification according to the movement control trajectory of the machine tool
(1) CNC machine tool with point control
Point control only requires the precise positioning of moving parts of the machine tool from one point to another, and the requirements for the movement path between points are not strict. No machining is performed during movement and movement between the coordinate axes is irrelevant.
To achieve fast and accurate positioning, the displacement between two points generally moves quickly first and then slowly approaches the positioning point to ensure positioning accuracy. The figure below shows the motion track of the point position control.
Machine tools with point control function mainly include CNC drilling machines, CNC milling machines, CNC punches and more.
With the development of numerical control technology and the reduction in the price of numerical control systems, numerical control systems used only for point control are becoming increasingly rare.
(2) Linear control CNC machine tool
Linear control CNC machine tool, also known as parallel control CNC machine tool, has the characteristic of not only accurately positioning between control points, but also controlling the movement speed and trajectory between two related points.
However, its movement route only moves parallel to the machine tool coordinate axis, which means that only one coordinate axis is controlled at the same time, eliminating the need for the interpolation operation function in the CNC system. During travel, the tool can cut at the specified feed rate and can normally only process rectangular and stepped parts.
Machine tools with linear control function mainly consist of relatively simple CNC lathes, CNC milling machines, CNC grinding machines and so on. The CNC system of this machine tool is also known as linear control CNC system. Likewise, CNC machine tools used exclusively for linear control are rare.
(3) Contour control CNC machine tool
Contour control CNC machine tool machining diagram
Contour control CNC machine tool, also known as continuous control CNC machine tool, has the characteristic of controlling the displacement and speed of two or more motion coordinates simultaneously.
To meet the relative movement path of the tool along the workpiece contour and the machining contour requirements of the workpiece, the displacement control and speed control of each coordinated movement must be precisely coordinated according to the specified proportional relationship . Therefore, the NC device in this control mode requires the interpolation operation function.
Interpolation involves describing the shape of the line or arc through the mathematical processing of the interpolation calculator in the NC system, based on input of basic program data such as line end coordinates, arc end coordinates, and center coordinates or ray. The pulses are then distributed to each coordinate axis controller based on the calculation results, controlling the link offset of each coordinate axis to meet the required contour.
During movement, the tool can continuously cut the workpiece surface and process all kinds of straight lines, arcs and curves. These machine tools mainly consist of CNC lathes, CNC milling machines, CNC wire cutting machines, machining centers and so on.
The corresponding NC device is called a contour control NC system, which can be classified into the following forms based on the number of linkage coordinate axes it controls:
① Two-axis articulation
It is mainly used for machining rotating surfaces on NC lathe or NC milling machine for machining curved cylindrical surfaces.
② Two-axis semi-linkage
It is mainly used for controlling machine tools with more than three axes. Two shafts can be linked and the other shaft can be fed periodically.
③ Three-axis articulation
It is generally divided into two categories. One is the three-axis linkage of X/Y/Z linear coordinates, which is mainly used in CNC milling machines, machining centers and so on.
The other is to control the rotation coordinate axis by rotating around one of the linear coordinate axes in addition to the two linear coordinates in X/Y/Z simultaneously.
For example, in a turning center, in addition to linking the longitudinal (Z-axis) and transverse (x-axis) linear coordinate axes, it also needs to control the linkage of the main spindle (c-axis) rotating around the z-axis at the same time. time.
④ Four-axis articulation
It simultaneously controls the link between the three linear coordinate axes of X/Y/Z and a rotary coordinate axis.
⑤ Five-axis articulation
In addition to simultaneously controlling the linkage of three X/Y/Z coordinate axes, it also controls two coordinate axes in A, B and C coordinate axes by rotating around these linear coordinate axes, forming the linkage to simultaneously control five axes .
At this time, the tool can be positioned in any direction in space. For example, the tool can be controlled to rotate around the x and y axes simultaneously, so that the tool maintains a normal direction with the machined contour surface at its cutting point, ensuring the smoothness of the machined surface, improving machining accuracy. and efficiency, and reducing the roughness of the machined surface.
2. Classified by servo control mode
(1) open loop control CNC machine tool
The power servo drive of this type of machine tool is open loop, which means there is no sensing feedback device. Generally, its drive motor is a stepper motor. The main feature of a stepper motor is that every time the control circuit changes the command pulse signal, the motor rotates through a step angle and the motor itself has a self-locking capability.
The power command signal issued by the NC system controls the drive circuit through the pulse distributor. It controls the coordinated displacement by the number of transformation pulses, the displacement speed by the frequency of the transformation pulses, and the displacement direction by the distribution order of the transformation pulses. Therefore, the most significant feature of this control mode is its convenient control, simple structure and low price.
The command signal flow sent by the NC system is unidirectional, so there is no stability problem with the control system. However, because the mechanical transmission error is not corrected by feedback, the displacement accuracy is not high. Early CNC machine tools used this control mode, but the failure rate was relatively high.
Currently, it is still widely used due to improvements in the driving circuit. In China in particular, this control mode is often used in NC transformation of general economical NC systems and old equipment. Furthermore, this control mode can be configured with a single-chip microcomputer or single-board computer as the numerical control device, reducing the price of the entire system.
(2) closed-loop control machine tool
The feed servo drive of this type of NC machine tool operates using a closed-loop feedback control mode. The drive motor can be a DC or AC servo motor, and the position feedback and speed feedback must be configured.
During machining, the actual displacement of moving parts is constantly detected and sent back to the comparator in the NC system in a timely manner. This value is then compared with the command signal obtained through the interpolation operation. The difference between the two is used as a control signal for the servo drive, which drives the displacement part to eliminate any displacement errors.
The installation position of the position feedback sensing element and the feedback device used determines whether it is a fully closed loop or semi-closed loop control mode.
① Full closed-loop control
As shown in the figure, the position feedback device adopts a linear displacement sensing element, with grid rulers being commonly used at present. It is installed on the saddle of the machine tool, allowing it to directly detect the linear displacement of the machine tool coordinates.
Through feedback, the transmission error in the entire mechanical transmission chain from the engine to the machine saddle can be eliminated, resulting in high static positioning accuracy of the machine tool.
However, within the entire control circuit, the friction, stiffness and backlash characteristics of many mechanical transmission links are not linear. Furthermore, the dynamic response time of the entire mechanical transmission chain is much longer compared to the electrical response time, which poses significant difficulties in correcting the stability of the entire closed-loop system. As such, the design and tuning of the system is also very complex.
This full closed-loop control mode is mainly used for CNC coordinate machines and CNC precision grinding machines that have high precision requirements.
② Semi-closed loop control
As shown in the figure, position feedback uses an angle sensing element, with encoders being the main type used today. It is directly installed on the end of the servo motor or lead screw.
Since most of the mechanical transmission links are not included in the closed loop of the system, it is called upon to obtain more stable control characteristics.
Mechanical transmission errors, such as those present in the lead screw, cannot be corrected at any time through feedback. However, the software configuration compensation method can be used to improve accuracy.
At present, most CNC machine tools adopt a semi-closed loop control mode.
③ Hybrid Control CNC Machine Tool
The characteristics of the above control modes are selectively combined to form a hybrid control scheme.
As mentioned previously, the open-loop control mode has good stability, low cost and low accuracy, while the full closed-loop stability is poor.
Therefore, to complement each other and meet the control requirements of some machine tools, a hybrid control mode must be adopted.
Open-loop compensation and semi-closed-loop compensation are widely used.
3. Classified according to the functional level of the CNC system
According to the functional level of a CNC system, it is generally divided into low, medium and high grades.
The limits of low, middle and high grades are relative and the division patterns will be different in different periods.
According to the current development level, various types of CNC systems can be divided into low, medium and high grades based on certain functions and indicators.
Among them, the middle and high grades are generally called full-function CNC or standard CNC.
4. Classification according to the type of processing technology and use of machine tools
(1) Metal cutting
It refers to CNC machine tools with various cutting processes such as turning, milling, drilling, grinding, reaming and planing.
It can be divided into the following two categories:
① Common CNC machine tools
Such as CNC lathes, CNC milling machines, CNC grinding machines, etc.
② Machining centers
Its main feature is a tool magazine with an automatic tool change mechanism, where the part passes only once.
After clamping, when automatically changing all types of cutting tools, various processes such as milling (turning), keying, hinge, drilling and tapping are continuously processed on each machining surface of the workpiece on the same machine- tool, such as (construction/milling) machining centers, turning centers, drilling centers, etc.
(2) Metal Formation
It refers to CNC machine tools that adopt extrusion, punching, pressing, drawing and other forming processes. Commonly used are CNC presses, CNC bending machines, CNC pipe bending machines, CNC turning machines, etc.
(3) Special processing
There are mainly CNC WEDM forming machines, CNC EDM, CNC flame cutting machines, CNC laser machining machines, etc.
(4) Survey and drawing
There are mainly CMM, NC tool setting instrument, NC plotter, etc.
