Machine tool refers to the machine that makes the machine, also known as the working machine or tool machine, which is traditionally referred to as the machine tool.
Generally, machine tools are divided into metal cutting machine tools, forging machine tools and woodworking machine tool and so on.
There are many ways to process mechanical parts in modern mechanical manufacturing: in addition to cutting, there are casting, forging, welding, stamping, extrusion and so on.
All the parts with high precision and fine surface roughness generally need to be machined by the cutting method on the machine tool.
Machine tools play an important role in the construction of national economic modernization.
Types of machine tools
There are many varieties and specifications of CNC machine tools, and the classification methods are also different.
Generally, it can be classified according to the function and structure according to the following four principles.
Classification according to the control trajectory of machine tool motion
(1) CNC machine tool with point control
Point control only requires the accurate positioning of the moving parts of the machine tool from one point to another, and the requirements for the motion trajectory between points are not strict.
No machining is carried out during the movement, and the motion between the coordinate axes is irrelevant.
In order to achieve fast and accurate positioning, the displacement between two points generally moves quickly first, and then slowly approaches the positioning point to ensure the positioning accuracy, as shown in the figure below, which is the motion track of point position control.
Machine tools with point control function mainly include CNC drilling machine, CNC milling machine, CNC punch, etc.
With the development of numerical control technology and the reduction of the price of numerical control system, numerical control system only used for point control is rare.
(2) Linear control CNC machine tool
Linear control CNC machine tool is also called parallel control CNC machine tool. Its feature is that in addition to the accurate positioning between control points, it also controls the moving speed and route (trajectory) between two related points.
However, its movement route only moves parallel to the coordinate axis of the machine tool, that is, there is only one coordinate axis controlled at the same time (that is, there is no need for interpolation operation function in the CNC system).
In the process of displacement, the tool can cut at the specified feed rate. Generally, it can only process rectangular and stepped parts.
The machine tools with linear control function mainly include relatively simple CNC lathes, CNC milling machines, CNC grinders, etc.
The CNC system of this machine tool is also called linear control CNC system.
Similarly, CNC machine tools simply used for linear control are rare.
(3) Contour control CNC machine tool
Machining diagram of contour control CNC machine tool
Contour control CNC machine tool is also called continuous control CNC machine tool.
Its control feature is that it can control the displacement and speed of two or more motion coordinates at the same time.
In order to meet the relative motion path of the tool along the workpiece contour and meet the requirements of the workpiece machining contour, the displacement control and speed control of each coordinate motion must be accurately coordinated according to the specified proportional relationship.
Therefore, in this kind of control mode, the NC device is required to have the function of interpolation operation.
The so-called interpolation is to describe the shape of the line or arc through the mathematical processing of the interpolation calculator in the NC system according to the basic data input by the program (such as the end coordinates of the line, the end coordinates of the arc and the center coordinates or radius).
That is, while calculating, pulses are distributed to each coordinate axis controller according to the calculation results, so as to control the linkage displacement of each coordinate axis to meet the required contour.
In the process of movement, the tool can continuously cut the workpiece surface, and can process all kinds of straight lines, arcs and curves.
Such machine tools mainly include CNC lathes, CNC milling machines, CNC wire cutting machines, machining centers, etc.
The corresponding NC device is called contour control NC system, which can be divided into the following forms according to the number of linkage coordinate axes it controls:
① Two axis linkage
It is mainly used for NC lathe machining rotating surface or NC milling machine machining curved cylindrical surface.
② Two axis semi linkage
It is mainly used for the control of machine tools with more than three axes. Two axes can be linked, and the other axis can be fed periodically.
③ Three axis linkage
It is generally divided into two categories. One is the linkage of three linear coordinate axes X / Y / Z, which is mostly used in CNC milling machines, machining centers, etc
The other is to control the rotation coordinate axis rotating around one of the linear coordinate axes in addition to the two linear coordinates in X / Y / Z at the same time.
For example, in a turning center, in addition to the linkage of the longitudinal (Z-axis) and transverse (x-axis) linear coordinate axes, it also needs to control the linkage of the main shaft (c-axis) rotating around the z-axis at the same time.
④ Four axis linkage
At the same time, control the linkage between the three linear coordinate axes of X / Y / Z and a rotating coordinate axis.
⑤ Five axis linkage
In addition to simultaneously controlling the linkage of the three X / Y / Z coordinate axes, it also controls two coordinate axes in the A, B and C coordinate axes rotating around these linear coordinate axes to form the linkage of simultaneously controlling five axes.
At this time, the tool can be set in any direction in space.
For example, control the tool to swing around the x-axis and y-axis at the same time, so that the tool always maintains a normal direction with the machined contour surface at its cutting point, so as to ensure the smoothness of the machined surface, improve its machining accuracy and machining efficiency, and reduce the roughness of the machined surface.
2. Classified by servo control mode
(1) open loop control CNC machine tool
The feed servo drive of this kind of machine tool is open-loop, that is, there is no detection feedback device.
Generally, its driving motor is stepping motor.
The main feature of stepping motor is that every time the control circuit changes the command pulse signal, the motor rotates a step angle, and the motor itself has self-locking ability.
The feed command signal output by the NC system controls the driving circuit through the pulse distributor.
It controls the coordinate displacement by the number of transformation pulses, the displacement speed by the frequency of transformation pulses, and the direction of displacement by the distribution order of transformation pulses.
Therefore, the biggest feature of this control mode is convenient control, simple structure and low price.
The command signal flow sent by the NC system is one-way, so there is no stability problem of the control system.
However, because the error of mechanical transmission 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.
At present, due to the improvement of the driving circuit, it is still widely used.
Especially in China, this control mode is often used in the NC transformation of general economic NC system and old equipment.
In addition, this control mode can be configured with single chip microcomputer or single board computer as numerical control device, so as to reduce the price of the whole system.
(2) closed loop control machine tool
The feed servo drive of this kind of NC machine tool works according to the closed-loop feedback control mode.
The driving motor can be DC or AC servo motor, and position feedback and speed feedback need to be configured.
During machining, the actual displacement of moving parts is detected at any time and fed back to the comparator in the NC system in time.
It is compared with the command signal obtained by interpolation operation, and its difference is used as the control signal of servo drive, which drives the displacement part to eliminate the displacement error.
According to the installation position of the position feedback detection element and the feedback device used, it is divided into full closed-loop and semi closed-loop control modes.
① Full closed loop control
As shown in the figure, the position feedback device adopts linear displacement detection element (grating ruler is generally used at present) and is installed at the saddle of the machine tool, that is, it directly detects the linear displacement of the machine tool coordinates.
Through feedback, the transmission error in the whole mechanical transmission chain from motor to machine saddle can be eliminated, so as to obtain high static positioning accuracy of machine tool.
However, in the whole control loop, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the whole mechanical transmission chain is very large compared with the electrical response time
This brings great difficulties to the stability correction of the whole closed-loop system, and the design and adjustment of the system are also very complex.
This full closed-loop control mode is mainly used for CNC coordinate machines and CNC precision grinding machines with high precision requirements.
② Semi closed loop control
As shown in the figure, its position feedback adopts angle detection element (encoder is mainly used at present) and is directly installed at the end of servo motor or lead screw.
Since most mechanical transmission links are not included in the system closed loop, it is called to obtain more stable control characteristics.
Mechanical transmission errors such as lead screw can not be corrected at any time through feedback, but the software setting compensation method can be used to appropriately improve its accuracy.
At present, most CNC machine tools adopt semi closed-loop control mode.
③ Hybrid control CNC machine tool
The characteristics of the above control modes are selectively concentrated to form a hybrid control scheme.
As mentioned earlier, the open-loop control mode has good stability, low cost and poor accuracy, while the full closed-loop stability is poor.
Therefore, in order to make up for each other and meet the control requirements of some machine tools, hybrid control mode should be adopted.
Open loop compensation and semi closed loop compensation are widely used.
3. Classified according to the functional level of CNC system
According to the functional level of CNC system, CNC system is usually divided into low, medium and high.
The boundaries of low, medium and high grades are relative, and the division standards 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-grade according to some functions and indicators.
Among them, medium and high-grade are generally called full-function CNC or standard CNC.
4. Classification according to the type of processing technology and machine tool use
(1) Metal cutting
It refers to CNC machine tools with various cutting processes such as turning, milling, collision, reaming, drilling, grinding and planing.
It can be divided into the following two categories.
① Ordinary CNC machine tool
Such as CNC lathe, CNC milling machine, CNC grinder, etc.
② Machining center
Its main feature is a tool magazine with automatic tool change mechanism, and the workpiece passes through once.
After clamping, by automatically changing all kinds of cutting tools, various processes such as milling (turning) key, hinge, drilling and tapping are continuously processed on each machining surface of the workpiece on the same machine tool, such as (building / milling) machining center, turning center, drilling center, etc.
(2) Metal forming
It refers to CNC machine tools that adopt extrusion, punching, pressing, drawing and other forming processes. Commonly used are CNC press, CNC press brake machine, CNC pipe bending machine, CNC spinning machine, etc.
(3) Special processing
There are mainly CNC WEDM, CNC EDM forming machine, CNC flame cutting machine, CNC laser machining machine, etc.
(4) Surveying and drawing
There are mainly CMM, NC tool setting instrument, NC plotter, etc.