The classification of CNC machine tools is as follows. The more complex the structure, the higher the accuracy and efficiency can be achieved by reducing clamping:
1) Ordinary CNC machine tools, such as milling machines and lathes
2) Machining center (compared with ordinary CNC milling machine, with automatic tool change device)
3) Turning center (compared with ordinary CNC lathe, its tool holder can carry rotary tools in addition to turning tools, and the spindle can be positioned)
4) Turning milling composite machining center (replace the cutter tower of the turning center with the spindle head and increase the degree of freedom)
Compared with ordinary machine tools, CNC machine tools have advantages in precision, efficiency, etc. Its composition structure includes:
1) Mechanical parts: bed, spindle, screw guide, chuck, tool rest, tailstock, cooling
2) Electrical control: CNC system (running on industrial control computer or controller, external driver and plc), driver (external servo motor), plc (external spindle drive, turret, cooling and other components), feedback device (grating and encoder)
The electrical control is divided into logic control (starting and stopping of spindle, positive and negative rotation, starting and stopping of cooling) and motion control (displacement, speed, acceleration and their combined control). The motion control of the CNC system has three more important functions, which are often related to various application anomalies (especially various cambers and turns). They are interpolation (fitting arcs with multiple small segments at the turning point is easy to make mistakes due to resolution limitations), acceleration and deceleration (starting and stopping, tool path turning point, severe acceleration and deceleration may trigger structural vibration), Compensation (tool compensation, reverse clearance, pitch error, thermal drift compensation, and accuracy loss can be significantly reduced after correct setting). It should be noted that the compensation is not universal. First, the repeated positioning error of the machine tool cannot be too large or it cannot be compensated, and the friction, inertia force and cutting force cannot be compensated. Therefore, full closed loop is required to pursue accuracy.
Control mode:
1) Open loop control: without feedback, the CNC system sends out pulses after calculation. Each pulse makes the stepper motor rotate at a certain angle after drive conversion and amplification. The workbench is driven by the ball screw to move a certain distance. The advantage is simple and cheap, but the disadvantage is that the precision is not as good as that of the closed loop system.
2) Semi closed loop control: position and speed of the servo motor are controlled by combining the displacement command sent out after the calculation of the CNC system and the angular displacement signal of the encoder in the servo motor. The precision and cost are between the open loop and the closed loop. More satisfactory results can be obtained by increasing the compensation for errors such as screw pitch, which is more widely used.
3) Closed loop control: In addition to the angular displacement feedback of the servo motor, there is also a linear position detection feedback on the moving parts. Combining these feedback and the home position shift command, the drive signal of the servo motor is obtained. It has high accuracy but is complex and expensive. It is mostly used for ultra precision lathes and ultra precision grinders.
Mechanical stiffness and servo stiffness: In addition to the mechanical stiffness of the structure itself, the control system also has servo stiffness. The higher the stiffness, the stronger the anti load capacity, and the lower the impact of external forces in theory. In fact, the full closed loop can better ensure the accuracy.
Follow error: In order to avoid large form and position errors in contour machining, in addition to the accuracy of the machine tool meeting the requirements, it is also required that the dynamic response meet the requirements. Otherwise, in multi axis linkage, the response lag of individual axes due to mechanical or electrical characteristics will lead to follow errors, which will directly lead to contour deformation.
electric machinery:
1) Stepping motor (there are multi-phase single stator radial structure and multi-phase multi stator axial structure, and different pulses are sent to each phase for speed regulation),
2) DC servo motor (the stator magnetic field is fixed, the rotor is continuously operated through the brush and commutator, and the speed is generally adjusted by adjusting the voltage),
3) AC servo motor (Simple structure and wide application, it can be divided into permanent magnet synchronization and AC induction. The rotation frequency of the latter is obtained by dividing the driving frequency by the number of pole pairs, and there must be slip. Generally, the speed is adjusted by a sine wave pwm converter. Open loop vf control, speed slip control, and vector control can be selected. The advantages of vector control are response and accuracy, but the disadvantages are lower efficiency. In the low speed segment, constant torque control increases the voltage with the increase of speed A voltage offset will be given at the time of international startup. Constant power control at high-speed section, constant voltage and torque drop)
Drive:
1) The driver structure can be divided into three parts, high voltage board, low voltage board and other accessories. The high voltage board is used to convert the input fixed 3-alternating current into DC and then convert it into the designated 3-alternating current through the chip according to the input control signal. The low voltage board is used to communicate with the system, store and operate the preset algorithm and provide the control signal to the high voltage board. The algorithm is the core of the driver.
2) The evaluation key of the driver is accuracy, response and low speed load. For the main shaft, it includes the speed rise time, overspeed amplitude, steady speed slip and fluctuation, low speed operation, and the rigidity against the load (cutting instantaneous speed drop). For the feed shaft, it includes the moving positioning accuracy and vibration after the gain is increased,
Position detection:
1) The structure of a resolver is similar to that of a motor. A transformer is formed between its stator and rotor. In the process of rotation, the close or far distance between the poles of the stator and rotor will affect the conversion coefficient of the transformer. Therefore, the angle of rotation can be inversely deduced by detecting the change of the rotor voltage. This is an incremental encoder rather than an absolute encoder. Compared with an optical encoder, the structure of a resolver is simpler and more reliable, with lower environmental requirements, However, the available speed and accuracy are not as good as the encoder. The machine tool generally uses an encoder instead of a rotary transformer (the electromagnetic encoder with lower requirements for the use environment of the spindle is generally used, and the optical encoder is generally used for the turntable and servo motor). Instead, Yamaha has a single axis rotary machine (with harmonic reducer) that uses a rotary transformer.
2) Inductosyn is a linear displacement sensor formed by expanding the resolver into fixed length and sliding length.
3) Grating, used for high-precision position control in closed-loop control.
4) Encoder: photoelectric encoder is commonly used on machine tools.
Machine tool structure:
The machine tool pursues high rigidity and large damping to ensure the accuracy under load. It can be adjusted by reinforcing rib and molding sand filling, or the load resistance can be enhanced by symmetrical structure (which will reduce the impact of temperature rise at the same time). Generally, vertical is inferior to horizontal and gantry.
According to the movement mode, it can be divided into worktable movement and spindle box movement. Generally, the lighter parts are selected for movement. If the workpiece is heavy, the spindle will be moved. In order to pursue high speed and high accuracy, the horizontal machining center will even adopt the frame in frame or box in box structure (multiple supports have good rigidity, and the symmetrical structure has little impact on thermal deformation).
Coupling and timing belt:
In addition to the electric spindle, there are other spindle transmission modes. For example, the vertical drilling and tapping machine commonly installs a low-speed and high torque motor above the spindle box, the lower box installs the spindle in the form of a flange, and the spindle is driven with a coupling between the spindle and the motor. For another example, the common synchronous belt transmission of the lathe is to arrange equidistant teeth on the transmission belt, which has a more accurate transmission ratio than the ordinary v-shaped V-belt, Compared with chain drive, it can save lubrication and reduce maintenance.
Ball screw nut pair:
The ball forms rolling friction between the screw rod and the nut to improve the creeping phenomenon. Because there is a clearance in the axial direction, it needs to be pre tightened, otherwise there will be impact and error in the positive and negative movement. The pre tightening method can be double nuts with gaskets, double nuts with nuts for locking, or single nuts with variable lead for self pre tightening.
Linear motor:
In order to pursue high speed and fast response, some machine tool manufacturers have changed the ball screw nut pair into a linear motor. Simply speaking, after the ordinary rotating motor is expanded, the corresponding part of the stator is called the primary, and the corresponding part of the rotor is called the secondary. The AC linear motor is mainly used on the machine tool. In order to control heating and cost, the high-speed machine tool generally adopts a short primary structure. The secondary is installed on the machine bed and the primary is installed under the workbench, It is equipped with a grating for closed-loop control and a guide rail for movement. At present, it has been used in imported machine tools to a certain extent, such as Shandong Ruizhi, which is less used in domestic machine tools, but is accelerating its promotion. Some linear motor suppliers integrate motors and gratings into modules to facilitate customer installation.
guide:
The guide rail is used to support and guide moving parts. To a large extent, the rigidity, accuracy and precision of the machine tool can be maintained. Common guide rails are divided into sliding guide rails with sliding surfaces machined directly on the machine bed, and linear rolling guide rails that are installed after the installation surface is machined on the machine bed (there is a rolling body between the guide rail strip and the slider, which needs to be adjusted and pre tightened). The Taiwan standard is also known as hard rail and line rail. The former is highly impact resistant, and most common in heavy machine tools, The latter is fast and easy to install (some machine tool plants can greatly improve production efficiency by using large gantry milling instead of guideway grinder or manual scraping), which is widely used in small and medium-sized machine tools. There is also a hydrostatic guide, which is relatively rare.
Five axis and turntable swing head:
The five axis configuration includes double turntables, double swing heads, and turntables plus swing heads.
Compared with the indexing table that can only convert between several fixed angles, the grating is used for closed-loop control. The rotary table driven by dd motor or worm gear is more flexible in application. It can add degrees of freedom outside the xyz three axis of the machine tool (such as the cradle rotary table) to achieve processing.
Workpiece installation:
The common installation method of lathe is three jaw chuck and center tailstock. When the chuck is to install large diameter workpieces, the jaw should be reversely installed.
If it is necessary to process according to a specific angle, an indexing head will be used, similar to adding indexing ring and vernier, locating pin, indexing plate and exchange gear on the chuck or tailstock, to divide the workpiece clamped between the centers or on the chuck into any angle.
Spindle:
slightly
Tool installation:
There is no need to replace multiple cutters repeatedly in the process of punching Pcb plates, and straight shank chucks are often used. For metal processing, it is necessary to use a tool magazine to repeatedly replace multiple types of tools. A 7:24 taper universal tool handle (conical surface contact, claw external and internal holding the rivet, iso international standard, bt Japanese standard) or a 1:10 hsk tool handle (at the same time, the cone surface contacts with the flange surface, the claw expands to the external buckle, which can adapt to faster processing) is often used. Both types of tool handles need to be equipped with a nut clamping elastic sleeve (also called a bobbin clamp, with various specifications such as er32).