There are many varieties and specifications of CNC lathes, and the classification methods are different. Generally, they can be classified according to the following four principles based on function and structure
First, according to the control track rapid prototyping cnc of machine tool movement
(1) Point control CNC lathe
Point control only requires accurate positioning of the moving parts of the control machine from one point to another, and the requirements for the trajectory between the point and the point are not strict, no processing is done in the moving link, and the movement between the axes is not relevant. In order to achieve both fast and accurate positioning, the movement of the offset between the two points generally moves quickly first, followed by a slow convergence to the selected point to ensure the accuracy level,, for the motion trajectory of the point control.
With point control function of machine tools are mainly CNC drilling machines, CNC milling machines, CNC machine tools, etc.. With the development of CNC technology and CNC system price reduction, purely for point control of CNC system is not common.
(2) linear control CNC lathe
Linear control CNC lathe is also known as parallel control CNC lathe, its characteristics are in addition to the accurate positioning between the reference point and the point, but also to control the movement speed and line (track) between the two relevant points, but its movement route is only parallel to the machine tool coordinate axis movement, in other words, while controlling only one coordinate axis (that is, the role of interpolation operations within the CNC system), in the process of shifting the tool can be specified feed Speed cutting, generally only processing rectangular, stepped parts.
Its linear control function of the main machine tools are relatively simple machining centers, CNC milling machines, CNC grinding machines, etc.. This machine tool CNC system is also known as linear control CNC system. As well, simply cnc plastic prototype for linear control of CNC lathes are not common.
(3) contour control CNC lathe
Contour control CNC lathe is also called continuous control CNC lathe, whose control feature is to be able to control the displacement and rate of 2 or more motion coordinates at the same time.
In order to realize the relative motion trajectory of the tool along the contour of the workpiece in line with the requirements of the workpiece processing contour, it is necessary to precisely integrate the offset control and speed control of each coordinate movement in accordance with the prescribed proportional relationship.
Therefore, in this type of control, the CNC device is required to have the role of interpolation operation. The so-called interpolation is based on the basic data input program (such as the end coordinates of the line, the end coordinates of the arc and the center coordinates or radius), according to the mathematical solution of the interpolation operator in the CNC system, the shape of the line or arc description out, that is, while calculating, according to the value of the axis controller to allocate pulses, so as to control the axis of the linkage offset with the required contour in the movement The tool continuously cuts the surface of the workpiece in the motion loop, and can process various straight, curved, and curved lines. Contour-controlled machining trajectory.
Such machine tools are mainly machining centers, CNC milling machines, CNC wire cutting ice, machining centers, etc., the corresponding CNC device called contour control CNC system based on the number of linked axes it controls, and can be divided into the following categories of ways
① two-axis linkage: mainly used for CNC lathe processing rotary surface or CNC milling machine processing curve column surface.
② Two-axis semi-linkage: mainly used for the control of more than three axes machine tools, where two axes can be linked, and the other axis can be used for cycle winning tool walking.
③ three-axis linkage: generally divided into two categories, one is the X / Y / Z three linear coordinate axis linkage, more for CNC milling machines, machining centers, etc.. Another category is in addition to the control of X / Y / Z in the two linear coordinates, but also control around which a linear coordinate axis rotation axis.
Such as turning machining center, it is in addition to the vertical (Z axis), horizontal (X axis) 2 linear coordinate axis linkage, but also need to control the spindle around the Z axis rotation (C axis) linkage.
④Four-axis linkage: It controls three linear coordinate axes of X/Y/Z and a rotation coordinate axis linkage at the same time.
⑤ Five-axis linkage: In addition to simultaneously controlling the linkage of the three X/Y/Z Yukline coordinates. It also controls two of the A, B and C axes of rotation around these linear axes, resulting in simultaneous control of five axes of linkage.
For example, the tool is controlled to wobble around the x-axis and the y-axis at the same time, so that the tool is always normal to the machined surface at its cutting point to ensure the smoothness of the machined surface, improve its machining accuracy and efficiency, and reduce the roughness of the machined surface.
Second, according to the way of servo control categorized
(1) open-loop control CNC lathe
This kind of machine tool servo drive is open-loop, that is, there is no inspection feedback equipment, generally it drives the motor for the stepper motor, the main feature of the stepper motor is that the control circuit converts each command pulse signal, the motor rotates a step angle, and the motor itself has the ability of self-locking.
CNC system output go tool command signal through the pulse regulator to control the drive circuit, it to convert the number of pulses to control the amount of coordinate offset, to convert the frequency of the pulse to control the displacement speed, to convert the pulse distribution order to control the direction of offset.
Therefore, the biggest characteristic of this type of control method is convenient control, simple structure and cheap. The command signal flow issued by the CNC system is one-way, so there is no problem of stability of the control system, but the offset accuracy is not high because the deviation of mechanical transmission is not calibrated by feedback.
Early CNC lathes are using this type of control, only the failure rate is relatively high, at present, because of the improvement of the rapid cnc machining drive circuit, so that it is still gaining more use. Especially in China, the general economic numerical control system and the old equipment of numerical control transformation mostly use this control method. In addition, this type of control can be equipped with a single-chip or single-board computer as a CNC device, prompting the price of the entire system to reduce.
(2) closed-loop control machine tools
This kind of CNC lathe tool walking servo drive is according to the closed-loop feedback control mode of work, its drive motor can be used DC or AC two kinds of servo motor, and need to be equipped with position feedback and rate feedback, in the processing at any time to check the specific offset of the moving parts, and timely feedback to the comparator in the CNC system, it and the interpolation operation obtained by the command signal for comparison, and the difference between the value as the servo drive control signal, so as to promote the offset parts to the control system. The difference is then used as the control signal of the servo drive, which drives the offset part to clear the offset deviation.
According to the position feedback detection element installation site and the use of different feedback devices, it is divided into fully closed-loop and semi-closed-loop two control methods.
①Fully closed-loop control
Its position feedback equipment selection of linear offset detection element (currently generally using the scale), installed in the bed saddle part of the machine tool, that is, direct inspection of the machine coordinates of the linear offset, according to the feedback can be cleared from the motor to the machine bed saddle of the entire mechanical transmission chain in the transmission error, so as to get a high level of static accuracy of machine tools.
However, because in the entire control loop, many mechanical drive link friction characteristics, stiffness and clearance are non-linear, and the entire mechanical drive chain dynamic response time compared to the electrical response time and particularly large. This brings great difficulty for the stability calibration of the whole closed-loop system, the system design and adjustment are also quite complicated thus, this fully closed-loop control method is mainly used for high precision requirements of CNC coordinate block bed, CNC precision grinding machine, etc..
Its position feedback selection corner detection elements (currently mainly using encoders, etc.), directly mounted on the servo motor or screw end. Because most of the mechanical drive stage is not included in the system closed-loop loop, and thus called to get more stable control characteristics. Screw and other mechanical transmission errors can not be calibrated at any time through feedback, but can be used to moderately improve the accuracy of the software fixed value compensation method. At present, most CNC lathes choose semi-closed-loop control method
(3) Mix and control CNC lathe
The characteristics of the above control methods can be selectively concentrated to form a mixed control scheme. As mentioned before, because the open-loop control method has good stability, low cost and poor accuracy, while the full closed-loop stability is poor, so in order to fill each other to meet the control requirements of some machine tools, it is appropriate to use the mixed control method. Selected more open-loop compensation type and semi-closed-loop compensation type two ways