4, several problems to be solved
The response of the X-axis of the lathe must be fast and reliable. The 1mm feed rate of 1mm is equivalent to the processing speed of 9600mm/min, which is obviously higher than the corresponding technical index of the general economical numerical control. Because it is repeated with load, it must be guaranteed to be foolproof, which puts higher requirements on the drive unit of the actuator.
At the same time, the X-direction movement path is 2mm, and the Z-direction is 2/90mm. This violates the rule that the X-direction feed rate cannot be excessively larger than the Z-direction when the thread is broken. Obviously, this problem cannot be solved by the controller. And look for a peripheral solution.
This kind of thread not only divides dozens of knives for turning, but each knives can also be divided into dozens of sections. The programming has no ready-made instructions, and only special compound cycle programs can be considered. This requires us to develop a kind according to the existing programming habits. Special instructions allow the operator to fill in the necessary size data and process data as required during programming.
Taking into account the scientific and practical processing, the user should be allowed to reasonably allocate the amount of knife in the X direction for each tool. When starting the machining, in order to avoid the knife tip falling, the amount of the knife should be arranged flexibly; in the middle stage, in order to avoid the vibration, the amount of the knife should be arranged reasonably, and the tool should be fed in a single direction (a few CNC systems have been developed). Features). In order to produce a lower surface roughness, the amount of snack knife or zero knife is allowed to be "tidyed".
5, the solution
In order to solve the X-direction fast and reliable response problem, we used a full digital AC servo motor and driver as the drive unit. In the selection, the power is increased to ensure the output torque, and a large inertia motor is used to improve the load capacity. Use a motor with a higher limit speed to meet high speed requirements. Because the AC servo system is an execution unit with self-testing ability to self-detect the position, speed, drive current, torque and other indicators and can be adjusted in real time, the reliability and accuracy are guaranteed.
The control part of the AC servo drive is actually a microcomputer unit that integrates software and hardware. It can perform various operations and set various parameters. We have specifically selected a driver with a dynamic electronic gear ratio function. For example, we have set an electronic gear ratio. Under this electronic gear ratio, when the driver receives a pulse signal from the CNC system controller, it drives Actuator movement 1μm. If we set the second electronic gear ratio, the ratio is X/time of the first ratio, then, under its action, if the driver receives a signal, the actuator is no longer running 1μm, but 20 μm. In the process of processing, it can be automatically switched as required, which is called dynamic electronic gear ratio function. We use this function to send a switching signal to the driver by the CNC system when the threaded end is finished. In the CNC system controller, the software pre-processes, depending on 20, to achieve less pulse and ensure X-pulse. However, the purpose is greater than the Z direction, and after the driver receives the switching signal, the second electronic gear ratio is called, see 1 running 20, and restored to the desired motion size. In other words, the problem is handled by the driver, and after the thread is processed, it is restored.
When designing the machining instructions specifically for special threads, the various problems as mentioned above in the thread turning process are repeatedly considered, including three aspects: the first is the X-axis electronic gear switching signal and the programming size is “20. "Processing and restoration"; the second is the processing of the running and deceleration sections of each knives and the closing and ending of each section, the segmentation processing and the cyclic processing of the normal processing and vacant stages; the third is the division of the multi-knife processing The knife process, the splitting phase, the intermediate phase, and the ending phase, how to deal with the amount of cutting. The design steps are to list the problem list first, and sort out the solution one by one, and then compile it into the internal support software of the numerical control system. We finally use G77 as a special CNC machining instruction for such special threads. The specific instruction format given is as follows:
G77 P(a)(b)(c) Q(d)(e)(f) R(g)(h)(i)
G77 X(j) Z9(r) P(m)(n) Q(o)(p) F(q)
The description is as follows :
G77 is a special instruction code. After reading this command, the CNC system first sends the electronic gear ratio switching signal to perform data processing of 20 to 1 and then starts processing a series of special tasks and recovers after the end. A, b, c, ... in "()" respectively represent the following size data or process data (not in order, nor in all). Such as: the center angle of each segment of the thread, the middle part (normal cutting part), the closing part, and the vacant part; the X-axis feed amount when the end is closed; the number of repetitions per knife segment; the first knife eats the knife Quantity; the last knife to eat the knife; total knife amount; finishing allowance; finishing repetition times; thread outer diameter; thread radius difference; pitch and so on.
After giving the above special instruction code, if the operator needs to ask for help during the specific programming, there will be prompt content on the system display, and the programming is as convenient as filling the form.
6, test results
In response to the above problems, a lot of work has been done from various aspects, including trials and revisions, as well as cooperation and support from relevant manufacturers. Now, this kind of special thread CNC turning equipment has been marketed and put into operation, and the effect is good, basically achieving the expected goal.
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