There are many kinds of threads: fixed pitch thread and variable pitch thread, single thread and multi-thread thread, external thread and internal thread. The tool setting method for machining threads on the original spiral line by using CNC lathe is one of the common methods for machining threaded parts. Turning is one of the most widely used methods in mechanical processing, which is mainly used for machining rotary parts. Lathe is the equipment for turning. The main movement of turning is usually the rotary movement of the workpiece, and the feed movement is usually realized by the linear movement of the tool. Thread processing is one of the basic functions of lathe.
Steps of NC lathe machining threaded parts:
When machining threads on a CNC lathe, the machining feed is not realized by a mechanical transmission chain, but by feeding the motor screw tool holder through the CNC system of the spindle encoder. According to the detected spindle rotation signal, the numerical control system controls the feed of the motor, realizes the proportional relationship required for turning threads, and cuts threads that meet the requirements. To solve this problem, three problems should be solved: first, the spindle rotates once, and the tool holder drives the thread turning tool to move a pitch t accurately in the Z direction; Secondly, thread processing can only be completed after many cuts. In order to prevent screwing in, the position of each feed must be the same. Finally, when cutting multi-head threads, it should be able to accurately index. In order to solve these three problems, the incremental photoelectric encoder is used as the spindle pulse generator in the CNC lathe, which is installed in the lathe spindle box and driven by the spindle through gears or synchronous toothed belts to realize the transmission of 1: 1. When the spindle rotates, the encoder rotates synchronously with the spindle, and at the same time, it sends out a pulse signal corresponding to the spindle angle, which is an important signal to control the tool movement in the thread processing process. Incremental photoelectric encoder is a digital goniometer which converts angular displacement into corresponding digital pulse signal, integrates sensor and analog-to-digital conversion, and its output pulse signal is ttl level, which is compatible with computer interface circuit. Incremental photoelectric encoder is mainly composed of photoelectric disk, photoelectric element, condenser and light source.
Photoelectric elements a and b are staggered by 90? Installation, the photoelectric disk rotates by a pitch, and under the irradiation of light source, photoelectric elements A and B get waveform output, which is 90? A sine wave with phase difference can be obtained by amplifying and shaping phase A and phase B at 90. . Output phase difference square wave. According to the phase relationship between phase A and phase B, the numerical control system determines the rotation direction of the encoder, thus obtaining the rotation direction of the lathe spindle. The pulse generated by phase C is used as the reference pulse, which is called zero pulse. Every time the encoder rotates, it generates a zero pulse at the fixed position C phase, which can be used as the synchronous control signal of multi-wire cutting. When turning threads, the spindle rotates once, and the encoder C generates a zero-pulse synchronous signal. Before each feed cutting, scan the C synchronization signal. The CNC system starts cutting when it detects the arrival of the C-phase signal, otherwise it is in a waiting state. This ensures that the starting position of each cutting is at a certain point on the circumference of the workpiece to be processed, and prevents the phenomenon of multiple cutting and screwing.
For multi-head thread cutting, phase A signal and phase C signal can be combined for multi-head indexing. Let the spindle rotate once, and phase A outputs n pulses. If the thread of the K head is cut, it is divided by n/k ... The concrete realization is that after cutting the first thread with the C-phase signal as the cutting starting point, when cutting the second thread, the C-phase signal is scanned, and then the n/k pulse of the A-phase signal is scanned, and this position is taken as the cutting starting point of the second thread, and so on. When cutting K thread, according to (k- 1) and A phase of C phase signal? (n/k) pulses are taken as the cutting starting point until all k threads are cut.
The spindle pulse generator rotates synchronously with the spindle, and the CNC system can control the cutter to feed in the Z direction according to the thread lead T and the spindle pulse signal, so as to ensure that the spindle rotates once and the cutter feeds a pitch in the Z direction. The principle is that the ratio n/l (calculated by the numerical control system) between the number n of encoder A-phase pulses corresponding to each spindle rotation and the number l of equivalent feed pulses required for the corresponding pitch t is stored in the counter as a counting constant. When turning threads, the spindle rotates. Every time the CNC system receives (n/l) A-phase pulses from the spindle encoder, it sends out a feed pulse, so that the cutter can feed the lamp L in the Z direction, so that the spindle rotates once and the thread turning tool can accurately feed a pitch in the Z direction. When NC lathe processes threads.
The lathe spindle drives the workpiece to rotate at a fixed speed. The CNC system first moves the tool rest to a fixed coordinate position according to the thread cutting process, and then sends the X-direction cutting feed command to start the thread processing cycle.
Step 1 of the cycle: the tool holder feeds in the X direction to reach the cutting position. At this time, the Z-direction servo controller waits for the zero-pulse synchronization signal sent by the spindle encoder, the X-direction and Z-direction servo controllers of the tool rest are in the state of electrical locking and zero positioning, and the tool rest is still.
Step 2: cnc receives the zero pulse of the spindle encoder, and the Z-direction servo controller immediately starts the tool rest, and performs thread cutting according to the Z-direction feed speed and displacement calculated by the A and B pulses of the spindle encoder and the pitch and thread length sent by the CNC system.
Step 3: When the thread length coordinates are reached, the Z-servo stops braking immediately, and the X-servo drives the tool holder to exit quickly.
Step 4: the Z-direction servo drive tool rest returns to the initial coordinate position of machining, and is ready to enter the next working cycle. In order to obtain higher thread machining accuracy, it is generally necessary to carry out multiple cycles. In each cycle, the Z coordinates of the four points remain unchanged, and the X coordinates increase by an increment of the cycle cutting depth. The purpose of zero pulse synchronization is to ensure that the feed point of each round thread cutting is consistent.
Using CNC lathe to process threaded parts can not only ensure the accuracy of machined parts, but also improve the machining efficiency by times. Numerical control lathe can replace traditional lathe when machining threaded parts.