Mass-produced gears generally adopt the rolling-shaving-turning process. If the gear hobbing machine has a large error, the gear quality will be difficult to guarantee. For this reason, people continue to explore simple and effective hobbing machine error correction compensation technology, so that the old machine tool has been refined transformation.
First, measurement and control type refinement technology
This solution uses the principles of real-time testing and real-time correction. A sensor (raster, etc.) is installed on the hobbing machine tool bar and the workbench, and a test system is formed with a computer (single-chip microcomputer); a stepping motor is also mounted on the differential axis, and a control system is formed with the computer. Measurement and compensation control are performed while the machine tool is processing the movement. According to the measured transmission error, the computer translates into the corresponding number of drive power pulses, which causes the stepper motor to drive the differential shaft to rotate, so that the workbench gets an additional compensation motion that is opposite to the transmission error, which can correct the machine tool error and improve the workpiece processing. The purpose of precision.
The advantages of this program are flexible working methods and convenient conversion of teeth; the disadvantages are: (1) high cost, a complete set of inspection and control system is required for each machine tool transformation; (2) poor reliability, and it is necessary to ensure that the complete system is continuously accompanied by production. Proceeding without error for a long period of time is almost impossible under the harsh environment of the workshop; (3) The installation, location, and shielding of the sensors are very troublesome. Therefore, this scheme has not been popularized in production.
Second, mechanical refinement technology
This program uses a one-time test, according to the test results of eccentric cam machining, adding the drive chain to make the transmission error correction and compensation. The advantage is that only one or two cams are added to each machine that is modified. The cost is low, the work is reliable, simple, and maintenance-free. The disadvantage is that the flexibility is poor. Each refinement can only be targeted to one or a few teeth. Gears, if you want to change the number of teeth, we must remeasure and redo the cam. Therefore, this method is suitable for the production of gears with large batches and few varieties such as automobile and motorcycle gears. The specific implementation of this program has the following forms.
1. Two-way double eccentric gear. One eccentric is transversely cut to two, and the eccentricity is designed based on the error values ​​of the correction axis in both directions. In the forward and reverse motion test, the phase is selected under the monitoring of the instrument, marked, find the optimal position of the respective error compensation and tangential misalignment, and turn a distance to ensure that the forward and reverse rotary motions are carried by only one gear, respectively. At the same time to ensure a certain backlash, and finally with the pin and the two teeth fixed. In this way, the two eccentric gears actually occupy the original one gear position. This device can solve the error correction of forward and backward movement.
2. E wheel eccentricity correction. As shown in the following figure, some machine tools start from the gear behind the hob until the differential mechanism has a total of 5 pairs of bevel gears with a 1:1 transmission ratio. In this case, their respective errors are combined into a single error of the same frequency. The e-wheels are machined to eccentricity, and the instrument can be calibrated to determine the phase, which can be offset.
Gear hobbing machine chain diagram
3.d wheel eccentricity correction. At this time, the object to be corrected is the periodic error of the worm gear, and the principle and method are the same as those of the above-mentioned 2. This method will make the d wheel (or e wheel) fixed, thereby reducing the choice of gear with matching gear, which limits the range of changes in the number of teeth processing.
4. Use the differential chain for correction. This method has similarities with the principle of the above-mentioned measurement and control scheme, namely, using the machine differential system, according to the error size to provide an additional counter motion to cancel. However, only one cam can be added here. As shown in Figure 1. The gears a′, b′, c′, and d′′ are differential suspension gears, where a′ is replaced by a cam, b′ is taken as a swinging rod, and c′ and d′ are amplifying ratio adjustment gears. a"-b" is the feed hanging wheel group, and a to e are the gear tooth hanging wheel group. Adjust the feeding hanging wheel, make the axis of the differential hanging wheel a′ rotate synchronously with the machine table, install the cam on the a′ axis, install a swinging rod on the b′ axis, and use the spring to ensure that the swing rod and the cam are unidirectional. Snap tightly, the increase of the cam will increase the rotation of the b' axis by c', d'. The differential mechanisms e, f, a, b, c, d and the worm bring an additional motion to the table. To offset the previously measured transmission error.
The cam machining is based on the test error curve. The shape of the cam envelope is the shape of the test curve. In the early stage, it was limited to processing methods. It only processed cams based on the long-period error test curve, and required a certain margin. Then, the cams were mounted and tested again. According to the test results, the cams were corrected with a guillotine (the cam material was made of organic glass. For ease of processing), so repeated several times. This method is mainly to correct the long-term error of the machine tool. With the development of computer technology and numerical control processing technology, corrections have been started based on transmission error (including long period error and short term period error) curves. The test curve obtained by using the FMT test system [1] [2] is composed of 1024 test points in a week. After converting these points into a CNC milling machine, a cam with a very accurate shape can be machined. Therefore, the long-period error and most of the short-cycle errors of the machine tool (a few high-frequency errors cannot be reflected due to the interference between the wheel and the cam on the pendulum) can be compensated and corrected, and the accuracy is even more significant.
Third, the conclusion
The premise of realizing mechanical refinement is precise measurement, and after measurement, firstly, according to the results of measurement and analysis, some coarse error links in the drive chain, such as shaft bending, hole misalignment, missing teeth, loose shaft sleeves... ...and so on before the replacement, this work using the FMT test system is very easy to do. On this basis, the refinement correction processing can be achieved for the inconvenient replacement parts, such as the cumulative error caused by the worm pair and the periodic error caused by the bevel gear.
Also note that once the machine tool is adjusted, it should not be disassembled. Otherwise, once the gear phase is disordered, the accuracy will deteriorate immediately.
The author once reconstructed an old gear hobbing machine at a certain factory, and the accumulated error was reduced from 13.6′′ to 5.2′′ in two days. The plant immediately moved the machine tool to the constant temperature workshop as the mother machine’s meticulous maintenance. It can be seen that this scheme is simple and easy to use, and it is very popular among users, and this technology is also easily extended to the transformation of other gear processing machines.
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