Hard turning is an accepted process for finishing hardened workpiece materials. Compared with other processing technologies, it has the characteristics of flexibility, high efficiency and good economy. Since its launch, with the assistance of machine tool manufacturers, the development of new PCBN material grades and the improvement of tool manufacturing methods, hard turning has continuously achieved significant performance improvements. The transmission parts are easy to hard turn, and the automotive synchronous meshing gears in this article are processed using different tool concepts. The number of different processing of this part highlights the advantages and limitations of the different concepts used.
Although the options for hard turning were quite limited a few years ago, as shown in this article, there are many options available today. Therefore, the processing technology should focus on the needs of users, and the machine tool technology is designed according to the user’s output, which is very important. Basic technology of hard turning When it comes to the basic technology of hard turning, the reference benchmark is to use a standard ISO blade geometry angle on a standard arbor to machine hardened parts to form the required part profile. Typical hard turning parts are shown in Figure 1. The machining process usually involves a variety of machining, such as the inner hole of the car, the end face of the car, the end face of the reverse turn and the conical surface of the turning synchronous meshing gear. Since the machining can be completed in one clamping, the direct benefit compared to grinding is the reduction of parts scrapped due to excessive position accuracy. Dimensional accuracy and surface roughness can be satisfied by adjusting processing parameters. Part clamping and machine tool stability also play an important role. Since the late 1980s, this technique has been used very successfully, and continues to be the basis for alternative grinding processes. High-productivity processing technology For all developing technologies, once the basic concepts have been accepted, it is inevitable that there will be a modified concept. As far as hard turning is concerned, the current emphasis is on increasing productivity. It is interesting that one process is low-feed processing, and the other process you expect is high-feed finishing technology.
Plug-in technology Plug-in (Plunging)
Plug-in technology Plug-in (Plunging) is essentially using a considerable part of the cutting edge length to generate the machining surface. This concept is not completely new, as it has been used very successfully in the valve seats of plug-in engine cylinder heads. However, with the successful development of the world’s first and currently the only integrated PCBN material grade CBN100 for finishing, the concept of plugging has expanded into other application areas. The overall CBN100 is much more economical. For example, a triangular blade provides six cutting edges for plunge turning, which makes the product ideal for plunge turning and traditional turning techniques. Compared with traditional hard turning, the main advantage is to shorten the machining cycle, which is about 75 to 90%. Represents the basic principle of plugging. The process relies on many key factors: first, the quality of the cutting edge of the blade is very important to obtain good surface roughness and the longest tool life. It is also necessary to increase the cutting speed and reduce the feed. This reduces the cutting force and ensures excellent dimensional accuracy. In order to maintain dimensional accuracy, the blade is allowed to feed empty during the last 2 to 3 revolutions of cutting. In order to avoid the cutting edge profile affecting the surface roughness, a small axial movement was made. With the application of these technologies and a good machine tool configuration, it is feasible to obtain highly consistent surface roughness and part accuracy. In gear turning, plunge turning has been used to machine tooth surfaces and synchromesh cone surfaces. The maximum length that has been inserted by PCBN is 16 mm. Sharpening technology The smoothing technology has been tried and tested in cemented carbide tools. The advantage of using a wiper blade is the machining capacity at a higher feed rate. . In fact, the principle of the smoothing blade is to place a large arc or multiple large arcs after the tip arc. Because the contact area is wider and the ratio of the peak to trough of the hard turning surface can be reduced, this allows the insert to have the same effect as a large circular arc or round insert. This also makes the surface roughness not worse after the feed rate is increased.
With the advancement of tool manufacturing technology, it is feasible to apply this principle to PCBN blades. The benefits of using wiper technology for hard turning are twofold:
- Improve productivity,
- Shorten contact time and provide longer tool life. In gear machining, wiper blades are usually used for machining inner holes.
One of the latest advances in PCBN wiper blade technology is the development of the Secomax CBN100 Crossbill ™ wiper blade. This unique blade combines the advantages of a one-piece PCBN and a split-handed wiper blade design. Until recently, the design of the wiper edge on the monolithic PCBN blade limited its ability to process steps. This is because the design of the smoothing edge on one cutting edge is opposite to the design of the opposite corner fillet. The introduction of the CBN100 Crossbill ™ insert has solved this problem. It is available in both right-hand and left-hand inserts. It can be used for axial turning towards the step (making full use of the wiper blade technology), and due to this design, it can also produce a perfect arc. Application of high productivity technology The wiper blade and plug-in process can be used in a series of different mass production applications. In gear turning, the combination of plunge turning and wiper blades is usually the best solution. Typical synchronous meshing gears including technical requirements such as dimensional accuracy and surface roughness. The first key point is the conical surface of the synchronous meshing gear. There are three options for processing this surface:
- (i) Traditional turning
- (ii) Plug in
- (iii) Smoothing blade technology
Traditional turning is a trial cut method, and serves as a benchmark for comparison with plunge turning and smoothing edge technology. In traditional turning, the cutting speed and feed used are 150-200 m / min and 0.1mm / rev, respectively. Parameters used in plug-in: Vc = 300m / min, f = 0.04mm / rev. As mentioned earlier, successful plunge turning depends on increasing the cutting speed and reducing the feed rate. The insert used is a one-piece Secomax CBN100 triangular insert with a negative angle groove, each insert providing six cutting edges. In plug-in machining, the positional accuracy of the tool is crucial because it will be copied onto the workpiece. When machining the taper surface, a special ‘customized’ arbor is required to provide a taper of 6.5 °, and the final adjustment of the taper needs to be completed on the machine tool. The main benefit of plugging is to shorten the processing cycle. The feed of the plug-in car is 0.04mm / rev, the cutting depth is 4 revolutions and the minimum air feed is completed, and the total contact time is completed within 0.04 seconds, while the traditional turning requires 4.96 seconds, the gap is more than 100 times. Surface roughness results-cone surface Turning machining time of the synchronous meshing gear end surface Analysis of the residual stress on the machined surface has also shown that insert turning has real benefits, depending on many factors, such as cutting edge status, machining parameters, etc. It is feasible that the entire surface of the machined part is under compressive stress. This is of course interesting for parts that bear dynamic loads. The surface of the insert turning also removes the problems related to the spiral surface that affect the surface roughness. Traditional turning generates a spiral tool path. Using the wiper technology on the taper surface is an option when the machining cycle needs to be shortened compared to traditional turning; however, this will require the blade to be adjusted to match the taper to ensure the effectiveness of the wiper effect. The inner hole of the gear can be processed using traditional blades and smooth blades. It is precisely the reason of the length of the inner hole that makes the insertion of the car not a viable solution. The main benefit of using a wiper blade is to increase the margin removal rate without affecting the surface roughness of the part. There is little or no advantage in using a wiper blade at a lower feed rate. Depending on the design of the wiper blade, the feed rate can be three times higher than traditional turning. It has two potential advantages. The first is to shorten the machining cycle, and second, less contact time provides potentially longer tool life. A variation on the principle of the wiper blade is to use a blade with a larger tip radius, such as a round blade. This kind of modification is acceptable for through-hole or unobstructed cylindrical turning, and it is not feasible to encounter a step during processing, which limits its application. The use of smoothing blades or round blades has a larger contact area, which does increase the pressure during cutting, but due to the low cutting force of hard turning, this usually does not cause the problem of dimensional accuracy.
Usually, the front face and / or the rear face are processed last. All three options are feasible, and of course because of the small area, the machining cycle is lower in all cases, but there is still an opportunity to minimize cutting time by applying plug-in technology. Conclusion As the industry places great emphasis on improving productivity by shortening the processing cycle time, both processing technologies discussed in this article have made important contributions. Although both technologies have some limitations, it has been proven that when the geometric angle of the part allows the use of plunge and wiper technology, the machining cycle is significantly improved and therefore the productivity
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Hard turning is an accepted process for finishing hardened workpiece materials. Compared with other processing technologies, it has the characteristics of flexibility, high efficiency and good economy.