Gear parts net forming forging technology

Forging forming is one of the most basic technologies of mechanical manufacturing; with the rapid development of various industries, the forging industry has long entered the micro-profit era. The development trend of forging technology and forging processed products is moving towards the direction of high quality and lightweight, precision net forming, high efficiency and low cost. With a series of national production capacities, cost reduction control policies and green mountains being the mountains of environmental protection policy, forging enterprises must establish technology, market-oriented green innovation system, must rely on technology support, innovation-driven and high-quality development in order to realize the rebirth of adversity.
Therefore, material-saving forging improves the accuracy of forgings and reduces the amount of subsequent machining to material. Even part of the material to forging process to replace has been more and more enterprises to pay attention to and physically implement. Only to improve the material utilization of forgings in order to reduce the consumption of raw materials, thereby reducing the heating of electricity consumption and subsequent forging in the machining of tool consumption. Precision forging net forming instead of the subsequent part of the machining process can greatly improve machining efficiency, eliminating tool consumption and equipment investment, saving labor costs, etc., to increase the economic benefits of enterprises, and strive to achieve or close to the “green forging”.

1. Planetary, half shaft differential gear non-tooth surface part of the functional surface of the net forming forging

(1) Planetary, half-shaft gear mounting surface on the oil groove structure
A differential planetary, half-shaft straight bevel gear is a key part of the vehicle transmission system; whether the vehicle’s two-stage axle or single-stage axle, its internal differential assembly has set or several sets of differential planetary, half-shaft straight bevel gear in the transmission system plays a vital role; in order to ensure that the differential straight bevel gear in the mesh rotation process is in a good working condition, the mounting surface of its gears with the differential case within the lubrication between the mounting surface of the gear and the wearing parts (shims) in the differential housing is extremely important. Different axle manufacturers have adopted different methods, either in the planetary gear spherical surface to increase a ring-shaped eccentric oil groove (Figure 1) or three nearly equal radial oil grooves (Figure 2) or in the half-axle gear mounting datum surface to increase the two hyperbolic isosceles trapezoidal or semi-circular oil grooves (Figure 3) or butterfly oil grooves (Figure 4).
If the planetary gear spherical surface of the ring-shaped eccentric oil groove program, the actual machining process only needs to design a set of special tilting eccentric tooling, turning processing is relatively easy, and processing efficiency is low. However, the three radial straight oil groove programs, which are approximately equal, are very difficult to process, and the efficiency could be higher. Usually, the three radial straight oil grooves use special tooling, milling in the horizontal milling machine using the molding disc milling cutter milling shape. After each milling of a straight oil groove, the gear should be removed, converted to a certain angle, and then mounted again through compression, milling another straight oil groove.

Figure.1 Eccentric ring-shaped oil groove on the spherical surface of straight tooth bevel planetary gear

Fig.2 Straight radial oil groove on the spherical surface of straight bevel planetary gears

Fig.3 Hyperbolic oil groove on the mounting surface of straight bevel half shaft gears

Fig.4 Butterfly-shaped oil groove on the mounting face of the spur bevel gears
In such a planetary gear sphere on the three nearly equal radial straight oil grooves, milling is completed on the need for loading and unloading, fixed compression three times, the efficiency is extremely low, and because the tooling is fixed, in the milling of straight oil grooves in the process of the tooling did not rotate around the center of the planetary gear sphere. Hence, milling out of the radial straight oil grooves is generally slightly wider in the middle, slightly deeper, the two ends of the slightly narrower, slightly shallower, not in line with the requirements of the drawing, and even more difficult to retain the straight oil grooves according to the drawing requirements. It is difficult to retain the transition angle R between the two beveled surfaces of the straight oil groove and the spherical surface according to the requirements of the drawings, which makes the spherical surface form a hard edge similar to a knife blade at the junction of the spherical surface and the oil groove after heat treatment and final grinding, and is extremely unfavorable to the relative rotation of the concave side of the spherical spacer and the spherical surface of the planetary gear. It is easy to cause scraping and research of the concave side of the spherical spacer, which produces unnatural early wear on the concave side of the spherical spacer and causes the gear to prematurely disengage from normal meshing, making the gears of a set of differential gears. This causes a set of differential gears to disengage from the normal meshing state prematurely, increasing the meshing clearance and generating noise or premature damage.
Similarly, suppose the oil groove on the mounting surface of the half-shaft gear is of a double-curve shape, according to the conventional machining process. In that case, it can also be realized by turning. Still, it is necessary to design and produce special tooling fixtures, clamping and fastening manually. After turning one groove, it is necessary to dismantle and switch to turn the other groove. Although the design of the tooling, depending on the size of the arc radius of the hyperbola and the radius of the center of the circle from the centerline of the gear different sizes, can be a one-time clamping of 3-5 pieces, this only saves the machining time, and auxiliary time accounted for a larger proportion of the time, the efficiency is still very low, and belongs to the intermittent cutting, the tooling cost will also increase. However, if the oil groove on the mounting surface of the half shaft gear for the butterfly is difficult to achieve in accordance with the conventional machining process, only through special processes, either by using CNC milling, in accordance with the three-dimensional modeling of the program generated by the milling process, or through the electro-erosion method using an EDM machine tool, the use of copper or graphite electrode electro-erosion processing, the two methods of low efficiency, high cost, in the enterprise in the large-scale production of extremely unsuitable.
(2) Planetary gear sphere on the oil groove net shape forging
Planetary gears on the spherical surface of the radial straight oil groove net forming forging, in order to overcome the shortcomings of the existing technology, and to provide high efficiency, saving raw materials, but also to reduce tool consumption of one-time forging molding process new methods. The process is realized in this way: according to the user’s product drawings of the structure of the spherical and radial straight oil groove, with reference to the coefficient of linear expansion of the forged material and the forging temperature, calculate the shrinkage rate at the desired forging temperature, taking into account the shrinkage coefficient of the hot forging and the subsequent machining of the spherical surface margins, calculate the product on the spherical straight groove of the one-time forging of the concave mold cavity parameters, the use of three-dimensional modeling technology, the design of the precision forging concave mold cavity parameters, the use of three-dimensional modeling technology, the use of three-dimensional modeling technology. Design the three-dimensional modeling of precision forging concave mold cavity (Figure 5), using high-speed milling machine in accordance with the three-dimensional modeling generated program, first rough milling and machining of the cavity before heat, heat treatment and then precision milling and machining of the concave mold cavity entity, concave model cavity set on the spherical surface of the inner camber, the left and right two bevels and the concave model cavity of the inner camber and the angle between the concave model cavity are both 120 degrees, the inner camber there are three, the three internal camber angle of 120 degrees between the three.

Figure.5 Radial oil groove cavity molding
Made planetary gear precision forging concave die can be corresponding to the product tooth mold, according to a certain assembly, connection relationship, in the press for a fire two forging forming forging. A forging using pre-forming, that is, the degree of full tooth shape in 80% -90%, no oil grooves on the gear sphere, the total thickness of a certain amount of downward pressure reserved; the second forging to achieve the tooth shape is full of oil grooves on the surface of the sphere fully formed to complete the planetary gear sphere on the radiating straight oil groove forgings of a one-time precision forging molding (Figure 6). This precision forging, as long as the size or weight of the material is strictly controlled, the heating temperature and strike force during forging are constant, and the machining allowance of the spherical surface can be stably controlled within the range of 0.5-1.0mm. This can be done by adjusting the size of the R angle of the transition between the ball surface in the concave model cavity and the three convex bars at an angle of 120 degrees so as to pre-preserve the approximate R angle required for the final spherical surface after heat treatment and fine grinding of the sphere, so that the final product of the spherical surface of the ball surface and the three radiating grooves on the side of the transition to put an end to the formation of hard edges similar to knife blades, to protect the concave side of the spherical spacer in the process of rotation, to avoid the difference in the reduction of the assembly. This protects the inner concave surface of the spherical spacer during rotation and avoids abnormal wear of the spherical spacer during the operation of the differential assembly, thus improving the service life of the whole set of gears and even the differential assembly.

Figure 6 Radial oil groove forging on the spherical surface of planetary gears

(3) Net-forming forging of hyperbolic or butterfly-shaped oil grooves on the mounting surface of half-shaft gears

Net-forming forging of hyperbolic oil grooves on the outer flats of half-shaft gear journals and mounting surfaces is realized by the following steps:

• 1) According to the product drawing and structure of the differential half shaft gear, strictly in accordance with the dimensions of the drawing (especially the dimensions of the outer circular flat and the double-curved oil groove on the end face), use three-dimensional software to carry out three-dimensional solid modeling of the part drawing to generate a cold state of the art three-dimensional entity.
• 2) Combined with the forging conditions and machining conditions, reserved appropriate machining allowances on the outer circle and the mounting datum, with reference to the coefficient of linear expansion of the forged material and the forging temperature, calculate the shrinkage rate at the desired forging temperature, expand the three-dimensional solid forging of the cold state of the part in step (1) to generate a hot solid forging, and then from the hot solid forging to generate the hot state of the special precision forging dimple die solid modeling (Figure 7).

Figure 7: Hyperbolic oil groove and inner cavity of the flat concave mold
The use of a high-speed milling machine in accordance with the program generated by the three-dimensional modeling, the first before the heat rough milling cavity, heat treatment and then precision milling cavity inside the concave die solid. Processed half shaft gear precision forging, the concave die can correspond to the product tooth mold, according to certain assembly connection relationships, in the press for a fire two forging forming forging. A forging using pre-forming, that is, tooth mold on the top, no oil groove in the bottom of the concave die for pre-forging, so that the degree of tooth filling in 75% -90%, half-axis gear mounting surface to leave a certain amount of pressure, the total thickness does not leave the amount of pressure; two forging to achieve the tooth filled and mounted on the surface of the double-curved groove is completely shaped to complete the installation of the half-axis gear mounting surface of the double-curved groove on the forgings of a one-time precision forging (Figure 8).

Fig.8 Half shaft gear outer circle flat and mounting surface hyperbolic oil groove forgings
In the same way, the half-shaft gear mounting surface of the butterfly-shaped oil groove is also the first according to the precision forging concave mold three-dimensional solid modeling (Figure 9), the production of precision forging concave mold (cavity in the plane of the two similar to the butterfly-shaped tab), the use of a fire two forging, forging in the press with a butterfly-shaped groove on the mounting surface with the precision forgings (Figure 10). This precision forging should strictly control the size or weight of the material, forging to maintain a constant heating temperature and strike force; the mounting surface of the machining allowance can be stabilized in the range of 0.8-1.2mm so that you can adjust the concave model cavity height of the two tabs, and indirectly ensure that the final machined half-axle gear mounting surface butterfly shaped grooves on the depth of the size of the range of requirements of the drawings.

Figure.9 Butterfly-shaped oil groove cavity modeling in the concave die

Fig.10 Butterfly-shaped oil groove forging on the mounting surface of half shaft gear

2. Gear spoke groove net forming and primary hole and weight reduction hole forming in one operation

(1) Gear rear end teeth and spoke groove net forming forging

The active external helical cylindrical gears for inter-axle differentials have a special structure: one end has an involute straight-toothed conical tooth profile, and the other end has end teeth with inverted bevels on the tooth side (Fig. 11). The structure of this product is complex, difficult to process, especially the other end of the end teeth with inverted cone. The conventional domestic production process is straight tooth conical teeth by net forming precision forging, directly as a subsequent machining process of positioning reference, turning the outer contour and bore and related dimensions. The other end of the end face meshing teeth, some radial straight teeth, and some arc-shaped straight teeth structures can be completed by CNC milling. But the tooth side containing an inverted cone of the end face teeth, general milling is difficult to realize, either through the five-axis CNC milling or through the arc tooth milling machine and special cutter, cutter tooth processing, the above two processing solutions, not only the equipment investment is large, but also the processing efficiency is not high, the enterprise is difficult to withstand the huge processing costs, more difficult to meet the needs of the market. Combined with the characteristics of this product and the successful experience of the previous development of end face meshing sets, technical personnel, after countless discussions and various forming simulation programs, finally succeeded in breaking through the barriers to the final shape, using one end of the bevel teeth, the other end of the end face meshing straight teeth, the first two forging one-time molding, and then take a cold correction of the end of the end face meshing teeth to make the inverted taper approximate shaping method, basically to achieve the requirements of the drawings to meet the needs of the user.

Figure.11 External helical cylindrical gears
In the production of this forging blank, the most important quality control index is to control the radial runout of the back-end combined tooth within 0.5mm based on the bevel gear as the detection benchmark. Therefore, it is necessary to design and manufacture a special mold structure-occlusion mold (Fig.12). The mold structure enables the heated and pre-forged blank to be forged in a variable cavity from semi-closed to closed. After more than one year of many tests and explorations of various products, and the assembly clearance tracking and adjustment between the guide rail and slider of the press, the guide column and guide sleeve of the mold base, and the parts of the mold, it has been successfully applied to mass production for several years, and the main and driven external helical gear forging blanks of the inter-shaft differential, the gear forging blanks of the gearbox, and the non-flash forging of various meshing sleeves have been realized.

Figure.12 Schematic diagram of occlusion mold structure
The entire forming process, on the upper and lower die center line of the misalignment requirements, is very strict (generally misalignment control within 0.25mm is appropriate), which must be strictly controlled on the press guide and the gap between the slider (generally controlled in the range of 0.10-0.15mm), this gap is too large, will inevitably result in the upper and lower die center of the misalignment is too large, beyond the theoretical design of the upper die and the lower die The gap between the cavity wall of the upper die and the lower die, will eventually cause forging “bite mold” phenomenon, damage to the upper die and the lower die cavity wall; press guide rail and the gap between the slide is too small, it will cause excessive friction between the guide rail and the slide, not only lead to the guide rail. The slide wear is too fast but also offsets some of the press’s striking force. In addition, for occlusion forging, must use the matching high-precision guide column, guide sleeve die holder or special structure of the self-locating die holder so that the equipment in the best guide clearance and high-precision die holder under the dual control to ensure that the upper and lower die centerline misalignment of 0.25mm or less. Controlling the temperature of the forging blank in the final molding between 880-950℃, the forging basically does not appear longitudinal burr, so the radial allowance of the forging can be well controlled, and the best can be controlled within 1mm. The thickness tolerance is controlled by the following factors, as long as the weight of the material is stable, the heating temperature range is constant, the equipment strikes a certain force, the mold cavity in a certain range of wear and tear, the thickness of the forgings is easy to ensure. Hot forging, as shown in Figure 13, cold correction of end teeth, a special hot forging end teeth cavity structure and cold pressing die structure so that the end teeth side inverted cone approximate realization.

Fig.13 Hot forging of bevel teeth, end teeth and spoke groove net-shaped hot forgings
The successful application and popularization of this process method fills several gaps in the country, reduces the investment of high-end equipment, saves raw materials, reduces machining processes, shortens the process route, greatly reduces the manufacturing cost of the product, creates economic benefits for the enterprise, and improves the market share. The mold structure has been authorized as a utility model patent, and the process method has been declared as a patent for invention. Gear and spoke groove net forming forging and intermediate holes, weight reduction holes hot punching forming with the world’s traditional energy sources continuing to reduce and control carbon emissions, as a manufacturer of automotive parts and components, forgings, material saving, improve the utilization rate of forging materials, reduce the weight of forgings, be placed in the work of the first place at all times. Axle and a variety of gearbox manufacturers, most of the larger diameter cylindrical gear design for both ends of the structure containing spoke grooves, and even sometimes the spoke grooves will be set up in the process needs or dedicated to the weight reduction of a number of holes. The forging blanks of such parts are usually reserved for machining allowances in the spoke slots, and the holes in the spoke slots for process or weight reduction are drilled one by one after machining the spoke slots at both ends. Because the traditional forging method cannot guarantee that the forging of two ends of the spoke groove coaxiality is within a certain range, it must be followed by turning corrections to ensure that the final processing of the finished product meets the requirements of the drawings. Our company utilizes an innovative occlusion die structure for the spoke groove net forming forging of inter-axle differential driven gears and gearbox gears and sustained mass production. The spoke groove net-forming die structure is shown in Fig.14, and the net-formed forgings are shown in Fig.15.

Fig.14 Schematic diagram of spoke groove net forming die structure

Fig.15 Spoke groove net forming forgings
After the forging of disk parts, in order to reduce the pressure of the drilling machine when subsequent machining drilling, our company gradually adopts the process of hot punching and cutting of intermediate holes to eliminate the subsequent drilling process and save the cost of tools. The hot-punched forging parts are shown in Fig.16 and Fig.17.

Fig.16 Hot-punched forgings for the inter-axis differential main wheel

Fig.17 Hot punched forgings for gearbox gears
The spoke groove is set with several process holes or weight reduction holes in the inter-axial differential driven wheel or transmission gear forgings, which can also be designed through a multi-hole punching hot punching die (Fig. 18, 19), the spoke groove of the net forming of the hot forgings slow cooling for some time (650 ℃ or so), the following spoke groove positioning, one-time forgings in the middle of the main holes and spoke groove of the weight reduction holes in the several hot punching, before and after the punching Forging modeling as shown in Figure 20, Figure 21.

3. Cylindrical gear outer straight teeth near net forming forging

Cylindrical gear teeth of the traditional process are through the traditional rolling, shaving, honing, and grinding to achieve, in recent years, domestic and foreign gear equipment renovation and automation to enhance. However, the efficiency of the gear has been improved, but ultimately through the de-materialization to achieve. With the high price of various raw materials in recent years, coupled with poor market demand and the wave of price reductions in the manufacturing industry, as well as the impact of a variety of factors such as epidemics, it is difficult for enterprises to develop benignly, and only by tapping the potential to reduce the manufacturing cost so that the market and the user can be barely maintained operation.

Figure 18 Porous Hot Stamping Die Pre-Punching Schematic

Figure 19 Schematic diagram of porous hot-die punching process

Figure.20 Three-dimensional modeling of spoke groove net-forming forgings

Figure.21 Three-dimensional modeling of forging after porous hot punching

In the past two years, our company has put material-saving forging on the top of the agenda, making full use of new technology and new methods to develop new products or improve traditional old technology, taking material saving as the starting point, aiming at less and no cutting, and combining or substituting work processes as the ultimate, so as to realize the leaping up of forging technology and technique. At the same time, in response to the trend of straight tooth cylindrical gear external straight tooth forging – grinding process or external straight tooth cold extrusion – shaving process, that is, the subversion of the external gear manufacturing process, for small specifications (OD 120mm, thickness of 70mm or less), large specifications (OD 260mm, thickness of 40mm or less) of the straight tooth cylindrical gears of the hot forging Forming, that is, the innovation of the gear manufacturing process of cylindrical spur gears. Through the comprehensive consideration of various factors on the tooth shape of the hot forging inner gear mold, such as the size and direction of the influence of the processing method on the tooth shape parameter, the subsequent method of finishing the outer gear and the amount of retention, optimize the required tooth shape parameter of the inner gear mold, use the slow-feeding wire cutting machine tool to process the straight tooth cavity and get the qualified inner gear cavity mold through the strict tooth polishing procedure to realize the closed hot forging forming of the straight-toothed cylindrical gears with outer gears on the high-precision die, holder. Gear closed hot forging forming, hot forging Figure 22, and cold forging see Figure 23. through the closed cavity hot forging outer straight tooth forging, not only to make the metal organization more dense, more importantly, the hot metal in the mold cavity molding, with the cavity to generate a fixed metal flow line, only in the tooth surface of the unilateral 0.15-0.20mm of the hot grinding amount, far different from the conventional hobbing cut off the Unlike conventional hobbing, the metal flow line is cut off, which greatly enhances the strength of the outer straight tooth and makes the service life of the gear greatly improved.

Fig.22 Hot forging of external straight-toothed cylindrical gears

Fig.23 Cold forging of external straight-toothed cylindrical gears
It can also be through the above methods, combined with the elastic deformation of forgings molds, to avoid too many factors affecting the consistency of the final forgings, poor stability, and the use of cold extrusion tooth shape-shaving process to complete the processing of external straight teeth. Cold extrusion process using the outer teeth, the need for cold extrusion before the material blocks phosphorylation saponification in order to improve the lubricating effect of cold extrusion and metal mobility; at the same time, the mold should be sufficiently cooled before continuous production. Through the cold extrusion molding of the outer straight tooth forgings, tooth shape only in the tooth surface unilateral left 0.05-0.08mm before the heat shaving amount of subsequent hot pre-turn machining bore, end face process, must be outside the straight tooth shape positioning, shaving only the processing of the involute tooth surface, and the top of the tooth chamfer, tooth root rounded corner in the cold extrusion mold cavity design and consider the same, without secondary machining. The product in the heat treatment strictly controls the heat treatment process and the parameters to prevent excessive deformation and affect the quality of the final product. Only in this way can we minimize the loss of tooth precision to meet the use of less stringent requirements of the working conditions. Cold extrusion of solid forgings see Figure 24; cold extrusion of hollow forgings see Figure 25.

Fig.24 Cold extruded solid forgings for straight-tooth cylindrical gears with external straight teeth

Fig.25 Cold extruded hollow forgings for straight-tooth cylindrical gears with outer straight teeth

4. Conclusion

The above examples are in the long-term production practice, should the user demand or market competition under the enterprise inward tap the potential of innovation results. As an enterprise, we should keep our eyes on the market, keep innovating, always adhere to the concept of “technological improvement, there is no best, only better”, explore in practice, perfect in the exploration, and always do the low-carbon economy, green manufacturing practitioners. In short, on the road of science and technology innovation, technical improvement never ends. As long as forger dedicated to scientific research, dedicated to practice, with a down-to-earth spirit of craftsmanship (the pursuit of excellence in the spirit of creativity, the spirit of excellence in the quality of the spirit of the user-oriented service), not forgetting the original intention, plowing in the forging industry in this piece of the vast fertile ground, China’s industry from the manufacturing power to the wisdom of a strong manufacturing country, it must be just around the corner.
Author: Meng Jiangtao