Gears are widely used in the transmission of mechanical devices due to their superiorities of high efficiency, compact structure and stability. Modulus of a gear indicate the size of gear teeth. Generally, the gear with the modulus larger than 2.5 mm is called a large modulus gear , which is difficult to manufacture for its large dimensions. As one of the primary manufacturing method for large modulus gears, cutting process has weaknesses of low material utilization rate, discontinuous material fiber and poor mechanical properties. Gear shaving is a high-efficiency process on gear work, but the tooth profile concave error of it is difficult to eliminate and has been one of the main factors causing vibration and noise of the gear transmission . The gears produced by sintering process have good dimensional accuracy and surface roughness . But sintering has inherent technical limitations, resulting in lower density of the sintered gears especially the large gears, which seriously affects the mechanical properties of gears such as strength and hardness. Advanced plastic forming methods, such as precision forging and extrusion forming, are capable of achieving high production efficiency as well as good mechanical properties of products which have complete material fiber . However, the large forming load in plastic forming brings lots of unavoidable problems, including the difficulty of mold unloading, the insufficient filling of teeth corner, the short service life of dies and so on.
Researches on reducing forming load in precision forging has been carried out for many years, the main methods of which were divided flow and floating die process . Kondo and Ohga  applied the process of dividing material flow to produce ring gears and it was proved to be accuracy and efficiently. Alves et al.  developed a flexible tool system for performing cold forging of gear parts based on finite element method, tool design and experimental expertise. Ohga et al.  examined the optimum process parameters combination of a two-step precision forging method to enlarge the possibility of applying the process utilizing divided flow with intention of reducing the contact pressure. Relief-cavities with different sizes on the top of die teeth which were used in hot precision forging to promote metal filling were designed . For the same purpose of reducing the difficulty of material filling of gear teeth, Tan et al. [9, 10] promoted the precision forging process and put forward an improved scheme combined with floating die structure based on the theory of restrained divided-flow, which could ensure the decrease of forming load and acquire standard gear teeth.
Gear rolling is a method of partial loading and local deformation which have advantages including low forming force, high material utilization and good surface strength of gears. At present, cold rolling process is only appropriate for small modulus tooth-shaped gears or splines on account of the work hardening phenomenon of metals . Wang et al.  studied the problems of non-uniformity teeth graduation in initial forming phase and formation of rabbit ears in later forming stage. Afterwards, Li et al. [13, 14] investigated the main factors affecting slippage problem and analyzed the formation mechanism of rabbit ear, which provided a scientific basis to further explore of controlling rabbit ear defects. Then, a gear rolling process using conical gear rollers was proposed, in which the axial feed applied on the blank replaced the radial feed applied on the roller, achieving better uniform tooth graduation and refining . Landgrebe et al.  employed a cross-rolling process characterized by round tools with outer gearings to achieve hot rolling of large spur gears and mathematically analyzed the forming force and momentum to determine the forces and torques required here.
Merklein et al.  presented a new approach for the direct forming starting from blanks named “Sheet-Bulk Metal Forming (SBMF)”. The material flow which concern the geometric accuracy and the form filling in SBMF was enhanced by a local increase of the friction as well as the process adapted blanks . Sieczkarek et al.  characterized the plastic flow and proposed a closed-form analytical framework to estimate the through-thickness pressure and force in sheet-bulk indentation. An incremental gear forming process by means of double-wedge gear tooth punch was provided . Then, his group analyzed the differences between incipient and repeatable material flow in incremental SBMF of gears, and investigated three influencing factors aiming for a formfilling progress of the first tooth element and an improvement of the teeth heights[21, 22].
The existing cold rolling and no-flash precision forging process are limited by plastic deformation and forming load restrictions in the manufacture of large modulus spur gears. Strong strain hardening during cold working also hinders the application of SBMF. In comparison, these problems can be effectively solved by hot-working and partial-molding methods. Thus, a successive tooth forming process is proposed based on the hot rolling and SBMF, which can form gears by a certain number of rational-pressing passes in the heated state. This process has superiorities of smaller forming load, higher material utilization, simpler tooling set and better mechanical properties of formed gears such as bending fatigue strength, showing a good potential in fabricating large modulus spur gears, especially for those with large axial dimensions.
Finite element method (FEM) was applied to study the influences of total pressing depth and distribution of per-pass feed in the preforming stage on final forming quality. The optimal process parameters were defined by numerical simulations, and the experiments were conducted to verify the feasibility of this newly proposed forming process.