2.1.1. Processing Principle
Figure 2 illustrates the synchronous rolling process with two rolling dies. The TSSR process is developed based on thread and spline rolling processes with radial feed-in, which is similar to the thread or spline rolling process, except the die structure is different. The TSSR die is composed of a threaded section and a splined section, and at the same time, the die should be able to meet the motion coordination of the threaded section and the splined section during the TSSR process and the phase difference requirement among rolling dies before rolling [
1,
2]. The geometric parameters of the threaded section and the splined section should meet the basic conditions Equation (1) for the motion coordination [
1].
where,
and
are the teeth of the splined section for the formed part and the rolling die, of the roller, respectively.
and
are the start of the threaded section for the formed part and the rolling die, respectively.
i is the ratio of relationship between the roller and the formed workpiece, and the value is the ratio between the teeth or the starts of the rolling die and formed part.
The phase difference of the TSSR die before rolling is relatively difficult to adjust. In some cases, it is necessary to adjust the die structure of the TSSR die. Therefore, it is not suitable to use more than two rolling dies in the TSSR process [
2]. The principle of synchronous rolling process using two dies is described as follows: a pair of synchronous rolling dies are mounted on the two drive spindles, the dies are made of the threaded and the splined sections; the two spindles are synchronously rotated in the same direction (rotational speed
nd), the two spindles simultaneously perform the radial feed-in motion (feeding speed
v), and the workpiece rotates in the opposite direction; and the threaded splined sections on different portions of the workpiece are rolled at the same time.
2.1.2. Characteristic of the Process
The TSSR can form the threaded and splined sections in one process, and it not only reduces the rolling time effectively, but also ensures the stable relative position between the threaded and splined sections. The meshing motion of the threaded section during a synchronous rolling process can promote the rotation of the workpiece and improve the dividing of splined section [
1]. In addition, theoretical analysis [
10] and experimental research [
7] indicated that the meshing motion of the threaded section dominates the rotation during the TSSR process. Therefore, the threaded section of synchronous rolling dies should contact the workpiece first, and then the splined section contacts the workpiece.
During the rolling process using
N rolling dies to form complex profiles (such as threads, splines, gears, etc.) the rolling dies feed-in radially, and the center distance changes continuously. The same deformation area, in other words, the contact area between the workpiece and one rolling die, will be contacted and compressed by the next rolling die after the workpiece is rotated by 1/
N revolution. A parameter such as compression amount (decrement
) is defined to reflect the degree of deformation of the same area during one rolling unit which is between the rolling die contact with this area and when it separates from this area. The
is the difference root radius of spline or thread before the rolling die contacts the workpiece and the root radius of spline or thread after the rolling die separates from the workpiece, as shown in
Figure 3. When it reaches the final rolling position, the radial feed-in motion of the rolling die stops.
According to the change of the compression amount during the rolling process, the spline rolling process using two rolling dies is divided into four stages [
9,
12]. Thus, it can be further inferred that the complex profile rolling process using
N rolling dies, such as the thread rolling process and spline (gear) rolling process, can be divided into the following four stages (shown in
Figure 4), and the variation of compression amount (
) and related processing parameters are shown in
Figure 4.
During the complex profile rolling process, the radial feed-in speed and the rotational speed of rolling die are constant in general, and the forming process can be divided into four stages, and each stage has the following characteristics:
The first stage is from the beginning of the rolling die contact with the workpiece (i.e., workpiece beginning to rotate) to 1/N revolution of the workpiece rotated. At this stage, the is gradually increased from zero to the value of the compression amount at the stable rolling stage. The rolling die feed-in radially and rotates, the radial feed-in amount of the rolling die increases linearly, and the center distance between the workpiece and the rolling die keep decreasing.
The second stage is from the 1/N revolution of the workpiece rotated to the rolling die reaching the final rolling position. At this stage, the remains unchanged at a stable value, and the rolling die feed-in radially and rotates. The feed-in amount of the rolling die increases linearly and increases to a maximum, as well as the center distance between the workpiece and the rolling die reduces continuously and reduces to a minimum.
The third stage is from the rolling die reaching the final rolling position to the 1/N revolution of workpiece rotated again. At this stage, the gradually reduces from the stable value to zero and the rolling die only rotates. The radial feed-in amount of the rolling die is constant, and the center distance between the workpiece and the rolling die is constant.
The fourth stage is from the end of the third stage to the end of the whole rolling process, which is a finishing rolling stage. At this stage, the is zero and the rolling die only rotates. The radial feed-in amount and the center distance are unchanged.
During the synchronous rolling process, the deformation of the threaded section or the splined section has little influence on the axial adjacent region, and this does not affect the deformation of the adjacent splined section or threaded section [
13]. Therefore, the rolling processes of the threaded section and the splined section can also be divided into the above four stages, but the first and second forming stages of the threaded section and the splined section may not coincide on the time axis, and the first forming stage of splined section may be incomplete. The compression amount
is determined by the rotating speed and radial feed-in speed of the rolling die, and it can be used as a comprehensive indicator of the motion parameters of the rolling die. The
in the steady rolling stage (second forming stage) can be approximately determined by Equation (2), where
i is calculated by Equation (1).
where, the
and
are the angular velocity and the rotational speed of rolling die, respectively;
v is the radial feed-in speed of rolling die;
N is the number of rolling die and
N = 2 is often adopted in the synchronous rolling process.