There are numerous methods for manufacturing gearwheels. In the case of cutting soft pre-machining, a differentiation is made between hobbing, gear shaping, generating planing, and power skiving. Hobbing and power skiving are so-called continuous methods.
The gear shaping method can be described or illustrated by a cylinder wheel gearing, since the intersection angle (also called the intersection angle of axes) between the axis of rotation R1 of the shaping tool 1 and the axis of rotation R2 of the workpiece 2 is 0°, as schematically shown in FIG. 1. The two axes of rotation R1 and R2 extend in parallel when the intersection angle of axes is 0°. The workpiece 2 and the shaping tool 1 rotate continuously about the axes of rotation R2 or R1, respectively, thereof. These rotational movements are shown by the reference signs ω1 and ω2. The shaping tool 1 performs a stroke movement in addition to the rotational movement, which is identified in FIG. 1 by the double arrow shx, and removes chips from the workpiece 2 during this stroke movement.
Some time ago, a method was revived, which is referred to as power skiving. The fundamentals are approximately 100 years old.
In the case of power skiving, as shown in FIG. 2A, an intersection angle of axes Σ between the axis of rotation R1 of the power skiving tool 10 (also referred to as the skiving wheel) and the workpiece axis of rotation R2 of the workpiece 20 is specified, which is not equal to zero. The resulting relative movement between the power skiving tool 10 and the workpiece 20 is a spiral movement, which can be decomposed into a rotary component (rotational component) and a thrust component (translational component). A cylindrical screw gearing can be considered to be a drive technology analog, wherein the rotary component corresponds to the rolling and the thrust component corresponds to the sliding of the flanks. The greater the intersection angle of axes Σ is with respect to absolute value, the more the translational movement component required for the machining of the workpiece 20 increases. Specifically, it causes a movement component of the flank cutters of the power skiving tool 10 in the direction of the tooth flanks of the workpiece 20. In the case of power skiving, the sliding component of the meshing relative movement of the engaged gear wheels of the crossed helical equivalent gearing is utilized to execute the cutting movement. In power skiving, only a slow axial feed sax (also called axial feed) in parallel to the axis of rotation R2 of the workpiece 20 is required and the so-called impact movement is omitted, which is typical for gear shaping. Therefore, no reverse stroke movement occurs in the case of power skiving.
The cutting speed in power skiving is directly influenced by the rotational speed of the power skiving tool 10 or of the workpiece 20 and on the intersection angle of axes Σ used of the axes of rotation R1 and R2. The intersection angle of axes Σ and therefore the sliding component are to be selected so that an optimum cutting speed is achieved for the machining of the material at given rotational speed.
The movement sequences and further details of a power skiving method can be inferred from the already-mentioned schematic illustration in FIG. 2A. FIG. 2A shows the power skiving of external gear teeth on a cylindrical workpiece 20. The workpiece 20 and the tool 10 (a cylindrical power skiving tool 10 here) rotate in opposite directions, as can be seen in FIG. 2A, for example, on the basis of the angular velocities ω1 and ω2. The cylindrical tool 10 is inclined away from the workpiece 20 to generate kinematic clearance angles.
Further relative movements are added thereto. The above-mentioned axial feed s is necessary to be able to machine the entire gear tooth width of the workpiece 20 using the tool 10. The axial feed causes a displacement of the tool 10 in relation to the workpiece 20 in the parallel direction to the workpiece axis of rotation R2. The direction of this movement of the tool 10 is identified in FIG. 2A with sax. If spiral gear teeth are desired on the workpiece 20 (i.e., angle of inclination β2≠0), a differential feed sD is superimposed on the axial feed sax, which, as shown in FIG. 2A, corresponds to an additional rotation of the workpiece 20 about the workpiece axis of rotation R2. The differential feed sD and the axial feed sax are adapted at the calculation point AP to one another by computers such that the resulting feed of the tool 10 in relation to the workpiece 20 occurs in the direction of the tooth gap to be generated. In addition, a radial feed srad can be used, for example, to influence the crowning of the gear teeth of the workpiece 20.
In power skiving, the vector of the cutting speed {right arrow over (v)}c essentially results as the difference of the two velocity vectors {right arrow over (v)}1 and {right arrow over (v)}2, which are inclined in relation to one another by the effective intersection angle of axes Σeff, of the axes of rotation R1, R2 of tool 10 and workpiece 20. {right arrow over (v)}1 is the velocity vector on the circumference of the tool 10 and {right arrow over (v)}2 is the velocity vector on the circumference of the workpiece 20. The absolute value of the cutting speed {right arrow over (v)}c of the power skiving process can be changed by the intersection angle of axes Σ and the rotational speed in the crossed helical equivalent gearing. Then, as already mentioned, relatively slow axial feed sax only has a small influence on the cutting speed {right arrow over (v)}c in power skiving, which can be neglected. The axial feed sax is therefore not considered in the vector diagram having the vectors {right arrow over (v)}1, {right arrow over (v)}2, and {right arrow over (v)}c in FIG. 2A.
FIG. 2B shows the power skiving of external gear teeth of a workpiece 20 using a non-inclined conical power skiving tool 10. In FIG. 2B, the intersection angle of axes Σ, the vector of the cutting speed {right arrow over (v)}c, the velocity vectors v, on the circumference of the tool 10 and {right arrow over (v)}2 on the circumference of the workpiece 20, and the angle of inclination β1 of the tool 10 and the angle of inclination β2 of the workpiece 20 are again shown. The angle of inclination β2 is not equal to zero here. The tooth head of the tool 10 is identified in FIG. 2B with the reference sign 4. The tooth face is identified in FIG. 2B with the reference sign 5. The two axes of rotation R1 and R2 do not intersect, but rather are arranged skewed in relation to one another. In the case of a conical power skiving tool 10, the calculation point AP is typically selected on the shared perpendicular line of the two axes of rotation R1 and R2, since tilting of the power skiving tool 10 is not required to provide clearance angles. The pitch circles of the spiral rolling equivalent gearing touch in the calculation point AP.
A tool 10, which comprises at least one geometrically defined flank cutting edge, is used in power skiving. The flank cutting edge/flank cutting edges are not shown in FIG. 2A and FIG. 2B. The shape and arrangement of the flank cutting edges are among the aspects which have to be taken into consideration in practice in a specific design.
In the example shown in FIG. 2A, the power skiving tool 10 has the form of a straight-toothed spur gear. The external contour of the main body in FIG. 2A is cylindrical. However, it can also be conical, as shown in FIG. 2B.
In power skiving, the (tooth) gaps are typically generated in multiple cuts with respect to the tooth depth. This is therefore also referred to as a multiple cut strategy. The computer design of the power skiving tool is essentially performed in this case in view of optimum chip conditions for the final cut, during which the final flank surface is generated.
The individual cuts in different tooth depths are typically executed by a corresponding infeed of the power skiving tool in the tooth depth direction. In power skiving using a non-inclined power skiving tool, the infeed corresponds solely to a change of the axial distance between the axes R1 and R2.
Several studies of power skiving using multiple cut strategy have shown that different chip or cutting conditions result for the individual cuts (at the different infeed depths). However, this is the case in particular if the same power skiving tool is used for the individual cuts in power skiving using multiple cut strategy. The chip or cutting conditions are different, since in particular the respective active pitch circle on the workpiece shifts in relation to the design position for the final cut.
The chip or cutting conditions even change significantly in this case, as precise simulations have shown. If inclined power skiving tools are used, in the extreme case, skiving cannot be performed without collision using an optimum power skiving tool for the final cut at a machining depth of approximately 25%.
These differences of the chip forming conditions at the various cutting depths not only result in wear of different levels on the power skiving tool, but rather they can also have an influence on the quality of the generated tooth flanks.
Furthermore, it is characteristic for power skiving that the part of the chip removed from the incoming tooth flank eF is thinner than the part of the chip removed from the outgoing tooth flank aF. This method property of power skiving can also be problematic in the case of a multiple cut strategy.