1、13FTM01 AGMA Technical Paper Power Skiving of Cylindrical Gears on Different Machine Platforms By Dr. H.J. Stadtfeld, The Gleason Works2 13FTM01 Power Skiving of Cylindrical Gears on Different Machine Platforms Dr. Hermann J. Stadtfeld, The Gleason Works The statements and opinions contained herein
2、are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract Skiving is a cutting process which was first patented in 1910 1 as an efficient process to manufacture internal ring gears. Like honing, Power Skiving uses th
3、e relative sliding motion between two “cylindrical gears” whose axes are inclined. The skiving cutter looks like a shaping cutter with a helix angle for example, 20 different than the helix angle of the cylindrical gear to be machined. The skiving process is multiple times faster than shaping and mo
4、re flexible than broaching, due to the continuous chip removal in skiving, but it presents a challenge to machines and tools. While the roll motion between the cutting edge and the gear slots occurs with the spindle RPM, the relative axial cutting motion is only about one third of the circumferentia
5、l speed of the cutter. The cutting components of rolling and cutting which result in a “spiral peeling” are represented with the process designation skiving. Because of the relatively low dynamic stiffness in the gear trains of mechanical machines as well as the fast wear of uncoated cutters, skivin
6、g of cylindrical gears never achieved a breakthrough against shaping or hobbing until recently. The latest machine tools with direct drive train and stiff electronic gear boxes present an optimal basis for the skiving process. Complex tool geometry and the latest coating technology were required to
7、give the soft skiving of cylindrical gears a breakthrough. Gleason has developed a line of dedicated power skiving machines, which apply solid HSS cutters for small to medium modules. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314
8、September 2013 ISBN: 978-1-61481-058-2 3 13FTM01 Power Skiving of Cylindrical Gears on Different Machine Platforms Dr. Hermann J. Stadtfeld, The Gleason Works The Power Skiving machine setup definitions The geometric setup of a skiving cutter relative to an internal ring gear is shown in Figure 1. T
9、he front view of the generating gear system is shown in the upper left graphic. The ring gear is oriented in the main coordinate system with its axis of rotation collinear to the Y4-axis. The cutter center (origin of Rw) is positioned out of the center of Y4in the X4-Z4plane by a radial distance vec
10、tor Ex. The pitch circles of the cutter and the ring gear contact tangentially at the lowest point of the pitch circle. The top view, which shows the tool inclination angle or shaft angle is drawn below the front view. In case of a spur gear the stroke motion is directed in line with the Y-axis. The
11、 relative velocity required as cutting motion is generated with a shaft angle around the X4-axis of the coordinate system shown in Figure 1. In case of a helical gear, the cutter inclination can be chosen independently from the helix angle. However, a helix angle of 20 or larger offers the possibili
12、ty to be matched with the shaft angle and use a simplified spur gear style shaper cutter for the skiving operation. Also in this case, the stroke motion is oriented in Y direction but an incremental rotation 2, which depends on the stroke feed has to be added to 1. The shaft angle can also be define
13、d differently than the helix angle and it will still require the same incremental 2, but the tool front face orientation and side relief angles have to be calculated from the difference between helix angle and the shaft angle . The side view to the right in Figure 1 shows a second possible tool incl
14、ination which is called the tilt angle. This tool tilt angle can be used to increase the effective relief angles between the blades and the slots, and it can also be utilized to eliminate interferences between the back side of a long spur gear style shaper cutter with minimum relief angles (see sect
15、ion “skiving tools”). Within limits, it is also possible to utilize the tilt angle for pressure angle corrections. Figure 1. Basic geometry and kinematic of Power Skiving 4 13FTM01 The three dimensional side view in Figure 2 shows an internal helical gear with a shaft angle between work and tool. Th
16、e graphic shows the base angular velocities of the work 1and the formula for its calculation. Figure 2 also includes the incremental angular velocity 2and the formula to calculate it from the helix angle and the axial feed motion (stroke motion). The cutting velocity is calculated as the difference
17、vector between the circumferential velocity vectors of work and tool in the cutting zone. Figure 3 shows a top view of the configuration between tool and work with the velocity vectors. Figure 2. Pitch cylinders of work and tool Figure 3. Calculation of cutting velocity 5 13FTM01 The reference profi
18、le of the tool is determined from the reference profile of the work applying the procedure shown in Figure 4. The reference profile of the work with pressure angles 1and 2and point width Wpis drawn as a trapezoidal channel, and it is cut with a plane under the shaft angle (Figure 4, top, right side)
19、. The profile is defined by the intersecting lines between the plane and channel, and it represents the reference profile of the tool. This tool reference profile is used in order to generate the involute in the tool cutting front (Figure 4, bottom right side). The machine setting calculation is sho
20、wn in Figure 5 (top) on the example of a bevel gear cutting machine. The explanation of the formula symbols are: X, Y, Z Machine axis directions (Y is perpendicular to the drawing plane) Shaft angle between cutter and work CRT Cutter reference height B Cutter swing angle PZPivot distance to spindle
21、front in Z-direction if B = 0 PXPivot distance to spindle center line in X-direction if B = 0 Z1, Z2Components in Z-direction Depending on the helix directions in work and cutter, the cutting takes place below or above the work gear center line in order to keep the B-axis angle below 90. In case of
22、no corrections, the crossing point between the cutter axis and the work axis lies in the cutter reference plane. The bottom section of Figure 5 shows the cutting blade definitions as reference. Chip formation and optimization of chip load Although the chip formation process of skiving appears differ
23、ent when compared to traditional gear cutting operations, understanding it a fundamental task in recognizing weaknesses or strength of the skiving process. Power Skiving has been called a combination of “cold forming” and cutting Is not every metal removing process such a combination? The task of a
24、successful process is an economical combination of speed, part quality, tool life (tool cost per part), and of course the investment in the machine tool. The plane in Figure 6 which shows a segment of an internal ring gear to be skived, and it is defined along the face width at the point where the l
25、ast generating occurs. The second cutting tooth from the left has just entered into a part of the slot which is already rolled out from the previous cutting action. The third cutting tooth is advanced towards the observer and just begins a generating cut (see top view of unrolled partial slots). The
26、 generating cut continues to cut tooth number six. The lowest scallop in the top view was generated between the second and sixth cutting blade position. Cutting blades seven and eight finish the form cut end section of the slot. Figure 4. Procedure for the calculation of the tool reference profile 6
27、 13FTM01 Figure 5. Calculation of machine settings (top) and cutting blade definitions (bottom) 7 13FTM01 Figure 6. Cutting sequence from engagement to exit A photograph of the tooth sequence form Figure 6 is shown in Figure 7 as a close up. As one blade rotates through the cutting mesh the orange c
28、olored scallop is generated, and the green colored section is form cut. The entire cutting action of one blade produces one chip which includes the material removal from generating and form cutting. Beginning from its addendum, the right side of the blade gradually engages with the gear slot during
29、the generating motion. It approaches the tip, and at this point it smoothly peels the chip up to the top of the gear. As the generation works its way from the right flank of the gear to the tip and from this point up to the flank, every section that had at one point chip removing contact with the wo
30、rk now stays in contact to the end of the cut. Only the generating chip removal (within the scallop) converts quickly into form cutting. The form cutting section might seem like an interesting phenomenon but it can be found in a similar form in all other generating cylindrical or bevel gear cutting
31、methods. It is also common in all generating and form cutting processes, that the tip region of the blades have the longest exposure to chip removal action within the green form cutting region. Since the front face of Power Skiving blades is a plane which connects the cutting edges for both flanks,
32、the side rake angle cannot be positive for both cutting edges. An acceptable compromise is a side rake angle of zero degrees. In order to enhance the cutting performance of the blade tip, a significant top rake angle can be introduced. However, peripheral cutter heads with carbide blades and top rak
33、e angles above 4 seem to fail with blade chipping in the tip area (see cutting blade definition in Figure 5). Figure 7. Top view of unrolled partial real slots (top) and graphics (bottom) 8 13FTM01 Power Skiving chips with a 5 times magnification are shown in the top section of Figure 8. The first c
34、hip “a” is a side chip from the first roughing pass of a module 4.0 mm gears with a 5 mm in-feed. The second chip “b” is U-shaped, which means unlike in the first chip, the side chips and the bottom chip have not been separated. This chip is from a second roughing pass with 3 mm in-feed. The third c
35、hip “c” in Figure 8 is from a finishing pass with 1 mm in-feed. The result of a not rolled up and not compressed chip which is just sheared off is a curved channel like indicated in the drawing at the bottom left in Figure 8. The two side walls as well as the bottom section of the channel are rolled
36、 up individually and mostly without breaking in separate pieces. The microscope photo at the bottom right in Figure 8 shows a cross section through the left wing of one chip with a magnification of 100x. At the beginning of cutting (center of the chip spiral) the chip thickness is small and increase
37、s slightly during the rather short generating section (orange scallop in Figure 7). A proportional increase of the chip thickness occurs from the beginning to the end of the form cutting section (green area in Figure 7). The right sides of the chips are thinner than the left sides, and a crack in th
38、e middle of the rolled up side wall can be observed. As the bottom left image indicates, the right channel wall has a more complex shape than the left side, and the skiving kinematic provides slightly different cutting conditions on both flanks. This explains differences in chip thickness between th
39、e two sides and the additional crack of the right side of the chip. In order to avoid U-shaped chips, Gleason has developed an in-feed strategy. After each stroke, an in-feed amount and a work angle setover is applied in order to generate L-shaped chips (Figure 9, left side). L-shaped chips reduce t
40、he wear on the cutting blades. It is possible to alternate the in-feed work angle setover direction from part to part in order to achieve even tool wear. Another possibility is to apply a positive side rake (e.g. 2) to the left side of the blade in Figure 8 which will enhance the cutting action on t
41、his side and generate L-shaped chips. If the resulting feed direction in Figure 8 is not exactly parallel to the right flank, but about 3 “steeper”, then a perfect clean up of the right flank is guaranteed, and the surface finish of the right flank will be equal or slightly better than the finish on
42、 the left flank. Figure 8. Analysis of chip formation 9 13FTM01 Figure 9. In-feed strategy for optimized chip formation 2,3 HSS cutters for Power Skiving and surface speed calculation Traditionally Power Skiving is performed with typical shaper cutters. A variety of different tools used for Power Sk
43、iving is shown in Figure 10. The first cutter (to the left) is a shaft type which is slightly tapered without helix angle in the cutting teeth. This cutter can be used for gears with a helix angle. The shaft angle between cutter and work will be set to the helix angle of the work. This also means th
44、at the helix angle of the work should be above 10 in order to generate sufficient cutting speed. Due to the straight nature of the cutting teeth, work pieces with small diameter and large face width might cause interferences between the slot and the far end of the cutting blade. The skiving cutter i
45、n the center of Figure 10 is a wafer cutter with only few re-sharpenings. The cutting teeth are straight, which makes this cutter only suitable for work pieces with a helix angle. The wafer cutter has very short relieved teeth, which will prevent interference problems in case of helical slots that w
46、ind around a small diameter work piece. The skiving cutter to the right of Figure 10 has serrated blade front faces and teeth which are oriented under a helix angle. The black coatings in Figure 10 are AlCroNite and the golden coating is TiN. Figure 10. Solid HSS PowerSkiving cutters, coated with Ti
47、N and AlCroNite 10 13FTM01 If the helix angle of the work piece is 15 and the tool helix angle is 20, then the shaft angle between skiving cutter and work has to be setup to 5 (same helix direction). If the helix directions are opposite, than a shaft angle of 35 has to be used. An interesting case o
48、ccurs if the gear helix angles of the work is identical to the cutter helix angle (same amount and same hand). In this case the shaft angle between cutter and work is zero, and no skiving motion is generated. The calculation of the cutting surface speed depending on the helix angle of the work and t
49、he shaft angle is shown in Figure 11. The upper graphic represents the unrolled pitch cylinder with teeth and slots indicated (see also right side graphic in Figure 11) for a spur gear. With = 0 the formula is simplified to the first special case. The lower graphic shows the formula simplification for the second special case, which occurs if the helix angle is equal the shaft angle . The cutting velocity formula considers next to the circumferential velocity at the work gear pitch diameter also the helix angle of the work and the s
