Also, sections of progressively increasing ease-off should not be ground without prepar- ing them in the previous cutting operation. This particularly applies to universalmotion heel or toe sections (UMC) as well as to second- order protuberance (blended Toprem) and flflank relief.
With heel or toe relief sections, a gear grinder sometimes has to remove 50% or more stock in some areas of the tooth if the sections are not prepared properly during soft cutting. For example, a green gear may have a regular stock removal of 0.13 mm per flflank. Variation from heat treat distortion may add 0.07 mm in certain areas. Also, the hardened gear may require removal of an additional 0.10 mm of stock within the relief section. If the green gear isn’t cut properly, a worst case could require the removal of 0.30 mm of stock in one grinding pass.
Possible results of such grinding include burn marks, new hardening zones, or a reduction of surface hardness due to the reduced thickness of the case depth. The case depth of bevel gears in the module range of 3–6 mm is recommended to be between 0.8–1.2 mm after heat treatment. The worst-case scenario would reduce the case depth during grinding to 0.5 mm, perhaps less, reducing the surface and subsurface strength.
Also, grinding of root relief, the so-called blended Toprem, leads to a critical condition on the grinding wheel because the small, sensitive tip of the grinding wheel might have to remove 10–30% more stock than the main profifile section. Heavy material removal at the tip of the wheel causes a deterioration of the protuberance section and the edge radius after grinding only a few slots. Subsequently, the remaining slots have reduced or no root relief, and an unacceptable blend into the root- fifillet radius. This effect cannot be cured by subsequent redressing.
An important part of the semi-fifinish strategy for modern bevel gear grinding is a root fifillet area which is not ground. The optimal protuberance of the cutting blades relieves the transition between flflank and root by a value between 60% and 100% of the stock allowance on the active flflanks. The cutting blades should have an edge radius 0.1 mm smaller than the edge radius of the fifinal grinding wheel profifile. They also should cut 0.1 mm below the theoretical grind depth.
The transition between the grinding profifile and the unground root area can be optimized on the drive side of both members, using a grinding wheel tip extension and a setover, to get a smooth blend of the ground to the unground root surface and clean up the root radius to the area of 30° tangent.
Figure 2 shows the superimposition of an outside blade profifile (blue), an inside blade profifile (green) and the fifinish grinding profifile (red). The cutting silhouette cuts 0.1 mm deeper, and in this example relieves 100% of the stock, in that a blend between cutting and grinding surfaces occurs below the active flflank working area. All parameters of the correct semi-fifinish blades follow a tight rule as soon as the amounts of relief on flflanks and root are defifined. For example, the edge radii of the blue and green cutting sides should blend seamlessly with each other, as should the clearance side radii with the same side (same color) cutting edge radii. This is necessary in order to achieve the maximal radii on the cutting blades but also to avoid fifins and grooves in the root bottom. Gleason has developed a software module called Semi-Finish Calculation. This module will automatically calculate blade parameters and new basic settings for semi-fifinish cutting from a few input items such as stock allowance percentage of relief, applied Toprem angles and amount of deeper cutting. If no input is given to the semi-fifinish input screen, default values that represent best practice in bevel gear grinding are instead used.
The distortions due to heat treatment cause an unequal cleanup along the face width and from slot to slot around the circumference. Also, the fifirst- and second-order corrections—applied after coordinate measurement to achieve correct flflank geometry—inflfluence the angle of the ground root line versus the semi-fifinish cut root line. This root-angle difference might result in a partially ground root bottom.
The rule is that the root bottom should not be ground in a section that contains 30% or more of the face width. Stock removal in the root bottom should also be contained to a range of 0 to 0.05 mm. The variation from slot to slot can change the unground section between 30% and 100% without disadvantage to the performance of the gear set. Figure 3 shows an example of a completely unground root—with no disadvantages for the strength performance of the ring gear.
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