Principle
The teeth and grooves of bevel gears are contractive, that is, they are wide at the big end and narrow at the small end. During machining, although the dividing head has been pulled up by a root cone angle, so that the large end of the conical surface of the outer circle of the bevel gear is slightly higher than the small end. During milling, the large end is cut deeper than the small end, and the width of the tooth groove is slightly wider than the small end, but this difference can not meet the requirements, so it is necessary to mill more on both sides of the tooth groove at the large end. When milling the bevel gear on the milling machine, after the middle tooth groove is milled for the first time, the tooth shape of the big end has been obtained, but the groove width does not meet the requirements. Therefore, each tooth groove generally needs to be milled with three cutters to achieve the purpose of milling more on both sides of the big end tooth groove. The machining of the allowance on both sides of the bevel gear groove is called offset milling. The principle of offset milling is: on the one hand, it deflects the bevel gear; On the other hand, move the workbench to realign the small end slot with the milling cutter. Using the offset difference between the large end and the small end perpendicular to the feed direction (transverse) when the bevel gear is deflected, the milling allowance gradually increases from the small end to the large end, and the large end is milled away.
At present, there are many offset milling methods for milling bevel gears, but due to the inconsistency of the ratio of cone pitch to tooth width (i.e. R / b), as well as the different parameters such as pitch cone angle and number of teeth, any method can not be applied to all bevel gears. Therefore, it can only be selected according to the specific situation and corrected in the trial cutting. The method of combining rotation and offset is often used for milling.
Method
The first method: first calculate the offset s, and the rotation of the bevel gear around its axis is determined by the trial cutting.
The second method: first count the rotation n of the dividing head handle, and the offset s of the worktable is determined by trial cutting.
The third method is to combine the deflection angle and offset of the indexing head in the horizontal plane for milling. First calculate the deflection angle of the dividing head and bevel gear in the horizontal plane, and the transverse offset of the worktable is determined by trial cutting.
Correction method of tooth thickness during eccentric milling
After the two sides of 2 ~ 3 teeth are offset milled by the above method, the large and small ends of bevel gear teeth shall be detected. If the actual measured value is inconsistent with the value marked or calculated on the drawing, the rotation and offset shall be corrected. The principles of correction are as follows:
- If the size of the small end of the bevel gear tooth is accurate and there is allowance at the large end, the rotation (or deflection angle) and offset shall be increased to increase the difference so that the small end will not be milled away.
- If the size of the large end of the bevel gear tooth is accurate and there is allowance for the tooth thickness at the small end, the rotation (or deflection angle) should be reduced to reduce the offset more. The small end is also milled away, while the large end is no longer milled away.
- If the dimensions of the big end and the small end of the bevel gear tooth have margin, and the margin is equal, it is only necessary to reduce the offset so that the big end and the small end are milled away.
- If the size of the small end of the bevel gear tooth is accurate and the size of the large end is too small, the rotation (or deflection angle) should be reduced, and the offset should be reduced appropriately, so that the small end is no longer milled, but the large end is cut less than the original.
- If the size of the large end of the bevel gear tooth is accurate and the size of the small end is too small, the rotation (or deflection angle) should be increased and the offset should be increased to make the small end milling less than the original. If the tooth thickness of the small end is too small when milling the middle groove, it is necessary to replace the milling cutter or manufacture a special milling cutter for processing.