Why are helical gears more difficult to do than spur gears?
(1) Mold processing factors. The straight toothed die ring can be cut by high precision slow-moving wire, the quality of which is very high, especially in surface roughness and cutting accuracy; and the helical tooth must be machined by discharge because of the existence of helical angle, so the discharge of the die ring must have electrodes, which is usually used by the copper male. Customized hobs are manufactured. According to the existing processing methods, two factors must be taken into account in the scaling of the helical-tooth copper coppers: one is the shrinkage of the plastic itself, but the discharge gap. Because plastic shrinkage is difficult to quantify accurately and discharge gap is inaccurate, it is difficult to fabricate accurate electrodes.
When EDM gear rings with electrodes, it is necessary to add spiral motion to make the electrodes work out gear rings according to spiral motion. The existence of this motion directly leads to the accuracy of machine tools to be added to the machining accuracy of gear rings. In other words, the accuracy of gear ring is affected by the accuracy of spark machine. Together, these factors determine that the processing difficulty of helical die kernels is higher than that of straight teeth.
(2) Injection factors. The straight teeth can be ejected directly, but the helical teeth must be rotated out. Because of the force factor, the plastic gear will bear a greater force when the helical gear rotates out of the die, so it will cause a larger deformation in the process of the helical gear out of the die. These deformation factors lead to the plastic gear products produced by injection moulding to be lower than straight teeth in tooth profile accuracy and radial runout index.
Generating method for cutting teeth
The cutter (or abrasive tool) whose profile is conjugated with the profile of the cut gear is used to cut out the teeth by generating motion between the wheel blank and the cutter. Generative gear cutting can process gears with different teeth in the same module with one cutter (or abrasive tool). It is widely used in gear processing, such as pinion, hobbing, shaving, honing and grinding.