According to the current processing method, two factors should be considered in the processing of helical gears: the shrinkage of the plastic itself and the discharge gap. Fabrication of precise electrodes is difficult due to the difficulty of precisely quantifying plastic shrinkage and the inaccuracy of the discharge gap. When the electrode discharges the ring gear, it is necessary to add a helical motion, so that the electrode can process the ring gear according to the helical motion. The existence of this movement directly causes the accuracy of the machine tool to be added to the machining accuracy of the ring gear.
Reasons why helical gears are more difficult to process than spur gears:
1. Injection molding factors: Straight teeth can be directly ejected through the enterprise at the same time, but the helical teeth must be screwed out. Due to the force factor when the helical tooth is rotated and ejected, the plastic gear problem causes the force to be greatly affected during ejection, so there may be such a relatively large deformation in the control work of the helical tooth ejection process. These deformation factors lead to lower profile accuracy and radial runout index of plastic gears than spur gears.
2. Factors of mold processing: The straight tooth core gear ring can be cut with high precision and slow wire, and the wire cutting quality is very high, especially the surface roughness and cutting accuracy. However, due to the existence of the helix angle, the core gear of the helical teeth must be processed by electrical discharge machining, and the electrical discharge must have electrodes, which are usually processed by a custom hob.
That is to say, the precision of the precision gear ring gear will be affected by the precision of the spark machine. These factors together determine that helical teeth are more difficult to machine than straight ones.