Number of hob heads for automobile gears

Although most automotive gear hobs still have only one head number, multi head hobs have great advantages under appropriate working conditions. Generally speaking, most single head hobs belong to finishing hobs. The number of teeth of automobile gear hobs is relatively small, and the position of head number is required to be very accurate. Although multi head hobs also have the same finishing procedures, multi head hobs are mainly used for rough machining and semi finishing.

The main factors that determine the number of automotive gear hobs are the production schedule and accuracy requirements of the product. Improving product efficiency is the decisive factor of adopting multi head hob.

Automobile gear single head hob cuts 1 tooth per revolution, double head hob cuts 2 teeth per revolution, three head hob cuts 3 teeth per revolution, etc. Therefore, the speed of workpiece is related to the speed of automobile gear hob, and also directly related to the number of automobile gear hob heads. Therefore, when the workpiece speed is increased, the indexing worm gear speed is also increased, which is likely to exceed the maximum speed limit of gear hobbing machine manufacturing. During machining, check whether the speed of indexing worm gear exceeds the range required by the equipment, especially when the number of teeth of automobile gear hob is small.

In actual production, the increase of speed does not mean the increase of production efficiency. The diameter of multi head hobs is larger than that of the corresponding single head hobs, but it does not increase in proportion to the number of hobs. Compared with the hob with double head number, the hob with single head number has more teeth to participate in the tooth profile correction, which can better ensure the tooth profile. However, due to the large load on the teeth, the feed of automobile gear hobbing must be reduced. Another reason is that increasing the diameter of automobile gear hob will affect the time required for tool production. In general, the overall production time of producing the same double head hob is 50% to 60% longer than that of single head hob, but the increased time proportion is significantly lower than that of automobile gear hob.

Although the production advantages of multi head hobs are often desirable, their application conditions must be carefully considered. Due to the characteristic tolerance of automobile gear hobs, multi head hobs strictly require the tolerance of thread spacing. Compared with single head hobs, other multi head hobs must have greater machining tolerance in the axial direction. These tolerances will vary according to the number of teeth, heads and spiral grooves of automobile gear hobs. There must be complete analysis when changing single head hob to multi head hob.

There are three ratio types between the number of teeth of parts and the number of automobile gear hob heads, which affect the accuracy of tooth groove and tooth profile, that is, even number, common multiple and prime number. The number of hob heads of automobile gears can be divided by the number of teeth of the part. Each tooth groove is corresponding to a special spiral groove tooth. The helix tolerance of any hob will be transferred to the tolerance of tooth profile and tooth groove of the part. However, the tooth profile tolerance of only one spiral groove hob is matched with that of the spiral groove group of multi spiral groove hobs, and the results are determined by the maximum tooth profile error. If the number of spiral grooves is common for the number of teeth, the same conditions can be expected.

When the number of teeth and the number of spiral grooves are prime numbers, each spiral groove corresponds to each tooth groove. Eliminating the error deviation of spiral groove can control the maximum accuracy of tooth groove. However, in order to eliminate the cogging error, the manufacturing of automobile gear hob should reduce the deviation of spiral groove between teeth as much as possible. Therefore, if it is required to improve the error accuracy between teeth as much as possible, chase teeth and common multiple should be used. If higher profile accuracy is to be ensured, non chasing teeth, even teeth and common multiple should be applied.

There are usually three relationships between the number of teeth and the number of automobile gear hob heads, as follows: even number of teeth, hob head number is 4, and the number of teeth is 12, which can be divided by head number 4, and the same spiral groove engages the same teeth in each cycle. With the same common multiple, the number of teeth is 10, which cannot be divided by head 4, but with the same common divisor, they can be divided by 2. The tooth array has a common divisor of 2 and can be divided into two groups of slots. The number of spiral hobs on the automobile gear line is not different, and each gear hob can chase different gear lines for multiple times. Each tooth of the part is machined from all spiral teeth of the automobile gear hob.

The size and position of the generated surface are proportional between the number of hob heads and the number of spiral grooves of automobile gears. If the number of teeth can be divided by the number of automobile gear hob heads, the generated workpiece plane is formed by the maximum deviation teeth and the minimum deviation teeth on the helix, and all double teeth or even the teeth on the helix have the same position. If the automobile gear hob is in the theoretical center, the tooth profile of each tooth is uniform on both sides. If there is a common divisor between the number of teeth of the part and the spiral groove, the dimension of the plane is the same as the common divisor of the number of hobs, but the positions of all every two teeth are different. Therefore, for this kind of gear with common divisor, it is impossible for all spiral grooves to be in one center. Therefore, if the shape is required to be symmetrical, the number of teeth of the workpiece must be divided by the number of heads.

In short, when using automobile gear multi head hob to produce parts, we must weigh accuracy and efficiency. At the same time, in the short term, the automobile gear multi head hob will increase the unreasonable cost in the product production stage. This paper will discuss the special application of automobile gear multi head hob later.

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