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1、Chapter 6 Gear Mechanisms 6.7 Manufacturing Methods of Involute Profiles 6.8 Addendum Modification on Involute Gears Cutting of Tooth Profiles There are various methods for manufacturing tooth profiles , such as die casting , precision forging, powder process , moulding , cutting and so on . Among t
2、hem , cutting is the most widely used . There are generally two types of cutting methods , i.e . form cutting and generating cutting . Form Cutting In form cutting, the desired tooth profile is produced by passing a cutter through the blank . Form milling with a disk milling cutter is shown in Fig.
3、The cutter rotates and the blank is fed axially . Form Cutting End milling cutters are used for large modules and double helical gears . The section of the cutter has the shape of the tooth space of the gear to be cut . Form Cutting Indexing is needed after a tooth space is cut so the accuracy of th
4、e gear depends on the accuracy of the index plate as well as the section shape of the cutter . Form Cutting As the shape of an involute depends on its base radius , a different cutter is required for each number of teeth even for the same module and same pressure angle . rb=r cos=(mz/2)cos Form Cutt
5、ing This is divorced from reality : in practice , only 8 to 26 cutters are available for each module and pressure angle . Each cutter is used for several different numbers of teeth and an error is therefore introduced . Form Cutting The cutting process is interrupted by the indexing and the producti
6、on rate is low in form milling . Its main advantage is that it can be accomplished on commonly available milling machines . Broach(拉刀)-Form Cutting Generating Cutting In a generating cutting method , the edges of a cutter take the form of a gear ( or rack ) with the same module and pressure angle as
7、 the gear to be cut . There is a generating motion between the cutter and the blank as if the cutter engages as a gear with the gear to be cut . By adding the cutting motion between the cutter and the blank , the profile of the teeth is formed gradually by a series of cuts . Generating Cutting If z0
8、 is the number of teeth of the cutter and z the number of teeth of the gear to be cut , their transmission ratio should be which defines the generating motion to be provided by the kinematic chain in the cutting machine. Generating Cutting The cutting motion is the reciprocation of the cutter while
9、the feed is the movement of the cutter toward the blank . Generating Cutting External gears can be cut with a rack-shaped shaper cutter. The edges of the cutter are now straight lines which can be made accurately . That is why involute gears are easy to manufacture . Generating Cutting The cutting m
10、ethod shown in Fig is not continuous in the shaping of a gear . In mass production , hobbing is used instead of shaping . Generating Cutting The shape of a hob is like a screw with some axial slots to form the cut ting edges . The cutting edge takes the shape of a rack and the hob is therefore a rac
11、k-shaped cutter . Generating Cutting The rotation of the hob provides the cutting motion and is also equivalent to the continuous translation of the rack-shaped cutter . At the same time , the blank is driven at a certain transmission ratio to the cutter by the kinematic chain in the cutting machine
12、 and this gives the required generating motion . Generating Cutting Meanwhile , the hob is fed along the axial direction of the blank to cut the teeth for the whole facewidth . Cutting is continuous and the production rate is therefore improved . Generating Cutting In the generating method , one cut
13、ter is enough for each module and pressure angle, in spite of the number of teeth of the gear to be cut . It is therefore the most widely used method to cut gears . Cutting a Standard Gear with Standard Rack-shaped Cutter The profile of a standard rack-shaped cutter is similar to that of a standard
14、rack but the tip line is c * m higher than the addendum line for cutting the profile at the root to provide the bottom clearance . Cutting a Standard Gear with Standard Rack-shaped Cutter The cutting edge between the tip line and the addendum line is not a straight line and the corresponding gear pr
15、ofile cut is not involute . Cutting a Standard Gear with Standard Rack-shaped Cutter Therefore, in discussing the generating of an involute profile , we simply regard the standard rack-shaped cutter as having the same profile as the standard rack . Cutting a Standard Gear with Standard Rack-shaped C
16、utter To cut a standard gear , the transmission ratio of the cutter and the blank in generating motion is the same as that of the rack and the gear. Cutting a Standard Gear with Standard Rack-shaped Cutter Furthermore , the reference line of the cutter should be tangent to the reference circle of the gear after the feeding has finished. Cutting a Standard Gear with Standard Rack-shaped Cutter Thus the reference line of the cutter rolls wit
