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Tooth-shaped hobbing is generally used as a semi-finishing process before finishing such as shaving or grinding. Generally, gears with a precision of 8 to 9 can be obtained after hobbing. The surface quality of the hobbing is low, and the roughing and finishing can be divided into two processes to improve the machining accuracy and the tooth surface quality and reduce the processing cost.
Hobbing machining accuracy analysis The machining error analysis that affects the accuracy of the transmission The main reason for the accuracy of the transmission is the relative position and relative motion of the hob and the machined gear during machining. The relative position change (geometric eccentricity) produces a gear radial error, which is evaluated by the ring gear radial runout $Fr; the relative motion change (motion eccentricity) produces a gear tangential error that varies by the fractional length of the $Fw As assessed, the eccentricity error accounts for a large proportion of the gear machining error.
Geometric eccentric geometry eccentricity means that the center of the workpiece does not coincide with the center of rotation of the machine table during installation. The factors that cause geometric eccentricity when the inner hole is centered and the end face is positioned are: (1) the axis of the clamp mandrel does not coincide with the axis of rotation of the table; (2) the inner hole of the workpiece and the mandrel are worn due to long-term use, so that the gap is increased; (3) The positioning end surface of the clamp is not perpendicular to the axis of the spindle; (4) the end surface of the workpiece is not perpendicular to the axis of the inner hole; (5) the spindle is deformed after clamping; (6) the end faces of the washer are not parallel and the positioning surfaces are not Eccentricity caused by various factors such as wiping or burr.
The left and right meshing lines caused by geometric eccentricity are incremented by $FLR=±egsin(U±A) where eg-geometry eccentricity; U——the angle at which the gear rotates; A—the combination of the tooth angle at the left and right meshing line increments The radial error caused by the gear is: $H=$FL $FR2sinA For various factors that cause geometric eccentricity, the following measures can be taken to improve the accuracy of the transmission 1 of the machined gear. (1) Guarantee the manufacturing accuracy of the fixture. (2) Ensure the accuracy of fixture adjustment.
The eccentric motion eccentricity refers to the gear first harmonic error caused by the uncoordinated motion between the tools of the table. This is mainly caused by the eccentricity of the indexing worm wheel. When the hobbing, the teeth between the workpiece and the tool are not coordinated with the forming motion, and the rotation speed of the table changes, so that the cut tooth-shaped base circle is eccentric. The left base circle is right A angle, the right base circle is left A angle, the eccentricity is em=rbewrw where rb is the base circle radius; ew—the worm wheel eccentricity; rw—the worm wheel index circle radius affects the transmission stability analysis. If there is an error in the involute tooth shape of one tooth, it will cause its speed ratio to change instantaneously during the meshing process. The base section has an error, which will also affect the impact of the pair of tooth meshing transitions to the other pair of teeth. Therefore, the main reason that affects the smoothness of the transmission is the gear tooth error of $ft and the base deviation of $fpb.
Tool tooth profile error When the tool has a tooth angle error dA, the base circle radius of the cut gear is increased to rCos(A-dA) from the theoretical base circle radius rCosA, and the pitch circle angle is reduced from A to (A-dA), the resulting base deviation is $fpb=-PmtanAdA, the tooth profile error is $ft=-EPmsinAdA where E—the degree of coincidence of the tool and the gear is visible, and the tool pitch error is directly reflected to the gear teeth. Shape tool pitch error, spiral error, lead angle error, pitch error tool pitch error, spiral error, lead angle error, corner error, etc., will cause the cutting edge of the tool to be separated from the basic worm helicoid, resulting in medium The rack translation speed changes, causing a knuckle deviation of $fpb and a pitch deviation of $fpt.
Adopting new technologies, new processes and new methods to improve machining accuracy and prolonging service life. CNC high-efficiency hobbing technology has been developed. At present, China has developed a new type of gear hobbing machine with high rigidity and high cutting power, and applied numerical control technology to coordinate various movements of machine tools. Reduce machine adjustment time and machine tool transmission error, thus reducing geometric eccentricity, motion eccentricity and machine tool drive chain error. The development of numerical control technology makes the measurement of gears in the machine simple and easy; the application of computer precision compensation software improves the accuracy of gear measurement in the machine. The application of new high-strength, high-hardness tool materials increases cutting speed. For example, the common cutting speed is 30~50m/min, and the cutting speed of the cermet hob can reach 600m/min. The use of the new tool extends the life of the tool and improves the cutting efficiency and precision.