Gears, as one of the most common transmission components, are indispensable parts in all machines and instruments. With the development of human society and the advancement of science and technology, people have increasingly higher requirements for gear transmission systems to minimize vibration and noise. The quality of gears directly affects the overall quality of the transmission, especially its noise performance. Therefore, controlling gear errors is currently the main way to improve the quality of gear systems.
I. Gear Machining Methods
1. Gear hobbing
Gear hobbing is a machining process based on the meshing principle of helical gears. The hob resembles a worm gear in shape, but its essence is a helical gear with a large helix angle. The number of threads on the hob is equal to the number of teeth. Due to the difficulty in manufacturing involute hobs, approximate Archimedes hobs and normal straight-profile hobs are actually used in practice.
The hobbing machine tool has the following basic movements: 1) The rotation of the hob, i.e., the cutting motion. For each rotation of the hob, the cutting teeth move one lead along the axial direction; 2) The rotation of the workpiece, i.e., the indexing motion. The generating motion between the workpiece and the hob is ensured by the machine tool's indexing motion chain. When the hob teeth move one tooth pitch, the workpiece rotates one tooth; 3) The slow movement of the hob holder along the gear axis, i.e., the axial feed motion; 4) When machining helical gears, during the axial feed process, an additional rotation corresponding to the helix angle β is obtained through the differential mechanism and the indexing worm gear pair. When the hob feeds one lead axially, the workpiece rotates one additional revolution.
2. Tooth
Gear shaping is a machining process based on the meshing principle of gear pairs, where one gear is the gear shaper cutter and the other is the workpiece. During the generating process, the profile of the gear being cut is the envelope of the motion trajectory of the gear shaper cutter relative to the workpiece. The gear shaper cutter is actually a modified gear; to form the cutting edge and perform cutting, one end face is ground into a conical surface to form the rake angle of the cutter tooth, and the involute tooth profile in the tooth width direction is ground into a conical shape to form the clearance angle of the cutter tooth.
During gear shaping, the machine tool undergoes the following five movements: 1) Rapid reciprocating cutting motion of the gear shaper cutter, with the stroke number adjusted using a gear changer; 2) Generating motion between the gear shaper cutter and the workpiece, adjusted using a gear indexing gear changer; 3) Radial feed motion of the gear shaper cutter. After the gear shaper cutter begins to contact the workpiece, it feeds radially towards the workpiece simultaneously with the circumferential feed until the full tooth height is cut, while the circumferential feed continues until the gear is machined when the workpiece completes one revolution; 4) Retreat motion. During the upward idle stroke of the gear shaper cutter, to avoid friction between the workpiece and the gear shaper cutter, the workpiece should retract a distance relative to the gear shaper cutter; 5) When machining helical gears, the gear shaper cutter also undergoes an additional rotation corresponding to the tooth helix angle β during its longitudinal movement along the gear axis.
3. Shaving teeth
Gear shaving is also based on the meshing principle of helical gears, and it is a free meshing generating method. This means there is no rigid kinematic connection between the workpiece and the cutting tool. The shaving cutter itself is an involute helical gear with grooves cut from the tooth tip to the tooth root to form cutting edges. During shaving, because the axes of the cutting tool and the workpiece intersect, a sliding velocity component (i.e., cutting motion) is generated on the tooth surface along its generatrix. The cutting edge of the shaving cutter then shaves off a very thin layer of metal from the tooth surface.
II. Gear Machining Errors
The various errors generated during gear cutting can be summarized into the following four types: ① Radial machining error: the change in radial distance between the cutter and the gear; ② Tangential machining error: the machining error caused by the disruption of the generating motion between the cutter and the workpiece or inaccurate indexing; ③ Axial machining error: the error in the movement of the cutter along the axis of the workpiece; ④ Error in the profile of the gear cutter.
1. Radial machining error
The radial error in gear cutting is caused by the positioning error of the gear blank on the machine tool, the radial runout of the tool, and the periodic variation of the position of the gear blank axis or tool axis (table oscillation).
(1) Geometric eccentricity caused by gear blank positioning error: Geometric eccentricity refers to the degree of misalignment of the rotation axis of the gear during processing and use (or inspection).
(2) Tool installation errors: ① Hob installation error: When machining spur gears, hob installation error results in tooth profile error, disrupting the gear's smooth operation and reducing the tooth height contact area. When machining helical gears, hob installation error creates waviness in the contact line, reducing the tooth contact area. ② Gear shaper installation error: The radial runout of the gear shaper cutter shaft, the eccentricity of the gear shaper installation, and the radial runout of the gear shaper itself combine to cause radial runout of the gear shaper, or geometric eccentricity of the gear shaper. This eccentricity not only causes radial error in the gear but also has a significant impact on the common normal length. In addition to the above errors, the radial runout of the gear shaper will also cause errors in the cutting gear's contact area.
2. Tangential machining error
Tangential errors in tangential machining, particularly in gear hobbing and shaping, are primarily caused by disruption of the generating motion of the tool and gear blank. The errors in the machine tool's kinematic chain, especially the final indexing worm gear pair, are the root cause of tangential errors. The error in the indexing worm gear causes eccentricity in the meshing of the gear being cut and the hob.
3. Axial machining error
The axial error in gear cutting is mainly caused by the inaccuracy of the machine tool guide rail and the skewness of the gear blank axis. In some cases, the machine tool kinematic chain also has an impact.
4. Errors in the shape of the cutting tool surface
Errors in the tool's profile surface arise from the approximate shape of the tool's profile surface, as well as manufacturing and grinding errors. Tooth profile angle errors in the tool's profile surface cause base pitch deviations and contact line direction errors in the workpiece, thus affecting the smoothness of spur gear operation and disrupting the tooth surface contact between spur and helical gears.
III. Gear Quality Control
1. Gear blank quality control
Gear blank machining is the first stage of gear machining, and it directly affects the gear profile, so it must be taken very seriously. Traditional gear blank machining methods for disc-shaped parts often involve drilling—rough turning—broaching—multi-tool turning—finish turning the end face with a tapered mandrel. For shaft-shaped gears, the process generally involves drilling a center hole flush with the end face—profiling turning—grinding. It can be seen that this arrangement involves many steps, and the reference position is prone to bumps and scratches. Currently, CNC lathe machining of gear blanks is widely adopted, allowing disc-shaped parts to be completed in just two steps: first, finish turning the inner hole and one end face, and then using this as a reference to finish turn the remaining parts; while for shaft-shaped gears, using a specialized chuck, finish turning can even be completed in one setup. This not only reduces the number of tooling and machine tool footprint, and reduces the probability of workpiece bumps and scratches, but also ensures good workpiece dimensional consistency and greatly improves the accuracy of the gear blank.
2. Tool quality
The quality of the manufacturing, installation, and regrinding of cutting tools (including hobbing, slitting, and razor blades) directly affects gear cutting. The manufacturing precision of cutting tools is generally guaranteed by the tool manufacturer, while the installation and regrinding require the technical expertise of the gear manufacturer. Tool installation must be strictly performed according to operating procedures to ensure installation accuracy; tool regrinding requires not only professional technical guidance but also high-precision specialized equipment. The regrinding of razor blades is particularly critical.
3. Control of tooling design
The design and manufacturing quality of gear cutting fixtures are crucial for ensuring the success of gear machining. Fixtures for disc gears employ a double-cone elastic expansion sleeve structure. This structure ensures uniform elastic deformation during clamping, effectively achieving backlash-free centering and eliminating geometric eccentricity in gear machining. Fixtures for shaft gears are designed to adhere to the principle of maintaining a constant machining datum throughout the machining process, resulting in high machining accuracy and stability for shaft gears.
4. Machine tools
The precision of gear-making equipment is another guarantee of gear-cutting precision. The machining precision of a machine tool is determined by the manufacturing of its various moving parts and the maintenance of their motion precision. Therefore, it is very important to be familiar with the machine tool's performance, make appropriate adjustments, and perform regular maintenance to keep the machine tool in good working condition.
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