The Importance of Single Tooth Thickness Inspection in Gear Machining Process

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The Importance of Single Tooth Thickness Inspection in Gear Machining Process

Core Conclusion

Intermediate inspection during rough hobbing is crucial for ensuring gear machining quality, which can promptly detect deviations and reduce scrap rates. Two key inspection points (chordal tooth thickness and common normal length) are essential to control groove width, pitch, and tooth profile accuracy.

1. Inspection of Normal Chordal Tooth Thickness (sn) When Hob Center Axially Feeds 30mm

Meaning and Background

" Hob center axially feeds 30mm" refers to the state when the hob moves 30mm axially from the reference position of the gear's datum end face during rough hobbing. Chordal tooth thickness is the chordal tooth thickness value measured at the reference circle of the gear, a key parameter reflecting the correctness of the relationship between feed depth and tooth thickness. Inspecting it in the rough rolling stage prevents over-cutting or under-cutting and ensures uniform finishing allowance.

Why Inspection is Necessary

Rough hobbing removes material quickly with large machining volume, making it prone to vibration or deformation. Inspection can early identify machining errors (such as tool wear, machine tool clearance, or workpiece clamping issues) and avoid error accumulation in the finishing stage. The inspection data can adjust radial feed or tool compensation.

Inspection Method

Use a gear tooth thickness caliper to measure chordal tooth thickness at the gear's reference circle. The actual measuring tooth height is corrected based on the measured value of the addendum circle. Calculate the theoretical chordal tooth thickness value according to gear parameters (module, number of teeth, pressure angle), compare the measured value with the theoretical value, and the allowable tolerance is set according to gear accuracy grades (e.g., AGMA or ISO standards).

Implementation Suggestions

The 30mm axial distance is an empirical value for most gear sizes. For gears with large modules and helix angles, recalculate to determine the appropriate value. In actual machining, optimize the inspection point when the radial feed reaches 30% to 50% of the total tooth height (total tooth height h ≈ 2.25 × module for standard gears).

2. Re-inspection of Common Normal Length When Hob Cut Tooth Height = WK × SIN(β) + 10

Meaning and Background

"Hob cut tooth height" refers to the value of the cutting tooth length achieved during axial feeding. WK is the designed (theoretical) value of the gear's common normal length (in mm), a core parameter for measuring gear accuracy that reflects cumulative pitch error and tooth profile deviation. β is the gear's helix angle (in degrees), and sin(β) corrects the height position to adapt to the characteristics of helical gears. The minimum tooth width height h = WK × sin(β) defines the key point for re-inspecting the common normal length, usually related to the gear's base cylinder or the critical area of tooth profile formation.

Why Re-inspect Common Normal Length

Re-inspecting at height h = WK × sin(β) is because the common normal length is most sensitive to changes in helix angle and pitch at this position. During rough hobbing, large material removal easily leads to cumulative errors in tooth profile and pitch. Re-inspection can quickly detect tooth profile deviations (such as tooth direction error or helix deviation) for compensation before finishing.

Re-inspection Method

Use a common normal micrometer or gear measuring instrument to measure the common normal length in the gear's normal plane (perpendicular to the tooth direction). Calculate the theoretical common normal length WK based on gear design parameters, measure the actual value when the radial tooth height reaches h = WK × sin(β), and compare the deviation between the measured and theoretical values. Tolerances are set according to AGMA or DIN standards, usually ±0.02～0.05mm.

Implementation Suggestions

Obtain accurate WK and β values from gear drawings or process documents. If β=0° (spur gear), sin(β)=0, and this re-inspection can be omitted. The calculated tooth height h should be less than or equal to the total tooth height; otherwise, re-inspect near the total tooth height. The re-inspection point is usually in the later stage of rough hobbing but earlier than the final size to leave room for correction.

Overall Process Significance and Quality Control Suggestions

Importance of the Two Inspection Points

Rough hobbing aims to quickly remove allowance, and intermediate inspection prevents batch errors and reduces finishing risks. Chordal tooth thickness inspection controls "groove width", and common normal length re-inspection controls "pitch and tooth profile" accuracy. Their combination ensures stable gear machining quality.

General Implementation Guidelines

Prepare by obtaining process inspection parameters (module, number of teeth, pressure angle, etc.) from gear drawings. Conduct the first inspection when the hob center axially feeds 30mm, and the second when the axial machining length reaches h = WK × sin(β) (usually around 70% of rough hobbing feed). Use calibrated measuring tools (tooth thickness caliper, vernier caliper, etc.). If the inspection value is out of tolerance, check feed depth, tool sharpness, machine tool accuracy, etc., and adjust the process accordingly.

Notes

If WK is not clearly defined, use the theoretical common normal length. WK in the formula h = WK × sin(β) is the designed value, not the measured value. In actual machining, these values should be calculated and corrected by process engineers based on drawing parameters.

Pub Time : 2025-11-03 09:39:52 >> News list

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