An In-depth Analysis and Solution to Gear Shaving Surface Scratches

Abstract
This paper presents a comprehensive analysis of the root causes of gear shaving surface scratches and formulates reasonable solutions to achieve control objectives. Through practical implementation, the long-standing issue of gear surface scratches at our company has been completely resolved, effectively enhancing gear surface quality and reducing gear mesh noise.

1. Introduction
Noise control in gear manufacturing has become a crucial quality control aspect. It not only reflects the quality level of a gear manufacturer but is also constrained by relevant environmental regulations. Gear shaving is a widely adopted precision finishing method for gears, particularly in transmission gear processing. It offers high processing efficiency, low costs, and significantly improves gear accuracy and surface quality. However, gear surface scratches frequently occur during shaving, significantly impacting surface quality and, consequently, gear mesh noise control. Therefore, addressing and resolving gear surface scratches during shaving is crucial.

2. Analysis of Gear Shaving Surface Scratches
Our company occasionally experiences varying degrees of gear surface scratches during shaving. Scratches are concentrated at the top of the gear teeth, absent in the middle, with consistent texture orientation. Each scratch is parallel, deviating at a certain angle along the involute direction, and has varying depths but remains consistent across the same curvature radius on the same gear. Scratches are more prone under similar processing conditions, particularly with soft material gears, gears with large modification coefficients, prolonged cutter usage, large shaving allowances, and prolonged unreplaced cutting fluids.

3. Theoretical Analysis of Causes of Gear Shaving Surface Scratches

3.1. Principle of Gear Shaving
Gear shaving is a precision finishing method where a gear shaver and workpiece engage as a non-side-clearance crossed-axis helical gear pair, producing relative sliding on the tooth surface. The gear shaver can be seen as a cylindrical gear with small chip removal grooves on its side, forming cutting edges where the grooves intersect the tooth sides.

3.2. Tangential Sliding Motion During Gear Meshing
Due to the shaving principle, the direction of the cutting motion is perpendicular to the comb teeth groove of the cutter blade. The shavings are pushed into the groove by the rake face of the cutting groove. Normally, they cannot scratch the tooth surface. However, under the influence of cutting heat, if shavings adhere to the cutter, they may scratch the tooth surface.

3.3. Cause Analysis of Gear Surface Scratches
Gear surface scratches are fine groove-like marks. Since the rake and flank faces of the shaver are smooth, they do not scratch the tooth surface. The only object in contact with the tooth surface besides the cutter is the chip. However, how does the chip scratch the gear tooth surface?

According to the shaving principle, if the chip adheres to the cutting edge, it can scratch the tooth surface like a knife, particularly due to the tangential sliding motion during gear meshing. This motion, combined with the cutting motion vector, causes the scratch direction to deviate at a certain angle, aligning with the described gear surface scratch direction. Therefore, the cause of gear surface scratches is chip adhesion and scratching.

4. Experimental Verification of Theoretical Analysis

4.1. Experimental Design
Potential factors affecting gear surface scratches were identified through theoretical analysis and tested individually as single variables, with other conditions held constant, to verify their influence.

Table 1. Gear Shaving Results with Different Material Hardness

Shaving MethodBlank MaterialBlank Hardness (HBW)Production Quantity When Scratches First Appear (Pieces)
Axial ShavingFAS3420H150–1611652
Axial ShavingFAS3420H165–1722056
Axial ShavingFAS3420H190–1991985

Table 2. Gear Shaving Results with Different Cutting Parameters

Cutting Speed (r/min)Number of Cycles (Affecting Back Cut Depth)Shaving Allowance (Common Normal Line)/mmScratch Situation
18050.15Minor Scratches
18030.15Severe Scratches
24050.15Severe Scratches
24050.10Minor Scratches
10050.08No Scratches

Table 3. Gear Shaving Results with Different Cutting Fluids

Cutting Speed (r/min)Number of Cycles (Affecting Back Cut Depth)Shaving Allowance (Common Normal Line)/mmCutting FluidScratch Situation
12050.08Used for Over 2 MonthsMinor Scratches
12050.08Contaminated with Hydraulic OilSevere Scratches
12050.08Newly Replaced, No AdditivesScratches After 1536 Pieces
12050.08Newly Replaced, With AdditivesScratches After 2347 Pieces

4.2. Experimental Conclusion
Each variable’s influence on scratches aligned with theoretical analysis, confirming its correctness and laying a theoretical and experimental foundation for resolving gear surface scratches. All factors affecting shaving cutting heat also impact gear surface scratches, indicating a multifaceted cause.

5. Solutions to Gear Shaving Surface Scratches

The root cause of gear shaving surface scratches is cutting heat-induced chip adhesion and scratching. Factors affecting cutting heat and chip viscosity impact surface scratches. To address this, comprehensive measures were formulated:

Table 4. Measures to Prevent Gear Shaving Surface Scratches

Measure CategorySpecific Measures
Material SelectionSelect gears with uniform density, no impurities, local defects, or excessive toughness. Material hardness should be 170–197 HBW.
Cutting Fluid Selection and ControlEnsure purity, anti-foaming capability, concentration, and lubricity. Prevent contamination, especially oil mixing. Additives can enhance anti-foaming and lubricity.
Cutting Allowance ControlChoose the smallest cutting allowance ensuring complete shaving. Recommend 0.06–0.10 mm for gears with modulus ≤ 10. Adjust radial feed, cycles, and efficiency as needed.
Cutting Parameter SelectionBalance cutting efficiency and scratch prevention. Parameters should be selected based on theoretical recommendations and field tests.
Cutter ControlReplace dull cutters promptly to maintain sharp cutting edges. Optimize cutter design, including chip flute, balance point, and material selection.
Shaving Method SelectionRadial shaving improves efficiency, reduces back cut depth, and extends cutter life, addressing scratching issues.

6. Improvement Effects
By adopting radial shaving, selecting materials of appropriate hardness, optimizing cutter chip flutes, determining reasonable cutting parameters, controlling cutting fluid purity, and using additives, we completely resolved gear shaving surface scratches. Post-improvement gear surfaces are significantly enhanced, with stable performance maintained for over a year and over 2 million compliant products produced.

7. Conclusion
Through analyzing the root causes of gear shaving surface scratches and verifying influencing factors through controlled experiments, we formulated and implemented comprehensive solutions, effectively resolving the issue. This significantly improved gear surface quality and reduced gear mesh noise, reflecting a high level of gear manufacturing quality and environmental compliance.

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