Abstract:
Based on the gear drawings provided by customers, this paper analyzes gear parameters, accuracy grades, hardness of hobbing blanks, and other special requirements to propose a method for formulating gear hobbing processing plans and a calculation method for gear beats. This method can determine whether a specific gear hobbing machine meets customer requirements and has important guiding significance for the gear hobbing process.
1. Principle of Gear Hobbing and Movement Relationship
Gear hobbing processes gears according to the principle of generative method. Machining gears with a hob is equivalent to the meshing of a pair of spiral gears with交错 axes. During the hobbing process, the teeth of the hob distributed along the spiral line successively cut out a thin layer of metal in the tooth groove. Each tooth groove is cut out sequentially by several teeth of the hob as it rotates. The involute tooth profile is formed by the envelope of a series of instantaneous positions of the cutting edge, as shown in Figures 1 and 2. The tooth shape is formed by generative and cutting movements, and the tooth width is formed by axial feed movement.
Hobbing methods are classified into axial feed method, radial feed method, and tangential feed method based on the feed mode. Different gears use different processing methods.
<img src=”https://example.com/gear_hobbing_principle.png” /> (This is a placeholder image for the gear hobbing principle)
2. Formulation Method of Gear Hobbing Processing Plan
When customers purchase hobbing equipment, they are most concerned about whether the hobbing machine can process their gear products and the processing efficiency. To address these issues, the solution route is as follows:
2.1 Analysis of Gear Workpiece Drawings
Judge the gear type, gear parameters, gear accuracy, blank hardness, processing steps, clamping method, and other special requirements based on the drawings.
2.2 Understanding the Structure of the Hobbing Machine
Understand the limit dimensions, strokes, speeds, cutting spindle power, and torque of each motion axis of the machine tool.
2.3 Determination of Workpiece Tooling and Clamping Method
Determine the tooling height, tooling outer diameter, tooling axial, and radial positioning surfaces based on the workpiece size.
2.4 Determination of Hob Parameters
- Hob Linear Speed: Determine the hob linear speed based on the spindle linear speed of the hobbing machine. Select hob materials that can meet the required linear speed, including high-speed steel, high-performance high-speed steel, powder metallurgy high-speed steel, solid carbide, and welded carbide materials with coatings. Dry and wet hobbing machines use different hob materials based on different linear speeds. Wet hobbing spindles have low rotational speeds, generally with linear speeds below 120 m/min, and commonly use ordinary high-speed steel materials. Dry hobbing machines have high spindle rotational speeds, with linear speeds reaching above 200 m/min, and commonly use powder metallurgy high-speed steel.
- Hob Size: Increasing the number of hob heads can improve gear processing efficiency. The number of hob heads is preferably mutually prime with the number of teeth of the workpiece to ensure small tooth pitch errors. However, the number of hob heads is not the more, the better. It needs to be selected based on the maximum rotational speed range of the turntable. The determination of the hob outer diameter needs to ensure that the hob does not interfere with any other machine parts of the machine tool when cutting in and out of the tooth part of the workpiece.
- Hobbing Process Parameters: After determining the hob parameters, select the spindle rotational speed based on the hob linear speed using the formula:
n=d01000πv(1)
Where n is the hob spindle rotational speed, v is the hob linear speed, and d0 is the hob diameter.
The worktable rotational speed n工件 is calculated using the formulas:
n工件=Pz⋅(1+tan2β2)i⋅ni⋅z2(2)
Pz=z22πd2tanβ2(3)
Where i is the number of hob heads, f is the axial feed rate, z2 is the number of gear teeth, Pz is the spiral lead at the gear pitch circle, β2 is the gear spiral angle, and d2 is the gear pitch circle diameter.
Select the axial feed rate f (mm/r) based on the maximum chip thickness at the hob tooth tip:
f=(complex calculation involving multiple parameters, see original paper for detailed formula)
The selection of f also considers the feed ripple requirement:
f=sin2αn2cosβδ(5)
δ=desired ripple depth
A summary table of recommended ripple depths for different hobbing processes is provided:
| Hobbing Process Requirement | Trace Depth (μm) |
|---|---|
| Precision hobbing | 0–3 |
| Pre-honing hobbing | 5–10 |
| Pre-rolling hobbing | 8–12 |
| Pre-shaving hobbing | 10–20 |
| Pre-grinding hobbing | 15–35 |
2.5 Determination of Axial Stroke
Axial stroke = tooth width + cut-in stroke + cut-out stroke.
2.6 Determination of Tool Paths and Feed Amounts
During rough hobbing, a single pass (2.25 mn) is often used. During finish hobbing, two passes are typically used, with the first pass removing more than 95% of the material and the final pass leaving a 0.2–0.5 mm finish to improve surface finish.
2.7 Verification of Parameter Rationality
Verify the rationality of the parameters by calculating the required torque of the hob spindle during cutting and comparing it with the torque that the spindle can output using the torque calculation formula:
Mt=1.75⋅0.650.8⋅t1.23⋅m0.26⋅f0.27⋅z1.8⋅K1⋅K2⋅K3(7)
Where Mt is the hob spindle torque, t is the depth of cut (in %), m is the normal module, f is the feed rate, z is the number of teeth, and K1, K2, K3 are correction coefficients for workpiece material, hardness, and spiral angle, respectively.
Tables of correction coefficients for different materials, hardnesses, and spiral angles are provided:
| Material Hardness (HB) | K1 Correction Factor |
|---|---|
| 45 Steel, 220 HB | 1.00 |
| 40Cr, 221 HB | 1.08 |
| 18CrMnTi, 265 HB | 1.34 |
| 38CrMoAlA, 265 HB | 1.24 |
| Gray Cast Iron, 190 HB | 0.48 |
| Material Hardness (HB) | K2 Correction Factor |
|---|---|
| 180 HB | 1.05 |
| 200 HB | 1.08 |
| 220 HB | 1.00 |
| 240 HB | 1.18 |
| 260 HB | 1.13 |
| 280 HB | 1.07 |
| Workpiece Spiral Angle (β2/°) | K3 Correction Factor |
|---|---|
| 0° | 1.00 |
| 10° | 1.07 |
| 20° | 1.11 |
3. Conclusion
From a practical application perspective, the formulation process and method for gear processing schemes are summarized, the calculation method for processing cycle time is analyzed, and processing parameters are reasonably selected based on practical experience, which has guiding significance for actual gear processing production. This scheme is conducive to gear manufacturers’ assessment of equipment production capacity and optimization of production process parameters, and can provide theoretical support for the structural design of hobbing equipment.