Abstract:
Gear hobbing is a crucial technology in the manufacturing industry, widely applied in the production of gears for machinery, aviation, instrumentation, and other fields. This article delves into the principles, processes, equipment, and practical applications of gear hobbing, aiming to provide comprehensive insights and guidance for practitioners in the field.
1. Introduction
Gears serve as primary transmission components in numerous products, with their precision playing a vital role in determining subsequent product quality. As a result, the advancement of gear manufacturing technology is of significant importance. Traditional gear manufacturing methods relied heavily on specialized hobbing machines. However, with the development of numerical control technology, CNC gear hobbing has emerged, offering a more efficient and versatile processing option. Mastering the principles of gear hobbing and implementing CNC milling technology for gears opens up new methods for gear manufacturing, which is of great significance in expanding the machining range of machines and improving machining methods.
2. Gear Manufacturing Processes
2.1 Principle of Gear Hobbing
Gear hobbing is based on the generating method. The hobbing process is akin to the meshing and rolling of a pair of crossed helical gears, where the hob and workpiece engage spatially under a specific motion relationship, completing the gear manufacturing process through the envelope (generating) principle.
In essence, a gear hob can be regarded as a helical gear with a large helical angle, a small lead angle, a few teeth (commonly with a tooth number of 1, i.e., a single-headed hob), long teeth, and multiple turns wound around the cylindrical surface.
2.2 Characteristics of Gear Hobbing
- High Efficiency: Gear hobbing boasts the highest material removal rate among gear manufacturing methods, especially with the development of multi-headed hobs and hard-surface cutting hobs.
- Fixed Process Parameters: Due to the generating method, the process parameters controlled during gear hobbing are relatively fixed. Only a few fixed process parameters need to be controlled, such as the gear center distance, tool center point coordinates, tool speed, worktable speed, feed rate, and cutting conditions.
- Fixed Cutting and Feed Methods: Hobbing generally adopts several fixed feed methods. The cutting methods mainly include forward hobbing and reverse hobbing, while the feed methods include radial-axial feed, axial feed, and simultaneous radial-axial feed.
- Complex Gear Geometric Parameters: The cutting envelope path is complex, with different types of gears having different and relatively complex geometric parameters.
Table 1: Angle Adjustment for Hobbing Straight and Helical Gears
| Hob Type | Gear Type | Adjustment Angle α |
|---|---|---|
| Left-hand Hob (λ) | Left-hand Helical Gear (β) | α = λ (Counterclockwise) |
| α = β – λ (Clockwise) | ||
| Right-hand Hob (λ) | Right-hand Helical Gear (β) | α = λ (Clockwise) |
| α = β + λ (Clockwise) |
2.3 Tool Motion and Adjustment
In gear manufacturing, the machine tool must provide several basic motions: the generating motion for forming the involute generatrix of the workpiece gear, the feed motion for forming the axial guide line of the workpiece gear, and the differential composite motion for forming the helical guide line of the helical gear.
The adjustment of the hob involves setting the axial position on the tool bar to ensure symmetry of the hob teeth (or grooves) with respect to the workpiece axis and adjusting the installation angle to match the tooth direction at the meshing point.
2.4 Choice of Hobbing Methods
The selection of the hobbing path has a significant impact on processing time, accuracy, and energy efficiency.
Table 2: Common Hobbing Feed Methods and Their Characteristics
| Hobbing Method | Characteristics |
|---|---|
| Axial Hobbing | Smooth cutting process, low surface roughness, uniform load, high tool durability, suitable for large feed rates and high cutting speeds. |
| Radial Hobbing | Shortens the cutting-in time, suitable for cutting narrow-tooth-width workpieces with large-diameter hobs. |
| Radial-Axial Hobbing | Combines the advantages of both radial and axial hobbing, improving processing efficiency while protecting tool life. |
2.5 Selection of Cutting Parameters
The choice of cutting parameters significantly affects gear quality, production efficiency, and tool life.
- Feed Rate: To improve efficiency, the largest possible feed rate should be used while ensuring gear quality.
- Cutting Speed: Should be determined comprehensively considering feed times, feed rate, workpiece material properties, gear modulus, and other processing conditions.
2.6 Choice of Machining Equipment
Modern CNC gear manufacturing machines utilize electronic gearboxes to achieve synchronized control of multiple axes, replacing traditional mechanical gear trains.
Advantages of CNC Gear Machines with Electronic Gearboxes:
- Shortened transmission chain, improved transmission stiffness and accuracy.
- Flexible and accurate positioning, capable of processing parts that cannot be machined by ordinary gear machines.
2.7 CNC Programming
The CNC system of the turn-mill composite machining center uses Siemens 840D sl, with verified electronic gear functions (EDDEF, EGON) capable of achieving precise multi-axis coupling control.
EGDEF Definition:
EGDEF(Follower Axis, Guide Axis 1, Coupling Type, Guide Axis 2, Coupling Type 2, …)
EGON Function:
EGON(FA, “Program Segment Transition Mode”, LA1, Z1, N1, LA2, Z2, N2, …, LA5, Z5, N5)
2.8 Hob Clamping
A specific tool bar is designed for clamping the hob, ensuring a rotational accuracy of <0.01mm.
2.9 Machining Route Design
To balance tool durability and part accuracy, an axial feed and radial layering approach is adopted for both rough and finish machining.
2.10 Machining Program
The main program is a Siemens system source program, implementing the hobbing process through a series of G-code commands.
2.11 Machining Results
After testing on nearly 50 parts, straight gears and helical gears were successfully machined.
3. Conclusion
With the rapid development of numerical control technology, many traditional, manual tasks are gradually being realized on numerical control equipment. By continuously strengthening theoretical learning and improving engineering application capabilities, process engineers can explore more solutions to problems. Gear hobbing technology, with its high efficiency and flexibility, holds great promise for the future of gear manufacturing.