Current Status Analysis of Small and Medium Module Gear Hobs

Written by: CGMA Small Module Gear Working Committee, Yu Rennan

There is a Chinese proverb: "A good horse needs a good saddle." With the advancement of CNC, intelligent, and energy-efficient technologies in gear hobbing machines, the foundation for high speed, high efficiency, high precision, and environmental friendliness has been established. The progress in gear cutting tool materials, machining technology, and coating technology provides effective assurance.

Using hobs on machine tools (gear hobbing machines) to process toothed parts on rims by the generating method. The hob is an essential tool on the gear hobbing machine — it is a gear machining tool with teeth arranged in a helical pattern along a cylinder or cone, used for machining cylindrical gears, worm wheels, and other cylindrical toothed workpieces by the generating method. Depending on the application, hobs are classified into gear hobs, worm wheel hobs, non-involute generating hobs, and fixed-mount hobs.

Small and medium module gear hobs have certain unique characteristics and process features. Carbide hobs, full-depth hobs, multi-start hobs, worm hobs, and hard tooth surface cutting (skiving) have been widely applied in small and medium module gear machining.

I. Classification of Hobs

1. By Machining Nature

2. By Structure

3. By Material

4. By Number of Starts

5. By Accuracy Grade

II. Current Status and Hobbing Process Features of Small and Medium Module Gear Hobs

The involute gear hob (commonly called gear hob) is a complex cutting tool — the most typical and widely used type of hob. Gear hobs are generating tools that machine workpiece tooth profiles based on gear meshing principles. They machine spur and helical cylindrical gears based on helical gear meshing principles. It is equivalent to a helical gear with very few teeth and a very large helix angle, with an appearance resembling a worm.

1. Development Status of Gear Hob Materials and Coatings

The advancement in gear hob materials and coating technology is the "double-edged sword" for achieving high-speed, high-efficiency gear hobbing and the guarantee for hard tooth surface cutting. Gear hob materials have progressed from conventional HSS → high-performance HSS → powder metallurgy HSS (PM HSS) → carbide. Research achievements in multiple hob materials have met the requirements of various application fields.

Gear hob coating technology has also evolved from the "yellow knives" first applied in the 1970s to today's various "black knives." See the coating types table below.

Gear Hob Coating Types Table

No Coating Material Micro-Hardness (HV) Friction Coefficient Against Steel (Dry) Internal Stress (GPa) Max Service Temp (°C) Coating Color Coating Structure
1 TiN Titanium Nitride 2300 0.40 -2.5 600 Golden Yellow Monolayer
2 AlCrN Aluminum Chromium Nitride 3200 0.35 -3.0 1100 Blue-Gray Monolayer
3 TiCN Titanium Carbonitride 3000 0.40 -4.0 400 Blue-Gray Multilayer
4 WC/C Tungsten Carbide/Carbon 1000/2000 0.10-0.20 -1.0 300 Dark Gray Laminated
5 CrN Chromium Nitride 1750 0.50 -1.5/-2.0 700 Silver-Gray Monolayer
6 Polycrystalline Diamond 8000-10000 0.15-0.20 600 Light Gray Monolayer
7 TiAlN Titanium Aluminum Nitride 3300 0.30-0.35 -1.3/-1.5 900 Purple-Gray Nano-Structure
8 TiCN+TiN Titanium Carbonitride + Titanium Nitride 3000 0.40 -4.0 400 Golden Yellow Multilayer, Graded
9 TiAlN+WC/C Titanium Aluminum Nitride + Tungsten Carbide/Carbon 3000 0.15-0.20 -1.7/-2.0 800 Dark Gray Multilayer, Laminated
10 DLC(a-C:H) 2500 0.10-0.20 350 Dark Gray Monolayer
11 TiAlN Titanium Aluminum Nitride 3300 0.25 -1.3/1.5 900 Purple-Gray Nano-Structure
12 TiAlN Titanium Aluminum Nitride 3300 0.40 -3.0/-3.5 900 Blue-Gray Nano-Structure
13 TiAlN Titanium Aluminum Nitride 3500 0.40 -4.0 800 Purple-Gray Monolayer

The advancement of gear hob materials has increasingly achieved excellent grinding performance, excellent heat treatment dimensional stability, excellent toughness, excellent red hardness, and excellent wear resistance. The progress in coating technology has improved coated tools' anti-friction and anti-adhesion capabilities, extending tool life by 2-3 times and increasing cutting speed by 25%-50%.

With the progress of carbide materials, high-hardness material machining technology, and carbide tool coating technology, gear tool durability has been improved, providing effective assurance for implementing high-speed cutting, dry cutting, and hard tooth surface cutting processes in small and medium module gear manufacturing.

From the following table showing the relationship between hobbing speeds before and after use and before and after coating (using hobbing of 40Cr, 35CrMo, 20CrMo alloy structural steel as examples), the importance of gear hob material technology advancement and coating can be clearly seen.

Gear Hob Hobbing Speed m/min Skiving Speed m/min
Carbide + Coated 120-160 100-140
Carbide Reground Uncoated 100-130 80-110
PM HSS + Coated 90-120 -
PM HSS Reground Uncoated 70-100 -
High-Performance HSS + Coated 70-80 -
High-Performance HSS Reground Uncoated 60-70 -
High-Performance HSS Uncoated 50-60 -
Conventional HSS 30-40 -

2. Application of Full-Depth Gear Hobs (Topping Hobs)

Full-depth hobs have been widely applied in small and medium module gear machining.

Conventionally used hobs are non-full-depth (also called standard hobs). Non-full-depth hobs only have top edges and side edges participating in cutting. In addition to top and side edges, full-depth hobs also cut at the hob slot bottom simultaneously. Because of this hobbing method, the tooth tip and tooth profile are generated simultaneously, ensuring concentricity between the tip circle and pitch circle. This highlights three major advantages: first, the precision requirement for gear blank outer diameter can be reduced; second, after hobbing, there are no burrs at the tooth tip and tip rounding is performed, playing an important role in reducing gear transmission noise; third, after gear heat treatment, outer diameter positioning can be used for bore machining, improving machining efficiency.

Regarding full-depth hobs, they were discussed in the book "Small Module Gear Machining" published by National Defense Industry Press in 1972. However, due to the limitations of hob manufacturing equipment and grinding wheel manufacturing technology, widespread application was difficult for many years. In recent years, with the improvement of hob manufacturing equipment and grinding wheel technology, manufacturing full-depth gear hobs is no longer a challenge. The widespread application of full-depth gear hobs has played an important role in reducing costs and ensuring precision in small and medium module gear manufacturing.

3. Application of Multi-Start Gear Hobs

Multi-start gear hobs were also discussed in the 1972 book "Small Module Gear Machining" published by National Defense Industry Press: "Multi-start hobs have very high production rates, so if the manufacturing precision of multi-start hobs can be improved, their use in gear machining will become widespread." In recent years, with the improvement of hob manufacturing equipment, multi-start hob manufacturing precision has significantly improved, leading to expanded applications, especially in small module gear manufacturing where multi-start hobs are commonly used for thin gears with many teeth.

4. Application of Worm Hobs

It can be said that six years ago, "worm hob" was still a new term in China. There were only "worm milling cutters," not "worm hobs." It was private enterprises, small and medium enterprises — Jiangyin SETE Precision Tools Co., Ltd. (Jiangyin Tool Factory) and Ningbo Langmanda Tools Co., Ltd. — that collaborated with gear manufacturing plants to produce worm hobs, filling the domestic gap and replacing imports. However, it must be acknowledged that China currently cannot manufacture machine tools for worm hobbing.

5. Application of Hard Tooth Surface Cutting and Skiving Technology

Small and medium module carbide gear hobs have been widely applied in gear manufacturing enterprises. Especially the application of hard tooth surface cutting technology (gears with tooth surface hardness of 48-52 HRC have been widely applied, and gears with tooth surface hardness up to 62 HRC are beginning to be applied). The application of new processes has compensated for the challenge of gear heat treatment deformation, playing a decisive role in ensuring gear manufacturing precision and reducing gear transmission noise (which can be combined with crowned teeth — lead modification technology). Hard cutting processes are mainly applied to gears with module m ≤ 1.25. The application of hard tooth surface skiving technology, on one hand reflects the progress of hob manufacturing technology, and on the other hand benefits from the national patent "Precision Automatic Tool Setting Device for Gear Hobbing Machines" applied and approved in 2006 by Zhejiang Jiaxue CNC Machine Tool Co., Ltd. (a private enterprise, SME), which changed the situation where gear skiving could only be performed on foreign hobbing machines. According to domestic users, Jiaxue's automatic tool setting device's tool setting time is even better than similar foreign devices. Currently, hard tooth surface skiving is more commonly applied to gears with module m ≤ 3.

Manufacturing precision of hard-cut and hard-skived gears: can stably achieve Grade 6-7 per GB/T 10095-2008. It should be noted that in small and medium module gear manufacturing processes, due to the rapid advancement of gear hob manufacturing technology and CNC hobbing machine manufacturing capabilities, high-speed, high-efficiency, and high-precision hobbing has been achieved. As a result, precision hobbing processes are challenging traditional hob-shaving processes (already demonstrated in small and medium batch gear manufacturing), and the development of precision hobbing processes is also competing with gear grinding processes in terms of machining precision, cost, and practicality.

Currently, the domestic localization rate of small and medium module gear hobs has reached over 80%.

6. "Basic Types and Dimensions" in Small Module Gear Hob Standards Can No Longer Meet Requirements

In gear hobbing, to extend the usage time of a single tool installation, reduce the frequency of tool changes, shorten auxiliary time, and improve the utilization efficiency of hobbing machines, both domestically and internationally, it has become common practice to increase the diameter and extend the length of gear hobs. With the progress of gear hob manufacturing technology, manufacturing capabilities can fully meet user requirements.

The "Basic Types and Dimensions" in "JB/T 2494-2006 Small Module Gear Hobs" and "JB/T 7654-2006 Solid Carbide Small Module Gear Hobs" can no longer meet requirements, but there are no updated standard specifications. It is proposed to select according to the "Basic Types and Dimensions of Small and Medium Module Solid Carbide Gear Hobs" in the following table.

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9/15/2016 12:00:00 AM