Gear manufacturing process: a precise exploration from traditional molding to intelligent manufacturing

Gears are the core components of mechanical transmission. The quality of their manufacturing process directly affects the performance, life and reliability of the equipment. From tiny gears in watches to heavy-duty gears in aerospace, different application scenarios have put forward diverse requirements for the accuracy, strength and production efficiency of gears.

With the breakthroughs in CNC technology, additive manufacturing and green manufacturing, gear manufacturing has moved from traditional basic molding to a new stage of intelligence and composite manufacturing. This article will systematically sort out the basic molding process, gear finishing, finishing process and special process of gear manufacturing,to provide a reference for everyone to choose the appropriate gear manufacturing process.

1. Gear manufacturing: molding process (basic molding)

(1) Casting

Principle:

The metal (such as cast iron, aluminum alloy, etc.) melted to liquid state is injected into the gear mold cavity under the action of gravity or pressure, and the metal liquid is solidified and formed in a predetermined direction by controlling the cooling rate. The mold types include sand mold (sand shell), metal mold (die casting) or ceramic shell (lost wax method). The shape of the cavity is used to give the blank a prototype of the tooth shape. After condensation, the blank is demolded and cleaned.

Methods: sand casting, die casting, precision casting (lost wax method).

Features:

Suitable for mass production, low cost.

Low precision (IT10-IT12), requires subsequent processing.

Commonly used for large, low-load gears (such as agricultural machinery, watch gears).

Material: cast iron, cast steel, aluminum alloy, etc.

(2) Forging

Principle:

Pressure is applied to the metal blank heated to above the recrystallization temperature (hot forging) or room temperature (cold forging), so that it undergoes plastic deformation in the mold cavity and fills the mold cavity, and the gear blank is formed by metal streamline reorganization. Hot forging uses the high plasticity of the metal to reduce deformation resistance, and cold forging improves the tooth surface strength through work hardening. The blank size needs to be controlled by mold precision.

Methods: hot forging, cold forging, warm forging.

Features:

Gears have high strength and good wear resistance.

Suitable for heavy-load gears (such as automobile transmission gears).

Medium precision (IT8-IT10), requires subsequent finishing.

(3) Powder metallurgy

Principle:

After mixing metal powders according to the proportion, they are pressed under high pressure to form a gear blank with initial strength. After high-temperature sintering (1000-1300℃), atomic diffusion and densification occur between the powder particles, directly forming gear parts. The mold accuracy during the pressing process determines the size of the blank. After sintering, the performance can be improved by oil immersion or heat treatment.

Features:

Suitable for small and complex gears (such as home appliances and power tool gears).

Material utilization rate is high, but the strength is low.

The accuracy can reach IT7-IT8, and the surface roughness is good.

2. Gear manufacturing: cutting processing (tooth shape finishing)

(1) Gear hobbing

Principle:

Using the hob (a tool shaped like a spiral worm) and the gear blank to generate motion (the speed ratio is equal to the inverse ratio of the number of teeth), the hob blade cuts the blank tooth by tooth through the hob blade, so that the surface of the gear blank forms an involute tooth shape. The helix angle of the hob matches the helix angle of the gear. During processing, the hob feeds along the axial direction to achieve full tooth width cutting.

Features:

High efficiency, suitable for mass production.

Precision can reach IT7-IT8 (economical type).

Widely used in cylindrical gear and helical gear processing.

(2) Gear shaping

Principle:

The gear shaping cutter (involute gear with blade) performs up and down reciprocating main motion, and at the same time rotates with the gear blank according to the tooth number ratio. The blade cuts the tooth groove material when moving downward, and quickly returns when moving upward to avoid friction. The precise tooth shape is formed by tooth-by-tooth enveloping, which is suitable for processing internal gears and multi-gears.

Features:

Suitable for internal gears, multi-gears and step gears.

Precision is similar to hobbing, but the efficiency is lower.

(3) Gear milling

Principle:

Use a forming milling cutter (disc or finger-shaped) that matches the gear tooth groove shape, and position it tooth by tooth through the dividing head. The milling cutter rotates to cut the blank, processing one tooth groove at a time, and the entire gear processing is completed after multiple indexing. The milling cutter profile directly determines the tooth shape accuracy, which belongs to the forming method processing.

Features:

High flexibility, suitable for single piece or small batch production.

Medium precision, often used for repair or large gears.

(4) Gear pulling

Principle:

Use a special broach (a tool with a series of teeth of increasing size) to make a linear feed motion, and cut the metal blank in sequence through the teeth. Each tooth bears a small amount of cutting (tooth lift is about 0.02-0.1mm), and a complete tooth shape is broached in one go. The broach motion trajectory is directly related to the tooth shape accuracy, which is a high-productivity forming process.

Features:

High precision (IT6-IT7), good surface quality.

High cost, only suitable for mass production (such as automotive synchronizer gears).

3. Gear manufacturing: finishing process (improving precision)

(1) Gear grinding

Principle:

Use a high-speed rotating grinding wheel (disc, worm or forming grinding wheel) as a cutting tool, and grind the tooth surface through the expansion method or forming method. The grinding wheel accuracy reaches the micron level, which can correct heat treatment deformation and improve tooth shape accuracy. During the generating grinding, the grinding wheel and the gear simulate meshing motion, while the forming grinding directly grinds according to the tooth profile shape.

Features:

The accuracy can reach IT3-IT5, and the surface roughness Ra≤0.4μm.

Used for high-precision gears (such as machine tools, aerospace gears).

High cost and low efficiency.

(2) Gear shaving

Principle:

The shaving cutter (helical gear with cutting edge) and the unhardened gear rotate at high speed in a free meshing state, and the relative sliding between the tooth surfaces is utilized. The thin layer of metal on the tooth surface is scraped off by the tiny serrations on the blade of the shaving cutter, the tooth profile error is corrected and the surface roughness is reduced. It is a finishing process without forced generating.

Features:

Improve the tooth surface roughness (Ra≤0.8μm) and correct small errors.

Suitable for the final processing of unhardened gears.

(3) Gear honing

Principle:

The honing wheel (resin-based abrasive gear) and the quenched gear mesh under elastic contact. Through the high-speed rotation (1000-2000rpm) and axial feed of the honing wheel, the micro-cutting effect of the abrasive particles is used to remove tooth surface defects, improve surface roughness and eliminate slight heat treatment deformation. It belongs to the finishing process.

Features:

Reduce surface roughness and eliminate heat treatment deformation.

Commonly used for correction of gears after quenching.

4. Gear manufacturing: special process

(1) 3D printing (additive manufacturing)

Principle:

Metal powder (such as titanium alloy, stainless steel) is melted layer by layer by laser or electron beam, or resin material is solidified layer by layer, and the gear is stacked and formed according to the gear three-dimensional model data. Complex structure gears can be manufactured without traditional molds. After forming, heat treatment is required to eliminate internal stress and improve strength.

Features:

Suitable for complex structure gears (such as topology optimization gears and internal cooling channel gears).

Material utilization rate is high, but the strength is low and post-processing is required.

(2) Cold extrusion/hot rolling forming

Principle:

At room temperature (cold extrusion) or metal recrystallization temperature (hot rolling), high-precision dies are used to apply static pressure to the blank, causing the metal to undergo plastic deformation through the die hole and directly forming the gear tooth shape. Cold extrusion relies on die precision to ensure size (tolerance ±0.02mm), while hot rolling reduces deformation resistance through temperature control and improves material utilization.

Features:

High material utilization and good tooth surface strength.

Suitable for mass production of small precision gears (such as automotive steering system gears).

(3) Electrospark machining (EDM)

Principle:

Using the pulse discharge phenomenon between the tool electrode and the gear blank, high-frequency electric sparks (voltage 100-300V, frequency 10^4-10^6Hz) are used to corrode the metal material and form the tooth surface according to the electrode shape. Suitable for hard materials (such as hardened steel and cemented carbide), with a processing accuracy of ±0.001mm, and no complex mechanical cutting force is required.

Features:

Suitable for processing superhard materials (such as carbide gears) or micro gears.

5. Selection logic and typical cases of gear manufacturing processes

(1) Core decision dimensions

Precision: Gear grinding is required for IT6 and above (such as machine tool spindle gears);

Batch: Gear hobbing/gear drawing is selected for large batches (the automotive gear line produces millions of pieces per year), and gear milling/3D printing is selected for small batches;

Material hardness: Gear grinding/gear honing is required after quenching (HRC58-62);

Cost: Casting/powder metallurgy parts are selected for low-load scenarios.

(2) Typical application combinations

Automobile gearbox gears: hot forging blank → gear hobbing rough machining → carburizing quenching → gear grinding finishing (precision IT6, load-bearing torque 5000N・m);

Home appliance gears: direct powder metallurgy forming (no cutting required, production cycle <5 minutes/piece);

Aviation gears: precision forging → five-axis gear grinding → coating treatment (surface hardness HV1200, life increased by 3 times).

6. Future trends in gear manufacturing: intelligence and composite processes

With the development of intelligent manufacturing, gear manufacturing is making breakthroughs in two major directions:

(1) Digital processing:

5 axis linkage machine tools integrate online detection to achieve a closed loop of "processing-measurement-correction" (e.g., the stability of gear processing accuracy in a smart factory has increased by 90%);

(2) Composite processes:

3D printing + grinding combination to manufacture complex structure gears, forging + cold extrusion to achieve less cutting and forming, and material utilization rate increased from 60% to 85%.

7. Conclusion

The choice of gear manufacturing process is a comprehensive balance of accuracy, efficiency and cost. From traditional casting to additive manufacturing, each process has its irreplaceable application scenarios. This article has listed in detail the various processes of gear manufacturing, and explained their processing principles and characteristics. The most suitable processing process combination can be selected according to actual applications and own needs.