Manufacturing gears with the required precision and quality is a complex challenge. Without the right processes, your gears may fail prematurely or not perform to spec.
Choosing the wrong gear manufacturing process can lead to increased costs, delays, and unhappy customers.
In this post, we’ll break down the key gear manufacturing processes including forming, machining, finishing, and heat treatment. You’ll learn the pros and cons of techniques like hobbing vs shaping, carburizing vs. tempering, and more.
Gear Forming
Casting
In casting, molten metal is poured into a mold cavity that defines the gear shape. Various casting techniques like sand casting, investment casting, and die casting can produce near-net shape gears. Casting allows forming complex shapes, but dimensional accuracy and surface finish are usually lower compared to machining processes.
Forging
Forging forms gears by plastically deforming heated metal blanks using high pressure in dies. The forging process can be done in open or closed dies, with the latter providing better dimensional control. Forged gears have a fine-grained microstructure with superior strength and toughness compared to cast gears. Additional machining is often needed after forging to achieve the final gear geometry and tolerances.
Extrusion
In extrusion forming, metal is forced through a die opening to create a continuous gear profile. The extruded gear profile is then cut to the required face width. Extrusion can efficiently produce long, slender gears like spline shafts. It results in a favorable grain flow pattern and eliminates transverse welds, leading to high strength and uniform material properties.
Powder Metallurgy
Powder metallurgy (PM) involves compacting metal powder in a die under high pressure to form a “green” compact, which is then sintered at elevated temperatures to bond the particles. PM can produce net shape or near-net shape gears with little to no machining. It allows using unique alloy compositions and can form gears with integrated hubs or shafts. PM gears have good dimensional accuracy and surface finish, but may have lower strength than wrought gears.
Gear Machining
Gear machining processes cut the gear teeth profile from a gear blank to achieve the required geometry, dimensions, and surface quality. Machining can produce external or internal gears, splines, racks, and other gear types from metallic or non-metallic materials.
Gear Generating vs. Form Cutting
Gear Generating
In generating processes, the gear tooth profile is progressively generated by a relative motion between the tool and workpiece that simulates the meshing action of a gear pair. The tool has a specially designed profile that corresponds to the space between gear teeth. As the tool rotates and translates relative to the workpiece, it gradually cuts the full depth of the tooth space. Hobbing and shaping are examples of generating processes. Generating can produce both external and internal gears, usually with involute profiles.
Gear Form Cutting
Form cutting processes use a tool with a profile that matches the complete tooth space geometry. The tool is fed linearly into the workpiece to cut the full tooth depth in a single pass, without any rotation between the tool and workpiece. Broaching and some milling operations are examples of form cutting. Form cutting is suitable for both involute and non-involute gear profiles, and can produce external or internal gears.
Common Gear Machining Processes
Hobbing
Hobbing is one of the most widely used gear manufacturing processes, especially for cylindrical gears. In hobbing, a hob (cutting tool) is fed across the face width of a gear blank as both the hob and blank rotate in a synchronized motion. The hob has helical cutting teeth that progressively generate the gear teeth profile and lead on the workpiece with each pass. Hobbing can produce both external and internal gears with high precision and efficiency.
Shaping
Gear shaping utilizes a reciprocating pinion cutter to generate gear teeth. The cutter, which is a toothed rack, is mounted on a ram and cuts the gear blank as the ram moves linearly up and down. Simultaneously, the workpiece rotates to allow the cutter to create the full gear tooth profile. Shaping is well-suited for both external and internal spur and helical gears, especially for large diameters and coarse pitches that are difficult to hob.
Broaching
Broaching is a machining process that uses a long, multi-toothed cutting tool called a broach to remove material in a single pass. For gears, broaching is primarily used to cut internal splines or gear teeth. The broach is pulled or pushed through the pre-machined hole in the gear blank, with each tooth on the broach progressively cutting the gear profile. Broaching offers high productivity and consistency but is limited to simpler gear geometries.
Milling
Gear milling involves using a multi-toothed milling cutter to machine the gear teeth. The cutter, which has a profile matching the gear tooth space, rotates as it is fed across the face of the gear blank. Indexing is used to rotate the blank to the correct position for each tooth space. Milling is versatile and can handle larger gears and more complex tooth geometries than hobbing or shaping. However, it is generally slower and less economical for high-volume production.
Gear Finishing Processes
Grinding
Gear grinding is an abrasive machining process used to improve gear accuracy, surface finish, and reduce noise. Grinding corrects distortions from heat treatment and removes nicks or burrs. The two main methods are form grinding, which uses a profiled wheel to grind the whole tooth form at once, and generation grinding, where the grinding wheel traverses across the rotating gear blank to generate the tooth profile. Grinding is crucial for precision gears used in demanding applications.
Lapping
Lapping is a low-speed, low-pressure abrading process that improves surface finish and removes minor imperfections on gear teeth. The gear is run in mesh with a master gear or lapping compound at low speeds to polish the tooth surfaces. Lapping can correct small errors in tooth profile, spacing, and lead. It is often used after heat treatment to achieve the required surface quality and reduce gear noise.
Honing
Gear honing is a fine finishing process that uses abrasive stones or rings to improve surface finish, remove nicks or burrs, and correct minor irregularities in tooth profile. The honing tool is run in mesh with the rotating gear at low speeds and light pressure. Honing is similar to lapping but uses fixed abrasives instead of loose compounds. It is commonly employed to achieve very smooth surfaces and tight tolerances.
Shaving
Shaving is a free-cutting gear finishing operation that removes a small amount of material to improve tooth accuracy and surface finish. A hardened and ground shaving cutter, similar to a rack, is run in mesh with the rotating gear blank under light pressure. The cutter has serrated teeth that shear off a fine layer of metal, correcting errors in tooth spacing, lead, and profile. Shaving is often performed after heat treatment but before grinding.
Burnishing
Burnishing is a cold-working surface finishing process that uses hardened rolls or gears to compress and smooth the tooth surfaces of a softer gear. As the burnishing tool and gear are run in mesh, the high contact pressure plastically deforms surface irregularities, improving surface finish and durability. Burnishing also imparts residual compressive stresses that enhance fatigue resistance. It is commonly used on non-ferrous or case-hardened gears.
Heat Treatment Processes
Carburizing
Carburizing is a case-hardening heat treatment that diffuses carbon into the surface layers of low-carbon steel gears. The gears are heated in a carbon-rich environment, allowing carbon to absorb into the surface. After quenching, the high-carbon surface layer transforms into hard, wear-resistant martensite, while the low-carbon core remains tough and ductile. Carburizing improves gear strength, durability, and fatigue resistance.
Hardening
Hardening is a heat treatment process that increases the hardness and strength of steel gears. The gears are heated to a specific temperature (austenitizing temperature) and then rapidly cooled, usually in oil or water. This transforms the microstructure into hard martensite. Hardening is essential for gears to withstand high stresses and wear. However, it can also cause distortions that may require post-hardening finishing processes.
Tempering
Tempering is a heat treatment applied after hardening to relieve internal stresses, increase toughness, and reduce brittleness. The hardened gears are reheated to a specific temperature below the critical point, held for a certain time, and then cooled. Tempering allows a controlled amount of martensite to transform into tempered martensite, which has a better balance of hardness and toughness.
