Spiral bevel gears transmit power between intersecting shafts at right angles, and their curved teeth engage gradually, which gives them high load capacity, smooth running and low noise. They are the standard choice for differentials, machine tool spindles, marine drives and heavy gearboxes. Manufacturing them is a specialist process, because the tooth geometry is three-dimensional and every gear must ultimately work as part of a matched pair.
The process starts with the blank. Forgings or cut bar stock are turned to the required blank dimensions, including the mounting bores and locating faces that determine how the gear sits in the housing. Because all later operations reference these faces, the blank tolerances must be controlled from the first operation, and the bore is usually finished to a high grade before the teeth are cut.
Tooth cutting is the heart of the process. Spiral bevel teeth are generated on specialist gear cutting machines using face-mill or face-hob processes, where the cutter and the workpiece roll together to create the curved tooth profile. The machine settings define the pressure angle, spiral angle, tooth depth and crowning, and the two members of a pair must be cut with matching machine settings so that they mesh correctly.
After cutting, the gears are case-hardened, typically by carburizing, to reach a tooth surface hardness in the range of 58 to 62 HRC. Hardening changes the tooth geometry slightly, so precision gears are then finish-ground on the tooth flanks. Grinding restores the accuracy of the tooth profile and spacing, and is essential for high-speed or high-torque applications where noise and contact stress must be controlled.
The most distinctive step is the matching run. The pinion and gear are assembled in a testing machine and run together under a light load with marking compound, so that the contact pattern, backlash and running noise can be checked. The contact pattern is adjusted by shifting the pinion position or by fine correction of the tooth flank geometry, and the final gears are marked as a matched set so they stay together in assembly.
For buyers, the practical takeaways are: ask for the contact pattern report if the application is noise or load sensitive, confirm whether the teeth are ground or lapped after hardening, and make clear that pinion and gear must be delivered and marked as a matched set. A supplier with specialist cutting machines and in-house testing can save you weeks of trial-and-error compared with a general machine shop.