“Scaling” in gear manufacturing refers to the oxide layer or surface decarburization that forms on gear surfaces during heat treatment. When gears are carburized or quenched in a furnace, high temperatures cause surface oxidation, leaving a brittle, dark-colored scale layer typically 0.1–0.5 mm thick.
This scale resulting from gear scaling is problematic for several reasons:
- First, it must be removed before final grinding or assembly—otherwise, it causes premature tool wear, dimensional inaccuracies, and poor fit with mating parts. Removing scale usually requires shot blasting or chemical pickling, which adds both cost (typically $10–$50 per gear) and lead time (2–5 days).
- Second, if the shop does not adequately control the furnace atmosphere, decarburization can occur, reducing surface carbon content and resulting in a soft, low-hardness outer layer. This is a hidden quality issue that only becomes evident during hardness testing—and often requires scrapping the entire batch.
- Third, the scale can distort the gear enough that even after grinding, the tooth case depth may be insufficient, compromising fatigue life. To avoid scaling costs, some shops use vacuum heat treatment or controlled-atmosphere furnaces (with endothermic gas), which minimize oxidation. However, these technologies are more expensive.
Always ask your supplier how they handle gear scaling and whether they use shot blasting as a standard process—some shops quote one price but later add a “descale service” as an extra.