Lapping removes so little material that it has no measurable effect on case depth or surface hardness. Let me explain why.
The lapping process removes material in micrometers. A typical lapping cycle removes between 2 and 5 microns from the tooth surface. Some aggressive cycles might remove 8 to 10 microns, but that is rare. To put that in perspective: one micron is 0.001 mm. Your carburized case depth is 0.6 mm, which is 600 microns. The material removed by lapping is less than 1 percent of the total case depth. Even after lapping, the tooth surface remains fully hardened. The hardness at the surface is still 58-62 HRC because you have only removed a tiny fraction of the case depth.
What about smaller gears with thinner case depth? For a very small gear with module 1.5, case depth might be 0.3 mm (300 microns). Lapping removes 2 to 5 microns – still less than 2 percent of the case depth. Even if you lapped the same gear ten times, you would still have over 80 percent of the original case depth remaining. In reality, gears are lapped once. So there is no risk of exposing the soft core.
However, there is one situation where you need to be careful. If your heat treatment process produces a very shallow case depth – say, below 0.15 mm (150 microns) – and you use an aggressive lapping cycle that removes 10 microns, then you are removing about 7 percent of the case depth. That is still acceptable for most applications, but you should be aware of it. The general rule is this: as long as your case depth is at least 10 times the lapping removal amount, you have no problem. For standard carburized gears with proper case depth (0.5 to 1.5 mm), lapping removal is less than 1 percent of the case depth. The gear remains fully hard.
There is another benefit that is often overlooked. The lapping process actually work-hardens the tooth surface slightly. The abrasive particles roll and slide between the teeth, creating compressive residual stresses in the surface layer. This is similar to a mild shot-peening effect. Some studies have shown that lapping can increase surface fatigue life by 10 to 15 percent – not because it makes the gear harder, but because it removes stress-raising high spots and introduces beneficial compressive stress. So lapping does not weaken your gear. It makes it stronger, smoother, and longer-lasting.
The real risk is not lapping removing too much case depth. The real risk is poor cleaning after lapping. If abrasive particles remain embedded in the tooth surface, they will act as cutting tools during operation and rapidly wear through the case depth – exposing the soft core in weeks or months instead of years. Thorough cleaning after lapping is far more important than the tiny amount of material removed during lapping itself.