A custom external thread 0.04 mm deep with a 0.1 mm fit on a d8 shaft should cut to a root radius of 3.86 mm or less but cut to 3.9466, because the kernel guessed the cutter side from the profile's extents and, when depth plus fit was smaller than the cutter overshoot, took an external thread for an internal one and moved the relief outward, with no rebuild error.