Sewing
Thread that melts in the eye is usually blamed on the thread. It is nearly always the needle temperature, and that is a speed setting.
— °C
Where heating and cooling balance
A lumped steady-state balance: it ignores the needle's own thermal mass, so it will not describe the first seconds of a seam or a short burst of high speed. The cooling coefficient dominates the answer and depends heavily on whether needle cooling air is fitted — measure it rather than assuming, and note that a blunt needle raises penetration force sharply.
Sewing Needle Heat Build-up & Thread Fusing Predictor — free, with the formula and a worked example, at Textile School.