How hot do TB6600 drivers run normally ?

[Neale]

One thing to remember with stepper motors is that they are still taking current when stationary - this is to hold the motor in position. So the driver will still be working as well. However, a lot of drives switch to "half-current" mode when the motor is stationary so that reduces power consumption and heat dissipation a bit. However, 40C or so doesn't sound that hot to me. My motors get hotter than that in normal operation!