Stepper Motor Holding Torque

This is the maximum torque that the motor can provide with both windings energized at full current before it starts jumping steps. The holding torque with one winding energized at the rated current is about 1/sqrt(2) times that.

Torque

The torque is proportional to current (except at very low currents), so for example if you set the drivers to 85% of the motor rated current, then the maximum torque will be 85% * 0.707 = 60% of the specified holding torque.

Motor Windings

Torque is produced when the rotor angle is different from the ideal angle that corresponds to the current in its windings. When a stepper motor is accelerating, it has to produce torque to overcome its own rotor inertia and the mass of the load it is driving. In order to produce this torque, the rotor angle must lag the ideal angle. In turn, the load will lag the position commanded by the firmware.

Microstepping

Microstepping can reduce torque. What this really means is that when the lag angle is assumed to be equal to the angle corresponding to one microstep (because you want the position to be accurate to within one microstep), higher microstepping implies a smaller lag angle, hence lower torque. The torque per unit lag angle (which is what really matters) does not reduce with increased microstepping. To put it another way, sending the motor a single 1/16 microstep results in exactly the same phase currents (and therefore the same forces) as sending it two 1/32 microsteps, or four 1/64 microsteps, and so on.