Torque and HP are essentially the same thing. HP is just torque over time, and the relationship is linear.

Not exactly. Power is torque times rotational speed. And saying that the relationship is linear is a bit confusing as the speed (RPM) is not a constant itself. Take a stepper motor for instance - peak torque is at zero RPM, where the power is zero, while peak power is at some mid point RPM where torque is substantially less than its peak value.

Hence, the way Terry put it, is the correct way. You are essentially transferring torque. Power has absolutely no meaning here. I could have a 6kW high speed spindle putting out the rated power at 30,000RPM, which means only 2Nm torque, whilst a tiny NEMA 34, 25 watt stepper would have twice that torque at zero speed. Power is the amount of work you can do over time, so obviously the 6kW spindle would do more work in a given time at its lower torque than the high torque stepper.

Dan