You need the motor torque constant. It's sometimes called Kt and/or Ke. This parameter will let you relate current to torque. Then you can work backwards, starting with the torque you want, figure out what the current must be, and then ohms law will tell you what voltage will push that current.
That driver will work fine, provided you use a proper power supply.
The allegro stepper-drivers are current-limited chopper stepper drivers. As such, you only have to ensure the power-supply voltage for the driver is > then the rated voltage on the stepper, and you have set the current limit properly.
Basically, chopper-stepper-drivers actually modulate ("chop") the drive voltage to the stepper in real-time to maintain a fixed coil current.
The ratings for your motor are steady state. Basically, it says that if you apply 2.55V DC, 1.7A of current will flow though the motor coil.
However, the Allegro drivers don't apply DC, they apply a duty-cycle modulated square wave, which limits the overall power delivered to the motor.
Functionally, the driver will vary the applied voltage to the stepper to maintain a fixed current (it's not quite that simple, motor inductance is involved, but it's a reasonable simplification). As such, as long as you're not applying more then 1.7A of current to the motor, it will work fine.
Basically, the simple version is the motor ratings are basically constrained by the thermal behaviour of the motor. If you apply too much power, it'll get hot enough to damage the motor.
With the A4988 driver board you link, you can vary the motor current by adjusting the tiny pot, which allows you to adjust the motor power to whatever you'd like.
If you run the driver off input DC within it's operating range, you will be fine.
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But on the line right under the amps/phase of 2.1A, there's a line that says the coil resistance is \$1.6\Omega\$. \$2.1\mathrm A \cdot 1.6\Omega = 3.36 \mathrm V\$, so there you go.
This is only correct if the motor is at rest. Stepper motors, like any other permanent-magnet motor, generate a torque that has a component that just comes from the motor construction, and a component that's proportional to current, not voltage. They also generate back EMF. So if you apply a constant-voltage step and move the motor rapidly, the available torque would go down.
How much back EMF you ask? Well, fortunately, you can figure this out. If a motor is perfect, then its torque constant is exactly equal to its speed constant -- this actually falls out of conservation of energy, because voltage * amps = speed * torque. If the holding torque really is only from coil current, then the torque constant is 0.65 N-m / Amp, which means the speed constant is 0.65 Volt / (radians/second) -- so you can work out what the back EMF is at any speed you may be driving the motor.
Note that this only gives you a rough estimate -- stepper motor datasheets usually leave a lot to you to figure out. I'd figure that number is correct to +0%, -20% or so, and if I really needed it to be something specific I'd get a sample motor and test.
That depends. If you use a simple motor driver that just switches the supply voltage, and the motor is moving slowly, you'll drive way more current (around \$12\mathrm V / 1.6 \Omega\$, in fact) through the motor. It'll be way more than it's designed for and the motor will burn up.
Torque will be greater, though -- briefly. Then it'll be much less -- permanently.
No. The equation isn't "either 2.1A or 3.36V", because the motor generates back-EMF. If you drive the motor with a constant-current stepper driver (which you can design yourself, build from chips designed to do the job, or buy as standalone boxes) then the driver will pulse the motor with 2.1A pulses to the best of its ability. "The best of its ability" means that it'll drive 2.1A up to some voltage that's limited by your supply voltage and the capability of the driver.
That, in turn, means that you'll get maximum torque at any speed up to some maximum limited by the driver and its supply voltage, at which point the torque will drop off.
Using my back-EMF estimate from above, if you're supplying 12V to a driver that has zero overhead voltage (i.e., it can deliver 12V pulses at 2.1A), then you could drive the motor at 120 RPM at maximum torque.