Torque curves are useless.
Sounds a far reaching statement but lets break it down.

For a start under what testing criteria was the motor under when the curves were produced as regards the driver ? This is never published and different drivers will give different results.

Some of the charts compare torque to PPS [ pulses per second ] and some compare torque to revs. There is no standard to any of this, for a start what voltage was used? Even a different of 10 volts will make all the difference to the output.

Add to that these motors are on a continual improvement path and what ships in one container isn't guaranteed to be in the next container even though they have the same part numbers.
How can you guarantee that they even put the right motor with the right data sheet ?

99% of people at home shop user level has no way of ever checking these statements. I have literally spent hours and hundreds of pounds trying out different motor / driver combinations on various applications and just because it works for me it may not work for the next guy if he's got a slightly different setup / design.

I have seem and had large 34 motors what should have oodles of torque crawling away only to perform brilliantly when the motor has been swapped for a smaller higher speed type 23.

Compare the torque curves on these and it doesn't make any sense, that 34 has 3 times the torque of the 23.

The secret with steppers is voltage, you must have 20 to 25 times the motor voltage to get anything like performance out of them.

Many of the so called high torque motors are high voltage low amps like 5.4 volts at 2.1 amps, this puts that motor expecting 108 to 135 volts, something that's not obtainable with current drives.

Now the smaller motor rated at say 2.7 volts and 4.2 amps needs 54 to 67 volts, obtainable with the larger more expensive drives and workable with the lower voltage drives but not at peak torque.

Factor in the worse axis like the Z with it's weight and you now have a whole load of figures that even a mathematician can't work out.

It's no secret that ARC's X3 CNC kit that later morphed into the Sieg KX3 took over a year to just run all the tests to get the right motor / driver / power supply combination.

There is also a move to used switched mode supplies as these are cheap and easily imported. These are not really suited to CNC motor operation as they do not have the capacitance to hold the voltage stabe and still keep the power required on tap.

A simple power supply rated at 60 volts for something like an X3 requires a capacitor of 100 volts working and a minimum of 22,000 uf to be rated correctly. This component will cost about £30 to £35 on it's own so you can see that switched mode supplies must cut corners to arrive at the price they do.

John S.