As a general rule you should use the lowest step rate possible to get the most torque out of your steppers and maintain the resoulution needed (most gantry type routers only need .001 or less, most mills need .0005 unless you are doing super precision work)
Torque drops off rapidly from full step and by the time you reach 1/8 step you are only getting @ 20-25% of your rated torque.
The best solution is to gear your motors thru zero backlash belts and pulleys to use full or half step.
Dean |