Hello all:
I am trying to choose a stepper motor and reduction gearing setup for a 9x49 BP clone. I am attempting to approach this from an engineering perspective as opposed to a guess-and-check or see-what-others-did standpoint, then see how my results compare to what works for others.
Design Constraints:
My goal is 20ipm minimum with a 30ipm optimum.
My power supply will be dedicated for the z-axis and will be matched to the motor.
Total weight of knee/saddle/table/vise/workpiece ~ 1100lbs.
I would like to keep the setup under $1k (I'm designing and building the brackets and guards).
Before I start:
1) Yes I understand that using the knee is not optimal for drilling and certain milling/contouring operations.
2) Yes I understand that the cast iron knee and cast iron column way/gib interface is not designed for high speed repeated movement and can accelerate wear and/or seize if driven too hard too often.
3) I am designing for use without gas springs deliberately as a sort of insurance policy. If I find that I have under-designed the system, I will add the springs to assist.
My understanding thus far:
-It is more important to match a stepper's power output (wattage) to the application as opposed to matching the torque.
-The maximum power output (wattage) for most steppers is just past the corner speed.
-Bipolar parallel wiring extends the corner speed out farther along the RPM range, effectively creating a longer flatter torque curve and more usable high-torque RPM.
-Increasing the power supply voltage in multiples of the stepper's rated voltage will extend out the torque curve with greater multiples providing diminishing returns.
-It is optimal to use a constant wattage/variable voltage power source as opposed to a fixed voltage power source.
-High amperage low impedance motors are designed for higher RPM use.
-Higher RPM at power (wattage) peak = higher gearing = more usable torque.
My approach to motor selection and gearing:
1) Find the PPR at the stepper's power peak just past the corner speed.
2) Calculate the RPM at peak, and determine the pulley ratio necessary to drive the gearshaft at 200RPM (20ipm at 10:1 shaft ratio).
3) Multiply torque output by pulley ratio to give approximate effective torque.
4) Calculate upwards force on the table using bevel gear/acme screw drive ratio, effective torque, and approximate drive assembly efficiency (acme screws are 20-40% efficient, bevel gears are 98-99% efficient + some losses in the bearings, friction in the ways, etc). I used 25% as a (hopefully) real world estimate.
5) Use the torque/force results to select motor and gearing.
I have compared NEMA 34s rated from 960-1810oz-in thus far as well as some NEMA 42s up to 2830oz-in. I am somewhat restricted in that not all stepper retailers publish legible torque curves (and even fewer publish power curves).
Thus far my standout system (keeping within my price range) seems to be an Anaheim automation 34Y314S 1700 oz-in NEMA34 at 9.9amps and 3.5mH using their constant power driver MLA10641.
This setup puts out an effective torque of 3965oz-in at 20ipm and 2643oz-in at 30ipm (3.5:1 and 2.4:1 ratios respectively). This provides a lead screw upwards force of 3892lbs for 20ipm and 2594lbs for 30ipm.
This is contrasted to lower amperage higher inductance offerings such as a NEMA 34 1810oz-in Keling setup at 8.8amps/5mH putting out 2098oz-in at 20ipm at 100V (1.6:1 & 2059lbs force).
Also contrast a Keling NEMA 42 2830oz-in 6amps/26mH running at peak power at 20ipm & 100V will only put out 1614oz-in (0.76:1 & 1584lbs force). Direct drive will only net 15ipm at peak power.
So I ask some of you veterans out there:
Am I on the right track?
Am I missing anything major? Elephant in the room perhaps?
Thanks much for any and all constructive input!