6436
« on: October 05, 2017, 03:00:07 AM »
Hi,
two phase steppers which are by far and away the most common move 1.8 degrees per full step, or 200 full steps per revolution.
If Mach3 or some other device pulsed the Step pin of your drive the motor would turn 1 revolution. If your stepper driver can execute half steps,
and most can, then 400 pulses are required for 1 revolution. If your drive is set to execute '8 step microsteps' then 8 x 200 =1600 pulses are
required to turn 1 revolution.
Most of us would like to believe that because we can turn an 8 microstep motor just one pulse at a time it can achieve 1/1600 th of a revolution
per pulse or 0.225 degrees per step, that is a nice fine resolution. In fact it doesn't work that way at all but nice to think that it does. Resolution is
reliably increased at half step but at quarter step it loses its reliability and accuracy. So why does everyone microstep... because steppers are smoother
when operated at 8 or 16 step microstepping.
For your application do you require very fine control? Given the gear reduction of the screw jack even 1/2 step will give you plenty of resolution.
1.8 degrees x 1/2= 0.9 degrees. 0.9degrees/6 (gear reduction)=0.15 degrees. With a pitch of 16 TPI that results in a resolution of your presshead
of 0.02 thousandths of an inch per pulse. Yeah, that's right 50 pulses for 1 thousandth of an inch...plenty good!
Steppers lose their guts when going fast, they have something like half their torque at 500 rpm. Assume you want your press to have plenty of ompf
so decide a maximum speed of your stepper is 500rpm. The maximum speed of the presshead would be 500/96=5.2 inches per minute, not really fast
but useful. Trying to work out how much ompf it got is a bit of a guess because with a large gear reduction and fine thread a large part of the input
torque it lost to friction. I would make a stab and say your 1600 oz.in will be 800 oz.in at 500 rpm and half of that lost to friction ie the actual torque
available for useful work is 400 oz.in If the diameter of the screw jack is 1 inch then the tangential force at the outer periphery of the screw is 800oz
or 50lb. With the thread mechanical advantage of (2 x PI x 1/2(radius))/ 1/16(pitch)=50.2 Therefore your presshead thrust should be in the region
of 50 x 50.5 =2513 lb. DON'T stick your finger in it! Bit of a sketchy calculation but probably close enuf for you to decide if you wish to pursue the idea.
Just goes to show what decent mechanical advantage can do.
Craig