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Gear Reduction question.
« on: October 04, 2017, 07:53:17 PM »
Hi ,

Trying something new , Mach3 controlled single axis (Nema-34 1600/oz) Press.
I thought with all these extra parts I have laying around I would try to fabricate a stepper driven press. Unlike most using the HF 20ton H-press/bottle jack method
I would try driving a 5-ton screw jack w/gear reduction. Using a 15amp 24vdc power supply and DQ860MA microstep driver running full 7.8amps I just need to drive this
single axis a few inches up/down and get enough tonnage to bend 16ga mild steel.  My problem is the screw jack has a mighty 6:1 gear ratio and the screw needs to spin
16 revs to equal 1.0inch.  This is where things get sketchy for me. So I figure the steeper motor would need to spin 96 revs to move the screw one inch. Thats ok but I'm new to mach3
and stepper drives , I'm looking for advise on what would be the best and most accurate way to figure out what would give me the most torque and get the desired accurate displayed motion.
My drive can run bewteen 400 and 51200 pulses/rev.  Thank you in advance.
Re: Gear Reduction question.
« Reply #1 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
My wife left with my best friend...
     and I miss him!
Re: Gear Reduction question.
« Reply #2 on: October 05, 2017, 07:01:15 AM »
Craig,

Thank you for the very informative response. 2500lb's should do the trick , I was hoping to achieve 3T , but this powers the pursuit anyway. I will continue the build.
I was playing around with Mach3 last evening and it was difficult to find to the sweet spot of speed and torque without loosing steps. Has anyone played around with the
HT (High Torque) Nema-34 motors , I could boost the tonnage slightly with 1800+oz motor. running the servo drive at full current really got the motor hot !! little concerned
about that ?
Thanks again for your quick response.
-Dennis
Re: Gear Reduction question.
« Reply #3 on: October 05, 2017, 07:11:12 AM »
Your motors have a current rating. Exceeding that continuously can damage the motor by demagnetizing it. It will still run but with way less torque. Steppers are continuous power devices so they consume the same power whether driving a load or not. So they heat up. Most decent stepper drivers reduce the current when not actually moving the motor.
Re: Gear Reduction question.
« Reply #4 on: October 05, 2017, 07:31:23 AM »
Hi,
2500 lb is just an informed guess. The actual stall pressure could well be more, much more.

If using half stepping the motor motion will be pretty harsh. The main reason for using microstepping is for smooth motion and the
illusory sense of increased resolution is very seductive however false it may be. You could try using 8 or 16 microstepping, the increase in smoothness
of motion may mean that its easier on the ear and may also be cooler running.

Steppers do run pretty warm, I back the current off on my motors to have them run a little cooler but they still get to 50-60 degrees C on the outside
and I imagine closer to 100 degrees inside....doesn't seem to do any harm. Whether they would stand that 24-7 as in industrial use I suspect not
but they have given me about 1000 hrs a year for several years so can't complain.

Craig
My wife left with my best friend...
     and I miss him!

Offline ger21

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Re: Gear Reduction question.
« Reply #5 on: October 05, 2017, 06:55:43 PM »
Quote
and it was difficult to find to the sweet spot of speed and torque without loosing steps.

The most torque (the rated torque) is when they are not spinning. The faster you go, the less torque they will have. Those motors probably lose their torque faster than any other motors you can buy, due to their high inductance.
To get more torque at higher speeds, you need to increase the voltage. To get the most out of those motors, ideally, you'd need well over 120V. 24V will give very poor performance at all but the slowest speeds.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Gear Reduction question.
« Reply #6 on: October 06, 2017, 06:56:19 AM »
Thank you for this input. I currently have a 15amp 24vdc supply source at the stepper driver. What your saying is go to say a 48vdc supply voltage and I can achieve more RPM's and torque ?

Cheers.
Dennis

Offline ger21

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Re: Gear Reduction question.
« Reply #7 on: October 06, 2017, 07:10:30 AM »
Yes, but if possible, I would go to 60-72V. The more the better.
And you only need about a 6 amp supply for the one motor.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Gear Reduction question.
« Reply #8 on: October 06, 2017, 09:48:52 AM »
The stepper has max 1600 oz-in torque.
More volts will make it accelerate faster .. but will never pass the stall torque.

1:6 gear reduction = 1/6" lift on a screw, be it acme or ballscrew.
Acme/etc == 30% efficient.
So you get about 500 oz-in / 1/6", around 25.4/6 = 4.23 mm screw rise.

This is what I had:
At 3 Nm on 1:3 on a 5 mm ballscrew 90% efficient thrust is == 600 kgf, or 6000 N.
About 1500 lbs of force.

Thrust force is about (efficiency, 30% on acme or gears-your case) x rise x torque, all numbers normalised to same units.
I use Nm / rise in mm, etc. in metric.

On 500 oz-in == 4 Nm effective, and 4.2 vs 5 mm effective, about 1.3 x 1.2 == 936 kgf force, 2000+ lbs lift force.
Re: Gear Reduction question.
« Reply #9 on: October 06, 2017, 10:09:09 AM »
I actually do have inventory to try , located 80vdc 6amp supply.
The Joycedayton screw jack specs:
Screw Dia:1.5" Not ACME actual ballscrew and Nut.
thread picth:1.0"
Grear Ratio : 6:1
worm shaft turns for 1.0"  = 6turns
This by no means will be a quick press , My target was 3Tonnage , I would be happy if this setup yields 2T.
Ideal speed 10ipm , realistic will likely be in the 5-7ipm ?

I appreciate everyone's input.

Thanks
Dennis