Motor Power Woes (Please Help!)

[basskitcase]

Im am almost finished with my first router. It is a steel framed moving gantry design with chrome plated supported shafting and open pillow block bearings. The lead screws are ACME 1/2" diamter, 10 TPI and the nuts are standard type ACME nuts. (hope to get a build log put up soon with pictures )

I hooked up my stepper motors today (Hobby CNC 305oz steppers driven by the Hobby CNC Pro board, chain drive to the leadscrews 3:4 ratio (3 motor turns per 4 screw turns)) and I am having problems with the motors not having enough power to turn the leadscrews. This is especially aparant when trying to run a program (using Mach 3) as i see an axis trying to slowly move, but it just sits there vibrating slightly instead of slowly spinning. I can usually jog ok, but it seems like it wants to delay a little bit when starting

I don't have my motors set at an excessive speed. Actually, the velocity and acceleration are set very low.

I am trying to figure out why this is happening, because it seems like many people here have successfully used steppers in this capacity range. It happens with all 3 axises. The axises all slide freely when the nuts are detached.

If i turn up the amperage to the motors, would that help? (set on about 2 amps right now).

I am willing to redo the lead screws if thats what you all think needs to be done (would like to go with 2 or 5 start for speed, but if i dont have enough power to turn a 1 start, how can i turn a faster screw?)

Please Help!!! I'm feeling very disappointed right now.

[judleroy]

I'm not sure if it might be a hobby cnc issue but others here i'm sure can help you with that. I would try direct driving the screw first and see what happens. If that doesn't work try a ratio for your gearing of say 3-1. That would be 3 turns of the motor to 1 turn of the screw. With the gearing you have now you are actually decreasing the motor torque at the screw. With those motors i'd say thats a bad idea. If the motors and driver can handle more amps go as high as you can and see how much that increases your torque before you mod anything.
Judleroy

[jsheerin]

What are your speed and acceleration settings?

Roughly how heavy is your gantry?

If you remove your motors, can you turn easily move the gantry by turning the chain drive by hand? Of if you have the electronics unpowered, just turn the motors by hand to move the gantry? This would test for binding in that portion of the machine.

[basskitcase]

I don't know what the actual numbers are, but in Mach3, the velocity and acceleration bars are almost all the way at the bottom. I'd say i have it set to take at least 1 - 2 seconds to reach full acceleration, and I'm moving no more than 15 IPM.

The gantry probably weighs 30-40 pounds.

if i remove the motors, i can turn the lead screws by hand, but they turn harder than what i would expect, but i guess i was a little surprised that the motors had such a hard time with it. As far as the gantry itself, its not binding. Before i put the screws on, i made sure it would slide easily.

All this leads me to believe i have excess friction in the screw assembly, which i assume is coming from the nut. Would it be a wise choice to replace the steel nut with a bronze or delrin one?

[ger21]

Maybe the screws are binding on the nuts? If you remove the motors, you should be able to turn the screws from the ends with hardly any effort at all. If they are tight, that's oart of the problem.
As for the 2 amp setting, torque is proportional to current. I don't know what your motors are rated at, but if it's 3 amps, you're only using 65% of the motors. What voltage is your power supply?

Try setting the accel to around 4 and see if that helps. And make sure the pulse width is at least 2.

[jsheerin]

Personally I would go with a plastic nut from dumpster cnc. That's what I use on my 1/2-10 5 start screws. Those were a little stiff when I first got them, but after running them for a while they loosened up and were easier to turn. In any case, if you can turn the screws by hand, I'm a bit surprised you're having issues. If it takes 1-2 seconds to reach full speed and you're only go 15ipm, that should be a reasonable acceleration setting and your low pitch screw should be giving you plenty of force to move a gantry that heavy. So my guess would be the nuts assuming your motors and driver are working properly. I don't personally have experience with that drive though, so maybe someone else can confirm you've got that working right. The only thing I'd suggest you check there is that the motors move fine when not attached to the screws, but it sounds like you've done that already.

[hemsworthlad]

Try disconnecting the gantry from the nut and see if screws turn ok then just hold the nut and feel the resistance, you should be able to hold the nut.
If it try's to spin in your hand or stalls then it's the nut.

If still stalls when gantry not connected and not being held then just work your way back throu the drive chain checking for other cause's or miss alignment etc.
It doesnt take much miss alignment to rob you of power, I had a similar problem but my problem showed at speed when motors were at the end of there torque and the miss alignment was only 3/10th mm or so.!!

[mlabruyere]

Yeah...misalignment will get you....mine was off my a hair and the machine worked great until I got to the extreme's then it would stall.....

Setup and alignment took longer than building my machine.....but all that test, measure, align....repeat paid off...

[ilya]

15 IPM on 305 oz-in motors is WAY too low.
This is either misalignment issue or your motors not getting enough juice. What's your power supply voltage/current is?

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My DIY CNC machine plans at www.8020CNC.com

[basskitcase]

Thanks to all who answered! I found out the power I was supplying was less than half of the rated max. I guess i forgot to set it higher after the initial board testing. Now I'm running ok at 30 IPM with no lost steps as far as i can tell. I think in the future I will upgrade to Roton ballscrews, but for now i have a working machine Check out my build in the build log!

Edited:
http://www.cnczone.com/forums/showthread.php?t=98950

[hemsworthlad]

Originally Posted by basskitcase Check out my build in the build log!

Cant see it post a link.!!

[ilya]

Originally Posted by basskitcase Now I'm running ok at 30 IPM with no lost steps as far as i can tell. I think in the future I will upgrade to Roton ballscrews, but for now i have a working machine

30 IPM is still too slow. I'm using the same 1/2-10 ACMEs as you do, and my nuts are from dumpstercnc (but this really doesn't matter for now). I'm getting 150 IPM on rapids. And this is with 280 oz-in motors.
I'd check the alignment. It looks like your screw and nut are off.

_____________________________________________________

My DIY CNC machine plans at www.8020CNC.com