My MDF CNC, need help with steppers

[untenops]

Ive designed and got most of my MDF CNC machine built. I thought I might be able to get away with using some unknown steppers I took out of some old epson dot matrix printers. But they only seem to be able to move the axis sometimes, at other time they skip steps. Of course right now I'm still using 1/4-20 all thread, not very efficient. I plan to switch to 1/4-16 or 3/8-12 ACME but I still don't think these steppers will work very well. But I have found some PK266-02A for a good price, about $25 each. Does anyone have and experience with these motors in a similar style machine? Any thought? Should I have any hope that they will work? Ive obviously skimped a few places, (all thread, steppers, and even driver board) but right now I'm building on a tight budget. Need to prove that I can get this thing to work before I spend a lot on better part.

Thanks for any help, advice or comments.

[gmfoster]

Originally Posted by untenops Ive designed and got most of my MDF CNC machine built. I thought I might be able to get away with using some unknown steppers I took out of some old epson dot matrix printers. But they only seem to be able to move the axis sometimes, at other time they skip steps. Of course right now I'm still using 1/4-20 all thread, not very efficient. I plan to switch to 1/4-16 or 3/8-12 ACME but I still don't think these steppers will work very well. But I have found some PK266-02A for a good price, about $25 each. Does anyone have and experience with these motors in a similar style machine? Any thought? Should I have any hope that they will work? Ive obviously skimped a few places, (all thread, steppers, and even driver board) but right now I'm building on a tight budget. Need to prove that I can get this thing to work before I spend a lot on better part. Thanks for any help, advice or comments.

Well for proof of concept you might get away with those motors but you are going to have to go real slow. With all thread like you have you have to spin motors fast to get anywhere and steppers lose almost all their torque at speed.

One thing to do is take the motors and place a piece of tape on them as a flag. Now step them using your cnc program and return to Zero. You wan find what speed you can spin your motors with no load. You may well be surprised at how slow you have to go to keep from losing steps.

Now resonance can enter into this but all in all steppers have their torque at quite low rates. So with all thread you have to spin fast to go anywhere and with small steppers you have a double whammy. I suspect to keep from losing steps with that setup you may well be at 10 IPM....I could be way wrong....

I was referring to the motors from your Epson I am not familiar with the motors you are thinking about.

Garry

[pfred2]

You mean like these?

http://www.mpja.com/prodinfo.asp?number=17748+MS

I have a few PK266-02B motors myself. I think they're a little on the weak side.

http://img683.imageshack.us/img683/6498/stepproto1.jpg

I plan on using two to drive my Y axis. The A is stronger than my Bs are though.

And two of these on the X:
http://img580.imageshack.us/img580/8...hqnoflash1.jpg

Which are twin stack bipolar and at 220 oz/in are stronger than the 266s. They cost me $14.95 each. So, if you're looking for the most power for the buck I think you can do a bit better than you've found so far. Though Vextas are nice motors.

[untenops]

Originally Posted by pfred2 You mean like these? http://www.mpja.com/prodinfo.asp?number=17748+MS I have a few PK266-02B motors myself. I think they're a little on the weak side. http://img683.imageshack.us/img683/6498/stepproto1.jpg I plan on using two to drive my Y axis. The A is stronger than my Bs are though. And two of these on the X: http://img580.imageshack.us/img580/8...hqnoflash1.jpg Which are twin stack bipolar and at 220 oz/in are stronger than the 266s. They cost me $14.95 each. So, if you're looking for the most power for the buck I think you can do a bit better than you've found so far. Though Vextas are nice motors.

Wow, thanks for the link I thought I was getting a good deal at $25. oh well, only bought one. Gives me a chance to test it I guess. What model are you talking about with 220 oz/in?

[untenops]

Originally Posted by gmfoster Well for proof of concept you might get away with those motors but you are going to have to go real slow. With all thread like you have you have to spin motors fast to get anywhere and steppers lose almost all their torque at speed. One thing to do is take the motors and place a piece of tape on them as a flag. Now step them using your cnc program and return to Zero. You wan find what speed you can spin your motors with no load. You may well be surprised at how slow you have to go to keep from losing steps. Now resonance can enter into this but all in all steppers have their torque at quite low rates. So with all thread you have to spin fast to go anywhere and with small steppers you have a double whammy. I suspect to keep from losing steps with that setup you may well be at 10 IPM....I could be way wrong.... I was referring to the motors from your Epson I am not familiar with the motors you are thinking about. Garry

Thanks, I got one motor installed now and the little Epson motors have a hard time running at 6 IPM. Now thats slow. I'm taking a guess and thinking they maybe 60oz motors, maybe less there 1.5" square. As for the all thread, yeah I know, later I plan to switch to ACME, 1/4-16 or 3/8-12, not sure yet.

[untenops]

Ok I have another question, this time about my driver board. I have a 3 axis stepper control board that is based on the TA8435. But the little info I have on it is not answering my question. So Ive been looking at the data sheets for that chip.
http://www.toshiba.com/taec/componen...//382/3537.pdf
But for some reason I can not wrap my head around the current sensing mentioned on page 21 of that data sheet. Is this chip capable of limiting the current supplied to the stepper motors? If so how do I make use of this? Does pin 10 need to be held low for this to work? What size resistors do I need to use on pins 18 and 21? Right now the board has 5 ohm resistors there. If I'm reading the data sheet right, I think that limits current to 1 amp. Maybe? Using some common sense I would think I would need a 0.8 ohm resistor there to limit 1 amp.
Here is a little more information on what I'm trying to do. Right now I have a stepper motor hooked up that is 12 ohm and a 16 volt power supply. So I should be drawing round 1.3 amps. My meter is reading anywhere from .8 to 1.3 amps total from the power supply with one stepper active. Ok? And it works fine but I need to use some steppers with a bit more power. So Ive ordered one to try out that is 3.6ohms. Well at 16 volts that over 4 amps, to much for the 8435 to handle. But if it limits current I should still be able to use it, I think. Please help.

[pfred2]

It depends on what your reference voltage is called VREF in the datasheet (that'd be the voltage on pin 10). Usually there is a 5 volt source used that also supplies the logic power the chip needs.

I'm no math whiz but even I can handle an equation like this one:
IO = VREF/RNF

1 amp = 5 volts / 5 ohms

Something divided by itself is 1. That works out for me. And yeah, the reason the current sensing is going on is because the TA8435 does PWM current chopping among its other tricks. I hope that helps you.

[untenops]

Thanks for the info pfred2. Last night I got in the stepper I ordered to test a PK266-02A. It is rated at 3.6 ohms when run bipolar series. And yes I'm drawing only 2 amps stopped, even at 30 volts. So the TA8435 is set to limit current at 1 amp per phase with 5 ohm resistors. Now my driver is the week link it looks like, as its limiting to one amp and the stepper can run at 1.4 amps. Also I'm running pretty much at my drivers limits voltage wish as well. And since these motors are rated at 10 mH I should be running close to 100 volts if I'm figuring it right. Might be why I cant get the stepper to run at more then 40 rpm before it stalls. Just stops spinning and make noise. Or maybe my driver just stinks as the data sheet for this motor shows it still at 100 oz in when spinning at 200 rpm @ 24 volts.

[pfred2]

Maybe. I never used a TA8435. I don't really think you need to run at 100 volts to go faster than 40 RPM. I know my TB6560s can run 2500 RPM at 24 VDC and 2 amps. As can drivers I've made using SLA7026Ms, running PK266-02B motors. Those I actually limit to about an amp.

It sounds to me like you're fishing for excuses as to why things aren't working for you with this 10 mH 100 volts stuff. I'm sure there is another very real reason your motor isn't running to speed.

If you really want help you have to say how you are generating your step pulses, and what sequence mode you are trying to run your motor in (that'd be the states of pins 8,9 on your driver IC). Maybe with that information someone could make an informed guess as to what you are doing wrong.

You don't have to use 100 volts to go over 40 RPM though thats for sure! If that was the case no one would be going anywhere now would we? I'm going to go out on a limb and say unless there is a lot of drag on your motor it isn't even the current limit that is holding you back. Steppers won't draw lots of current running unless they are really loaded down.

[untenops]

Yes I'm fishing and grasping at straws. I figured the 10mH thing was not really the problem but thought I would mention it. Also I was way off with my math as well. When I took a look at it again yesterday, it looked like my motor was turning a lot faster then 40 rpm. Rechecked my math and my top speed seems to be 240 rpm not 40. So thats better. My software in setup mode will only let me test speed in .1in/s increments. So I seem to be able to run at .2in/s that .1 * 60 = 6 to convert to ipm. Then 6(ipm) * 20(for my thread pitch) = 120rpm. So when I run at .2in/s that should be 240 rpm. But If I step it up to .3in/s the motor stalls. I might understand this if I had a load on the motor, but right now its not connected to any load.
As far are how I'm generate my steps, I'm not sure what your asking for here. I'm using EMC2, if that helps. And for pins 8,9 I've tried every combination of high and low here. But usually have these set low, think thats normal 2 phase full steps. So am I expecting to much from this motor?

http://catalog.orientalmotor.com/ima...66-02b_stc.jpg

Thanks again for any help.

[pfred2]

my top speed seems to be 240 rpm

240 RPM then resonating sounds to me like you are running in full stepping mode. Switch to 1/2 or 1/8th mode and you should be able to run the way the you want to.

As far are how I'm generate my steps, I'm not sure what your asking for here. I'm using EMC2, if that helps.

Of course it does. In Stepconf under Driver Timing Settings, what step time do you have? My TB6560 doesn't run below 5096 ns. Step Space doesn't seem to matter for me.

am I expecting to much from this motor?

I don't think so. I think you're just learning about full step resonance. Like I said go to another mode selected by pins 8 and 9 on your IC. Full step runs out of gas right about where you are! Enjoy.

[untenops]

I finaly got the larger VEXTA PK266-02A stepper motor mounted and tested. Works a lot better, about twice the speed of the little ones I had and much more power.