Been lurking on a few forums for a while before taking the plunge and finally realising how little I know !
Odd, the more you know the more you realise what you don't know...
Anyway, have slowly been getting together a few bits and bobs for fleabay etc to see if I can mash together something resembling a 3 axis cnc rig.
So Arduino in hand, a couple of stepper motors, some H-Bridge L298N driver boards and a 12v power supply, I thought I'd see if I can get some movement out of my steppers. Which is where my problems started, I have to be honest, I did not realise that NEMAxx is a chassis standard (best way I can think of describing how I understand it), so when I saw a couple of NEMA17's on ebay I hit buy it now and happily sat looking at them, realsing I had some significant questions about them, voltage, steps, ampage, deg etc.. So a few hours of t'internetting later, and am not much wiser..
The steppers are 17PM-K112-02W from Minebea.
Regardless of my lack of getting enough knowledge, I hooked one of the badboy's up and did some digging around for some code, 30 mins later, erm.. well, ok, it "sort of" turns.. Heat sink on L298M gets too hot to touch, so have switched it off before I need to replace it !
Is anyone able to point (in simple terms) what I am doing wrong, apart from guesswork and lack of knowledge :)
How do I calculate the voltage of the power supply required ?
How many steps per rotation ?
Anything else I shoudl know that I have not asked ?
To be honest for now, I'd just be happy getting a few motors doing what I want (ideally spinning), as am on a budget so tight it's incredible but am hoping by summer to have something, even if it just draws on paper..
Thanks In Advance
Do you have a datasheet for those motors? The 112 designation seems to be a special, it's not in the catalogue. They are 200 steps/rev. Low torque motors by cnc standards. You got these off fleabay @ £6.50 each?
What L298 driver boards are they? Most of the shield ones have uselessly small heatsinks.
Sorry to say this, but you'll need to treat these steppers and maybe the shields as a throwaway learning experience. At best you can create a 'learning toy' but for any real cnc work they're useless.
However, happy to help you 'get them turning'. What electronics expertise do you have and what test equipment do you have to hand?
First thing we need to do is try and work out what spec those steppers are and whether you have wired them up right.
Last edited by irving2008; 25-03-2014 at 08:31 AM.
The Stepper Motors are : E-Bay item 271431291269, no datasheets that I could find, searching the links the seller put with them, as with you no 112, or for that matter the ending W which supposedlt designates Unipolar or Bi-Polar is not listed
And L298 Driver boards : E-Bay Item 301045130656
I did have a breakthrough last night and got the steppers moving using a 5v source, and the heatsink did get a "tad" warm ! So will look at upgrading it and maybe a small fan off an old graphics card put to use.
Perhaps useful for a 3D printer project?
Basically you're relying on the nominal resistance of the stepper motor winding to do the current limiting. I don't know the resistance or inductance of those motor windings (can you measure them?) but for an example calculation lets assume 1.5ohm and 10mH (educated guesses looking at other frame sizes with a 112 winding designation).
The L298 will drop about 3v @1A and 5v @2A. At 2A the motor winding will drop 1.5 * 2 = 3v so any supply voltage to the L298 >8v is going to fry it, so on 12v its going to get very hot very quickly!
At 1.2A the voltage across the motor is 1.2 * 1.5 = 1.8v and, on a 5v supply, across the L298 its 3.2v. The L298 will be dissipating (both windings active) 3.2 * 1.2 * 2 W = 7.7W. That tiny heatsink is probably no better than 5degC/W so the heatsink/case will be @ 20 + 7.7*5 = 60degC and the junction @ 60+ 3 * 7.7 =88degC
So 5v is do-able. But thats static holding torque, not motion.
As soon as you start stepping the inductance comes into play. Your torque will, crudely, be halved when the impedance of the inductor equals its resistance. At 1.5ohm and 10mH that's about 240rpm.
Last edited by irving2008; 26-03-2014 at 07:51 AM.
I've seen this route taken so mant times all with same negative outcome and To be honest Other than for the learning experience your just going to burn money messing around like this and again being honest the learning experience isn't really relative to using a CNC machine or getting one running has modern drives are simple to setup and very cheap for low spec drives which will still be far more advanced than anything you'll make.
Far easier and cheaper to just save up and buy the real thing.!!
Thanks very much for the replies gents, once I saw the motors running I sort of guessed the usability of the motors was going to be quite limited to 3d printing, or maybe laser cutting, being as am on a very tight budget and have the neccessary item for the route I will probably go down it and forget about the possibility of prototype PCB routing. So in a way the money isn't wasted, I should learn plenty on the way, especially for when I do get the funds to build something much better and able to handle loads.
But, being positive, I have already learned something :)
Being as am not expecting the table to move more than 150mm x 150 mm I am planning on keeping with the cheap/learning amd using drawer sliders, which yes I know are far from ideal, I will also using threaded bar with long nuts (freebies) which again, while not being highly accurate, should provide me with movement..
Originally I was hoping to build a RepRap of one sort or another, but due to illness, my daughter is taking a year out of Uni which is where I was going to get the parts done on their 3d printer for material cost only, for which I think the parts I have would use been quite servicable.. Was also going to be able to get use of their laser cutter which I could have made a nice frame etc.. but hey ho..
the learning experience isn't really relative to using a CNC machine or getting one running
Far easier and cheaper to just save up and buy the real thing.!!
those steppers have a torque of probably 0.4Nm @ 2.5A. You're running them at best at 1.2A so you'll get maybe 0.2Nm static torque. At 240rpm this is reduced to 0.1Nm as per my previous post.
With the frictional inefficiencies of drawer slides and threaded rod you'll be lucky to get any movement at any useful speed.
I'm not being negative here I'm being realistic and speaking from seeing countless others waste the same time and money. You have Wasted £30 in the usablity terms and learnt nothing other than electronic experience that isn't relative to building a Modern CNC machine.
You speak about draw runners and threaded Rod without even realising the impact those things have.? They make all the above and what's gone so far even more pointless because your just digging a bigger hole. They are so inefficient your motors and therefore weak home brew drives will need to be driven so Much harder and they won't stand that workload without even more money and time is invested. Very soon your at the same money as buying the real thing and still have piss poor drives and cronky frustrating machine.!!
Like I say I'm not not being Negative and I also see where your coming from with doing it for the learning experience which is great. . . . BUT CNC is bit like the "Lord of the Rings" it will soon become your "Precious" and your under it's control.! . . . All the time it's trying to draw you to it's Master while dragging you thru the wasted lands of Electronic Doom sapping your will and money.!! . . . . . . But you need to be a Strong Hobbit and resit the urge to wear the Ring and throw the Shite into Bin and buy decent bloody drives. .
Last edited by JAZZCNC; 26-03-2014 at 10:56 AM.
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