Modifying stepper driver circuit

Hey guys,


Is it possible to modify this:

[reprap.org]

to run this (the 425 oz/in stepper):

[www.hobbycnc.com]


Thanks for any advice...I am trying to build a CNC mill by using the readily available parts for a RepRap...

Honestly, if you're planning to use the hobbycnc steppers you may as well use one of their cheap driver boards as well. They're unipolar, but they've worked well for my repstrap so far. (admittedly, it's only milling polystyrene insulation for the moment, but it's moving just fine on 12 or 24 volts).

I bought their EZ Driver Kit, [www.hobbycnc.com] . It's actually cheaper than 3 axes of the Reprap board ($64 versus $25*3=$75). Don't get me wrong, I love the reprap board for a one-stepper driver board, especially if you need bipolar and are totally committed to 12vdc, but honestly it's not perfectly suited to the application from a cost standpoint, and it's a lot more work to assemble than the EZ board.

To answer the question without commentary, the boards will work perfectly well with the 8-wire motors, unmodified. You'll just have to figure out which wires you want to connect the windings properly.

But I am looking for a bipolar setup for more power as the steppers are only rated at 305 oz/in in unipolar as opposed to the 425 oz/in in bipolar. Also, I have no computer that supports parallel because I have no computer with that connector.

Unless if I am being stupid and don't realize that the reprap stepper driver is bipolar than please disregard my earlier comment. And just a last confirmation, the stepper driver L298 can handle the current for the steppers I plan on using correct?


Thankx so much for clearing this up.

You're right that the reprap driver is bipolar, which yields more torque. However, the motors that you are looking at are rated at 3 amps per phase, while the reprap drivers are 2 amps. This won't prevent you from using the motors with the reprap driver, but it does mean somewhat less torque and somewhat lower maximum rpm (it definitely means less power, since electrically that's P=IV, but I'm guessing you actually meant torque).

I should have been clearer about my setup, I have an Arduino controlling my driver board using the Gcode Interpreter standard firmware with no problems at all.

You should also take a look at [www.hobbycnc.com] from the hobbycnc site, it has a couple of tidbits in the comments about the ratings.

Like I said, the reprap drivers WILL work with the motors, but whether they'll get you the torque you need I don't know.

What are you driving that needs 425 oz-in versus 305 oz-in, btw?

I am designing my first CNC mill which is going to mill some soft aluminum/copper for CPU water blocks along with some acrylic/plexy and various plastics for modding etc. I am not concerned with speed but concerned about power and accuracy (accuracy for home built CNC is limited but will take the best I can get). I am putting it in a lead screw setup, I looked at ball screw and rail but I am under tight budget and thought that the best way would be the lead screw method with all the backlash precautions. Also I am planning on using a dremel with its end mill bits which would be cheaper and I can use the dremel for other purposes as well...

Since the stepper is rated at 3 amps thats why I asked if I could modify the driver circuit a bit to get me the rated power I need. So is there any modification that can be done?

I almost forgot, the maximum working area I hope to have is 60 cm by 120 cm so its tiny...I think...

As far as I know there are no pin-compatible 3 amp replacements for the hbridge in the reprap stepper driver, so I don't see any reason to try to modify it directly.. the logic chip could be used in a new design, but that's about it. 60 cm by 120 cm is pretty small but that's not particularly relevant - there's not much difference for the motor load between 60cm and 300cm, really. If you were doing a plywood sheet router, then the size would definitely matter XD. If you're milling metal, leadscrew is a good choice - high resolution and low motor torque work well, though your speed won't be amazing. I don't have any experience with CNC milling metal, though, so I don't know exactly what you'll be needing, but at very low feed rates (not the best for a dremel tool, but oh well) even fairly weak motors should be able to drive the mill with a high reduction leadscrew (ie 20tpi to 32tpi). I would be willing to bet that your biggest issue is going to be structural rigidity, not motor torque.

I am some what sure of what to do, since I want to have water-soap lubricant loop. Keeping in mind the loop, I will have a basin underneath that catches the lubricant filters it and then loops back to the tooling bit. So I need a gantry type CNC. I was thinking of a frame made from plumbing pipe, the ones that are made out of iron or whatever, then have MDF board cut to the pieces and then cover that with some wax or plastic so that the MDF doesn't degrade from the lubricant/coolant splashing on it. May be I will even replace the MDF with PTFE plastic...

I have heard of people using 2 L298s in parallel that would give you more amperage, or theres the more powerful l293e driver chip to give you the amperage you need. Or if your willing to go with unipolar you could get the stepgenie they only cost 5 bucks each and you use them with hexfets so you can easily get over 30 amps. Thats actually what will drive my repstrap after I get the software loaded.

so aside from using 2 L298s in parallel, any other modifications? Like changing the resistor values to changing the diode values?

Also, changing the L297 to an L293E, do I need to change the L298s to a parallel config? sorry if my questions either don't make sense or hav obvious answers, I am somewhat of a n00b at electronics...

Edited 1 time(s). Last edit at 02/26/2009 12:42AM by slisgrinder.

Im not exactly sure what you meant on the first question if you meant..

Will changing the resistor or diode values increase the amperage, the answer is no.

Will I have to change any resistor or diode values to use 2 l298s in parallel, I'm not sure. I'm not an expert but [www.cnczone.com]
is a schematic from the datasheet of either the l297 or l298 not sure which, i dont remember seeing that in the l297 datasheet its probably in the l298s......Anyway another probably easier but more expensive solution would be to use 2 reprap stepper motor drivers and wire the step signals from both together and the direction signals from both together. I haven't tried that but it sounds plausible...

I got my facts about the l293e wrong it is actually less amperage. I thought I read it was more somewhere maybe not. I hope that didn't cause any problems.

heres a somewhat similar thread on CNC Zone
[www.cnczone.com]
if your not a member there I suggest you join. Its probably the second best site for rapid prototypers (right after the reprap.org of course). I picked up 4 unipolar steppers with enough torque to drive my repstrap on wave drive mode for only $20 shipped.

Sorry about the rambling post if you need anything clarified I'm following this thread.

---

_______________________________________________________________________________________________________

Yes my Avatar is My RepStrap.....Isn't it pretty

For the latest Updates check my main site [www.repstrap.weebly.com] or my blog [www.pingiscoolest.blogspot.com].

I already signed up to cnczone a long time ago. I know basic electronics and how they work just wasn't sure to stepper driving since this is new to me.

Thanks for your help guys. I will be back with a working schematic of the stepper driver, right now I have to focus on the CNC mill itself, the design and stuff...

OK, Good Luck!

---

_______________________________________________________________________________________________________

Yes my Avatar is My RepStrap.....Isn't it pretty

For the latest Updates check my main site [www.repstrap.weebly.com] or my blog [www.pingiscoolest.blogspot.com].