- February 13, 2016 at 11:43 am #7492
Any suggestions on which would be best to go for? I want to try and use the machine for milling wood/cf/plexiglass and maybe a little light aluminium work but I’d like to use a laser too for cutting/engraving. I’m figuring the torque is useful for the milling and the resolution is useful for the engraving and maybe for doing PCBs so really not sure which way to go.February 13, 2016 at 12:37 pm #7493
I personally would go for the power but that’s just my $.02. With the micro-stepping drivers you’ll have plenty of resolution to do the projects you’ve described. If you’re even considering aluminum work then you’ll need the extra torque. Good luck on your build!February 14, 2016 at 9:03 am #7526
Ya the 1/32 microstepping on the DRV8825s will help with the resolution alright. The accuracy of microstepping probably won’t be great under heavy load compared to smaller real steps but for engraving/cutting with the laser it shouldn’t be a problem I guess.
I’ll probably go with the 1.8deg ones alright, the torque will be useful for milling and definitely for aluminium as you said.February 17, 2016 at 5:19 pm #7786
wow, only $54 shipped for 5 of the 85oz is a great deal. I just ordered from amazon last night and it was $75. I’m going to try and cancel and see if I can grab these instead. $21 bucks is $21 bucks. Thanks for that link.
I struggled with the same question, but saw everyone was getting great results with the standard 1.8 degree steppers that I thought the torque difference would be well worth it. If nothing else…an upgrade in the future.February 18, 2016 at 4:06 pm #7845
Ya it’s a very good deal alright. The shipping is fast too.
I ordered on Sunday and received them today.
I’ve been reading up on the best way to drive 2 of them from one DRV8825 and I’m not sure what to do yet. Connected in parallel I won’t be able to run the 2 of them at their rated current (not that I could anyway seeing as at that current they’d be above the glass transition temperature of the PLA mounts).
Connected in series they can be run at their rated current and there should still be enough overhead voltage as those motors are rated at 2.8V per phase and I’m using a 12v supply.
I think I’ll just test both configurations once the rest of my parts arrive and decide what works best.February 19, 2016 at 5:16 pm #7897
Why wouldn’t you be able to run them at full power?
Convert the rating into watts and then check it again. We are running 2 64oz/in per axis at full power and have enough room for two more.February 20, 2016 at 7:50 am #7923
I get where you’re coming from with the wattage calculation but you can’t apply it in that way to this situation from what I can see.
The drivers we’re using are chopping the input voltage at high frequency to try and maintain the current that we want. This means that the motor won’t receive your power supply voltage directly, so a wattage calculation using P=V*I with the supply voltage as V will give you the incorrect value.
The torque is proportional to coil current so you’ll lose a lot of torque if you use that method when setting up your stepper drivers. I’ll verify it once I get my DRV8825s but they’ll be a few weeks.
Saying that, we won’t be able to run the motors at full current anyway because they’ll be too hot for the PLA mounts. We should adjust them to get the best balance we can between motor temp/driver current+temp/torque though.
My motors are specified for upto an 80c rise above ambient so I know if I ran full current the PLA would soften.
Btw, absolutely love your design. I can’t wait to get it up and running, I’m printing off all the parts while I wait for some of the stuff to arrive from China.February 20, 2016 at 8:29 am #7927
This gets covered a lot here and other places. Be a little careful of making definitive statements until you try it. People will read it and get a little panicked and then email me.
Use watts, look at the pololu conversions for the drivers. Yes the steppers chop. That is why your steppers can be over powered by a lot. They aren’t always driving full power and never should heat all the way up. The 80C is a max temp you should never let them get that hot. When you adjust the voltage at the pot tin turn controls the total wattage.
You can easily drive 2-4 of these big steppers on an activity cooled 8825. There are hundreds of these out there Using this exact setup. And if you need proof I ran all 4 of the extruder steppers in parallel, http://www.vicious1.com/4-head/February 20, 2016 at 9:57 am #7943
I’m not trying to cause anyone to get panicked and none of this is meant to come off as confrontational or anything so please don’t take it as such. I’ve nothing but respect for your project.
I’m not saying that the steppers won’t work if you do it that way, I’m just saying that you would not be getting the full benefit of them if you calculate the current in the way that you are saying.
They’ll run fine but you’ll be running them at only part of what they are capable of. These stepper motors are current driven, so if you are not reaching the rated current then you will not get the rated torque. Also stepper motors do always run at full power even when stationary, the motor coils are constantly energised to develop their holding torque.
It’s quite safe to run stepper motors hot also as long as you don’t exceed the rated temperature. My motors when running at the rated spec would be quite hot (as I said, too hot for the PLA brackets).
e.g. My steppers are specified at 2A per phase, 2.8V with a coil resistance of 1.4 ohms.
If I was to use your method and assume that the driver would feed 12V to the motor the I would end up with 2.8Vx2A=5.6W —->> 5.6W/12V=0.47A. —->> 2 steppers =0.94A
That would work but you won’t be getting the power that you think you are getting. The motors won’t receive 12Vx0.47A per coil. They’ll get an initial surge and then the driver will chop the voltage to control the current. You won’t have 5.6W but a fraction of that.
The coil resistance is a constant so if you apply ohms law, 2.8V/1.4ohms=2A. For the driver to achieve the lower current than you have set it to, it must reduce the voltage across the coil, not increase it.
If you were to apply 12V to a 1.4ohm coil then 8.5 amps would flow.
I’ll verify it when I get my drivers but if you want to verify it yourself in the meanwhile all you need are 2 ammeters.
Do it with only 1 stepper and have it hold position. Put an ammeter in series with the output of your power supply and another ammeter in series with 1 motor phase.
Double the motor phase reading because the 2nd phase will have the same. You should find that the current on the supply side is much lower than the total motor current.
That’s because the input voltage is 12V and the voltage applied to the motor will be lower.February 20, 2016 at 10:20 am #7947
Your understand of the drivers seems to be greater than mine.
We do seem to be agreeing that the steppers can be driven from these drivers just fine, as many as you need. If I get some free time I will double check all my math, and the pololu equations.
You seem to be saying that the PLA mounts will limit you, easy solution. Under power your motors, cut some aluminum mounts, install them and power back up.
How much more power do you think you are going to get from the max you can run your steppers on a pla mount vs. the max that they can handle?
I have actually tried running the crap out of my steppers and don’t actually feel a difference above 0.8v setting in the driver that is why I ship them set to 0.7 so no one needs to have active driver cooling. If I can get my hands on a nice force gauge I will post actual numbers. At 0.8 the steppers might get 2 degrees above ambient. And the ones running on the aluminum mounts don’t seem to raise any because it acts as a really nice heat sink.February 20, 2016 at 4:55 pm #7991
Ya I agree you can run a few motors off the drivers no problem. For things where torque isn’t overly important like 3d printing/laser cutting etc. it’ll make little difference anyway if you run them a bit under spec.
The PLA mounts aren’t really what I was concerned about anyway, that’s just more a side-effect of pushing up the current.
I just want to get the most torque I can out of the motors that I bought, that’s all. If I didn’t want to try milling aluminium/tougher materials I wouldn’t even be that worried about it as for most situations you don’t need that much torque.
What I was looking at was whether to wire the 2 stepper motors in series or parallel. Wiring them them in series would allow me to push more current into each motor without overloading the driver.
e.g. 2 motors in parallel with a driver set for 1.5A only gives 750mA to each motor coil, where as in series each motor coil would receive 1.5A without pushing any extra current through the driver.
Good idea on the aluminium mounts, I guess that’s the beauty of these machines. They can make their own upgrade parts.
I’ll do that if I think it’ll be worthwhile but it probably won’t be necessary. The torque is proportional to current but the temperature will be proportional to the square of the current, so a small drop in current should lead to a large drop in motor temperature.
If you do end up testing with a force gauge I’d love to see the results. You have mpcnc’s with aluminium motor mounts at the moment?February 20, 2016 at 8:31 pm #8012
So that’s the other thing. Power/torque has never been our issue, it rigidity. the first three aluminum milling videos I put up were with 42oz/in steppers. The only reason I upgraded the kits was my supplier offered them at the same price. There are a ton of factors. The biggest one you will need to worry about since you seem to be going for aluminum milling machine is the shortest legs and z axis as possible to maintain rigidity.February 21, 2016 at 5:48 am #8025
Ya that’s the hardest decision about it really. I’d like a large work area incase I need it at some point but rigidity will be an issue then.
What sort of size do you recommend before rigidity would be a problem? And how helpful do you think midspan supports would be?February 21, 2016 at 7:10 am #8027
Completely up to the material you are cutting and the speeds and tolerances you expect.February 21, 2016 at 7:21 am #8028
That’s the thing, I’d prefer it to be a bit of an all rounder to try different things with. Speed isn’t a major issue for me but I would like to be reasonably accurate.
What size were the machines you tried with aluminium and how did you find the accuracy?
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