It’s possible that the reading is just incorrect if the DRV8825 is still changing the voltage/current at high frequency. The best way to be sure would be to use an oscilloscope to see the current/voltage over time.
That’s based on my assumption that the driver is still pulsing at high frequency when it’s holding the motor in position because it still has to chop the input voltage(12v) so that it can maintain the current limit. I could be wrong there, it’s just a guess on my part.
Ok so my DRV8825’s finally arrived so I decided I’d see if I found a similar VRef to current ratios to what you came out with.
In full step mode, at 0.5v VRef I was finding around 0.5A in the coil. I adjusted until the coil current was 1A and the VRef at that point was 0.97v.
I then put it into 1/32 mode with the same VRef and measured 1.43A
That at least shows that full step current being 70% of the microstepping current is true. 1.43Ax0.7=1.001A
So maybe it is the case that the ratio on the ones that we have are slightly different to the Pololu ones.
The resistors on the ones that I have are the same as yours. They have .1 ohm sense resistors and the 152 and 103 resistors in the same locations as yours. The 2nd set of resistors aren’t really relevant to us anyway as they’re not in place of the ones on the Pololu board, they connect to a different pin on the IC if you follow the track on the PCB.
edit: Just had a quick read over the datasheet for the IC. From what I see the only 2 pins that should affect the current ratio are pins 6 and 9, “ISENA” and “ISENB”.
That’s where the sense resistors are connected on both boards and both are .1 ohms.
I wonder if the reading we’re ending up with is an RMS value of the waveform being fed to the motor coil.
For the 1A current that I read when set to .97v that would work out as (1 x √2=1.41A). That’s only 71% though due to being full step, so (1.41/0.71= 1.98A)
That value is much closer to what I would’ve expected from the VRef setting. The RMS value would also be for a pure sine wave which isn’t what we have here so that would effect that final value a little also but I don’t by how much.
I just did another quick test to see if the RMS theory would hold true at different values.
I set VRef to 0.5v and went with full step mode. Measured current was 0.515A. —> (0.515A x √2) / 0.71= 1.02A.
(VRef x 2) would give 1A.
Seems to work but whether or not it’s a coincidence I don’t know.
So are you saying the output is something like a rectified sine wave, with the peak at Vref/2, and subsequently RMS is the DC component measured by the meters?
Did you try measuring the current with the meter in AC mode? I didn’t try that… I’m not sure what it would read, on a rectified wave, but if it’s 0A then that’s not it.
I’m inclined to not try that, because I like the solution you’ve come up with so far, and it makes me happy to think that’s the truth 
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Also, it probably doesn’t matter, but the R100 is 0.1 Ohm, not 100 Ohm, in case you are trying to calculate the loss of that resistor too.
Yes, that’s what I think is happening. I think it’s a high frequency PWM signal but we’re ending up with an RMS value rather thank the peak. It’s just a guess but seems to work out. Can’t be sure without a scope though 
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I didn’t try measuring in AC mode, I will next time I hook them up though. I’m not sure how it’ll show up when stationary though as I don’t think the current through the coil will reverse until you start stepping.
Sorry, meant .1 ohm. No idea why I said 100 ohm, will fix it in a sec. Brain fart I guess lol.
When I was trying to narrow down a problem one of the things I did was rewire so every motor went to it’s own stepper driver. The problem ended up not actually being related to the wiring, but since I had done all the running of the wire already anyway I went ahead and populated the RAMPs board with 2 extra stepper drivers and broke out every motor onto it’s own driver.
From what I’ve seen, I can go up to .8V and still have a stepper that is only at 60C. If I go up to .9V the stepper goes up to 70C which is too close to the 80C I have read that stepper motors start to lose magnetism at. I’ve ordered some 40mm x 40mm heatsinks I’m going to put on my Z axis and see if it can do .9V and still stay around 60C. For actual milling I’ve always found the Z axis having the proper torque is more important than any other motor. If the Z axis gets pulled down into the wood (like say if your bit becomes a bit dull in the middle of a large cut), things go very bad very quickly where if the X or Y axis lose steps it messes up your work but at least it doesn’t get pulled down into your spoilboard.
I know at .8V if I really try - I can move the Z axis while it’s sitting idle (holding torque), but at .9V I can’t physically do it with my hands - so there is definitely a noticeable strength difference even as high as .8 to .9V. Cooling the drivers is very important - I personally use a fan I pulled out of an enterprise level network switch - loud as can be, the noise coming from it could be heard through the rack cabinet, and through a wall - but boy does it push some air. When you’re milling you already have a ton of noise (granted the DWP611 is MUCH quieter than my DW660 ever was) so a loud fan doesn’t matter.
That’s interesting. I hadn’t thought of using the other two slots for two more driver boards.
0.8V for one motor means you are running it at 1.6A, which is more than the rated current for my motors. What motors are you using?
What method were you using to measure the temperature? I’m interested in both the guage, and whatever you were doing to stress it. I’ve used a 60C hotbed to heat up a PLA print so I could mold it a little, are your motor mounts changing shape at that temp?
Very good info though. I would feel better if I could measure the values for my machine, specifically. Right now, I’ve got bigger hurdles than maxing out the torque, but I’m gonna want to do that at some point.