Stock SD4 Vrefs

Recently I picked up a cheap multimeter and decided to take a peak at the voltages on my stock SD4 (Printrboard with A4988 drivers).

Here are the Vref (Suggested values from Solidoodle are in brackets):

E: 0.257V (0.25V)
Z: 1.201V (1V)
Y: 0.995V (1V)
X: 0.798V (1V)

To make some sense out of these numbers I looked up the rated voltages/amps for the Nema 14 (x) and Nema 17 (e,y,z). Here's what I found:

Nema 14 (SM35HT36-1004A) - 2.7V, 1.0A
http://www.smartautomation.com.cn/Produ … icleID=500
Nema 17 (SM42HT33-1334A) - 2.8V, 1.33A
http://www.smartautomation.com.cn/Produ … icleID=500

Finally I found the math to understand the relation between Vref and the rated values. http://reprap.org/wiki/Pololu_stepper_d … or_current

Using the, according to the wiki page, "Made in China" A4988 Polou knock-off drivers:

I_tripmax = V_ref / (8xR_s)..... in this case Rs is 0.1 ohms
*If you're using DRV8825 I_tripmax = V_ref*2

So for the x axis we have I = 0.8 / (8*0.1) = 1A

Which is what the Nema 14 is rated for.

I converted the rest of my Vref(rated current in brackets):

E: 0.32A (1.33A)
Z: 1.50A (1.33A)
Y: 1.24A (1.33A)
X: 1.00A (1.00A)

Now I was finally able to understand what all these numbers meant and hypothesize what to change.

E - This value is WAY too low and can go higher, however I noticed that the extruder stepper runs really hot already even with the heatsink/fan on the back. Maybe it needs another heatsink on top? I would be interested to know what Vref other people are running.

Z - Overdriven for more torque? However it might be okay since this motor isn't used for a long duration of time. I will probably end up lowering this anyway and see what happens.

Y - This is pretty much spot on.

X - Once again spot on. I do understand that other people have experienced skipped steps on the x-axis and that's why they added on a fan. I hypothesize their Vref must have calibrated to what Solidoodle recommends (1.0V = 1.25A) which is what caused the overheating.

(Power = I^2*R, and if you do the math it's 1^2*R vs 1.25^2*R which means you'd be looking at P = 1*R vs 1.5625*R !!!)

Didn't think this post was going to be this long, but I hope this helps the community. I'm really looking forward to feedback on what Vref people are running their extruder motors at. Thanks for reading!