An update from me: The new stepper is in place and seems to have cured my y skipping problems. I'm still using the same stepper driver with the same vref. I guess the old stepper was running at the limit and this new one can handle the same current but with more ease.

I did have to buy M3 x 50mm screws to mount it, and the longer length needed the belt gears moved down the top shaft slightly to align.

Some steppers are more efficient than others - in other words, some designs produce more torque for a given current than others.

I did have to buy M3 x 50mm screws to mount it, and the longer length needed the belt gears moved down the top shaft slightly to align.

More current  -- need cooling!

product-1182
"The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1 A per coil, a heat sink or other cooling method is required."

Black edition product-2128
"actual current you can deliver depends on how well you can keep the IC cool. The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1.2 A per coil, a heat sink or other cooling method is required (in our tests, we were able to deliver approximately 1.4 A per coil with air flow from a PC fan and no heat sink)."

"The drivers are still Pololu A4988 but come with little heatsinks."

It's my understanding that these drivers sink to the board and the
heat sink on top does little.

I got this from the Pololu website. http://www.pololu.com/catalog/product/1182
They rate it for higher amperage with the heat sink.

I bought the AD8825 they come presolder with heatsinks and supposedly can handle 2.5amps no problem

I am using the original servos with the SD8825 so I tried to set them a hair lower to see what happens and they run perfect now, no missteps and much smoother quiet operation. I have anew 1.2A higher torque stepper I am thinking of swapping out the Y to crank up the speed a little

It's my understanding that these drivers sink to the board and the
heat sink on top does little.

So for me that would be 80% of 1.8A = 1.44A, with the vref set to 0.576.

Thanks to all who have replied.

There's no harm to matching to stepper maximum, other than wasting energy.
No harm in shooting too low, other than missing steps.

My research has included the Polulu site, here and the reprap site, but I'm still not quite sure. Is it always just trial and error? I understand the maths but am not sure what the target should be short of matching it to the stepper maximum.

A gap still exists in my understanding, however. Should I simply be setting the vref on the new driver board to target the max current of 1.8A or is there more calculation that goes into it? I would have though the physical force the stepper was moving would come into it.

Sure, if the steppers are rated for it, and the drivers can handle it passively.

PS  Here is the link:

http://reprap.org/wiki/Sanguinololu#Final_Check

In the interests of solving my y axis skipping problem, I have purchased some replacement stepper motors and new stepper motor drivers.

The drivers are still Pololu A4988 but come with little heatsinks.

The steppers are upgraded 17HS8401 with 1.8 amps current/phase (compared to 1.3 amps for the stock y-axis).

http://create3d.com.au/shop/product_inf … ucts_id/49

The best thread I've found for information on understanding the required currents and rating is here:

http://www.soliforum.com/topic/459/soli … iciencies/

A gap still exists in my understanding, however. Should I simply be setting the vref on the new driver board to target the max current of 1.8A or is there more calculation that goes into it? I would have though the physical force the stepper was moving would come into it.

Any explanations on the topic people could post up would be greatly appreciated, both by me and no doubt by future searchers.