Prints are shrinking on Z axis for some parts but not others

Controllers Smoothie
1

I calibrated my printer, I used calipers moved Z axis 100 mm, and the Caliper shows exactly 100!
I printed test cube several times, and the cube is perfect size on all axis.
But when I printed an object that is 250 mm my part shrinked by almost 10 mm, and I can’t figure out why or how?
My printer is 400x400x400 mm, using smoothieboard with latest firmware. The build plate goes up/down, and it’s pretty heavy, so if it would loose steps, then the part would actually get bigger not smaller (it would fall down, make a gap and print in the air). I have 4 motors for Z axis (one in each corner), with an external 4.2 amps microstepping driver.
I use Symplify3D slicer, and I use the same profile for my test prints, and the part I need that is over 250 mm tall.
As I said, when I print tests, it’s fine, but when I print the part I need, it’s shrinking… I have no clue what could be wrong.

Any ideas?
Thanks!

Imported from wikidot

2

Can you print a ruler in the Z direction ( like this https://www.thingiverse.com/thing:109739/files or any of the many others ) and see exactly where you get the stretching by comparing it to an actual ruler ?

3

I found out it’s because of the external motor driver. Does anyone know a good external motor driver (4 amps or more) that I can quickly get (from Amazon maybe?) that works with the smoothieboard?

4

I use the CW5045 and an pretty happy with it. Pretty much anything from leadshine that matches your needs will be fine. Also look at the external drivers documentation on the wiki it uses a specific driver and lots of folks use that one and like it.