Hi, during the build of my 3Drag / Velleman, I see some problem on XY motor installation. 
First important think is to substituite the origial L staf of X motor.
The original L shaft is very bad, it oscillating when you move with hands the plate.
I replace with one more strong.
Second problem is the Y motor from original guide has positioned in bad manner.
The edge where the wires come out (yellow hole) does not allow a correct support of the motor to the plate.
For correct place you must rotate motor of 180° and the wire come out from top.
Watch video below .. to see you how to check if your motors work correctly.
Tuning XY printed measure
Now correct XY motor step on firmware to make perfect hole and cube.
The problem is generate from not perfect parallelism of the belt with
respect to its point of attachment. The true path of the belt is
trapezoidal .. not rectangular.
This introduce error on real printed measure.
For this work I use this formula:
DefaultStep/MeasureSide = NewStep/1mm
DefaultStep = 64,25 step/mm
MeasureSide = measure side of printed cube
NewStep = new value to set on firmware
Now for this I make this 3d model of 10mm cube with 5mm hole, you find on my Thingiverse account. The formula is reworked to this:
NewStep = 642,5 / MeasureSide * 10
OK .. this is jobs sequence for X axis, Y axis in same way.1) Print the STL file.
2) Measure the X side (my case I measure 9,8mm)
3) Calculate NewStep ( 642,5/9,8*10=65,56 )
4) Set new value on firmware and repeat the test to verify.
