Top Layer of print is squashed

Quote
Zavashier
For extruders, the formula is :
direct drive type : e_steps_per_mm = (motor_steps_per_rev * driver_microstep) / (hob_effective_diameter * pi)
wade (geared type) : e_steps_per_mm = (motor_steps_per_rev * driver_microstep) * (big_gear_teeth / small_gear_teeth) / (hob_effective_diameter * pi)

BUT, for extruders, that's not that simple. Because the tooth of the hob comes inside the filament and that modify the hob effective diameter. So you need to calibrate manualy your extruder after a setup with the formula :

Find your actual extruder step per millimeter settings, write the value
Mark accurately the filament 150mm above the extruder
Ask the printer to extrude 100mm of filament (repetier or so)
Whait the extruder to stop
Now measure the distance from the top of the extruder to the mark
Calculate the real amount of filament extruded (150-the last measurement), write the result
Calculate (last steps per millimeter*100)/the real amount of filament extruded
You've got your new steps per millimeter value
Open your configuration.h and make the correction
Upload the updated firmware to your arduino
Mark again at 150mm and ask again to extrude 100mm of filament
Measure if the result is ok within 2%, if not calculate the new steps per millimeter
When it's ok; your extruder extrude now the good length of filament
The first step of extrusion calibration is done. Congratulations

It's good, but it's not done. We now need to measure accurately the filament itself. Some filaments diameter is not the same all along the filament. You need a digital caliper, and you want to make several measurements to find an average value. When it's done, set your slicer parameters with that value. Remember you want to set this up with each new spool of filament, event with the same colour from the same brand. Now our system is able to extrude 100mm of filament, and we know as precisely as possible the filament diameter. So the software side is able to deliver the right amount of plastic. Good.

BUT, we will melt the plastic, that means we will transform it. That means that the volume of cold (unextruded) plastic possibly differs with the volume of plastic out of the extruder... So we need to check this out.

You need a STL file of a one wall cube, that wall of the same thickness of your nozzle diameter. For example you've got a 0.5mm nozzle, you need 0.5mm walls. Design it or get one at Thingiverse or so.
Print it with slow settings, and verity all repetier (or so) settings are standard or 100%
When it's done measure the thickness of the walls of your printed object, but at the top because the bottom of the part may be thicker.
Calculate (your print wall thickness/nozzle diameter)*100 = your new feedrate
Modify your slicer parameter with the new feed rate
Print another test thin wall cube and verify the thickness of the walls
If you're within 10% of the asked value, it's ok. Remember our repraps are not high precision machines.
If not, Calculate again a new feed rate, BUT calculation is this time different !
last feed rate * (nozzle diameter/measured wall thickness)
when you get an acceptable print, your calibration is done ! Congratulations !

Now you can print benchmark parts, you're as precise as your printer can be. It's good to keep in mind the best your machine can do. You don't need anything else to be calibrated. If the accuracy does not matches your needs, you need probably to upgrade your printer's components do get better prints.