General manual is here: https://github.com/openpnp/openpnp/wiki/Setup-and-Calibration Some specific points to check regular:

Essential to have the steps/mm setting right before starting to calibrate anything. The steps/mm setting directly derives from the math for GT2 belts, no need to measure it. One may double check by means of a ruler.

The machine has to be square in XY-layer. Otherwise board locations can't be located precise.

• Problem: Locations further away from a teached point might be off.
• How to measure: With downlooking camera and grid paper
• Solution A: If using two y-axis motors enable the dual endstop feature in marlin. Calibrate with M666 Yxxyy. (Marlin firmware only)
• Solution B: If using one dual-shaft motor fix the axis to be square to each other. (dual shaft y-axis motor only)
• Solution C: Setup squareness compensation in OpenPnP (make sure to use the right g-code-command!) or Marlin Firmware

• Problem: Fisheye effect on camera images
• How to measure: look at rectangles with the camera and see if they show up as proper rectangles.
• Solution A: Use OpenPnP internal compensation
• Solution B: Use Lenses with correction or lenses with higher focal length value (6mm and above).

Keep in mind, that rulers, pcbs and other stuff have a thickness. If it's used upside up and upside down, the holders for the device (e.g. ruler) have to be at different height to compensate for the thickness!

Keep that in mind when:

• Setting the Pixel/mm value for the cameras

Problems that may show up if not having the focal layer right:

• Offset for parts, that are aligned via bottom cam due to z-axis misalignment (pos_xy(z=0) may be different from pos_xy(z=-20)!)

Check for the 100% correct position. The layer to be referred to is the pcb top layer (see above). If there is a slight angle between z-axis and camera axis there is a deviation if bottom camera checks at z=0 and placement is e.g. at z=-20mm. Check bottom position by means of a transparent calibration ruler / via in pcb or similar. Top and bottom cam have to align perfectly.

Nozzle offset is related to the camera on the head (camera offset is 0). Measure with clay or flour. To remove the impact of runout: Average center by rotating tip in clay. Measure distance from this center to the center of the upper camera.

Visual Homing enabled? (Create Part “FIDUCIAL-HOME”)