The first step to calibrating a delta printer is setting the limit switch heights. Each tower has a limit switch at the top, and they should all be the same height from the print bed.
In the firmware (Smoothieware), set the printer's Z-max (total height) to something about 5 mm longer than you think the height actually is. This will allow you to move into negative Z territory if one tower is taller than the others.
All calibration steps should always be run with the nozzle and bed stably at typical printing temperatures to compensate for heat expansion.
Move down to just above the print bed at three points (for Rostock Max v2 good test points are X0 Y100; X-77.94 Y-45; and X77.94 Y-45. Be very careful and approach the print bed in 1 mm, then 0.1 mm increments. Note at what height the hotend nozzle touches the print bed; for this purpose, use a sheet of paper to test for resistance.
Adjust the nozzle touch point up or down using the screw located on top of each carriage.
When the nozzle hits the print bed at the same height at all three test points, proceed to the next step.
Next, set the Z-max height. Move the nozzle down slowly at X0 Y0 until it touches the print bed. Subtract whatever the current Z position is from Z-max to set the actual printer height.
Next, adjust the horizontal radius to adjust for any inconsistency across the bed.
Measure the height at the same three test points as the first step. The nozzle should touch the print bed at exactly zero. If the nozzle doesn't reach the print bed, adjust the horizontal radius down in 0.2 mm increments until it does. If the nozzle touches the bed too soon, adjust the horizontal radius up in 0.2 mm increments until it touches at zero.
The printer is now manually calibrated!
You can additionally run automated calibration AFTER manual calibration to correct for any mechanical build errors in tower position and/or angle.
A typical auto-calibration routine for Smoothieware is:
G28 (Home XYZ)
G32 (Calibrate the machine )
M500 (Save probe results)
G28 (Home XYZ)
Manually: jog down to touch the plate
M500 (Save homing offset)