The automatic calibration for delta is done in the DC42 fork of the RepRap firmware. It shall be noted that this fork have been adopted by Think3DPrint3D for the Duet 0.8.5 and by RepRapPro for the Fisher delta, however the development is done at fast pace and the branch is always ahead of the 'official' release of RRP and T3P3, particularly regarding the calibration. So, some comments and parameters below only apply to the original DC42 branch version. (development version). As 19 October 2015, the last version of this fork is [1.09k->https://github.com/dc42/RepRapFirmware]].
If using the Fisher, for good calibration, the calibration macros delivered with the Fisher shall
Number of parameters (Sn)
The calibration use defined test positions and adjust parameters according the
(according David Crocker)
Six parameters calibration
The 6 parameters add to the 4 parameters test the angular position of the X and Y columns (they search if the triangle is not equilateral), as this is a current default on "Kossel type" printers. This is why there is only X and Y angle, the Z angle being the reference angle.
The problem is that the calibration algorithm attempt to transform all these defaults in angular position fault. Generally speaking, the algorithm find a solution with angles which minimize the hotend/bed delta, hence helps to have a nice first layer. These found angles may be way off the reality.
On the Fisher, it is reasonnable to consider there is no angular fault.
For other printers, you shall print delta calibration part or this part to research your angular faults, then enter the corrected angles in the M665 command in the configuration file (or in the G-code windows, but this is lost at restart). After enter of the corrected angles, reprint a new delta part till finding the good angle.
Then use 4 parameters calibration. There will be a larger delta between the hotend and the bed, but your parts dimensions will be more accurate.
Number of measured points
Accurate results are only obtained if you have at least 3 or 4 measured points more
On the Fisher, the measure of the bed position need force and create a measurement offset because
You will find below extract of my own Fisher calibration macros, but it is only supplied
Extract of my bed.g macro, with 7 measurement points and 4 parameters
; Probe the bed and do auto calibration G1 X-64.95 Y-37.5 F12000 G4 P300 G31 Z-0.12 G30 P0 X-60 Y-37.5 Z-99999 ; X tower G4 P300 G30 P1 X64.95 Y-37.5 Z-99999 ; Y tower G4 P300 G30 P2 X0 Y75 Z-99999 ; Z tower G4 P300 G31 Z-0.15 G30 P3 X-32.48 Y-18.75 Z-99999 ; half way to X tower G4 P300 G30 P4 X32.48 Y-18.75 Z-99999 ; half way to Y tower G4 P300 G30 P5 X0 Y37.5 Z-99999 ; half way to Z tower G4 P300 G31 Z-0.17 G30 P6 X4 Y0 Z-99999 S4 ; centre, and auto-calibrate
Extract of my main calibration routine, with 10 measurement points and 6 parameters
; Probe the bed and do auto calibration G1 X-64.95 Y-37.5 F12000 G4 P300 G31 Z-0.15 G30 P0 X-60 Y-37.5 Z-99999 ; X tower G4 P300 G30 P1 X64.95 Y-37.5 Z-99999 ; Y tower G4 P300 G30 P2 X0 Y75 Z-99999 ; Z tower G4 P300 G31 Z-0.18 G30 P3 X-32.48 Y-18.75 Z-99999 ; half way to X tower G4 P300 G30 P4 X32.48 Y-18.75 Z-99999 ; half way to Y tower G4 P300 G30 P5 X0 Y37.5 Z-99999 ; half way to Z tower G4 P300 G31 Z-0.22 G30 P6 X-52 Y30 Z-99999 ; between X and Y towers G4 P300 G30 P7 X52 Y30 Z-99999 ; between Y and Z towers G4 P300 G30 P8 X0 Y-60 Z-99999 ; between Z and X towers G4 P300 G31 Z-0.20 G30 P9 X4 Y0 Z-99999 S6 ; centre, and auto-calibrate
2 iterations of 10 points/6 parameters calibration done after a cold start and
1 iteration of bed.g (7 points/4 parameters) after a cold start and hotend heated