I write some Octave (MATLAB) scripts to model the delta bot commands and compute the actual effector position of a Delta bot. I can introduce errors to the Delta bot definition, and plot the results of its response to an ideal command. In this case, I am just raster scanning the bed of the printer.


When the DELtA_RADIUS is too small, we see center-low distortion (bowl), and XY scale expansion.
When the DELTA_RADIUS is too large, we get a center-up distortion (bra), and XY scale shrinkage.


When the rod length is longer than reported, we get a bowl (and shift down).
Rod length errors seem to be roughly half as sensitive as radius setting errors.
I intend to measure rod length carefully, and calibrate the radius setting only.


Tower position error causes this curve in the direction of the misplaced tower.


XY distortions are harder to plot. The tower shift does make some lines curve.
The grid points should all be equally spaced at 5mm.

I would recommend setting the compiled in MAX_Z setting at slightly less than your actual endstop height. You can use endstop offsets (M666) to adjust proper center location and home z-height.
Calibrate the compiled in DELTA_RADIUS setting to remove any bowl distortions.
The M666 endstop offsets should be able to remove tilt and potato-chip shaped distortions.

It might be worth the effort to write a script that runs a fast-converging optimization search algorithm that can adjust these four parameters. Unfortunately, since DELTA_RADIUS currently requires a re-compile, I can’t write a simple procedure. I am giving serious thought to putting a DELTA_RADIUS adjustment in the EEPROM which can be altered by an M code. The drawback is that Marlin uses macros for about 10 quantitles, all dependent on DELTA_RADIUS, so it will require replacing some of these macro number definitions with variables that need to be initialized at start-up.

I have been having some trouble with the delta 3d printer endstop calibration procedures.
It seems like something more formal might converge faster.

I question weather putting the effector in the center of the print bed is correct, given construction errors. My printer is unlikely to be perfectly symmetric and squared.

It seems to me that if things are centered, any bowl distortion should be uniform.
Hence, we should be able to calibrate the endstops by measuring the height of points in a commanded circle about the “center”, then adjusting the endstops such that those points would be at the same height on a properly calibrated printer.
For this procedure, some bowl distortion is expected. Try to calibrate for a flat, symetric bowl.

Alternate this flat-bowl calibration procedure, with adjustments to the DELTA_RADIUS to try and remove the bowl distortion. I will assume that the rod length can be accurately measured, so it will not be adjusted.

Command the effector to equally spaced positions around a circle, and note the actual Z height error at each position. Using calculate_delta.m, convert these Cartesian error positions to the Tower XYZ heights that should have been required to command to this measured Cartesian position.

Compute the mean of all of these tower positions, and subtract them from the current endstop offsets as set by M666 (and reported by M503).

Iterate on this procedure until until the commanded circle is flat.

You may wish to adjust by something like 0.9 or 0.6 times the computed adjustment to avid overcorrection.

I would recommend sampling 4 points, at (50,0,z0), (0,50,z0), (-50,0,z0) and (0,-50,z0) since these positions are easier to command.
If you wish, you could calibrate at points neareast to each tower, which should be (-43.3,-25,z0), (43.3,-25,z0) and (0,50,z0)

I am not done calibrating my Kossel-Mini yet, but so far I’m finding a lot of coverage of the mechanical calibration procedure to be rather wordy and intimidating.
In retrospect, having made it part way through, it does not seem all that bad.

I moved the macros MANUAL_Z_HOME_POS and DELTA_RADIUS to the top of my Configuration.h, since they seem to be the main two variables that need tweaking.

MANUAL_Z_HOME_POS can be rather accurately estimated with a simple ruler. It is the distance from the extruder hot-end tip to the bed when in the home (top) position. I would enter a number a few mm larger than this into your MANUAL_Z_HOME_POS macro at first so that you have a little “working room” to adjust it.

DELTA_RADIUS appears to be the length of the XY projection of the diagonal rod, when the effector head is centered. I would estimate this at first with a ruler (or square).

I think that the setting of DELTA_DIAGONAL_ROD is fairly critical. This is the center-to-center distance of the holes at the ends of the diagonal push rods. Try to find somebody with good calipers to measure this as accurately as possible.

• End stop calibration
  
  If the extruder tip is not at(near) the center of the print bed after homeing and moving in pure Z to a position just above the bed :
  
  1. Adjust the position of the endstops so that the extruder tip would be closer to the center.
  2. Re-Home, then move to a position just above the plate, at X=0, Y=0.
  3. repeat
  
  
  
  
• Z-height calibration
  1. Home the effector.
  2. Command it to move a few mm above the center of the print bed
  3. Lower the print head (moving in -Z direction) until the extruder tip almost touches the glass (within 0.1mm or so).
  4. Read the extruder position (M114), and subtract the Z ordinate (which should be nearly 0) from the setting for MANUAL_Z_HOME_POS.
  5. Re-compile and upload with new MANUAL_Z_HOME_POS setting
  6. repeat until adjustment is (nearly) zero
  
  
• Radius Calibration
  
  1. Move the head to a position slightly above the print bed and near the center, and measure the actual height above the print bed.
  2. Move the head towards the edge of the bed. A pure X-Y move, constant Z. Measure the true distance to the bed.
     
     • If the head is farther from the bed at the edges than the center, decrease DELTA_RADIUS.
     • If the head is closer to the bed at the edges than the center, increase DELTA_RADIUS.
     
     
  3. repeat
  
  
  

When the height and flattness are pretty good the residual error can be compensated for by the autoprobe procedure.

If there is not an M-code to set MANUAL_Z_HOME_POS and DELTA_RADIUS and save them to the EEPROM, I think there should be. Every time you do maintenance on the frame, belts, etc… these will need to be rechecked.

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It may not be realistic to get the centering perfect by moving the endstops delta designs which do not have micro adjustment screws (like the Kossel-mini).
It would seem to me that once you have things centered fairly well, you could add G-code like:

G28 ; home
G91 G21 ; relative positioning in mm
G1 X-0.7 Y0.3 Z-0.4 ; adjust for error from imperfectly placed endstops
G92 X0 Y0 Z0 ; make this position the new home
G90 ; return to absolute positioning

to your homing operation.

Upon reading some discussions on “proper” generic G-code, I think that this should also be valid:

G21 ; make sure we are in mm
G28 ; home
G91 G28 X-0.7 Y0.3 Z-0.4 ; set proper HOME to this relative dist from G28 sensed HOME
G90 ; return to absolute positioning


If there is not a place in pronterface to define this homing procedure, it might be nice to get it into the delta firmware somehow. It would make sense that the above XYZ home offset would be among the parameters updatable by M-code, and saved in the EEPROM.