As promised, here is my procedure for calibrating a dual head printer.
Background.
The following procedure was developed for a ReliaBuild 3D Dual head printer (Previously Rigidbot) and printers of a similar design. The dual heads are two separate stepper motors and nozzles on a single carriage.
Mechanical precision is a key part of this procedure working. This particular machine is using the milled bed and extruder carriage, and heated bed for maximum flatness. No stamped parts. The bed is flat within 0.05mm or less across the surface.
The frame design allows for a closed loop leveling process starting with the bed rails (Y) being in plane with the mid rails and the Z rails being perpendicular and mounted 90 degrees to them. The top rails are in a parallel plane to the bed which aligns the X rods to also be parallel and 90 degrees to the Y rods. The flat bed is floating on the Y rods which allows it to be leveled parallel to the X gantry at the proper distance from the extruder nozzles closing the loop. See picture one.
I have extracted out and generalized the parts of the process that are more or less universal to this configuration of printer. I am assuming that the mechanics of the printer are already calibrated to the proper steps. I calculate those based on the mechanical parts and don’t change them from there. I also assume that the jerk, acceleration, and selected print speed are balanced. So here we go…
Step One (Picture 2)
I check that the bed is flat while hot. I use 60C for PLA. I use a level and test it in X and Y as well as each diagonal. I test for flatness by running a thin piece of paper along the edge and note that it does not slip under the edge of the level. This confirms there is less than 0.1mm of variation at this point.
Step Two (Picture 3)
I make sure the nozzles are hot and cooling fans are running. I level the bed with a piece of printer paper (0.1mm thick standard) to the lowest of the two nozzles. This time there was no difference between the two but if there is then I focus on the lowest one. The stock leveling routine for the printer provides 4 leveling points plus the center. I use the “Go Back” option to move back and forth on one side and them do the same on the other till I get the same amount of drag at all four leveling points. I make sure my hand is clear of the leveling bolts when testing the paper drag so I don’t put pressure on the bed. Then I move to the center of the bed and lower Z to zero to confirm I get the same drag on the nozzle at that location and confirm that the bed is indeed flat.
Step Three
I measure the difference in nozzle heights by homing the heads on a piece of paper and then sliding additional layers of foil under the nozzle that is the higher of the two till I get the same drag on the main paper for both nozzles. Splitting the paper into two long pieces, one under each nozzle can make this easier.
Step Four
I equalize the nozzles by putting the foil shims from the higher of the two nozzles under the mounting block of the lower of the two nozzles. This requires raising the gantry at least 170mm high so I can reach the mounting bolts underneath. Remember the nozzles are hot. Some printer designs will need to be cooled down. After the shims are placed and the mounting block tightened back down I re-level the bed to the two nozzles while hot. Should only require the same 1/8 counter clockwise turn on all four bed leveling bolts to true it up. I make sure I can go all the way around the bed without having to adjust the bolts.
Nozzles are now trued up to the same level above a flat bed.
Step Five (Picture 4)
Find the optimal melt temperature of the two filaments. The heads should already be hot from leveling the bed. I raise the nozzles about 35 mm off the bed and selecting nozzle 0 extrude out a constant stream of filament. If the filament pig tails and curls up under the nozzle it is too cold so I raise the temperature 5C. If it drops down, falls over and spider webs then it is too hot so I drop the temperature 5C. I want a rope coil on the bed with a straight vertical section rising to the nozzle. Switch to nozzle 1 and do the same thing. Note the temperature at which each filament produces a little cobra. See picture 5.
Step Six (Picture 6)
Print a pair of calibration clips. These clips adjust the physical measurements to compensate for the thermal properties of the plastic. I do this for each roll. I write the final properties I determine on each roll of filament along with the nozzle size used and possibly the layer height if I want.
I get the clips here: https://www.thingiverse.com/thing:1037301
I start with the measured diameter of the filament and the previously determined optimal melt temperature. When the clips are done I wait for them to cool to 40C and remove them from the bed. Flip one of them upside down (bed squish sides are facing away) and snap them together. If the clips are tight then I increase the nominal filament diameter or lower the extrusion multiplier. If the clips are too loose then I lower the filament diameter or increase the extrusion multiplier. I may need to print two or three sets to dial them in but it is worth the time. When there is no under or over extrusion the clips will fit together easily and stay put with no wiggle. See pictures 7 and 8.
The two sets of clips are interchangeable with the same fit. See picture 9.
Note that one filament required a size of 1.8 and a temperature of 210. The second filament required a size of 1.75 but a temperature of 215. Yet they each produce the same dimensional result on identically stepped extruders.
My filaments are now calibrated to my machine and each other. I do not skip this step. This is very important for when I print a dual color calibration object.
Step Seven
To determine the X and Y offset of the nozzles I roll a piece of clay between a folded piece of paper (See picture 10) and home the heads on it creating two dents. Any soft thick material that can be dented and hold a mark will work here. I then measure the distance between the dents as accurately as possible. See picture 11. I enter this value in my slicer.
While not shown I can get an initial Y offset by moving the heads in X double the distance they are apart and homing in Z again. I be very careful to not shift my paper. This will create two sets of marks that I can then draw a straight line between the dents made by the same nozzle. There will be a section where the lines are side by side. I measure the distance between the lines to get a starting point for my Y offset.
Step Eight
Print a calibration checker board.
I find it here: https://www.thingiverse.com/thing:2172438
I use an ooze shield with a skirt set to 2 mm away and at 4 lines so that the ooze shield has a better base to stick it to the bed and not get ripped off.
The long idle times of the non printing head will generate little arms and the ooze shield captures these. The part cooling fans cool the arms enough that they don’t stick strongly to the shield and hang up the nozzles.
I do not need to print the entire board while making fine tuning adjustments. Four or five layers above the bed is enough to get a clean measurement. Any offset in the alignment can be measured and adjusted in my slicer.
When I feel you have everything dialed in I print a complete calibration checker board. See pictures 12 and 13. If no further adjustments are required the colors should transition smoothly with no lip or catch in all directions. A testament to the effectiveness of calibrating the two filaments to each other and the alignment of my nozzles.
Step Nine (See picture 13)
I print a set of dual color objects to test my setup. In this case I printed traffic cones with the colors set opposite between the two. The ooze shields are a nice creamy blend of the two colors and the transition of the colors on the cones is smooth and crisp.
While the initial calibration steps do take some time I only have to do it once for a particular set up. From that point on it is load and go.
I hope this provides some useful tips to help others calibrate their dual head machines and get great prints more easily.
Picture 14 is the result of a first time user with the a new machine following these steps. Both sets in the image were printed on the same run, and on the first test attempt.
Happy printing!
