Best posts made by Peter120

After lots of testing I am finally ok with this version.
Turns out the seesaw concept is amazingly accurate for measuring the nozzle tip to probe height offset. The resulting measurement produces single digit micron errors. While this level of accuracy is overkill; all elephant foot is gone.

I used a ¼”-28tpi hex bolt and with a hack saw converted it into a T-bolt. The friction (rotation brake) results from ball bearing preload. Two belleville washers (cupped spring washers) axially squeeze the inner bearing races together as the nut is tightened which increases the braking force. The nut also secures the T-bolt to the 2040 extrusion. An aluminum pipe is used as a spacer between the 2040 extrusion and the belleville washer.

The seesaw "board" is bent to lower the T-bolt mounting position on the 2040 extrusion.

A paper clip was formed into a ramp that resets the seesaw height for the next offset probing cycle whenever the Y axis is homed.

For cleaning plastic from the nozzle tip, I used a 614 foil from an old Braun shaver (it didn’t have any big holes when I started). This is effective for cleaning plastic from a HOT nozzle tip over a few mm of Z height but it is not fully finished.

Some parts for a previous design that were not used.

Cheers -Peter

Motor acceleration settings are an accuracy vs vibration trade off and you just pick your poison. The stock Z axis on a Cetus shakes whenever the extruder accelerates. If you add a steel bracket and tie the Z stepper face to the aluminum base it improves dramatically. I upgraded to a 24V power supply and in the config.g below I turn off the steppers when the print finishes. If your Z axis brake/clutch slips fix or increase the stepper idle current so the extruder linear rail does not fall on the print!
The belts have glass cords and they establish the printer motion accuracy together with the steppers so no calibration is even possible aside from axis tramming.

; ****** X&Y axis swapped May 6 2020 origin now front left

; General preferences
G90 ; Send absolute coordinates...
M83 ; ...but relative extruder moves

; Network
M550 P"Duet" ; Set machine name
M552 S1 ; Enable network
M587 S"xxxxx" P"xxxxxxxxxxxx" ; Configure access point. You can delete this line once connected
M586 P0 S1 ; Enable HTTP
M586 P1 S0 ; Disable FTP
M586 P2 S0 ; Disable Telnet

; Drives
M569 P0 S0 ; X Physical drive 0 goes backwards
M569 P1 S1 ; Y Physical drive 1 goes forwards
M569 P2 S1 ; Z Physical drive 2 goes forwards
M569 P3 S1 ; E Physical drive 3 goes forwards
M350 X256 Y256 Z256 E256 I0 ; Configure micro stepping with no interpolation
M92 X1280.00 Y1280.00 Z1280.00 E1650 ; Set steps per mm, extruder calibrated white PLA
M566 X900.00 Y900.00 Z900.00 E120.00 ; set maximum instantaneous speed changes (mm/min), speed threshold for executing next gcode move
M203 X12000.00 Y12000.00 Z12000.00 E7000 ;E1200.00 ; Set maximum speeds (mm/min) about 200mm/s
;M201 X3200.00 Y7000.00 Z3200.00 E12000.00 ; Set accelerations (mm/s^2) for 1Kg,350gr,100gr X skipped a few steps
M201 X1500.00 Y1500.00 Z1500.00 E1000.00 ; Set accelerations(mm/s^2)for 1Kg,350gr,100gr https://wilriker.github.io/maximum-acceleration-calculator/
M906 X450.00 Y450.00 Z450.00 E400.00 I0 ; Set motor currents (mA) and motor idle factor in per cent, Z&Y steppers not quiet at 475ma PSU19.4V

M84 S30 ; Set idle timeout

; Axis Limits
M208 X0 Y0 Z0 S1 ; Set axis minima
M208 X190 Y191 Z277 S0 ; reversed X&Y axis May6 2020

; Endstops
M574 X1 Y1 Z2 S0 ; reversed X&Y axis May6 2020 X low end, Y low end, Z high end, S0 endstop triggered active low Vout

; Z-Probe
M558 P0 H5 F120 T6000 ; disable Z probe but set dive height, probe speed and travel speed
M557 X15:175 Y15:175 S20 ; define mesh grid

; Heaters
M307 H0 B0 S0.66 F400 ; disable heater bed bang-bang mode set max PWM, up Hz, 120W peak with extruder heater Amps jumping around
M305 P0 T50000 B2718 R4700 ; set thermistor + ADC parameters for heater 0 50K bed
M143 H0 S122 ; set temperature limit for heater 0 to 122C
M305 P1 X501 R47000 ; configure PT1000 for heater... per dc42 administrator suggestion with cetus PT100
M143 H1 S285 ; set temperature limit for heater 1 to 285C ....can be 300C

; Fans
M106 P0 S0.25 I0 F500 H0 T45 ; set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned on bed
M106 P1 S1 I0 F500 H1 T45 ; set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on extruder

; Tools
M563 P0 D0 H1 F0 ; define tool 0
G10 P0 X0 Y0 Z0 ; set tool 0 axis offsets
G10 P0 R0 S0 ; set initial tool 0 active and standby temperatures to 0C

; Automatic power saving
M911 S10 R11 P"M913 X0 Y0 G91 M83 G1 Z3 E-5 F1000" ; Set voltage thresholds and actions to run on power loss

; Custom settings are not configured

Thanks for the welcome and sharing your thoughts.
My underlying interest is just how accurate can a PLA object be printed or better perhaps is what is limiting print accuracy. Is SLA required for better print accuracy or can FDM be improved.

dc42 Point 1 is valid and it was not on my radar. I have a Cetus with a Z axis belt which I can coax it into showing micro stepping errors on selected printing tests. You picked a good printer geometry example. I thought about a springy leadscrew to motor shaft coupler supporting the build platform weight.

Point 2 I did consider but figured that internally the stepper is periodic every 4 steps or in my case 0.8mm and that one could choose a winding and polarity that resulted in some step position margin; however, I was thinking of belts and not screws. Your point 1 using a leadscrew might be hit and miss for Z, but probably not for the X and Y belts.
I found this link where they measured the stepper accuracy and it is obvious why thin Z layers have a problem with belts.
https://www.applied-motion.com/news/2015/10/stepper-motor-accuracy
Between two adjacent bad steps the difference error is 1/10 of a step which is 0.02mm for my belts. This is small until you think about the error as a percentage of a 0.05mm layer. Micro stepping errors would also subtlety vary the extruder nozzle flow rate.

deckingman. The interference fringe is sensitive to several printer errors but the trick would be separating out the individual variables and adjusting the printer. I posted an extrusion calibration test that you might like. It is based loosely on a micrometer. You print a nut and bolt and the snug final angular position measures the axial thread clearance which is known for the STL model printed. Flip the nut over and remeasure the angle and you get the overhanging thread sag.
https://www.thingiverse.com/thing:3390910