Suggestions on my layer shift issues?

Frank833

Large delta with Duet Wifi.

I am having layer shift issues. It seems it is ALWAYS:

Between 1CM and 2.5CM into the print
When my prints are large - as in 250mm across

If I print something smaller, say 50mm across, it never seems to happen
It does not always happen when I print large items
I am always printing slow (need to get it reliable first), typically running about 15-30mm/s

What I have tried to fix and or have changed for other reasons:

Checked for binding, found a bit and fixed it
Upped drivers to 2A with 60% holding
Switched from GT2 to GT3 (in case it was a belt slip)
Moved from Nema 17 1.8 to Nema 17 0.9 steppers (to not loose resolution with larger GT3 pulleys)
Added cooling fans to Steppers (they were VERY hot, now run about 80-85F [less than 30C])
Added fan for Duet (though driver chips never seemed hot)

The cooling fans on the steppers seemed to have helped as I could NOT get a complete large print before that point. Now I can…. sometimes.

What seems weird, is that if a single tower had an issue - belt slip or stepper issue, it should cause one end of the print to be high or low. But that is not the case.

In the pictures, I choose to let this print finish because I can 'fix' this part manually. This part is about 10in / 25cm across in X and Y on this print. Shift roughly 6-8mm. With a part this far across, it seems a shift would be catastrophic as it would lift one side a bunch, or grind the head into one side a bunch.

There is no way for my bed to shift without it being noticed - and that seems to me to be the 'logical' way for a shift like this to happen.

A bit more data that may or may not be relevant....

Parts designed in F360
Sliced with Slic3r Prusa edition (latest)
Printer uses THK 15 size linear rails (bit bigger than those used on small deltas)
Printer uses a flying extruder, but the weight of it is held from the frame of the delta, not from the carriages

The attached pictures show the shift about 20mm into the print, and a possible smaller shift about 20mm higher.

Suggestions on what to check/change/investigate from here?

Thanks,

Frank

dc42

If it only occurs on large prints then perhaps the travel speed is too high. Use https://duet3d.com/wiki/Choosing_stepper_motors#How_to_work_out_the_power_supply_voltage_you_need to work out whether you have a high enough supply voltage to maintain torque at the maximum speed you are using, and/or try reducing the maximum speed.

Frank833

The above print is at 25mm/s print, 40mm/s travel.

40mm/s gives me 5.7 volts on the calculation (7.7 if you add a couple of extra volts). I am running 12v and am happy to upgrade to 24 as needed. The power supply has ample amperage and my bed heater is running on mains.

Frank833

Here is what went into my calculation:

Target Speed 40 mm/sec

Delta Smallest Rod Angle 30 degrees
Belt pitch 3 mm
Pulley Size 20 teeth
Motor Current (Used) 2 amps
Motor Inductance 3.00 mH
Step Size 0.9 Degrees
Rated Current 2 amps
Rated Holding Torque 0.46 Ncm

Maximum Belt Speed 69 mm/sec
Max Motor Revs 1.2 second
Back EMF due to inductance 4.5
Back EMF due to rotation 1.2

Suggested voltatge 7.7

dc42

Looks like the problem isn't loss of torque during travel moves then.