What I have written down are absolute physicaly possible maximums for my machine (if I would allow it to run in SW to the very limits) ... and I'm not sure even they are achievable with my 24V power source (but they would be achievable with 48V power source). The reason I'm not sure here is because of the static moment charts for my steppers are missing or incomplete for 24V power source. I considered only 1/3 of the maximum static torque of my steppers but may be it should have been 1/5.
The point is when you actually want a high acceleration to lead to some error you must be able to sustain the acceleration long enough to actually move the carriages for a distance bigger than the error they should lead to and that means you must be able to run steppers at high enough speed with some torque still available. With high acceleration speed ramps up really quickly. The motor torque goes down with the speed because of wiring inductance. But to sustain the acceleration printer needs the torque.
Anyway I rerun my notebook with only 1/5 of the maximum torque and got maximum acceleration of 5.6g and maximum belt elongation of 0.3mm and maximum radial force on carriage of about 10N (and that should lead to rod bend of about 0.4mm - this is just "by eye" measurement - I'm lazy to pull the rods out of the printer and do it precisely). For these to be achievable the maximum printer speed limit can stay at probably common 0.3 m/s
but the maximum acceleration would need to be set to uncommon 56000 mm/sē (most people probably use only 10000 mm/sē or 9000 mm/sē, which was (and probably is) the Marlin default) or it can be somewhat smaller but then jerk must be non-zero.
Most people probably use the default acceleration limit which is about 10e3 mm/sē = 10 m/s^2 ≅ 1g. But they also will probably have non-zero jerk (probably about 20 mm/s).
If they would use also zero jerk (which probably nobody has set to 0 but lets assume it for a while) then that would mean that their maximum belt stretch error for T2.5 steel core belts is about 0.07 mm. 20 mm/s jerk will probably increase it only by about 0.005 mm (just an educated guess).
So really, if people run at low acceleration limits of about 1g and low jerk (at most few tens mm/s) then they can assume their belt stretch just below 0.1 mm. That is if they use steel core T2.5 belts. If they use glass core GT2 belts then the maximum error is about 3 times worse (i.e. 0.3mm). The rod bend error will be around 0.1 mm.For these numbers to be valid speed limit may stay rather low at about 300 mm/s. If they use some insane high jerk (but which still does not lead to stepper motor skipping) then that may increase their maximum error by at most half step (i.e. about 0.1 mm). This all assumes their frame is rigid and no resonance happens.
Now, do I typically hit these errors. Hah, not at all because most of the time:
- my maximum acceleration is 0.4g; limits belt stretching to 0.02mm at most (about 1 micro-step)
- my printing moves are limited to 60 mm/s (I still did not improve the extruder), which is effectively the same as maximum acceleration limit of about 1.8g (from the point of view how it can influence belt stretching); limits belt stretching to 0.1mm at most
- although I typically use non-printing moves of 200 mm/s these are mostly initiated with retract and ended with retract reverse, since these take about 0.1s that is plenty enough for position to settle to the precise spot
So if I would improve the printer then the order of improvements should be:
- stiffen the frame (slight vibration of the top plate can be seen while printing), I doubt the amplitude is more than 0.1mm, I could measure it with accelerometer in my mobile - easy and simple, but I'm probably lazy
- check whether there is a play between belt and pulley, people say a lot of pulleys are not precise enough
- and I should do something with the extruder so that I can print more quickly
As for as people claiming some insane speeds. Well maybe they citied their slicer limit and forgot about lower limit in firmware :-)