Harmonic vibrations (or: Go fast for nice prints)

Warning: Don’t try this at a stock TAZ, or at least at your own risk! The IGUS bushings may start to “smoke” at the speeds mentioned below!

I was thinking about harmonic vibrations (visible on external perimeters) today and it would be nice to hear your opinion on that.
Have a look at my post with picture from the Openbuilds X axis mod: https://forum.lulzbot.com/viewtopic.php?f=16&t=2252&start=280#p23904
I got straight walls, but at some speed there is a ripple in it which comes from harmonic vibrations. I have such ripple in nearly all of my prints when the perimeter speed is not <40mm/s. But on the other hand, there are piercets high speed trials at 400mm/s and his cube is perfectly straight. So I did some tests today to see where the vibration is coming from and how it is related to movement speed.

There is the very useful (but badly documentated) DryRun feature of Marlin which can be used to “print nothing” at different speeds sending “M111 S8” to the printer. So I sliced a test cube (100x100x20) with a single perimeter at 100mm/s. This way I was able to set the real print speed by the speed % on the TAZ display, 60% means 60mm/s for example.
I let it run, with a finger on the extruder body to feel whats going on (same is also true for the Y axis by the way):
.) Starting from low speeds (20mm/s), the extruder body is vibrating a lot and it gets worse as the speed is increased until 80mm/s. The Openbuild Rail and the stepper motor mount are not vibrating, but you can feel the same amound of vibration also on the stepper itself. So source = stepper, transmitted by the belt.
.) Going from 80mm/s to 100mm/s, the vibration on the extruder body gets lower rapidly. The stepper is still vibrating, but the X axis “sledge” seems to dampen this higher frequencies.
.) At about 100-120mm/s, there is no feelable vibration left on the extruder.

With this results, I did my first real high speed run, a test cube with 40x40mm at 150mm/s. To keep the extrusion rate in an acceptable limit, the layer height was set to 0.1mm (=7.2mm³/s). Tada, one of my nicest prints ever! Zero ripple on the walls, only my stock Y axis had a little bit of trouble while accelerating to 150mm/s which results in a short but heavy corner ringing on the Y side :laughing:

Somebody here who had a simmilar observation? It would be intresting to know how the aluminum backplate for the Openbuilds Mod compares to my printed one regarding to this vibration dampening.

In the high speed trial videos, my Taz also has the X and Z cable chain anchors on the backplate, which probably acts as a damping mass http://www.thingiverse.com/thing:1277441

Which extruder chassis are you using? My reinforced and altered chassis (http://www.thingiverse.com/thing:1113697) is about twice the amount of structure that you find on the Taz 6 stock prototype version. That comes at the cost of additional mass and the Taz 6 style is better for PLA, but mine is probably better suited for high speed runs, and the additional mass may be damping mroe of the vibrations you were seeing. It’s also possible you have an unbalanced stepper motor core. You might try swapping the X axis stepper and one of your other axis steppers, maybe Z and see if it has any effect.

Yes I can imagine that you X cable chain mod acts as a damper on the X belt mounting bolts.
I’m using the stock TAZ 5 chassis but with the reinforced extruder body using the top mounting screw, but I think that will not change much in this situation. If yours is more stiff, it should transfer the vibration from the belt even better…

Stepper core should be OK, the vibration that can be felt on the motor itself is the same on X and Y for equal speeds. The vibration level on the stepper is also quite equal on ever speed, only the level at the print bed and X carriage varies. It seems as low frequencies (low speed) are transfered 1:1 through the belts, with a resonance peak at about 50-60mm/s on the X carriage while higher frequencies are absorbed quite well by the construction.