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Technical explanation of ringing

Posted by brucehvn 
Technical explanation of ringing
August 02, 2019 07:48PM
I've just rebuilt my i3 variant from the ground up using vslot and I'm in the process of doing calibrations. Both with the old and the new printer I'm having some issues with ringing. I've read many articles about adjusting acceleration and jerk settings, but I really want to understand how the ringing happens from a technical point of view.

Let's say I'm talking about a top solid layer where I can see that coming off of the edges where a complete 180 degree change in movement happened, there some ghosting. And if I have say a bolt hole in the top of the object, I'll see the ghosting of a partial circle maybe 3 or 4 of them. Again that's where a complete change in direction happened. So, what physically is causing those ghost images? What's happening with the extruder motion that causes those? If I understand that, then maybe it can help me understand how to tweak acceleration and jerk. I know about the Prusa calculator for acceleration, but that doesn't really recommend any particular setting.

I also am assuming that the acceleration settings also means there is a deceleration that is the same so when the extruder is approaching a direction change, it slows down the movement to the point of reversal.
Re: Technical explanation of ringing
August 06, 2019 12:22PM
When the printer is moving, it has momentum, and you stop it suddenly (Say to make a 90 degree corner) there can be some flex in the frame and supports, which "rings" like the sides of a bell for a given period of time. While this happens, the print head has continued on its course, and the very slight ripples get left in the print path.

In my experience, ringing is generally a symptom of inappropriate movement in the printer's frame, and can be made better with stiffening of the printer's frame. My leadscrew printer has slight ringing in the Y axis, caused by the acryllic frame having a little flex where the Y drive mounts to the frame. I had thought hta tit would be stiff enough, but it obviously was not. I haven't been bothered by it enough yet to reinforce the weak link (And find the next weak link) but may end up rebuilding the printer with stiffer parts.

Decreasing acceleration helps, because it reduces the force causing the ringing, and reduces the span on the extruded filament where the ringing can show itself. Yes, the acceleration setting also governs deceleration, as in this case, it's defined as a change in speed, rather than as just an increase. One of the bigger changes that you can make, however is "jerk" which is an "instantaneous" change in speed. This is used so that the printer can make arcs or small changes in direction without needing to calculate the acceleration for each tiny change in direction each time, but still gets used for larger angles. If this is set too low, the printer has to slow down a lot in order to make curves, but it will also reduce ringing. Low jerk + low acceleration will dramatically increase print times, since the printer will hardly ever reach full print speed.


MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts
Re: Technical explanation of ringing
August 12, 2019 08:21AM
Ok, that's a pretty good explanation. In my case, I doubt frame flexing is an issue with the rebuild printer. On my old printer which was an i3 variant, I could see that could be an issue. It used a single metal plate for the vertical frame and of course that was held in place using threaded M8 rod and nuts. I always felt like that was not really stable.

The rebuilt printer, though is very solid with the entire frame built from aluminum extrusions. Plus, I'm using a Titan extruder with a pancake motor so there's a lot less mass than I used to have.

So, based on what you are saying, the ghosting is most likely happening during acceleration away from the edge, or bolt hole and the drive can't meet the acceleration requirements that I've imposed on it, meaning that the print head is not making a smooth speed up which results in some extra deposits of filaments. Kind of a stuttering effect though probably not visible with the human eye.
Re: Technical explanation of ringing
August 12, 2019 11:57AM
The motor's rotor is coupled to the coils by magnetic fields. The fields act like springs. The belt can stretch a little- it's a spring, too. Everything that's moving has inertia. So you have a spring driving a spring that's driving a mass (extruder carriage or bed). Even if the frame is a perfect solid, and there's no flex in pulley and motor mounts, you will have ringing.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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