Extruder Calibration

[Daid 306]

(if the counter nut is tightened properly).

Voila

Not sure how I caused it, but mine was slipping in the beginning. Afterwards I used some... excessive force to tighten it, and no problems anymore.

[mm_build 1]

When two gears inter-mesh, the force from the opposing gear keeps them turning together in time. The tooth spacing is such that they are forced into a steady increment of teeth-per-rotation. The extruder gear works in the same way, except instead of another gear to press against, there is a plastic filament. As the extruder gear bites into the filament, it creates teeth. However because the filament is soft, the gear is free to carve teeth that are loose and not a perfect fit. This tendency is caused by kinetic friction on the filament. As friction on the filament increases and becomes greater than the force required to compress the plastic, the path of least resistance is to carve a wider channel.

The gear and the teeth of the filament stay interlocked, but it is a sloppy fit, this is possible even with metal gears. It is possible for the teeth of the extruder gear overcome the integrity of the plastic altogether, which results in a situation where the filament is stripping. Because ABS is softer, it appears to not only strip more easily, but also yields to the extruder teeth sooner. At the forces required for extrusion, ABS is already reaching a point where it is slipping enough to cause a noticeable difference in rate of plastic from what the machine is supposed to be feeding. Settings such as "Packing Density" have been used to compensate for this.

On the left is an extruder gear working at 100% efficiency, it is essentially simulating a rack-and-pinion such as you would see in real life. On the right is the filament slipping by a small amount, causing the tooth marks in the filament to be closer together, and feeding less overall length. As the extruder gear turns it carves into the imprints of the plastic instead of pushing off of them, resulting in rounder grooves with overall less spacing.

The above example simulates a 10% rate of slip. Below are two similar sized pieces of filament fed at different tensions. You can visually identify a difference in the spacing.

[joergen 2]

Thank you, great explanation!

I am glad my (overly) complicated hypothesis of slipping steppers can be dismissed.

At least I got one thing right, that the extrusion calibration should be done with the right tension setting...