5.3 JKN Advance K

The Jetty-Kubicek-Newman (JKN) Advance parameters address extrusion issues associated with accelerating and decelerating molten plastic through the extruder. Left uncorrected, these issues can result in excess plastic at points of low speed (blobbing) as well as small bumps (blemishes) and small gaps. In truth, some of these issues are best addressed by using an acceleration-aware slicer or gcode post-processor. However, at present no such tools exist.

The first of the two JKN Advance parameters, K, addresses an issue associated with a plastic deficit when accelerating and a plastic surplus when decelerating. You will find a suitable value for K by looking at the corners of your calibration boxes printed with acceleration enabled and at reasonably fast speeds. The fill need not be solid. Indeed, printing with 20% infill will also be informative — make sure that it still looks good.

pictImportant Before proceeding and expending time and effort, realize that your printer ships with reasonably well-tuned JKN parameters. If you are experiencing printing defects, adjustment of these parameters will likely not be satisfactory nor resolve any issues you are attempting to address. The following tuning information is more here to satisfy the curiosity of idle, technical minds.

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Begin calibrating K by ensuring that acceleration is enabled and that the JKN Advance K2 parameter is set to 0 (Section 5.4). Also, make sure that the maximum X and Y axes acceleration values are at or below 500 mm/s2 so as to minimize overshoot effects. These values may only be set with ReplicatorG – Sailfish, MakerWare, or Desktop. To use the latter two, refer to Section 6.5.

Print a calibration box (Section 5.1.4) with the default K value of 0.005 (Replicator-style printer) or 0.007 (Thing-o-Matic or Cupcake). Then print two more boxes, one with K set to 0.0025 and another at 0.0075 (Replicator-style) or 0.005 and 0.009 (Thing-o-Matic or Cupcake). It can also be helpful to print a calibration box with K set to 0 for comparison.

What you are looking for is a value of K which helps reduce the amount of extra plastic at the corners but does not reduce it so much that the corners become bevelled (i.e., foreshortened) or gaps begin appearing in the solid surface layers (e.g., the box’s top face). As you decrease K below the optimal value, the top infill will be fine but the extra plastic at the corners will begin to increase.

Keep in mind that there is no single “ideal” value for K. You just want to get it into the right range.

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Figure 5.10: Calibrating JKN Advance K — calibration boxes printed on a Thing-o-Matic with ABS plastic

Examine the three calibration boxes shown in Figure 5.10. As you can see, the box on the right with K=0.01000 is showing some gaps, so the value of K is too large. The middle box with K=0.00500 has a reasonable top surface but the corners might do with a little less plastic, so K is too small. The leftmost box, K=0.00250 could stand to have a better top surface, again, K is too small. For Thing-o-Matics with an Mk7 extruder, a better value of K probably exists between 0.00500 and 0.01000. Note that the firmware’s default value for Thing-o-Matics is 0.00850. A different default value for K is used on Replicator-style printers since they use a different extruder and 1 / 16 microstepping — compared the Thing-o-Matic’s 1 /8 .