Gcode varies retraction value?

[sam2b 1]

Cura version 4.4 at least...

Hi, I am confused: when I specify 4mm retraction in Cura, why aren't all retractions 4mm within the Gcode? Read below for context.

 

I set Cura to have a 4mm retraction value. After slicing, out of curiosity I was browsing one of my Gcode files to verify the retraction values were indeed 4mm. To my surprise all (but one) G1 lines regarding retraction all have less than 4mm, and all vary in values!  I would enjoy a technical answer as to why, please.  Thanks in advance.

 

This is a search for all retractions in my gcode generated by Cura:

Edited November 23, 2019 by sam2b

[burtoogle 514]

Hi, can you please attach the gcode file to this thread. Even better, please save the project (File -> Save) and attach the .3mf file to this thread. Thanks.

[sam2b 1]

Sure, thanks.  Attached is a zip file with both the .3mf and .gcode files.

Retraction_Varies.zip

[burtoogle 514]

Duh! The answer is obvious now I think about it. You're using absolute extrusion so when the cumulative amount of filament extruded is less than 4mm, a 4mm retraction will require a negative E value. i.e. if the current E value is 0.5, a 4mm retraction will require an E value of -3.5. Note that every now and again, Cura puts a G92 E0 in the gcode to reset the absolute value back to 0. That is normal. Hope this helps.

[sam2b 1]

Thanks for the investigation.

I'm sorry I do not understand your example. How/why would a 4mm retraction require an E value of -3.5?

9 hours ago, burtoogle said:

i.e. if the current E value is 0.5, a 4mm retraction will require an E value of -3.5.

 

Edited November 23, 2019 by sam2b

[burtoogle 514]

At the start of the print, the absolute extrusion value is set to 0. As the print progresses, the value increments. When a retraction occurs, 4 is subtracted from the value. So, for example, imagine that after a line was printed the extrusion value was, say, 1026.35. If a retraction occurs at that point, the E value will be set to a value 4 less, i.e. 1022.35. When the travel move has finished, the un-retract occurs and the E value will return to 1026.35 (or even greater if an extra-prime amount has been specified). Therefore, until the cumulative amount of filament extruded is more than 4, a retraction will make the E value become < 0.