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I've been trying to print some M14 female threads. The male threads I've printed were perfect and fit into a female metal nut, but for some reason I cannot get the female threads to "stick". This means that they end up being hard to screw into, or the male end (both metal and my printed versions) goes in wonky.

I'm printing in PLA on an Ender 3 V2 at 200 °C on a 50 °C bed. I've tried various resolutions (down to 0.1 mm layer height) and they all exhibit the issue. Print speed is 50 mm/s, cooling is just the stock cooling and fan 100 % after the first layer.

The actual file can be found here (note that it's slightly oversized vs a metric M14 female to allow for printer tolerances) and the follow image shows what I'm talking about:

Example

It's as though the nozzle is pulling the filament away from the wall, but I've tried Z Hop Retraction and combing (infill only) and that gives the same result.

Answer: The accepted answer and the comments against the question pointed me in the right direction. A combination of turning off combing and reducing the hot end temp to 190°C gave me perfect threads in 3 consecutive prints.

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  • $\begingroup$ What size nozzle and what layer thickness are you using? In general, since threads are necessarily overhangs, thinner layers do better when printing threads. $\endgroup$
    – Davo
    Commented Jan 4, 2021 at 12:28
  • $\begingroup$ @Davo 0.4 mm nozzle. I've tried layer heights from 0.2 mm down to 0.1 mm and all have the same issue. $\endgroup$
    – djdd87
    Commented Jan 4, 2021 at 14:35
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    $\begingroup$ Great info. Print speed and any cooling? It looks like the deposited material isn't cooling (hardening in place) fast enough. $\endgroup$
    – Davo
    Commented Jan 4, 2021 at 14:46
  • $\begingroup$ Speed and cooling are key, please update the question. Note that PLA can generally be printed cooler than 200 °C. The overhangs are pretty large, part of the circumference is printed over air @ 0.2 mm layer height. $\endgroup$
    – 0scar ♦
    Commented Jan 4, 2021 at 15:08
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    $\begingroup$ Comments are for us to ask you for more details, you need to update the answers on the comments into the question (the more information, the better answers you receive on your question). I've updated the question for you. $\endgroup$
    – 0scar ♦
    Commented Jan 5, 2021 at 9:22

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I do a lot with printed threads, and find this problem creeps up mostly when the filament is wet. Higher temperature (like 220) can partly compensate but it's better to dry your filament and store it properly to keep it dry.

Assuming standard metric thread profile, though, these are also pretty severe overhangs for a concave extrusion (where motion of the head will tend to pull the way you're seeing) unless you use very thin layers. You probably need 0.16 mm or thinner to print reliably (independent of pitch, though finer pitch also needs finer layers for other reasons), and limited acceleration for outer walls (I use 500 mm/s²). If you slice with "outer walls first" turned off, you may be able to get by with thicker layers, provided you have everything else tuned perfectly to ensure the outer wall sticks to the previously extruded inner one (i.e. flow rate perfect).

A contributor to your problem could also be underextrusion due to oozing during combing. You might try setting max comb distance very low (like 0.6 mm, around what I use) or turning off combing and see if that helps.

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  • $\begingroup$ Hi, I was already using layers down to 0.10mm, which all had the issue. However, I did a slider flow rate test from Teaching Tech 3D (teachingtechyt.github.io/calibration.html#flow). It came out that I was under extruding, so I'm currently running a print at a higher flow rate (with a 0.12mm layer height and and combing off). Will report back later today when I've run some more tests. $\endgroup$
    – djdd87
    Commented Jan 5, 2021 at 8:37
  • $\begingroup$ Thanks - a combination of turning combing off and reducing the temp to 190°C solved it. $\endgroup$
    – djdd87
    Commented Jan 5, 2021 at 13:58

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