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Printing screw threads

peterb8f15b29
(@peterb8f15b29)
Famed Member

I am having trouble printing bolts and screws. The external threads always get filled and they are impossible to free up even using a die. The sizes range from M2 to M8.I have tried using a PVA filament for the supports but this does not seem to help - they are still filled.

Any ideas please.

Regards

Peter 

Quote
Topic starter Posted : 10/09/2021 8:11 am
Pablo
(@placherre)
Member Moderator

Hi Peter!

Great challenge you have ahead… 😀 

Would you send me some pictures of the models you have printed? What Slicer are you using? In a different topic I read you are trying to install Prusa Slicer, did you succeed with it? Because it will be good to have the settings you set up. If you were successful installing Prusa Slicer, you can export the project with the .3mf extension. The file with that extension will have the model and the settings as well. In Prusa, go to File > Save Project As... > Name it and save it. In Cura, click on File > Save Project… (a Summary window will open, click Save) > Name it and Save. Then, I would recommend you to save the file as a compressed file. Otherwise, you won't be able to attach it on the reply.

Also, would you send me the source of the files? So I can take a closer look at them in Fusion. 

I am looking forward to your response.

Kind regards,

Pablo

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Posted : 10/09/2021 6:40 pm
peterb8f15b29
(@peterb8f15b29)
Famed Member

@placherre

Hi Pablo, Thanks for the reply and appologies for not getting back to you sooner.

I have attached a photo of the prints

20210928 110327
  • The top one an 80mm M8 threaded rod printed vertically and after I started threading it with a Die

  •  the Middle one is the same 80mm M8 threaded rod printed horizontally with supports (the flat you can see)

  • the bottom one is an M5 Wheel Passive spindle printed vertically with an 8mm long thread on the end.

     

  • I know the strength of a print is in the direction of the grain IE the direction of the nozzle movement but if I print a rod horizontally I am faced with the support os the underside which just fill the thread! But the print seems to fill the thread anyway.

Hope that is enough to go by

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Topic starter Posted : 28/09/2021 4:41 am
peterb8f15b29
(@peterb8f15b29)
Famed Member

Hi Pablo,

Sorry you asked for the source of the file. They were created on AutoDesk Inventor and sliced  on Cura.

Kind regards

Peter

ReplyQuote
Topic starter Posted : 03/10/2021 3:16 am
Pablo
(@placherre)
Member Moderator

Hi Peter!

I apologize again for the late response…

After analyzing your description carefully, I would suggest some topics to consider. Generally, printing small thread sizes is not an easy task. But, I will help you to achieve the desired results.

The first thing to have in mind is the purpose of the bolt you are trying to print. Be aware that the loads you will be able to apply are very much lesser than industrialized metallic bolts. Also, there is not enough information about the mechanical properties of PLA. Due to this, I recommend you to print several models and test them with different loads in order to verify what is the appropriate load that your printed bolt can resist. If you have a luggage scale, you can experiment with different weights. If not, use weights that you know their values. Attach one end of the bolt to the scale and the other end to the test weight. Then, check at what weight the bolt fails. The following video describes the measuring procedure by comparing bolts printed with FDM or SLS (resin). But, the main idea about the video is to show you how to test your prints.

Link: https://www.youtube.com/watch?v=k5xxZKv7IRg&t=17s

Mechanically speaking, the recommended orientation should be horizontal. Because all layers will react perpendicular to the tension and torque direction of the thread. On the other hand, if you place the bolt vertically, the layers will be parallel to the tension and torque forces. Therefore, the thread will resist less tension and torque when placing vertically the thread axis compared to the build plate. However, if you place the bolt horizontal, a satisfying printing result will be hard to achieve (you already experienced it). The main issue of placing the model horizontally, is related to the thread crest size (see the following picture attached for reference). For instance, in a M5 thread, the width of this tip is below the nozzle diameter. Bearing this in mind, part of the thread (the one placed on the support) will be different from the rest. 

Thread Nomenclature

Another disadvantage is related to the support you have to place underneath the thread, because the bolt head is placed on the build plate. Not only because you have to remove it; but because part of the thread might come out with the support.

In conclusion, by printing the bolt vertically you will get better printing qualities in detriment of tensile and torque strength. In my opinion, I suggest you print the model with the thread axis perpendicular to the build plate.

Regarding the die, have in mind the heat generated when rectifying the thread. Because the friction between the printed model and the die creates temperature, which may end up damaging the print. Remember that PLA can go from a rigid to a flexible state when the temperature is close to 55-60°C. Due to the Glass Transition Temperature. On account of this, use the die only if necessary and turn it slowly.

When setting up the slicer parameters, if possible, use a 2 mm wall thickness (or 5 perimeters when using a 0.4 mm diameter nozzle). This will ensure a strong thread. Related to this, since infill will help to ensure a secure thread, I will suggest you to set up the infill to 25% at least. In addition, you could use the gyroid pattern, which gives you a better resistance with less infill compared to other patterns.

One of the most important settings is the layer height. Because the layer height is related to the thread size: the smaller the thread, the thinner the layer should be. As a general rule, for M12 and above bolts the optimal layer size is 0.2 mm. Therefore, in your case you should print your bolts with a 0.1 mm layer height. Bearing this in mind, remember the optimal layer height should be between 25-50% the extrusion width. For example, if you are printing with a 0.4 mm diameter nozzle, the optimal layer height should be between 0.1-0.2 mm. Another thing to consider is that the lower the layer height, the printing time increases. 

Furthermore, the layer height is related to the pitch of the thread (the smaller the thread, the smaller the pitch). Referring to the previous picture, the pitch is the distance between two counterpart points of a thread (between two thread crests, for example). For a M8 thread, the pitch is 1.25 mm. Having this in mind, the layers should be exact multiplies of the pitch. So, if you print with 0.1 mm layer height, there will be 12,5 layers in every pitch (which is not a round number). But, for a 1.25 mm pitch, for 12 layers, the layer height should be 0.104 mm, which is hard to achieve by the nature of the printer’s hardware. In conclusion, the for the M8 bolt print with a 0.1 mm layer height. For a M5 thread, the pitch is 0.8 mm, which will give you a total of 8 layers in the pitch distance.

Another important concept to consider is the part cooling configuration. In order to achieve a better layer adhesion, the part cooling is very important. First of all, run the part cooling fan at 100%. In addition, increase the cooling time on the model because you are printing a small model. Hence, you have two options: reduce the printing speed or print more models in the same batch. The main idea is to allow the material to cool down at every layer. Otherwise, the layer adhesion will not be satisfactory. When you place many models in the same printing batch, once the nozzle moves to another part, the latest printed layer will cool down properly before the nozzle comes back to print the following layer.

Additionally, if the bolt doesn’t have a full thread, you can optimize the printing time by setting up different layer heights. For example, you could print the head and the shank with a 0.2 mm layer height and the thread with a 0.1 mm layer height. 

Please, let me know if you have any questions about any of these topics… 😀 

I am looking forward to your response.

Kind regards,

Pablo

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Posted : 07/10/2021 1:31 am
peterb8f15b29
(@peterb8f15b29)
Famed Member

Thanks for the very comprehensive response. it is good to understand the why rather than just a "do this, do that" Thank you.

I will print them again using your logic.

As it happens there is virtually no load on the bolts so strength should not be an issue in this case but it may be more important on other projects.

Thanks again

Peter

ReplyQuote
Topic starter Posted : 09/10/2021 12:57 pm
Pablo
(@placherre)
Member Moderator

You are welcome Peter! 

If you have any concerns, just let me know.

Happy printing!

Kind regards,

Pablo

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Posted : 09/10/2021 1:44 pm
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