How to match threads of a bolt and a nut

[lapinsuo]

I designed a cylinder with threads and a matching nut with threads, both with the same settings (except "Class", which I cannot select to be the same). Yet when I 3D print them with PLA with 0.15 mm layer thickness and no supports, I cannot screw them together. I'd like to first make sure that the problem isn't with the model.

Settings for bolt:

Thread type: ISO Metric profile

Size: 14 mm

Designation: M14x2

Class: 6g

Direction: Right hand

Settings for nut:

Thread type: ISO Metric profile

Size: 14 mm

Designation: M14x2

Class: 6H

Direction: Right hand

Many thanks for any help!

[jeff_strater]

I am not completely sure what "class" is for threads - gotta be some thread experts out there.  But, I have printed threads in 3D before.  In my experience, you need to add some spacing for them to be able to screw together reliably.  Select the helix faces from one side or the other, and use Offset Faces to add that tolerance.  You'll have to play around with the offset value to get the ideal fit.  Here, I offset the male side:

[g-andresen · Accepted solution]

Hi,

for a thread arrangement you need to define a reasonable clearing. For example, start with a negative offset of 0.1 mm for the nut.

günther

[lapinsuo]

Thanks a lot for the fast replies! I tried -0.1 mm offset for 2 faces, but so far no luck. I redrew the nut and now applied -0.2mm. Will try to print tomorrow and let you know if I succeeded.

[g-andresen]

Hi,

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@lapinsuo  schrieb:

Thanks a lot for the fast replies! I tried -0.1 mm offset for 2 faces, but so far no luck. 

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I create offsets for all faces.

Have a look at the screencast.

günther

[TheCADWhisperer · Accepted solution]

I am surprised that none of the experts has mentioned that you Hex Nut is modeled all wrong.

There is no external access to the helical thread for the screw to engage.

3D Printing is not a particularly precise process.

You generally need to increase clearance compared to traditional machining processes.

Examine the Attached file...

Note the lead-in for the threads and the added clearance (adjust the clearance as needed for your printer).

Another option is to chase the threads with a tap and die after printing.

Examine the Attached file...

[g-andresen · Accepted solution]

Hi,

Unfortunately, since I didn't look at your file, I didn't find the errors that @TheCADWhisperer  pointed out.
The thread does not cut through the fillets/chamfers. So no bolt can be screwed in.
In the screencast and in the file you can see how to model the thread access. It is the same principle for the bolt.

günther

[lapinsuo]

Thank you so much everyone! Indeed, after remodeling through decreasing the size of the fillets, it works beautifully!

I also was able to find the sweet spot for clearance, which is 0.15 mm. At 0.2 mm it was easy to screw the nut in, but it wiggled a bit. At 0.1 mm I managed to do it with a bit of silicon grease, so 0.15 mm would probably be good.

[TheCADWhisperer]

@lapinsuo 

I hope you took the time to thoroughly investigate and understand my example. 
I would have expected additional questions from a beginner.

[lapinsuo]

Hi @TheCADWhisperer ,

Again, thank you for the time you took to model the right way. I did examine the model and I think the big mistake I made was to increase the fillet so much that it removed the entry to the nut's threads.

So, I learned to watch out for modeling actions that can ruin the entry plus increase clearance - I had never used that function before. 

As a beginner, I might not even realize what I should ask 😉

[TheCADWhisperer]

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@lapinsuo wrote:

Hi @TheCADWhisperer ,

As a beginner, I might not even realize what I should ask 

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@lapinsuo 
Can you recreate my model from scratch?

If not, you should be asking questions.  This will help on all of your future designs.

Did you catch the issue with @g-andresen example?