Why won't my threads line up?

[michael_ray_neal]

See model as no matter what I try, I can get the threads to line up.  Can anyone please help out.  Looking for what Ive done wrong or missed. Trying to 3D print when fixed.

[MRWakefield]

Add a rigid joint and set the rotation to a suitable value.

[etfrench]

 You only need to worry about that if the two pieces need to end up at a precise position relative to each other, for example, matching the grain in a wooden piece.

I'd recommend adding an entry chamfer to each of the threads. As currently modeled the pieces may not thread together.  Edit the thread features and turn Modeled off.  Add the chamfers.  Turn Modeled back on. 

You may also need to add clearance to 3d printed threads.  Normally .1mm to .2mm on three of the four faces works well.  I haven't tried this on threads this large so you may need to experiment to find the best value.

[michael_ray_neal]

OK, I did what you said and my model looks like yours now, but I guess I really don't understand how the joint command works with threads as just adding it and then setting the angle to 180 degrees lines up the threads perfectly....I hope when I print it.  But how did this work.  I always thought that you created the same exact threads on your two pieces and then did a small offset on the threads.  Going to read up on it I guess cuz me not understand right this minute...lol

[michael_ray_neal]

I don't think so as I think the threads need to line up and not overlap if they are going to work.  I understand offsetting the threads but that didn't work for me as the threads were so off. I understand chamfering the ends for sure but I don't think this is my only issue.  Can you maybe explain a bit more and even show how you do it your way?

[etfrench · Accepted solution]

As long as the threads use the same specifications, they will mate.   If you rotate the lid, you will find the position where they match.  No other changes are needed.

Fusion threads are not created properly with chamfers on the ends.  Chamfers help center the mating pieces as well as adding a gradual entry point.   Look at the entry points of the chamfered threads vs. the un-chamfered in this file (Just suppress the chamfers to see the un-chamfered).

3d printers usually make the threads slightly larger than specified, so offsetting the faces will give  you a better fit.

[MRWakefield]

Setting the joint angle rotates one component (including its thread) in relation to the other. In this case a rotation of 180° looks about right. As @etfrench has said, the rotational relationship really doesn't matter unless you need features to align on the two components when screwed together.

[michael_ray_neal]

OK, thanks for the help.  I printed the the model and the lid is still a bit tight (only screws on a little bit before real tight).  I understand offsetting the face of the threads but which threads.... the 2 thread faces of the lid or the 2 thread faces of the Top Ring?  You mention doing it on 3 of the 4 faces.  Can you explain which three and why only three and not all four?

[MRWakefield]

I made a post about this some time ago as there was a lot of misinformation being quoted regarding this subject.

https://forums.autodesk.com/t5/fusion-design-validate-document/general-guidance-on-adjusting-size-of...

Hopefully this should answer any questions you have.

[michael_ray_neal]

Added the offset.  Does this look better?

[MRWakefield]

Almost! Instead of selecting both flanks, just select one flank and both the major and minor diameters. The post I linked to above should explain all.

[michael_ray_neal]

OK...Getting close but take a look please....

I have attached 4 pictures.

1: Model without any offsets

2: Picture of me selecting the major, minor and one bottom flank

3: Picture of me selecting the major, minor, one bottom flank, and one top flank

4: Parts of a thread

Notice that when I select 3 faces (major, minor and one bottom flank), the threads look like they are going to be tight on the top flanks.

But when I select 4 faces (major, minor and one bottom flank, one top flank), the threads look like they have enough gap to work.

So what's the right way here....offset 3 or 4 faces?

Also love your post about creating threads.  Since I'm new to this, you might want to show a picture of what you call a flank, major and minor parts of a thread.  I think I figured it out but I used the picture attached for names

[michael_ray_neal]

Additional pic

[michael_ray_neal]

Also one last question...Do you want to offset faces on the male (external...Top Ring) threads or the female (internal...Lid). or does it matter as long as you do one or the other but not both.

[MRWakefield]

Ok, I can confirm that you've correctly identified the major, minor and flanks. As I mention in my linked post, if you select both flanks it will result in a distorted thread form. Crest widths will get narrower and root widths will get wider. This also results in a thinner and weaker thread form. The larger the offset the greater the distortion. Try entering a larger value and see what happens. Try the same exaggerated value when selecting the three recommended surfaces and see that the thread form keeps its shape.

It might be worth reading through my linked post a few times until it 'clicks'. Hopefully you'll reach a 'eureka!' moment when it just suddenly becomes obvious 😉

Don't worry about the fact that it appears to have more clearance on one side than it does the other. Don't forget that until the cap is screwed on tight it is free to float axially and radially all over the place. When the cap is screwed on fully only one pair of flanks (one on the cap and one on the Top Ring) will be touching anyway. This would be true whichever flank you choose. This is how screw threads work.

BTW: It's better to paste images directly into the body of the post (like above) rather than as attachments as it makes it much easier to view.

Hope this helps.

EDIT:

I've added some chamfers to the threads as suggested by @etfrench as it's good practise to have these. I've also changed the rotation value to show how the threads engage when screwed fully home. I settled on a value of 172.25° as this looked visually close to what I wanted to show. Feel free to play around with this value to help you visualise what happens as you screw the cap down. Values between 201.64° and 172.25° should show how the clearance moves from one flank pair to the other. Hopefully this demonstrates that it doesn't matter whether there's more clearance between one flank pair than the other. I have added the rotation angle as a user parameter to make it easier to edit the value and immediately see the effect.

Hope this makes sense.

See attached f3d file.

[MRWakefield]

Was my previous post useful @michael_ray_neal or do you require more assistance?

[michael_ray_neal]

Perfect.  Love the answers you gave.  Sorry it's taken me so long to test the results.  Great support from you.

[MRWakefield]

You're welcome, glad I could help. BTW, you appear to have marked your own reply as the solution! If you could mark one/some of mine and @etfrench's as the solution(s) that would be favourable. Thanks.

[MRWakefield]

Sorry to be a pain. I'm sure it's a slip-up but now two of your posts have been accepted as solutions rather than posts by myself and @etfrench !

[michael_ray_neal]

Crap. lol. 
how do I fix that as I don’t see a way to do that.