Threads too large

[john_meyerPN8R4]

I have created the world's simplest part: an extension tube for the recessed oil filler opening on a Honda EU2200i portable generator. Here's a rendered drawing of what it looks like:

Honda EU2200i Oil Filler Extension

I have selected ISO Metric Profile, Size 30.0 mm, M30x2, Class 6g, Full Length, Modeled.

The problem is that the threads print 0.3 mm too large and it therefore doesn't fit. The Fusion 360 inspector shows the diameter of the thread to be 29.8 mm. This is exactly the size my Ender 3 prints (it is nicely calibrated); my calipers measure precisely this 29.8 mm diameter. However, the Honda dipstick threads measure 29.5 mm.

I have verified from many sources that the dipstick opening is indeed 30mmx2. Therefore, is there a flaw in the Fusion 360 modeling? Is the Fusion 360 library correct? Have I chosen the wrong thread model (I have looked through all the other metric options, but maybe I missed something). Or, am I using the thread tool incorrectly?

Is there a way to modify Fusion to give me the threads I want?

Since this is such a simple print, I can obviously scale this down in my slicer, but I'd prefer to do this correctly.

I've attached my f3D file, in case anyone has a moment to look and advise me.

Thanks in advance, and Happy Holidays to everyone!

[seth.madore]

Printed threads seldom actually mesh up with manufactured threads. You have a couple of options;

1) It's often that the 60 degree portion of the thread is what's binding up. Use the Press/Pull command and adjust that a bit and reprint.

2) Edit the XML document to create a custom thread with the values you think would work best.
https://knowledge.autodesk.com/support/fusion-360/learn-explore/caas/sfdcarticles/sfdcarticles/Custo...

[john_meyerPN8R4 · Accepted solution]

Thanks for the quick reply. I'll check the thread angle more carefully. I may have a thread gauge which will check not only pitch but angle as well. The pitch (the 2mm portion of the 30x2 spec) looks to be perfect.

I'm printing just the threads (without the long cylinder) and have scaled the x & y down by 1% in my slicer. If it is simply a diameter issue, I should be able to screw these threads into the opening. However, if it is the 60 degree angle you mention, then it will still bind.

Thanks for the link to the custom thread design instructions. That will be my next step.

[edit]The part which I scaled in Cura to 99% (X&Y only, keeping Z at 100% to preserve thread pitch) worked perfectly. It gave me precisely the correct outside thread diameter. I suspect the Fusion 360 library may not be correct, but I am an EE, not an ME, so I am not familiar with these things.

[g-andresen]

Hi,
use the offset face tool and edit the thread a bit

günther

[etfrench]

The Fusion 360 values are right at the middle of the tolerances.  The Honda thread is below the minimum tolerance if you're measuring the external threads.  It is barely over the minimum if you're measuring the internal threads.

This site lists the specs for metric threads.

[john_meyerPN8R4]

I hadn't thought about the offset face tool. I selected Offset Face from the Solid-->Modify section. I zoomed in and selected the face which is the top of the thread. I was then able to reduce the diameter of the thread. Pretty interesting. However, this has the negative side effect that what it actually does is "saw off" the top of the threads, just as if you rotated the part in a lathe and ground off the tops of the threads. I don't see how this can work, because the threads won't seat very well. It's very much like transferring 78 rpm records with a truncated needle (something that is done by those who transfer and restore 78s). In that case it is what you want because the goal is to keep the needle from going all the way down into the groove which has been eroded by age and wear. In this case, you want contact for the entire depth of the thread.

So, perhaps I misunderstood the idea, but I don't think this will work.

[john_meyerPN8R4]

Thanks for the link to the thread specs. I was looking for that earlier today and now that I have it, I'm bookmarking it. Very useful.

[etfrench]

You can use Offset face on the flanks as well as the top and bottom of the thread. 

[john_meyerPN8R4]

Thank you.