Modeling 1 1/4 -8 UNC threads gets "unable to create tool body"

[sprior913]

I'm trying to model a woodworking lathe faceplate centering jig to be 3D printed.  My Nova DVR XP lathe spindle is 1 1/4"-8 TPI.  At first I selected "ANSI Unified Screw Threads" 1 1/4"-8 UN fully modeled threads and the 3D print didn't fit.  Then I checked and saw that I should have used 1 1/4"-8 TPI UNC, but when I try to create those threads Fusion gives me an "unable to create tool body" error.  Since I'm creating a design for 3D printing I believe I have to use fully modeled threads, so I don't understand what's the problem.  I've designed and 3D printed screw threads before down to 1/4-20 and they worked fine so I don't think the issue is on the 3D printing side.

Can anyone explain why Fusion doesn't want to fully model the UNC version of the threads. or correct how I'm specifying the threads for the spindle?

[aaronkcap]

Can you post the model please, I made a test piece and was unable to replicate the issue on my end. I would love to be able to test it and see whats happening.

[sprior913]

Attached, the model was so simple I didn't think to add it.

[sprior913]

I recreated the problem on a new drawing by:

1. creating a cylinder with a 1 1/4" diameter and some short length.
2. selected the outside of the cylinder and selecting create thread, checked the modeled box (it doesn't fail if this is not checked, but I need it), "ANSI Unified Screw Threads" 1 1/4"-8 UNC (not just UN).  That's when it fails.  It does not fail if I select 1" 1/4"-8 UN" or don't have modeled checked.

[kshea9RNL8]

Doesn't help any but clearly this is a bug you have found, same exact thing happened on my end as well, no work arounds helped that were tried.

Maybe for the sake of moving on, can you use the 1 1/4x7.

[sprior913]

Does anyone know how to submit this as a bug?

[kshea9RNL8]

This shou;d help you, you have done the first part 🙂

https://www.autodesk.com/products/fusion-360/blog/how-to-best-report-issues-and-get-help/

[Anonymous]

Just encountered the same bug today while trying to model something similar for a wood lathe. Did you ever find a workaround for this?

[TheCADWhisperer]

@Anonymous 

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

[etfrench]

1.25"x8 UNC threads are in the dialog, but I don't think they look quite right.  The screencast shows how to add the start chamfer as well as modify the fit.

[Anonymous]

@TheCADWhisperer @etfrench 

Both the f3d file and a short screencast has been attached. For some reason, Fusion simply refuses to create a modeled threaded hole at 1-1/4" 8TPI UNC. Things seem to work fine when I briefly clicked around other size and thread combinations. It's just this specific hole type that's having an issue.

Any ideas?

Screencast link: https://knowledge.autodesk.com/community/screencast/3d35d2cd-6ce5-4699-8f70-3e54d9a72107

[Anonymous]

-- deleted --

[etfrench]

Yes, female thread failure repros on my machine as well. The internal thread spec is in the thread xml:

<ThreadDesignation>1 1/4-8 UNC</ThreadDesignation>
<CTD>1 1/4-8 UNC</CTD>
<TPI>8.0</TPI>
<Thread>
<Gender>external</Gender>
<Class>2A</Class>
<MajorDia>1.2404</MajorDia>
<PitchDia>1.1361</PitchDia>
<MinorDia>1.0314</MinorDia>
</Thread>
<Thread>
<Gender>internal</Gender>
<Class>2B</Class>
<MajorDia>1.25</MajorDia>
<PitchDia>1.1232</PitchDia>
<MinorDia>1.0773</MinorDia>
<TapDrill>1.125</TapDrill>
</Thread>
</Designation>

A workaround is to model the male thread, then use Combine/Cut to create the female thread.  You can use Press/Pull as shown in the screencast to adjust the fit.  Press/Pull also works on the thread flanks, so you should be able to create a tight fitting 3d printed thread with a little trial and error.

@jeff_strater Is this a known bug?

[etfrench]

It looks like the bug is in the xml file or the code which uses the pitch diameter when calculating the internal thread.

Modifying the internal pitch diameter from 1.1232 to 1.1420 allows the internal thread to be created. 

Attached is a modified thread xml file.  See this site for instructions on installing the file.

I don't think the pitch diameter should affect the actual shape of the thread form, but it will probably affect simulations and stress analysis.

[Anonymous]

@etfrench @TheCADWhisperer 

Thanks foe your suggestions. The both seem like viable solutions. I’ll give them a try!

[etfrench]

The pitch diameter does affect the thread profile.  It should be at or close to the midpoint of the thread flanks.  Using Push/Pull on the top and roots of the threads will change the effective pitch diameters.  If you can measure your lathe's threads top width and root width accurately, you can change the settings in the xml to give you an accurate model.

[MRWakefield]

I think the presence of the 1¼-8 UNC thread in the F360 file is a mistake and it should probably be removed (or ignored!). As far as I can ascertain, the correct designation is 1¼-8 UN (which is also present in the thread file and with what look like correct figures to me - confirmed by Machinery's Handbook 26th ed. and other sources).  1¼ UNC is 7 TPI (not 8 ) and is also present in the thread file.

BTW, there's a thread calculator here if you want to verify the Fusion figures or if you need a custom thread combination.

So anyway, I'd just use the 1¼-8 UN and ignore the 1¼-8 UNC designation as it's clearly wrong and as far as I can tell shouldn't even be in there.

I hope this helps (and makes sense!).

[etfrench]

I agree with you. The 1 1/4x8 UNC threads are poorly formed.  When 3d printing, it would be better to start with the 1 1/4 UN threads, then resize until the printed thread fits properly.  The resizing is dependent on the individual 3d printer and how well it is calibrated.

[MRWakefield]

Absolutely, 3D printing is way less accurate than machining so it's pretty normal to have to 'adjust' the values to suit your individual printer. I'm afraid it's probably going to mean a bit of trial and error before he gets the required fit.