Threading w/ Offset Shrinks Original Diameter... how to calculate this?

[glencandle]

When I create a hole with diameter .875" and then add threads sized .875", with an offset (a pictured), I get a smaller diameter.  I understand why, because the threads are extruded from the diameter, and the offset is reflecting this newly created diameter.  

What I'd like to know is the math behind this. How is the thread thickness (not threads per inch, but the distance they are extruded from the surface) calculated?

The new diameter is .113" smaller than it was before adding threads.  Which means the threads are being extruded a distance of .0565" from the inner surface.  Where does this number come from? 

And do I have any control over this?  What if I want all things the same but I want the threads only to extrude to a distance of .04, resulting in a new diameter of .795"?

[TheCADWhisperer]

Out on the shop floor - what size drill do you use as tap-drill for a 7/8-9 UNC thread?

Do you own a Machinery’s Handbook?

For the math formulas check the Handbook or search Wikipedia for standard thread nomenclature (in this case Unified National Stanard - coarse).

Edit: Are you Extuding circles and then adding threads?  Why not use the Hole command with Tapped option?

[glencandle]

Thanks, I don't have a handbook, I have no machine experience at all, I am just making designs and sending them out to CNC shops.  Thank you for the suggestion of the handbook, I will have to get one of those!

I am extruding circles because my threads are only meant to begin about half way down the well.  But the end result would be the same regardless, no?

[jhackney1972 · Accepted solution]

If you are using the Hole Command, the data for the tap hole comes from, in your case, the ANSIUnifiedScrewThreads.xml file currently located (according to the latest version) in this folder. (Windows 10)

[glencandle]

Oh wow, thanks! 

Can this file be modified? 

Why aren't these settings accessible within Fusion?  Would Inventor have more options in this case?

[jhackney1972]

They are editable but I will remind you, that every time Fusion 360 updates, a new version is written.  The folder is that nasty part of the file path I posted earlier.  Some people who modify their thread data use an Add-In called ThreadKeeper which preserves the table between updates.

The XML file is accessible via the path I post earlier but it is not available to edit from the application.  This is the same with Inventor, it is an external file so modifications to the thread table are basically handled the same.  Inventor does not update automatically, that is the big difference.

[TheCADWhisperer]

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@glencandle wrote:

I am extruding circles because my threads are only meant to begin about half way down the well. 

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I recommend that you Attach your file here.

Split Face or cut the counterbore after the thread.

I strongly recommend that you Attach your file here.

Based on your level of experience and stated goal - i very strongly recommend that you DO NOT fool with the thread data.  I am surprised that @jhackney1972 would even make that suggestion without more information about your true Design Intent.

Where did you come up with the number .875 for the counterbore? (I think I know the answer to this - but I ask the question to promote critical thinking.)

[glencandle]

Thanks to your "use a hole with a tapped option" suggestion I realized I can do everything I need in one go.  So thank you for that! 

As requested here is the project:

https://a360.co/3wmPG8j

I would still like to be able to customize my threads though, I want a very coarse thread and 7/8-9 (the coarsest I can find in Fusion) is still not enough.  I would like a 5 or 6 option.  Ideally I would love to make a part that threads all the way down with one half turn.

Thinking I might mess with the XML file and just keep a copy of the original just in case I get into trouble.

The way I came up with the .875 size is because I'm digging a hole into a 1.125" cube and 7/8" is pretty much the coarsest thread I can work with that doesn't cut through my wall.  If you have a better idea I'd love to hear it!

[TheCADWhisperer]

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@glencandle wrote:

As requested here is the project:

https://a360.co/3wmPG8j

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A picture does no good.

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

I now understand your Design Intent for a high lead pitch.

Once I see your file we can discuss possible "solutions".

I recommend that you communicate with your shop as well as this will be a non-standard thread.

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@glencandle wrote:

Ideally I would love to make a part that threads all the way down with one half turn.

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Now you are talking about Pitch TPI (lead), not major or minor diameter of the thread.

[glencandle]

Sorry about that, I thought that was a link to the project file, not just an image.  Here ya go: