How does Inventor choose a cosmetic thread?

[ethan_howardMMXXIV]

I modeled an internal 1-1/2 NPT thread to the numbers shown below for "Tapered Pipe Thread NPT" but when choosing the cosmetic thread, it jumps to 2" NPT thread. Is there a reason for this? How does Inventor decide, given the thread.xls file, since there is a big dimensional difference between 1-1/2 and 2 NPT? Thank you.

Autodesk Inventor Professional 2024

64-Bit Edition

Build 272, 2024.2.1 Update - Date: Thu 2/8/2024

Autodesk Vault Professional 2024

Build 29.2.88.0

2024.2.1 Update

[CGBenner]

@johnsonshiue do you know the criteria used by Inventor for this?

[chris]

@ethan_howardMMXXIV This is a normal thing, IV usually presents the size two sizes larger for me. It's weird, but I've just learned to make sure I choose the right thread size in the dialog

[johnsonshiue]

Hi! I am sorry I don't know the logic behind it. I need to work with the project team to understand it better.

Many thanks!

[CGBenner]

@ethan_howardMMXXIV Hello, do you still need help with this question?

[ethan_howardMMXXIV]

Thank you for your question. If I understood how Inventor chooses a cosmetic thread, I could better model such that when I select the surface to be threaded, it would choose the correct thread. Currently, it chooses a larger thread. One person said that it chooses two threads larger. Why such a discrepancy?

[Mario.VanWiechen]

Inventor does better with convential threads as far as guessing which thread it should be. NPT is very different as you know, 1 inch pipe is 1 5/16 od so guessing becomes difficult it seems

[CGBenner]

@johnsonshiue Hi, Johnson.  Have we learned anything new about this?

[johnsonshiue]

Hi Folks,

I am sorry I don't have any further information. Let me work with the project team to understand it better.

Many thanks!

[WillL84]

Inventor never seems to guess threads properly lol. It always guesses but for me with standard threads it's ALWAYS at least 1 size too big.

[yulong_zhang5AYNC]

Hi @ethan_howardMMXXIV . 

After Dev's investigation, from the code design, the thread size by auto-selecting when picking up a conical/cylindrical face follows below rule:

1. Calculate the diameter of cone/cylinder face (if it is cone, the diameter is the mid diameter)
2. Choose the larger but closest thread size from the first Type family's group, compared with Thread Nominal Size.

So, for this case, the cone diameter is (1.7106+1.7446)/2 = 1.7276 inch. And larger but closest Thread Nominal Size is 2 NPT. That's the reason. This is by design.

Thanks.

[ethan_howardMMXXIV]

Hi Mr. Zhang, thank you for describing how Inventor works for threading. I appreciate how this community sticks to an issue. Could you tell me why choosing the closest larger thread size is "by design"? Designers ensure threads are modeled correctly by the Machinist's Handbook/standards. It would save a little time if Inventor gave us the correct cosmetic thread. Even if a Designer models the preparation of a drilled or conical hole imperfectly, it is close enough that Inventor should know which thread size to use?

[yulong_zhang5AYNC]

Thank you for your reply @ethan_howardMMXXIV . I discussed with developer. You are right about it, we also thought it should choose the closest size when the drilled or conical hole is imperfect. Developer did some fix. Thank you again for your comments.

[ethan_howardMMXXIV]

Thank you for fixing this Mr. Zhang