Incorrect NPT hole

[martins]

Hello,

It appears that the hole definition for 1-1/2 in. NPT holes is incorrect. I created a simple NPT hole in a body. The result is a top hole diameter of 1.753, a bottom hole diameter of 1.703 with a hole dept of .681. That is an effective taper of 2.1 degrees, not the required 1.78 

Oops. 

I assume that's a bug. Is there a proper place to post bug reports?

Martins.

[blair]

The taper for American Nations Standard Pipe Tread is: 1 in 16 or 0.75" per foot. The corresponding half-angle of taper or angle with the centerline is 1 deg, 47min.

Internal Thread Dia:

Minor Dia:

Min: 1.785

Max: 1.794

Machinist Handbook: 27th edition

[martins]

Blair,

I found a reference in the 29th machinery handbook for "American National Standard Straight Pipe Threads for Mechanical Joints, NPSM and NPSL, ANSI/ASME B1.20.1-1983" with the thread diameters you listed. I don't believe those apply to NPT thread. There is a standard diagram showing the basic dimensions of the NPT thread. The E1 dimension ( defined as 'the pitch diameter at the large end of the the internal thread end at the gaging notch" defines the big hole size. I see that is 1.82234. Subtract a thread height and you should get the required minor diameter.

Anyhow, I'm not too concerned with the exact diameter measurements. To mill a part that gages properly I can adjust offsets to get the target size. The wrong taper angle is troublesome because HSM ( the CAM program that integrates with Inventor) uses the taper angle of the hole to generate the tool path. So, in order to properly mill the part, I have to manually define the hole instead of using the built-in hole function. What a PITA.

Martins.

[blair]

For north American pipe thread, it will be the same taper for pipe as well as fittings. There is a standard for British pipe thread, which is very close (if not the same) to the N-A standard.

[martins]

Yes. I know that. The point is that the hole taper is improperly defined.

I'm confused about why are you bringing up information unrelated to the point of the conversation. Why mention BSP? I was also confused why you mentioned the hole diameters in your first comment when my contention was with the hole taper value. Granted there is a specification for the accuracy of the taper angle, but it's unrelated to the data you brought up. Please explain.

Martins.

[SBix26 · Accepted solution]

If I measure the angle of the 1-1/2 NPT hole directly (sketch plane centered in the hole, Project Cut Edges), it measures exactly the right angle within a few thousandths of a degree.

Sam B

Inventor Professional 2018.1.1
Vault Workgroup 2018.0
Windows 7 Enterprise 64-bit, SP1

[martins]

Sam,

Thank you for posting this. You are correct. It took me a while to find my error. It turns out that the hole has a little lip at the transition between the sloped section and the straight section. I was measuring the wrong diameters. The .007 in. difference in diameter caused the error. Again, thanks for pointing this out.

Thinking about this further, the lip is (probably) there because the minor diameter is cut .007 oversize. The numbers make sense that way. Apologies for the error. 

Martins.