Adjusting NPT Thread Depth

[jonadams2002]

Hi all,

I wanted to ask about the greyed out NPT thread depth. I've been trying to size holes for NPT Flare Fittings from McMaster, and all the McMaster fittings have thread engagement that is beyond what Inventor will allow me to model. See picture below:

My question is,  can I modify the 0.3945 inch threaded segment shown in the box? Ideally I'd like to be able to bring the bottom of the threaded section of hole all the way down to the length of the flare fitting 3D model.

My apologies if this has been asked before - there were a few similar ones but nothing that seemed to ask exactly this.

Thanks!

[Mario.VanWiechen · Accepted solution]

You need to look at how pipe fittings go together. The threads are tapered to make a seal. If the face of the hex hits the surface there will be no seal. The threads on both are as they should be. Use a thread engagement of approx the thread size, eg a 1/4 NPT will go in the female thread 1/4 inch

Go to a hardware store or your shop floor and play with some pipe fittings, will make more sense

[Frederick_Law]

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@jonadams2002 wrote:

My apologies if this has been asked before - there were a few similar ones but nothing that seemed to ask exactly this.

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You're asking to change an well established industrial standard.

You'll need a really good reason and some prove.

https://en.wikipedia.org/wiki/National_pipe_thread

If you need the fitting to tighten all the way, use a different thread.

Like SAE o-ring:

https://www.mcmaster.com/fittings/thread-type~un-unf-sae-straight-/specifications-met~jis/

[swalton]

McMaster Carr normally adds a Thread Engagement dimension to the 2d prints of their fittings.  That is a reasonable value to use when modeling.  I like it better than the Hand-Tight dimension from the ASME standard.

I normally offset a workplane at the Thread Engagement dimension for each male tapered thread in my models.  I use an axis mate and that workplane to constrain the fittings in my assemblies.

[Frederick_Law]

If you really, really, REALLY want to change that, it's in thread.xls.

It will become a non-standard tap which you'll need a process/QC to verify you are getting what you want.

You'll need to spec it on drawing so machinist know what they need to do.

Also you should know that usually a sealant is used to connect NPT threads so they'll seal.

Usually it's Teflon tape.

It'll make the fitting stick out further.

You can "try" to account for that.

It'll be a good learning experience 😎

[rmheffne]

Do you know which values add up to the modeled depth? I've found that the "Useful thread length" column appears to be the correct L3 depth to the standards, but when I model some of them, they dont all actually model to the depth shown in that column.  Also, when i change the "Useful thread length"  - it doesnt change the modeled depth of the thread either. 

It seems like perhaps its looking at a combination of the values there to determine the modeled depth??? maybe the Basic minor diameter at small end and the taper angle? 

The 3/8ths NPT is the biggest discrepancy right now, as its listing .407 deep in the chart, but only models to about .360" deep.   I use these models to program my thread milling cycles and need the taper to go down to the correct depth.

**Update: I had changed the Tap drill standard to be more correct/useful and that did change the modeled depth. Problem solved. 

@Frederick_Law 

[Frederick_Law]

Yes, there are more than one place to set everything up.

Machine one and check.

Some CNC stop advancing when pull out.

It make concentric circle instead of spiral.

I need to hand tap a few hundreds parts ..... lot's of screwing around.