Enlarging a 3/8npt thread.

[VSE]

Hi all,

I have a question related to tapping.

I have an aluminum part that has two 3/8NPT threaded holes. They work fine with Earl's NPT to AN adapters. The only problem is that most of these adapters (including Russell's and XRP's) have higher profiles. I need adapters that can mate closer to the part surface.

I can think of two ways around this:

1. Tap the 3/8NPT threads deeper so that I can tighten the 3/8NPT adapters deeper as well.

2. Enlarge/re-tap the 3/8NPT threads to, say, M18x1.5 threads and use metric adapters, which will mate with the surface, thus lower height profile.

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Option 1 is not feasible unfortunately due to a limited bottoming issue with the 3/8NPT tap. Drilling the hole further is not an option unfortunately.

I am hoping for Option 2. My reasoning is based on this. 3/8NPT threads have a 1.5 pitch, much like the pitch in M18x1.5. Also since 3/8NPT OD ranges from 16.4mm to 17.5mm, I am guessing tapping for a M18x1.5 should work.

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Does anyone has any prior experiences with this? Any advice,comments greatly appreciated.

Thank you for reading.

 

[hydtools]

Given the choice I would change to an SAE o-ring port. -8(1/2 inch), 3/4-16 thread.

http://www.zeroleak.com/specs/ports.html

Ted

 

[MikeHalloran]

Option 1 becomes possible, for re-tapping, if you trim the point of your tap, making it more closely resemble a bottoming tap.

Or I may have misunderstood the problem, again.



Mike Halloran
Pembroke Pines, FL, USA

 

[mrainey]

You can also buy short-projection pipe taps that let you tap deeper without bottoming out.

Software For Metalworking

http://closetolerancesoftware.com

 

[MikeHalloran]

OR,
if you are the producer of the part, you can use your CNC mill to mill the threads.


Mike Halloran
Pembroke Pines, FL, USA

 

[VSE]

@ MikeHalloran : you are spot on with grinding down the tap. I thought about that but just wasn't sure if it was feasible.

@ mrainey : I think the short projection is the solution! I didn't know it's called just that. I kept doing a search on bottoming npt tap and google must have missed the link to short projection.

Thanks all for the replies. I will post with results once I have the short projection tap.

 

[VSE]

All,

The short projection tap came in (mcmaster 2553A14) and from first look, this SP tap looks just like a standard tap. I don't see the 'shortness' of it.

I tried the SP tap and it didn't go any further than the standard tap.

Here's a pic. Guess which one is the SP tap?

http://farm3.static.flickr.com/2748/4479458925_0b3398a5cf_o.jpg

 

[VSE]

Think I found my answer...

I found this from Greenfield taps.

http://www.cuttingtoolsupply.com/Greenfield/Taps/PipeTaps/ShortProjection/ThreeProjectionLengths.htm

Turns out McMaster carries only the short version and that was what I purchased. What I really needed is a extra short version.

 

[JARUK]

I have had a similar problem and the easiest solution, as previously mentioned, is to grind some of the lead off the tap.
This means that, as it is a tapered thread, you will start cutting material before you hit the bottom of the hole.
Good luck and I hope that helps.

 

[11echo]

Well I concur on trimming off the nose of a NPT tap, BUT I wonder if you'll need to trim of the front section of your adapters? When you use a tap to cut pipe threads the tap "fully forms" a section which is the normal thread engagement. But now your shorting the tap to open up the mouth of the tapped hole ...but not chasing the threading down deeper. SO your adapter "may" bottom out on the threads in the same old spot!?? ...Just "thinking out loud", Good Luck!